A plane is standing on runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).
The question is:
Will the plane take off or not? Will it be able to run up and take off?
No, it will never take off.
Unless it is vertical take off plane
Unless it is vertical take off plane
The plan can't take off because it requires lift gained from the aerodynamics of the wings and the air effecting it. It gains lift by increasing speed and the air forcing it upward. If the plane is moving along a conveyer belt that doesn't allow any displacement of the craft itself, then the air speed required to lift the plane isn't there. Hence no take off.
Hi all,
My 2¢, the plane will be able to take off, as long as its source of propulsion is independent of ground speed (I'm not aware of any aircraft that use driven wheels to achieve flight). As the plane gains speed on the belt, the belt will cause a slight increase in drag from friction in the wheel bearings, but the load on the engine is largely due to the airflow around the plane. The plane could take off, it would just need to use a bit more power. Now if this were a sea-plane under similar conditions I'm not so sure it could take off. The friction of moving the floats and fuselage through the water is probably pretty substantial. Untill next time...
My 2¢, the plane will be able to take off, as long as its source of propulsion is independent of ground speed (I'm not aware of any aircraft that use driven wheels to achieve flight). As the plane gains speed on the belt, the belt will cause a slight increase in drag from friction in the wheel bearings, but the load on the engine is largely due to the airflow around the plane. The plane could take off, it would just need to use a bit more power. Now if this were a sea-plane under similar conditions I'm not so sure it could take off. The friction of moving the floats and fuselage through the water is probably pretty substantial. Untill next time...
Unlike automobiles, jet engine planes doesn't rely on friction force between wheels and conveyer belt. It will start moving and take off.
hell yes!
QUOTE (Guest+Jul 19 2005, 02:06 PM)
Unlike automobiles, jet engine planes doesn't rely on friction force between wheels and conveyer belt. It will start moving and take off.
How the hell will it start moving if the band will compensate for its movement???
How the hell will it start moving if the band will compensate for its movement???
This could make airports a whole lot smaller.
QUOTE (Turikon+Jul 19 2005, 05:11 PM)
How the hell will it start moving if the band will compensate for its movement???
It cannot compensate, because there is no force feedback between airplane jet engine and the belt, except small amount of friction in the wheels
It will take off! But I wouldn't give much for it landing safetly. The tires most likely would have burst/burned on take-off, having rotated at perhaps twice their normal speed on the belt. Don't try this one!
The plane will not take off, because for an airplane to take off, there has to be movement. This is just like a car going 60 and a conveyer belt going 60. Since the car is not moving, an airplane woudn't either. But for an aiplane to stay up, there has to be enough air to lift up the wings.
Yes, the plane will take off, practically as it normally would. If you couldn't see the "runway" moving, you would see no difference to a normal takeoff, unless you could discern that the plane's wheels were spinning twice as fast as normal.
The reason: The plane, even if on the ground, accelerates relative to the AIR! Thanks to its wheels there is very little friction between it and the ground it stands on, so what the ground does is of almost no importance.
Now, if you would put wings on a car, that would be different. The takeoff would not work, because the car uses its wheels to push itself forward relative to the GROUND. It would not achieve any speed relative to the air, so its wings could create no lift.
The reason: The plane, even if on the ground, accelerates relative to the AIR! Thanks to its wheels there is very little friction between it and the ground it stands on, so what the ground does is of almost no importance.
Now, if you would put wings on a car, that would be different. The takeoff would not work, because the car uses its wheels to push itself forward relative to the GROUND. It would not achieve any speed relative to the air, so its wings could create no lift.
The plane cannot take off. the lift on the plane is due to air speed. the plane must move through the air.
I'll add to my earlier post. (tires burn.....)
The plane most certainly will take off! Thrust from its engines acts on the surrounding air--has nothing to do with the ground or the belt the plane is on.
Said another way, the belt speed will always be to slow, will not hinder the planes acceleration ALONG THE BELT untill take-off speed is reached. The plane will ALWAYS ACCELERATE ALONG THE MOVING BELT, so long as the engine thrust is high enough to overcome belt/tire drag.
Of course the belt will have to be long enough for the plane normal take-off distance!
As I said earlier, kiss those tires goodby!
The plane most certainly will take off! Thrust from its engines acts on the surrounding air--has nothing to do with the ground or the belt the plane is on.
Said another way, the belt speed will always be to slow, will not hinder the planes acceleration ALONG THE BELT untill take-off speed is reached. The plane will ALWAYS ACCELERATE ALONG THE MOVING BELT, so long as the engine thrust is high enough to overcome belt/tire drag.
Of course the belt will have to be long enough for the plane normal take-off distance!
As I said earlier, kiss those tires goodby!
Imaging a rope attached to the plane, that pulls him forward. Whatever belt is beneath it, the plane will still move forward. Jet engine thrust is just like the rope.
The only thing action that enables the plane to fly is a laminar flow of air across its wings. If the plant is moving at the minimum take off speed relative to the wind, it will fly, however that happens. In this case if the plane, sitting on a conveyor, can be accelerated to that minimum speed the plane will fly. But what is the point. Can the conveyor accelerate the plan faster, ie in a shorter distance? The navy has used steam catapults on carriers to achieve that end. 
The plane may appear to have a ground speed of 400 mph, but it has an air speed of 0 knots, and air speed is what matters. It will not take off.
QUOTE (Catpewk+Aug 9 2005, 08:44 PM)
The plane may appear to have a ground speed of 400 mph, but it has an air speed of 0 knots, and air speed is what matters. It will not take off.
I never seize to be amazed at how many people dont read past the first post...
I never seize to be amazed at how many people dont read past the first post...
all i know is..........i'm not gettin on 'that' plane!! 
THE CONVEYER AND PLANE ARE GOING IN THE SAME SPEED BUT OPPOSITE DIRECTION SO WONT THE PLANE JUST SIT THERE WHEEL SPINNING? IT WONT TAKE OFF AT ALL
The Harrier achieves lift. Typical jet without air flow under wings it may be difficult to achieve lift. Enough thrust in upward direction will overcome lack of "laminar airflow".
The plane will not take off. PERIOD.
Taking off requires a upward net force. This is caused by LIFT. So, with Newton helping us out a bit here, we will prove that the plane cannot take off:
Starting out, at v=0, the plane is not moving through the air, hence the total net force is still a vector pointing DOWNWARD (this is the airplane's weight). When the airplane starts moving (by providing forward thrust) the conveyor belt reacts and does not let the plane roll forward RELATIVE TO THE AIR. Velocity relative to air is STILL zero.
Key point here: if the airplane does not move THROUGH THE AIR, the lifting force is NOT generated.
And thus the airplane does not take off.
Taking off requires a upward net force. This is caused by LIFT. So, with Newton helping us out a bit here, we will prove that the plane cannot take off:
Starting out, at v=0, the plane is not moving through the air, hence the total net force is still a vector pointing DOWNWARD (this is the airplane's weight). When the airplane starts moving (by providing forward thrust) the conveyor belt reacts and does not let the plane roll forward RELATIVE TO THE AIR. Velocity relative to air is STILL zero.
Key point here: if the airplane does not move THROUGH THE AIR, the lifting force is NOT generated.
And thus the airplane does not take off.
As I see it, basically what is happening is exactly the same as if a plane were susupended in the air by a rope, and the plane' wheels are turning.
This=NOTHING HAPPENS!
If a plane is able to liftoff without moving along the ground, why would we need runways at all? Would planes not just fly straight into the air? Or for that matter, burst right out of hangers?
Obviously, you should'nt talk about VTOL aircraft....
This=NOTHING HAPPENS!
If a plane is able to liftoff without moving along the ground, why would we need runways at all? Would planes not just fly straight into the air? Or for that matter, burst right out of hangers?
Obviously, you should'nt talk about VTOL aircraft....
Folks this is a no brainer.
The aircraft has engines that through action/reaction principles cause it to accelerate in relation to the AIR not the belt.As the aircraft starts to move the belt will move in the opposite direction, the only effect this will have is to increase the speed of rotation of the tires, the plane will continue to accelerate in relation to the air and the ground beside the moving belt no matter how fast the belt moves. This may have detrimental effects on the tires but will not detrementally affect the AIRSPEED of the plane. In fact, air entrained by laminar boundary layers on the belt may actually inhance the airflow to the wings, shortening the takeoff run. Depends on how long the landing gear legs are. In the 60's a plane was built with a hovercraft type skirt instead of regular gear, this plane wouldn't even notice the increasing speed of the belt
The aircraft has engines that through action/reaction principles cause it to accelerate in relation to the AIR not the belt.As the aircraft starts to move the belt will move in the opposite direction, the only effect this will have is to increase the speed of rotation of the tires, the plane will continue to accelerate in relation to the air and the ground beside the moving belt no matter how fast the belt moves. This may have detrimental effects on the tires but will not detrementally affect the AIRSPEED of the plane. In fact, air entrained by laminar boundary layers on the belt may actually inhance the airflow to the wings, shortening the takeoff run. Depends on how long the landing gear legs are. In the 60's a plane was built with a hovercraft type skirt instead of regular gear, this plane wouldn't even notice the increasing speed of the belt
QUOTE (dirak+Jul 19 2005, 09:53 AM)
A plane is standing on runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).
The question is:
Will the plane take off or not? Will it be able to run up and take off?
You have to undestand how an airplane flies: It needs an airflow on top and below the wings. (to be more precise: the air must flow a little bit faster on top than below the wing, so that the air pressure on top of the wing is slighter LOWER than the air pressure below the wing).
Guess what's going to happen if the air pressure below the wings is higher than the air pressure on top of the wing...well, the airplane will take off.
If you don't have that airflow, the airplane will NOT take off. No matter how fast the weels run on the belt. An airplane only accelerates on a runway TO CREATE THAT AIRFLOW ON TOP AND BELOW THE WING.
If, in your belt-experiment, the airplane is not moving relative to the air, then it will stay on the ground since there is no airflow on top and below the wings.
Those who think that the airplane will take off are REALLY REALLY freaking ignorants and should not be on this forum.
The question is:
Will the plane take off or not? Will it be able to run up and take off?
You have to undestand how an airplane flies: It needs an airflow on top and below the wings. (to be more precise: the air must flow a little bit faster on top than below the wing, so that the air pressure on top of the wing is slighter LOWER than the air pressure below the wing).
Guess what's going to happen if the air pressure below the wings is higher than the air pressure on top of the wing...well, the airplane will take off.
If you don't have that airflow, the airplane will NOT take off. No matter how fast the weels run on the belt. An airplane only accelerates on a runway TO CREATE THAT AIRFLOW ON TOP AND BELOW THE WING.
If, in your belt-experiment, the airplane is not moving relative to the air, then it will stay on the ground since there is no airflow on top and below the wings.
Those who think that the airplane will take off are REALLY REALLY freaking ignorants and should not be on this forum.
Nice try but it couldn't work. As several posters have suggested, it's not to do with speed compared to the ground, but speed compared to the air.
In other words, if you do the opposite, and keep the runway still and the wheels don't move at all, but you increase the airspeed (e.g. a very powerful wind machine), the plane would lift into the air.
This effect can in fact be seen in certain high wind situations.
In other words, if you do the opposite, and keep the runway still and the wheels don't move at all, but you increase the airspeed (e.g. a very powerful wind machine), the plane would lift into the air.
This effect can in fact be seen in certain high wind situations.
I'm sorry for those who think this sounds ingorant (REALLY, REALLY freaking ignorant to some) but the speed of the conveyor has nothing to do with the airspeed of the airplane once the thrust of the engines overcomes the friction of the wheel carriage. The wheels don't drive the plane forward so you could do just about anything with the conveyor and it will have very little effect on the airplane getting up to speed and taking off.
The plane is accelerating due to the engines pushing against the air which is not being moved by the conveyor. Since the air is still acting normally the plane will get up to speed and take off.
If you want to see how it works put a cart on a moving conveyor and stand off to the side and hold the cart. The cart doesn't move backwards because you are holding it independent of the conveyor. If you push forward on the cart (just like the engines of the plane pushing on the air, not the ground) it will move forward but the wheels will be turning much faster than normal.
An airboat in a river is another great example. It goes by pushing on the air, not the water so it mostly unaffected by the river flow as long as the thrust of the fan overcomes the friction of the hull on the water. They go upstream just about as fast as downstream.
The plane is accelerating due to the engines pushing against the air which is not being moved by the conveyor. Since the air is still acting normally the plane will get up to speed and take off.
If you want to see how it works put a cart on a moving conveyor and stand off to the side and hold the cart. The cart doesn't move backwards because you are holding it independent of the conveyor. If you push forward on the cart (just like the engines of the plane pushing on the air, not the ground) it will move forward but the wheels will be turning much faster than normal.
An airboat in a river is another great example. It goes by pushing on the air, not the water so it mostly unaffected by the river flow as long as the thrust of the fan overcomes the friction of the hull on the water. They go upstream just about as fast as downstream.
sorry Guest, but you are still missing the point. Yes the engines provide thrust which propels the plane forwards.
BUT putting the plane on a rolling conveyor negates that effect, it is akin to trying to run on a frozen lake in slippy shoes. The plane will not 'get up to speed' at all, it will remain stationary. There will be no airflow over the wings, there will be no lift, no Bernouilli effect - viz no take off.
Perhaps you are getting confused with VTOL aircraft, which do not use lift to take off. They do use the pure thrust of the engines to overcome gravity - but they work by pointing the engines down, against gravity. On a conveyor, the engines are pointing backwards, not against gravity at all.
BUT putting the plane on a rolling conveyor negates that effect, it is akin to trying to run on a frozen lake in slippy shoes. The plane will not 'get up to speed' at all, it will remain stationary. There will be no airflow over the wings, there will be no lift, no Bernouilli effect - viz no take off.
Perhaps you are getting confused with VTOL aircraft, which do not use lift to take off. They do use the pure thrust of the engines to overcome gravity - but they work by pointing the engines down, against gravity. On a conveyor, the engines are pointing backwards, not against gravity at all.
For the people who still think this idea could work, here's a question for you:
Remove the conveyor belt altogether, and put the plane on a stand which keeps the wheels a little bit off the ground. How fast would you have to spin those wheels for the plane to take off?
Remove the conveyor belt altogether, and put the plane on a stand which keeps the wheels a little bit off the ground. How fast would you have to spin those wheels for the plane to take off?
Nice-Try,
I'm sorry but I am not missing the point. The wheels do not provide any thrust or motion to the airplane (except a little friction and wheels are used instead of the skids the Wright Brothers used to reduce this as much as possible) so have no real effect on the take off. Take off is achieved by the air being forced out the back of the engines which moves the body of the airplane through the air. Again, the wheels only just roll along with the body to reduce the friction between the plane and the ground. The ground can be bumpy, smooth, moving or stationary and the plane will still move.
The rolling conveyor really does not change the speed of the plane against the air or the ground. If the wheels were what was moving the plane forward then it would but the wheels only freewheel around wherever the engines acting on the air push them.
If you put the plane on a stand that is fixed the plane wouldn't move. If you allow the stand to move the plane would take off though probably not very well due to the increased drag of the stand. The engines still push on the air, not the ground, and the stand would drag along the ground with the plane until the airspeed was enough to create the lift.
Your running on the ice is not a good example because your feet are pushing you along the ground so your speed is dependent on your thrust against the ice. If you stand on that same ice and a nice heavy wind comes along and you run against it you will get more of the effect of the plane on the runway. If you are sliding due to the wind and try to walk backwards (simulating the conveyor belt) you will still slide along if the wind is strong enough to overcome the little bit of friction between your whoes and the ice. The thrust of the airplane's engines create the wind by moving the plane forward through the air whether on the ground or up in the air.
Put the same plane on skis on the same ice and get the engines up to speed. The plane will take off just like normal.
Have you ever seen a bird (usually a gull near a lake or sea shore) take off simply by jumping up into the wind? This is a good illustration of this effect. If the gull is standing still and the wind is blowing, the ground is still in relation to the bird (the conveyor moving at the same speed as the plane) the the bird will still fly because the air is not still in relation to the bird.
It is completely different from a car that needs to push on the ground with its wheels to move or a person walking which need their feet to push on the ground to move.
I'm sorry but I am not missing the point. The wheels do not provide any thrust or motion to the airplane (except a little friction and wheels are used instead of the skids the Wright Brothers used to reduce this as much as possible) so have no real effect on the take off. Take off is achieved by the air being forced out the back of the engines which moves the body of the airplane through the air. Again, the wheels only just roll along with the body to reduce the friction between the plane and the ground. The ground can be bumpy, smooth, moving or stationary and the plane will still move.
The rolling conveyor really does not change the speed of the plane against the air or the ground. If the wheels were what was moving the plane forward then it would but the wheels only freewheel around wherever the engines acting on the air push them.
If you put the plane on a stand that is fixed the plane wouldn't move. If you allow the stand to move the plane would take off though probably not very well due to the increased drag of the stand. The engines still push on the air, not the ground, and the stand would drag along the ground with the plane until the airspeed was enough to create the lift.
Your running on the ice is not a good example because your feet are pushing you along the ground so your speed is dependent on your thrust against the ice. If you stand on that same ice and a nice heavy wind comes along and you run against it you will get more of the effect of the plane on the runway. If you are sliding due to the wind and try to walk backwards (simulating the conveyor belt) you will still slide along if the wind is strong enough to overcome the little bit of friction between your whoes and the ice. The thrust of the airplane's engines create the wind by moving the plane forward through the air whether on the ground or up in the air.
Put the same plane on skis on the same ice and get the engines up to speed. The plane will take off just like normal.
Have you ever seen a bird (usually a gull near a lake or sea shore) take off simply by jumping up into the wind? This is a good illustration of this effect. If the gull is standing still and the wind is blowing, the ground is still in relation to the bird (the conveyor moving at the same speed as the plane) the the bird will still fly because the air is not still in relation to the bird.
It is completely different from a car that needs to push on the ground with its wheels to move or a person walking which need their feet to push on the ground to move.
There's no doubt that the engines would provide thrust regardless of what the wheels are doing. This thrust would normally have the effect of making the plane roll forward on its wheels. It would gradually pick up speed, and as it does so, air moves over the wings, and as the wings are curved on top, the air has to travel further over the top of the wings than it does over the bottom.
in effect, the air is becoming 'rareified' above the plane, and this is what provides lift to counter the effect of gravity and allow the plane to fly. In a way, the pressure difference sucks up the plane by its wings.
When a bird takes off from standing, it does so by flapping its wings, to provide downward thrust. Once it is up and moving forward, it can glide using the same principle as a plane.
You said that if the plane is on a stand, it would still move forward but not very well, however this suggests that you do really understand that a plane must move forwards in order to take off if it is to use lift alone.
Do you agree that keeping a plane stationary and using a very strong wind machine to create airflow over the wings would allow that plane to take off without rolling forwards?
If you do, then you'll see that the reverse of having no airflow but movement over the ground (or conveyor belt) CANNOT give the plane the lift it needs to fly.
in effect, the air is becoming 'rareified' above the plane, and this is what provides lift to counter the effect of gravity and allow the plane to fly. In a way, the pressure difference sucks up the plane by its wings.
When a bird takes off from standing, it does so by flapping its wings, to provide downward thrust. Once it is up and moving forward, it can glide using the same principle as a plane.
You said that if the plane is on a stand, it would still move forward but not very well, however this suggests that you do really understand that a plane must move forwards in order to take off if it is to use lift alone.
Do you agree that keeping a plane stationary and using a very strong wind machine to create airflow over the wings would allow that plane to take off without rolling forwards?
If you do, then you'll see that the reverse of having no airflow but movement over the ground (or conveyor belt) CANNOT give the plane the lift it needs to fly.
If the plane wasn't physically moving there would be no movement and no airflow over the wings. The problem is that the plane has wheels and they will roll in the direction the thrust pushes them regardless of what the ground is doing.
I used the gull analogy to show that a bird can take off into the wind without flapping its wings. It is very easy to observe this at a beach when the wind is kicking around. There is no ground speed but plenty of relative airspeed so the bird just spreads its wings and pops up into the air. It's also what allows them to hover over the ground in the wind. Gulls are very good at this also. No ground speed but they are certainly in the air with the relative airspeed due to the wind.
If the conveyor was able to move the plane over the ground to create no airflow (which means the conveyor is stationary otherwise there is airflow in some direction) what stops the engines from pushing on the air and rolling the plane on its wheels? The wheels will still roll in the direction the engines push them. There is nothing to stop the plane from rolling and coming up to speed.
I'm not trying to sound like a *** here but there is more to airflow over a wing to create lift than most of us were taught. It wasn't until I was trying to teach my son about how planes fly that I found the information in some of my college texts. Using the theory you state (which is what we all learned in school) a wing with a curved upper side and straight lower side will generate lift like you say. The problem comes in when the wing isn't shaped like that. Most acrobatic capable planes (from little bi-planes to fighter jets) have symmetrical airfoils so the air travels the same distance over both top and bottom. Also, the Wright Brothers and most followers for many years didn't have bottom surfaces on their wings. They only had one layer of covering so the air travelled the same distance there also. Angle of attack into the oncoming airstream must be present in these circumstances for lift to occur. It is a combination of both things that make planes fly. I personally wish they would teach both halves of this theory in school.
I used the gull analogy to show that a bird can take off into the wind without flapping its wings. It is very easy to observe this at a beach when the wind is kicking around. There is no ground speed but plenty of relative airspeed so the bird just spreads its wings and pops up into the air. It's also what allows them to hover over the ground in the wind. Gulls are very good at this also. No ground speed but they are certainly in the air with the relative airspeed due to the wind.
If the conveyor was able to move the plane over the ground to create no airflow (which means the conveyor is stationary otherwise there is airflow in some direction) what stops the engines from pushing on the air and rolling the plane on its wheels? The wheels will still roll in the direction the engines push them. There is nothing to stop the plane from rolling and coming up to speed.
I'm not trying to sound like a *** here but there is more to airflow over a wing to create lift than most of us were taught. It wasn't until I was trying to teach my son about how planes fly that I found the information in some of my college texts. Using the theory you state (which is what we all learned in school) a wing with a curved upper side and straight lower side will generate lift like you say. The problem comes in when the wing isn't shaped like that. Most acrobatic capable planes (from little bi-planes to fighter jets) have symmetrical airfoils so the air travels the same distance over both top and bottom. Also, the Wright Brothers and most followers for many years didn't have bottom surfaces on their wings. They only had one layer of covering so the air travelled the same distance there also. Angle of attack into the oncoming airstream must be present in these circumstances for lift to occur. It is a combination of both things that make planes fly. I personally wish they would teach both halves of this theory in school.
Apologies if I was stating the obvious at all! I think we both actually are in agreement, just coming from opposite angles!
The way I understood the original post was "could a plane take off if you turn the engines up to full but stop the plane actually moving forwards by means of a conveyor belt underneath it?" The conveyor would roll backwards at the exact same speed as the plane would otherwise move forwards, so there's no net movement of the plane w.r.t. the air.
You make some interesting points. I have also seen some of those acrobatic planes in action, in one case the plane was deliberately stalled, and pointed vertically upwards. The sheer power of its engine was enough to actually allow it to just hang there, and then even start to climb vertically - power to weight and all that.
Also symetrical wings which can work if they are angled against the airflow. I guess that's a different form of lift - actually translating the horizontal airflow into a downward one? I'm no physicist though! Just an average curious human
The way I understood the original post was "could a plane take off if you turn the engines up to full but stop the plane actually moving forwards by means of a conveyor belt underneath it?" The conveyor would roll backwards at the exact same speed as the plane would otherwise move forwards, so there's no net movement of the plane w.r.t. the air.
You make some interesting points. I have also seen some of those acrobatic planes in action, in one case the plane was deliberately stalled, and pointed vertically upwards. The sheer power of its engine was enough to actually allow it to just hang there, and then even start to climb vertically - power to weight and all that.
Also symetrical wings which can work if they are angled against the airflow. I guess that's a different form of lift - actually translating the horizontal airflow into a downward one? I'm no physicist though! Just an average curious human
I agree that we sort of agree.
I see the question as can a moving conveyor stop a plane. I agree totally that if the plane isn't moving it won't take off. My side was that the conveyor is just like ground in that the plane just rolls along it. If it's moving with the plane it wouldn't take off any sooner than if it wasn't moving or moving backwards. Of course the little friction the wheels provide would cause some difference but that is negligible.
If you word it as you just did we would agree.
Wierd how a slightly different interpretation can put people at complete opposite ends of things.
I see the question as can a moving conveyor stop a plane. I agree totally that if the plane isn't moving it won't take off. My side was that the conveyor is just like ground in that the plane just rolls along it. If it's moving with the plane it wouldn't take off any sooner than if it wasn't moving or moving backwards. Of course the little friction the wheels provide would cause some difference but that is negligible.
If you word it as you just did we would agree.
Wierd how a slightly different interpretation can put people at complete opposite ends of things.
The plane WILL take off.
The previous comments are correct; whether or not the ground below it moves, the engines on the plane will make it move forward.
Think of the opposite: A plane flying at 200 MPH into a 200MPH headwind stands still over the ground, but is still flying perfectly fine.
Of more interest to me is whether the friction on the wheels of the moving belt is less than the induced airflow along the surface of the belt; the greater force will determine whether the plane gets off sooner or later than before.
The previous comments are correct; whether or not the ground below it moves, the engines on the plane will make it move forward.
Think of the opposite: A plane flying at 200 MPH into a 200MPH headwind stands still over the ground, but is still flying perfectly fine.
Of more interest to me is whether the friction on the wheels of the moving belt is less than the induced airflow along the surface of the belt; the greater force will determine whether the plane gets off sooner or later than before.
QUOTE (dirak+Jul 19 2005, 09:53 AM)
A plane is standing on runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).
The question is:
Will the plane take off or not? Will it be able to run up and take off?
The variable here is friction. If we assume that there is no friction from the wheel bearings, then it makes no difference what speed the 'ground' [conveyor] is moving: the aircraft will advance and ultimately take off. IF the friction of the moving surface is enough to overcome the thrust of the engines, then the craft will not move and not take off. I don't think the friction is that great, but I agree with an earlier post that there would be some hellacious wheel spin!
The question is:
Will the plane take off or not? Will it be able to run up and take off?
The variable here is friction. If we assume that there is no friction from the wheel bearings, then it makes no difference what speed the 'ground' [conveyor] is moving: the aircraft will advance and ultimately take off. IF the friction of the moving surface is enough to overcome the thrust of the engines, then the craft will not move and not take off. I don't think the friction is that great, but I agree with an earlier post that there would be some hellacious wheel spin!
To carry it further: if there is no friction in the wheels, and the runway starts moving, then the aircraft remains motionless. Then add thethrust of the engines, and the plane moves forward.
QUOTE (dirak+Jul 19 2005, 09:53 AM)
A plane is standing on runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).
The question is:
Will the plane take off or not? Will it be able to run up and take off?
are you sure you guys understand the experiment?
It says: "This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction)"
That means that someone, say on the control tower, watching the experiment would see the airplane ALWAYS on the same spot. That means the airplane has no velocity relative to the air.
No velocity relative to the air MEANS no airflow around the wings MEANS no lift MEANS NO TAKE OFF.
@ Guest: the exaust gas coming from the jet engine IS NOT pushing against the air, actually a jet engine would work better in a vaccum (assuming there would be a device to feed in oxygen).
The question is:
Will the plane take off or not? Will it be able to run up and take off?
are you sure you guys understand the experiment?
It says: "This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction)"
That means that someone, say on the control tower, watching the experiment would see the airplane ALWAYS on the same spot. That means the airplane has no velocity relative to the air.
No velocity relative to the air MEANS no airflow around the wings MEANS no lift MEANS NO TAKE OFF.
@ Guest: the exaust gas coming from the jet engine IS NOT pushing against the air, actually a jet engine would work better in a vaccum (assuming there would be a device to feed in oxygen).
Very simple, place an stationary observer next to the runway, if the plane has velocity in relation to the observer the plane will take off (assuming it is moving fast enough).
If the plane stays in the same spot in relation to the observer then it will simply stay in the same spot.
The one exception would be if somehow the moving conveyor increased the airflow in the direction of movement, in this case the plane may take off a small amount but since it would only be a localised turbulence the plane would crash as soon as it left the affected air pocket.
If the plane stays in the same spot in relation to the observer then it will simply stay in the same spot.
The one exception would be if somehow the moving conveyor increased the airflow in the direction of movement, in this case the plane may take off a small amount but since it would only be a localised turbulence the plane would crash as soon as it left the affected air pocket.
QUOTE (frasersteen+Aug 30 2005, 10:40 AM)
Very simple, place an stationary observer next to the runway, if the plane has velocity in relation to the observer the plane will take off (assuming it is moving fast enough).
If the plane stays in the same spot in relation to the observer then it will simply stay in the same spot.
The one exception would be if somehow the moving conveyor increased the airflow in the direction of movement, in this case the plane may take off a small amount but since it would only be a localised turbulence the plane would crash as soon as it left the affected air pocket.
If the conveyor belt is would create an airflow, the speed of the airflow would decrease with height. That means the speed of the airflow would be higher below the wing compared to the top of the wing. You would get the opposite of what we look for to make an airplane take-off (the airflow must be faster on top). So even on that case ( the conveyor belt creating an airflow) the airplane would not take off either.
If the plane stays in the same spot in relation to the observer then it will simply stay in the same spot.
The one exception would be if somehow the moving conveyor increased the airflow in the direction of movement, in this case the plane may take off a small amount but since it would only be a localised turbulence the plane would crash as soon as it left the affected air pocket.
If the conveyor belt is would create an airflow, the speed of the airflow would decrease with height. That means the speed of the airflow would be higher below the wing compared to the top of the wing. You would get the opposite of what we look for to make an airplane take-off (the airflow must be faster on top). So even on that case ( the conveyor belt creating an airflow) the airplane would not take off either.
A conveyor belt which can match the speed of an aircraft attempting to take off for sustained periods?
Yeah, hold your breath.
Aircraft tires, bearings, and/or struts which could hold up under such conditions?
Right.
Interesting thought experiment. Much like asking how many angels can dance on the head of a pin.
Yeah, hold your breath.
Aircraft tires, bearings, and/or struts which could hold up under such conditions?
Right.
Interesting thought experiment. Much like asking how many angels can dance on the head of a pin.
Wright Brother,
What is the exhaust pushing against to give the thrust if it is not the air? If it were a true vacuum how would you achieve thrust if the exhaust gasses hit nothing? There would be no reaction to the action of the gasses exiting the engine.
The pros use air jets and rocket engines in space but space is not a true vacuum and has lots of stuff floating around in it to be pushed against.
I don't mean to sound belittling but I may have a misunderstanding and want to clear it up.
What is the exhaust pushing against to give the thrust if it is not the air? If it were a true vacuum how would you achieve thrust if the exhaust gasses hit nothing? There would be no reaction to the action of the gasses exiting the engine.
The pros use air jets and rocket engines in space but space is not a true vacuum and has lots of stuff floating around in it to be pushed against.
I don't mean to sound belittling but I may have a misunderstanding and want to clear it up.
Wright Brother,
What I don't see is how the conveyor can stop the plane from moving at all. If it is a normal plane using a runway it has wheels. If the wheels roll the conveyor can go whatever direction it feels like and the engine thrust will still push the plane forward (like a float plane taking off heading upstream in a river). I don't see a way for a moving conveyor to move in the opposite direction of the plane fast enough to stop the plane from moving unless it was fast enough that the wheels seized.
What I don't see is how the conveyor can stop the plane from moving at all. If it is a normal plane using a runway it has wheels. If the wheels roll the conveyor can go whatever direction it feels like and the engine thrust will still push the plane forward (like a float plane taking off heading upstream in a river). I don't see a way for a moving conveyor to move in the opposite direction of the plane fast enough to stop the plane from moving unless it was fast enough that the wheels seized.
The plane can take off only under these circumstances.
If the engines were routed directly infront of the leading edge of the wing thereby creating pressure difference between the top and bottom (lift) of the wing efectively making it a vertical takeoff machine)
You could not pay me enough however to be the test piolet as systems like that have not been invented yet.
If the engines were routed directly infront of the leading edge of the wing thereby creating pressure difference between the top and bottom (lift) of the wing efectively making it a vertical takeoff machine)
You could not pay me enough however to be the test piolet as systems like that have not been invented yet.
Acid test for expirement
If the blane with its brakes fully engaged and engines at full throttle skids then....
Maybe
If the blane with its brakes fully engaged and engines at full throttle skids then....
Maybe
Aaron,
If the plane had its brakes on and the engines didn't have enough power to overcome the braking force the conveyor wouldn't move as there is no motion of the plane to compensate for.
If the plane could only slightly overcome the braking force then the conveyor could move fast enough to keep the plane on the ground.
If the plane could easily drag the wheels then it could get up enough speed to take off because dragging them on the moving conveyor would probably be easier than dragging them on the stationary ground.
If the plane had its brakes on and the engines didn't have enough power to overcome the braking force the conveyor wouldn't move as there is no motion of the plane to compensate for.
If the plane could only slightly overcome the braking force then the conveyor could move fast enough to keep the plane on the ground.
If the plane could easily drag the wheels then it could get up enough speed to take off because dragging them on the moving conveyor would probably be easier than dragging them on the stationary ground.
QUOTE (Guest (from yesterday)+Aug 30 2005, 05:07 PM)
Aaron,
If the plane had its brakes on and the engines didn't have enough power to overcome the braking force the conveyor wouldn't move as there is no motion of the plane to compensate for.
If the plane could only slightly overcome the braking force then the conveyor could move fast enough to keep the plane on the ground.
If the plane could easily drag the wheels then it could get up enough speed to take off because dragging them on the moving conveyor would probably be easier than dragging them on the stationary ground.
Sounds like a definate maybe to me.
If the plane had its brakes on and the engines didn't have enough power to overcome the braking force the conveyor wouldn't move as there is no motion of the plane to compensate for.
If the plane could only slightly overcome the braking force then the conveyor could move fast enough to keep the plane on the ground.
If the plane could easily drag the wheels then it could get up enough speed to take off because dragging them on the moving conveyor would probably be easier than dragging them on the stationary ground.
Sounds like a definate maybe to me.
I just stumbled across this discussion today, and all opinions considered....
The aircraft will not become airborne, friction on wheels or not.
Consider the following.
A rolling wheel moves over the ground in the same manner that a walking person does; some measurable distance is traversed. The only difference is that one is measured by the circumference of the wheel while the other is measured by the length of a person's stride. Both the aircraft and the person have a movable component that contacts the ground.
Now, place the person on a treadmill and have him walk forward at a speed of three miles per hour. If the treadmill is motionless, the person will walk off the front of the treadmill. If the treadmill is moving at a speed of greater than three miles per hour, the individual will fall off the back of it. If the treadmill is moving exactly three miles per hour, the individual will remain in the same relative location indefinitely (or at least until he gets very tired and thirsty). However, while the person is 'moving' forward at a perceived speed of three miles per hour relative to the treadmill, his movement relative to the air is 0.
The aircraft rolling forward on the conveyor (the treadmill) that is moving toward him at the same speed would also have no forward motion relative to the air, and hence no lift, no takeoff. Wheels? Legs? Its immaterial. Both 'devices' are covering a measurable distance that is moving under them. With no forward motion there is no lift.
The case of an aircraft flying at 200 mph into a 200 mph headwind is a different issue. While the ground speed of the aircraft is 0, the speed of the wind, which is not motionless, over the wing is 200 mph, and lift is obtained.
The aircraft will not become airborne, friction on wheels or not.
Consider the following.
A rolling wheel moves over the ground in the same manner that a walking person does; some measurable distance is traversed. The only difference is that one is measured by the circumference of the wheel while the other is measured by the length of a person's stride. Both the aircraft and the person have a movable component that contacts the ground.
Now, place the person on a treadmill and have him walk forward at a speed of three miles per hour. If the treadmill is motionless, the person will walk off the front of the treadmill. If the treadmill is moving at a speed of greater than three miles per hour, the individual will fall off the back of it. If the treadmill is moving exactly three miles per hour, the individual will remain in the same relative location indefinitely (or at least until he gets very tired and thirsty). However, while the person is 'moving' forward at a perceived speed of three miles per hour relative to the treadmill, his movement relative to the air is 0.
The aircraft rolling forward on the conveyor (the treadmill) that is moving toward him at the same speed would also have no forward motion relative to the air, and hence no lift, no takeoff. Wheels? Legs? Its immaterial. Both 'devices' are covering a measurable distance that is moving under them. With no forward motion there is no lift.
The case of an aircraft flying at 200 mph into a 200 mph headwind is a different issue. While the ground speed of the aircraft is 0, the speed of the wind, which is not motionless, over the wing is 200 mph, and lift is obtained.
QUOTE (dedawson+Oct 13 2005, 07:24 PM)
I just stumbled across this discussion today, and all opinions considered....
The aircraft will not become airborne, friction on wheels or not.
Consider the following.
A rolling wheel moves over the ground in the same manner that a walking person does; some measurable distance is traversed. The only difference is that one is measured by the circumference of the wheel while the other is measured by the length of a person's stride. Both the aircraft and the person have a movable component that contacts the ground.
Now, place the person on a treadmill and have him walk forward at a speed of three miles per hour. If the treadmill is motionless, the person will walk off the front of the treadmill. If the treadmill is moving at a speed of greater than three miles per hour, the individual will fall off the back of it. If the treadmill is moving exactly three miles per hour, the individual will remain in the same relative location indefinitely (or at least until he gets very tired and thirsty). However, while the person is 'moving' forward at a perceived speed of three miles per hour relative to the treadmill, his movement relative to the air is 0.
The aircraft rolling forward on the conveyor (the treadmill) that is moving toward him at the same speed would also have no forward motion relative to the air, and hence no lift, no takeoff. Wheels? Legs? Its immaterial. Both 'devices' are covering a measurable distance that is moving under them. With no forward motion there is no lift.
The case of an aircraft flying at 200 mph into a 200 mph headwind is a different issue. While the ground speed of the aircraft is 0, the speed of the wind, which is not motionless, over the wing is 200 mph, and lift is obtained.
All opinions considered, including my original one....
The aircraft will become airborne!
Return to the treadmill, but put a bicycle on it, with someone to hold it steady. As the speed of the treadmill increases, the wheels turn faster, but since it is being held steady. the bicycle does not move.
Now, have someone push the bicycle forward, off the front of the treadmill. The wheels turn faster to cover the additional terrain, but the bicycle does move forward.
The engines of the aircraft are the same as the individual pushing the bike. The wheels merely serve as a contact point with the ground. The aircraft will gain both ground and air speed and will take off.
The aircraft will not become airborne, friction on wheels or not.
Consider the following.
A rolling wheel moves over the ground in the same manner that a walking person does; some measurable distance is traversed. The only difference is that one is measured by the circumference of the wheel while the other is measured by the length of a person's stride. Both the aircraft and the person have a movable component that contacts the ground.
Now, place the person on a treadmill and have him walk forward at a speed of three miles per hour. If the treadmill is motionless, the person will walk off the front of the treadmill. If the treadmill is moving at a speed of greater than three miles per hour, the individual will fall off the back of it. If the treadmill is moving exactly three miles per hour, the individual will remain in the same relative location indefinitely (or at least until he gets very tired and thirsty). However, while the person is 'moving' forward at a perceived speed of three miles per hour relative to the treadmill, his movement relative to the air is 0.
The aircraft rolling forward on the conveyor (the treadmill) that is moving toward him at the same speed would also have no forward motion relative to the air, and hence no lift, no takeoff. Wheels? Legs? Its immaterial. Both 'devices' are covering a measurable distance that is moving under them. With no forward motion there is no lift.
The case of an aircraft flying at 200 mph into a 200 mph headwind is a different issue. While the ground speed of the aircraft is 0, the speed of the wind, which is not motionless, over the wing is 200 mph, and lift is obtained.
All opinions considered, including my original one....
The aircraft will become airborne!
Return to the treadmill, but put a bicycle on it, with someone to hold it steady. As the speed of the treadmill increases, the wheels turn faster, but since it is being held steady. the bicycle does not move.
Now, have someone push the bicycle forward, off the front of the treadmill. The wheels turn faster to cover the additional terrain, but the bicycle does move forward.
The engines of the aircraft are the same as the individual pushing the bike. The wheels merely serve as a contact point with the ground. The aircraft will gain both ground and air speed and will take off.
The plane would take off! For the plane to take off it has to create lift (lower pressure over the top of the wing, higher pressure underneath hence lifting the wing up) in order to achieve lift the fluid or gas (air in this case) has to accelerate over the wing! Well in the case of a plane the plane moves through the air (which is at a velocity of 0 reguardless of what the conveyor is doing) to create this acceleration of air over the wing. Now we know the conveyor has no negative effect on the air surrounding the plane so we address the propulsion of the plane; If its a propeller it creates the same effect as the wing ie. the props create lower pressure over the top of them and higher pressure underneath, pulling or accelerating the plane through the air. If its a jet then the engine creates thrust by Newton's third law of action and reaction. A gas, or working fluid, is accelerated by the engine, and the reaction to this acceleration produces a force on the engine pushing or accelerating it forward through the air.
So to sum it all up, for a plane on the conveyor belt not to be able to lift off the belt would have to affect the air surrounding the plane because thats where the plane derives its thrust which accelerates it through the air, and the planes acceleration through the air is how it creates lift.
P.S. The conveyor moving the ground underneath the plane at an equally negative velocity to the planes forward velocity would only affect the rotation speed of the wheels times 2.
Hope this helps a little bit,
Jeremi R.
Mechanical Design Engineer /
Project Engineer / Which ever one they want me to be for that day. lol
Manufacturing Engineer/
So to sum it all up, for a plane on the conveyor belt not to be able to lift off the belt would have to affect the air surrounding the plane because thats where the plane derives its thrust which accelerates it through the air, and the planes acceleration through the air is how it creates lift.
P.S. The conveyor moving the ground underneath the plane at an equally negative velocity to the planes forward velocity would only affect the rotation speed of the wheels times 2.
Hope this helps a little bit,
Jeremi R.
Mechanical Design Engineer /
Project Engineer / Which ever one they want me to be for that day. lol
Manufacturing Engineer/
The plane will not take of. It needs a certain amount of air volume to go thru the engine. If the plane is on a tread mill then its speed relative to the air is 0, hence no thrust, and no lift off.
QUOTE
No velocity relative to the air MEANS no airflow around the wings MEANS no lift MEANS NO TAKE OFF.
This is really the only relevant thing to consider. The ground speed of the aircraft is not important, nor is the amount of thrust the engines develop. If a plane's take off speed is 140mph, that is the speed the air must flow over the wing in order to develop the lift required to take off. If a plane has a ground speed of 140 mph, but also has a tail wind of 140mph, that means the plane will not be moving relative to the air. If the plane were to turn around, into the wind, it could take off with zero ground speed and zero thrust.
This is why aircraft carriers turn into the wind before launching any planes (and why they will generally get up to full speed, to add to the air speed over the deck). It allows the planes to take off with a lower 'ground' speed, so that the catapault can exert less effort. The plane may only be moving about 100mph relative to the deck, but the extra 35mph of the ship + 10mph wind will allow the planes to get airborne. Turn the catapault around, and now that 100mph deck speed is only 65mph air speed. You're gonna lose that aircraft.
The only way it could take off is if the conveyor was able to drag enough air along with it to develop that 140mph air speed. But anyone who's studied fluids will realize that's pretty much impossible.
This is a great thought experiment. I think the plane will take off provided it can generate enough thrust to overcome the (relatively small) friction caused by it's wheels spinning against the runway.
The key to understanding the problem is the bicycle on the treadmill example used a couple of posts back. If I'm on the bicycle pedaling as fast as I can and the treadmill keeps speeding up to match the speed of my pedaling I'm never going to reach the end of the treadmill. However, if my walks up to the treadmill with his feet firmly planted on the ground and gives me an almighty push I think he will be able to easily overcome the friction of the bike on the treadmill and push me off the end of the treadmill.
In the case of the aircraft, the source of the forward movement is a lot like my friend standing on the ground next to the treadmill. A lot of the people responding to this question seem to equate the source of the forward movement with the guy pedaling the bike on the treadmill instead.
In the plane experiment, I think the friction of the wheels on the runway could still be overcome by the thrust of the engines even if the conveyor belt were traveling at thousands of kilometers per hour. The real question is whether there is a limit to this (relatively small compared to the thrust of the engines) friction? What happens if the conveyor belt is moving infinitely fast?
Darren.
The key to understanding the problem is the bicycle on the treadmill example used a couple of posts back. If I'm on the bicycle pedaling as fast as I can and the treadmill keeps speeding up to match the speed of my pedaling I'm never going to reach the end of the treadmill. However, if my walks up to the treadmill with his feet firmly planted on the ground and gives me an almighty push I think he will be able to easily overcome the friction of the bike on the treadmill and push me off the end of the treadmill.
In the case of the aircraft, the source of the forward movement is a lot like my friend standing on the ground next to the treadmill. A lot of the people responding to this question seem to equate the source of the forward movement with the guy pedaling the bike on the treadmill instead.
In the plane experiment, I think the friction of the wheels on the runway could still be overcome by the thrust of the engines even if the conveyor belt were traveling at thousands of kilometers per hour. The real question is whether there is a limit to this (relatively small compared to the thrust of the engines) friction? What happens if the conveyor belt is moving infinitely fast?
Darren.
But as soon as the plane starts to overcome the friction and move forward, the conveyor speeds up to eliminate any forward motion - right?
QUOTE (Justavian+Oct 31 2005, 06:36 AM)
But as soon as the plane starts to overcome the friction and move forward, the conveyor speeds up to eliminate any forward motion - right?
Nope... the plane is not powered by it's wheels.. but by jet thrust on the static air.. so whoosh.. the plane is airborne!
Imagine you're wearing a jetpack (or rocketbelt, for the purists) and you are also running on the treadmill... all is fine...
Now bend over to get the right thrust vector and light it up.. the rocket pack will push against the static air and shove you right off the front of the treadmill.
Running faster will not help as the thrust is coming from the air/rocket interaction and not from the feet/treadmill interaction.
Now if you had a sports car on the conveyor and it had lift devices (wings) fitted, it would not be able to take off as the forward thrust is coming from push at the treadmill, it's "groundspeed" would be rather high but it's speed relative to the air around it would be nil.
Nope... the plane is not powered by it's wheels.. but by jet thrust on the static air.. so whoosh.. the plane is airborne!
Imagine you're wearing a jetpack (or rocketbelt, for the purists) and you are also running on the treadmill... all is fine...
Now bend over to get the right thrust vector and light it up.. the rocket pack will push against the static air and shove you right off the front of the treadmill.
Running faster will not help as the thrust is coming from the air/rocket interaction and not from the feet/treadmill interaction.
Now if you had a sports car on the conveyor and it had lift devices (wings) fitted, it would not be able to take off as the forward thrust is coming from push at the treadmill, it's "groundspeed" would be rather high but it's speed relative to the air around it would be nil.
I'm thinking of the force diagram, and i still can't see the plane taking off. Just as with a stationary runway, the thrust of the engines must overcome the friction of the conveyor. Since there is no lift, the normal force of the plane will ensure that the plane experiences no reduction in that frictional force once it gets moving. No matter how hard the engines push (and regardless of whether we're talking about a jet engine or a car apply its power to the wheels), the conveyor can still apply an equal force in the opposite direction.
The jetpack isn't the best example, since they have enough thrust to simply launch you directly into the air - there's no need for air to be moving across a wing, and there's no friction to push in the other direction. But if you had a somewhat weak pack - maybe just a big fan on your back - and you were on roller skates, you would never leave the conveyor. The fan applies a force in one direction, the conveyor applies an equal force (in the form of friction) in the other direction.
The jetpack isn't the best example, since they have enough thrust to simply launch you directly into the air - there's no need for air to be moving across a wing, and there's no friction to push in the other direction. But if you had a somewhat weak pack - maybe just a big fan on your back - and you were on roller skates, you would never leave the conveyor. The fan applies a force in one direction, the conveyor applies an equal force (in the form of friction) in the other direction.
I agree that the key concept here is friction. Regardless of where the forward propulsion is coming from - the wheels or pushing against the air - if the friction of the wheels is great enough, forward motion could be reduced to 0 by the conveyor belt. Because the friction of the wheels is generally pretty insignificant compared to the thrust generated by a jet engine, the conveyor belt would have to be moving at incredible speeds to cause the jet to remain stationary. This assumes that the friction of the wheels can be increased in an unlimited fashion. Is this the case or is there a limit to the amount of friction the wheels could produce (in a theoretical, not practical, sense)?
Another interesting point about this example is that in real life, I think the jet would take off. If you had an aircraft with the largest jet engines available (say 4 or 8 of them) complete with wheel hubs with the lowest amount of friction sitting on the fastest conveyor belt that could be constructed with modern technology, I think the plane would still take off. If however, we are talking about the imaginary and an imaginary conveyor belt that could move infinitely fast, then why couldn't we also have imaginary wheel hubs with no friction, in which case the plane would still take off.
Darren.
Another interesting point about this example is that in real life, I think the jet would take off. If you had an aircraft with the largest jet engines available (say 4 or 8 of them) complete with wheel hubs with the lowest amount of friction sitting on the fastest conveyor belt that could be constructed with modern technology, I think the plane would still take off. If however, we are talking about the imaginary and an imaginary conveyor belt that could move infinitely fast, then why couldn't we also have imaginary wheel hubs with no friction, in which case the plane would still take off.
Darren.
QUOTE (Dazweeja+Oct 30 2005, 11:59 PM)
This is a great thought experiment. I think the plane will take off provided it can generate enough thrust to overcome the (relatively small) friction caused by it's wheels spinning against the runway.
The key to understanding the problem is the bicycle on the treadmill example used a couple of posts back. If I'm on the bicycle pedaling as fast as I can and the treadmill keeps speeding up to match the speed of my pedaling I'm never going to reach the end of the treadmill. However, if my walks up to the treadmill with his feet firmly planted on the ground and gives me an almighty push I think he will be able to easily overcome the friction of the bike on the treadmill and push me off the end of the treadmill.
In the case of the aircraft, the source of the forward movement is a lot like my friend standing on the ground next to the treadmill. A lot of the people responding to this question seem to equate the source of the forward movement with the guy pedaling the bike on the treadmill instead.
In the plane experiment, I think the friction of the wheels on the runway could still be overcome by the thrust of the engines even if the conveyor belt were traveling at thousands of kilometers per hour. The real question is whether there is a limit to this (relatively small compared to the thrust of the engines) friction? What happens if the conveyor belt is moving infinitely fast?
Darren.
Put your friend on the treadmill with you. Now have him push you forward. The treadmill speeds up to match the speed of the bike. The bike still has no forward motion and your friend falls off the back of the treadmill.
If the plane never moves in relation to the air. The plane will not take off.
The key to understanding the problem is the bicycle on the treadmill example used a couple of posts back. If I'm on the bicycle pedaling as fast as I can and the treadmill keeps speeding up to match the speed of my pedaling I'm never going to reach the end of the treadmill. However, if my walks up to the treadmill with his feet firmly planted on the ground and gives me an almighty push I think he will be able to easily overcome the friction of the bike on the treadmill and push me off the end of the treadmill.
In the case of the aircraft, the source of the forward movement is a lot like my friend standing on the ground next to the treadmill. A lot of the people responding to this question seem to equate the source of the forward movement with the guy pedaling the bike on the treadmill instead.
In the plane experiment, I think the friction of the wheels on the runway could still be overcome by the thrust of the engines even if the conveyor belt were traveling at thousands of kilometers per hour. The real question is whether there is a limit to this (relatively small compared to the thrust of the engines) friction? What happens if the conveyor belt is moving infinitely fast?
Darren.
Put your friend on the treadmill with you. Now have him push you forward. The treadmill speeds up to match the speed of the bike. The bike still has no forward motion and your friend falls off the back of the treadmill.
If the plane never moves in relation to the air. The plane will not take off.
A stationary plane can maintain only its current height surely, and as the current height is ground level...
Since the aeroplane has no movement, the air (unless in a really strong headwind) will be non existant; all that happens is the air will be sucked through the engines which DOESNT create lift. Engines explode, scientists say "*** were we thinking"
Since the aeroplane has no movement, the air (unless in a really strong headwind) will be non existant; all that happens is the air will be sucked through the engines which DOESNT create lift. Engines explode, scientists say "*** were we thinking"
Sorry if this is a little after the fact and everyone is bored of this discussion but I just read the posts here and i found myself thinking of course it's clearly like this....then wait, no it's clearly like that....then finally I think I got it (i think) with another analogy so that i end up with the opposite opinion that i started with, and with (to me anyway...and obviously to some others as well) the counterintuitive answer.
look at the exact same set up of the experiment as originally mentioned and change one thing....the vehicle is a hovercraft.
so....it's propelling itself forward and as it does, the conveyerbelt moves at the same speed but in the opposite direction. now the original experiment does not say that the plane must some how be kept in one spot (relative to the control tower), only that the speeds of the plane (ie.air propelled vehicle....a hover craft in my analogy) and the speed of the conveyerbelt are in equal but opposite directions. so say the hovercraft accelerates to 5 mph (assuming it's a perfectly still day and it's speedometer it measuring air speed) and maintains that speed, the fact that the conveyerbelt is moving backwards (also relative to the control tower..not relative to the air propelled vehicle) at 5 mph is not preventing the hovercraft from moving forwards. the hovercraft and conveyerbelt are parting ways at 10 mph...this is true....but that's not preventing the hovercraft from moving forward through the air.
accelerate the hovercraft to 50 mph and the conveyerbelt is moving backwards at 50 mph...the hovercraft will just move right off the belt....... in essence this means that with the plane, it's landing gear will have to rotate at double the velocity they would normally have to in order to take off....so its propellers pull it through the air at 5 mph and it's wheels are turning at 10 mph....but the fact that it's wheels are turning at 10 mph is not preventing it from moving relative to the air (or the control tower) at 5 mph.....of course if you change it's propulsion system to it's wheels (make it a system where a glider is launched from the top of a car), then yes it stays in the same spot because it has to pull itself over the ground (and therefore relative to the ground) in order to gain air speed and generate lift. But the plane pulls itself through the air, irrespective of it's ground speed in order to generate lift.
The plane takes off!
it's wheels simply spin at double their normal velocity
look at the exact same set up of the experiment as originally mentioned and change one thing....the vehicle is a hovercraft.
so....it's propelling itself forward and as it does, the conveyerbelt moves at the same speed but in the opposite direction. now the original experiment does not say that the plane must some how be kept in one spot (relative to the control tower), only that the speeds of the plane (ie.air propelled vehicle....a hover craft in my analogy) and the speed of the conveyerbelt are in equal but opposite directions. so say the hovercraft accelerates to 5 mph (assuming it's a perfectly still day and it's speedometer it measuring air speed) and maintains that speed, the fact that the conveyerbelt is moving backwards (also relative to the control tower..not relative to the air propelled vehicle) at 5 mph is not preventing the hovercraft from moving forwards. the hovercraft and conveyerbelt are parting ways at 10 mph...this is true....but that's not preventing the hovercraft from moving forward through the air.
accelerate the hovercraft to 50 mph and the conveyerbelt is moving backwards at 50 mph...the hovercraft will just move right off the belt....... in essence this means that with the plane, it's landing gear will have to rotate at double the velocity they would normally have to in order to take off....so its propellers pull it through the air at 5 mph and it's wheels are turning at 10 mph....but the fact that it's wheels are turning at 10 mph is not preventing it from moving relative to the air (or the control tower) at 5 mph.....of course if you change it's propulsion system to it's wheels (make it a system where a glider is launched from the top of a car), then yes it stays in the same spot because it has to pull itself over the ground (and therefore relative to the ground) in order to gain air speed and generate lift. But the plane pulls itself through the air, irrespective of it's ground speed in order to generate lift.
The plane takes off!
it's wheels simply spin at double their normal velocity
conversly make the experiment such that the conveyerbelt moves forward (in the same direction that the plane is moving), but always at double the velocity of the plane. It's propellars spin...the plane is moving forward at 5 mph (relative to the control tower), the belt at 10 mph (relative to the control tower)....the wheels are spinning backwards at 5 mph! as the propellars are sped up, the plane pulls itself through the air faster then at take off ( i don't know...say 80 mph air speed) the belt is moving forward at 160 mph, the wheels are spinning on the belt backwards at 80 mph.....again...irrelevant (except for the mechanical specs on the wheels).
Having re-read the initial question, I'm actually going to say that I absolutely believe that the plane will take off, assuming the plane has a reasonable size engine and a decent wheel hub/ball-bearing assembly. I'm no expert on planes but I would assume there is no force being applied through the wheels by an engine like in a car, rather they would just be spinning "in neutral" as it were and all the forward propulsion would come from the jets/propellers. I can't see the point in having a separate engine plus differential setup just for the wheels in addition to the jets/propellers as this wouldn't be at all necessary for the plane to get airborne.
The initial question says that the speed of the conveyor belt always matches the speed of the plane. When the plane is traveling at 5mph, for example, the conveyor belt is traveling backwards at 5mph. All the conveyor belt does is increase friction on the tires which is primarily transferred to wheel hub/ball-bearing assembly. This friction will be pretty insignificant compared to the thrust of the plane's jets/propellers against the air and the plane will still be accelerating. At 10mph it's the same thing, the friction has increased slightly but the thrust of the plane pushing against the air will far outweigh this and the plane will still be accelerating. With large enough jets and no wheels, ie. the plane simply sliding on it's underside, I think the plane would still easily be accelerating in these conditions. Whether it actually gets airborne or not will ultimately depend on the thrust of the engines being able to overcome the lesser friction of the wheels to enough of an extent to reach take-off velocity but I would think that something like a jumbo jet would have no problem with this.
I originally misread the question and thought that the speed of the conveyor belt could be increased to whatever speed necessary in order to keep the plane stationary. A jumbo jet needs to reach around 220mph to takeoff. A conveyor belt traveling at 220mph in the opposite direction is going to cause quite a bit of friction in the wheels but I think the four jets would still easily overcome this. One thing is for sure, you would need a conveyor belt that is far, far longer than the average runway.
Darren.
The initial question says that the speed of the conveyor belt always matches the speed of the plane. When the plane is traveling at 5mph, for example, the conveyor belt is traveling backwards at 5mph. All the conveyor belt does is increase friction on the tires which is primarily transferred to wheel hub/ball-bearing assembly. This friction will be pretty insignificant compared to the thrust of the plane's jets/propellers against the air and the plane will still be accelerating. At 10mph it's the same thing, the friction has increased slightly but the thrust of the plane pushing against the air will far outweigh this and the plane will still be accelerating. With large enough jets and no wheels, ie. the plane simply sliding on it's underside, I think the plane would still easily be accelerating in these conditions. Whether it actually gets airborne or not will ultimately depend on the thrust of the engines being able to overcome the lesser friction of the wheels to enough of an extent to reach take-off velocity but I would think that something like a jumbo jet would have no problem with this.
I originally misread the question and thought that the speed of the conveyor belt could be increased to whatever speed necessary in order to keep the plane stationary. A jumbo jet needs to reach around 220mph to takeoff. A conveyor belt traveling at 220mph in the opposite direction is going to cause quite a bit of friction in the wheels but I think the four jets would still easily overcome this. One thing is for sure, you would need a conveyor belt that is far, far longer than the average runway.
Darren.
I should clarify that by accelerating, I mean moving at an increasing velocity away from my initial starting position, or away from an stationary observer standing next to the conveyor belt.
gmilam, I don't think in the treadmill analogy the thrust against the air would be characterized as someone standing behind you but still on the treadmill, more like someone in a fixed seat above you independent of the treadmill, attached to the ceiling for example.
justavian, are you suggesting that the frictional force of someone traveling at 5mph with the conveyor belt traveling in the opposite direction at 5mph wearing rollerskates and a big fan is the same as someone in the same situation wearing runners (with no wheels obviously) and a big fan for example? Surely in the same conditions, friction can be reduced relative to thrust by using different types of contact surfaces for example. I think the force diagram would only hold if we were talking about a car on the treadmill not a situation where the force is being applied on two separate planes (in the physical/engineering sense of the word plane, not as in air-plane), one plane at the level of the conveyor belt and one plane at the level of the jets/propellers. At the level of the jets/propellers there would be a separate force diagram with thrust vs drag + a small amount of friction from the wheel hubs. In the car on the treadmill situation, the forces being enacted at the level of the conveyor belt are all that require consideration.
gmilam, I don't think in the treadmill analogy the thrust against the air would be characterized as someone standing behind you but still on the treadmill, more like someone in a fixed seat above you independent of the treadmill, attached to the ceiling for example.
justavian, are you suggesting that the frictional force of someone traveling at 5mph with the conveyor belt traveling in the opposite direction at 5mph wearing rollerskates and a big fan is the same as someone in the same situation wearing runners (with no wheels obviously) and a big fan for example? Surely in the same conditions, friction can be reduced relative to thrust by using different types of contact surfaces for example. I think the force diagram would only hold if we were talking about a car on the treadmill not a situation where the force is being applied on two separate planes (in the physical/engineering sense of the word plane, not as in air-plane), one plane at the level of the conveyor belt and one plane at the level of the jets/propellers. At the level of the jets/propellers there would be a separate force diagram with thrust vs drag + a small amount of friction from the wheel hubs. In the car on the treadmill situation, the forces being enacted at the level of the conveyor belt are all that require consideration.
After more thought, i KNOW it wouldn't take off!
The plane turns on its engines, and starts to roll forward. This trips the sensor - that the plane is no longer centered over a certain point. So the conveyor begins moving, constantly trying to re-center the plane. This will continue for some time, until the plane's wheels are experiencing several hundred miles an hour worth of simulated movement. At some point the bearings fail, or the tires heat up and melt. Then the plane will then be experiencing so much friction that the conveyor will easily be able to keep it centered.
I win! haha.
The plane turns on its engines, and starts to roll forward. This trips the sensor - that the plane is no longer centered over a certain point. So the conveyor begins moving, constantly trying to re-center the plane. This will continue for some time, until the plane's wheels are experiencing several hundred miles an hour worth of simulated movement. At some point the bearings fail, or the tires heat up and melt. Then the plane will then be experiencing so much friction that the conveyor will easily be able to keep it centered.
I win! haha.
So you think you have won, do you? I think you have actually made an incorrect assumption that the purpose of the conveyor belt is to keep the aircraft stationary. It isn't. The conveyor belt merely moves at exactly the same speed as the aircraft but in the opposite direction. The conveyor belt isn't constantly trying to re-center the plane, just speeding up relative to the speed of the plane which may or may not re-center the plane. This is not the same as a mechanism which is designed to keep the plane in one spot. In fact, the position of the plane on the conveyor belt has no relation to the speed of the conveyor belt. The most obvious effect of ther speeding up of the conveyor belt is that the aircraft wheels would spin twice as fast but the overall effect on the aircraft's position relative to the control tower is debatable. Sure it will create more heat and more friction but even at takeoff speed for a jumbo jet for example, the wheels would be spinning at 440mph, which is not insurmountable considering as far back as 1970 jet-powered cars were traveling faster than this and their wheels didn't blow up or fall off. I think the increased friction of free-spinning wheels spinning at 440mph - in the direction they were designed to spin as well - would be pretty insignificant as a force acting against the thrust of the engines.
I look forward to your response.
Darren.
I look forward to your response.
Darren.
Darren,
When i first read your post, you had me convinced that i misunderstood the original problem. But after a little pondering, i'm not so sure...
I guess the answer to the question depends on one thing - how do you measure the speed of the aircraft, in order to have the conveyor match it?
It seems to me that the speed of aircraft must be the simulated ground speed, as measured by its tires. Any movement forward on the conveyor will result in a slightly higher calculated ground speed - and the conveyor will therefore speed up in the opposite direction. So any movement relative to some fixed position means the same thing. Regardless of how power is applied to move the plane, if it moves relative to that fixed point, the simulated ground speed of the aircraft has necessarily increased - and the conveyor can always compensate instantaneously.
I realize now that my comment about the wheels experiencing several hundred miles an hour is way off. They'll very quickly be simulating thousands of miles an hour or more.
When i first read your post, you had me convinced that i misunderstood the original problem. But after a little pondering, i'm not so sure...
I guess the answer to the question depends on one thing - how do you measure the speed of the aircraft, in order to have the conveyor match it?
It seems to me that the speed of aircraft must be the simulated ground speed, as measured by its tires. Any movement forward on the conveyor will result in a slightly higher calculated ground speed - and the conveyor will therefore speed up in the opposite direction. So any movement relative to some fixed position means the same thing. Regardless of how power is applied to move the plane, if it moves relative to that fixed point, the simulated ground speed of the aircraft has necessarily increased - and the conveyor can always compensate instantaneously.
I realize now that my comment about the wheels experiencing several hundred miles an hour is way off. They'll very quickly be simulating thousands of miles an hour or more.
The answer is simple. the plane will take off. Think of the conveyor that has a frictionless surface, it has no effect on anything on top of it no matter how fast it moves.
Why do we assume the conveyor be frictionless? If there's any friction at all, it will work like a poisitive feedback loop. The faster the plane's wheels turn, the faster the conveyor moves, which spins the wheels faster, which results in a higher calculated ground speed, which results in the conveyor moving faster. If there's any friction at all, the conveyor will be able to prevent the plane from taking off.
Why does an airplane fly?
http://www.physlink.com/Education/AskExperts/ae25.cfm
If there is no airflow over the wings, there will not be any lift.. The plane will not take off.
http://www.physlink.com/Education/AskExperts/ae25.cfm
If there is no airflow over the wings, there will not be any lift.. The plane will not take off.
QUOTE
If there is no airflow over the wings, there will not be any lift..
I think that's been established. The question is whether or not the moving conveyor can actually prevent the airplane from moving forward.
OK I think some people are getting confused about lift, friction, acceleration and thrust So I'll break it down like this;
Under the circumstances of the initial question, if you can understand the differences between the 2 variables in the question I'm about to ask you, then the concept behind the plane being able to take off will make sense.
Take that same conveyor system and place a Formula One race car on it and right next to it place a rocket powered drag car. One of these vehicles WILL accelerate FORWARD while one will be unable to gain any forward acceleration. Which one and why?
Answer: The Rocket powered drag car will accelerate forward, while the Formula One race car will not be able to. Why is that? Because the Formula One race car's motor uses the wheels to transfer the trust>(a force that propels a body of mass in motion) it creates to the ground (which happens to be a conveyor belt moving in a equally opposite velocity). BUT the rocket powered drag car's engine doesn't apply it's thrust to the ground, it actually uses the reaction of accelerating air through its engine to create thrust.
So here's the kicker for all those that say the plane would not be able to lift off; Suppose the conveyor was never there in the first place but use the same logic that you have been using to explain why the plane wont take off OK. My question for you to answer is how will the plane fly once it leaves the runway? based on your argument "the conveyor has the ability to counteract the planes ability to gain forward motion" then how will the plane maintain velocity or accelerate once in the air? What is it pushing against now that the conveyor is gone?
I look forward to seeing some of the answers
Under the circumstances of the initial question, if you can understand the differences between the 2 variables in the question I'm about to ask you, then the concept behind the plane being able to take off will make sense.
Take that same conveyor system and place a Formula One race car on it and right next to it place a rocket powered drag car. One of these vehicles WILL accelerate FORWARD while one will be unable to gain any forward acceleration. Which one and why?
Answer: The Rocket powered drag car will accelerate forward, while the Formula One race car will not be able to. Why is that? Because the Formula One race car's motor uses the wheels to transfer the trust>(a force that propels a body of mass in motion) it creates to the ground (which happens to be a conveyor belt moving in a equally opposite velocity). BUT the rocket powered drag car's engine doesn't apply it's thrust to the ground, it actually uses the reaction of accelerating air through its engine to create thrust.
So here's the kicker for all those that say the plane would not be able to lift off; Suppose the conveyor was never there in the first place but use the same logic that you have been using to explain why the plane wont take off OK. My question for you to answer is how will the plane fly once it leaves the runway? based on your argument "the conveyor has the ability to counteract the planes ability to gain forward motion" then how will the plane maintain velocity or accelerate once in the air? What is it pushing against now that the conveyor is gone?
I look forward to seeing some of the answers
QUOTE
What is it pushing against now that the conveyor is gone?
The plane is always pushing against the air. The question i'm posing is whether or not the conveyor can apply enough rolling friction to the wheels to have an impact on the forward momentum, either via a catastrophic failure in the wheels, or in the case of aircraft with less thrusts, enough rolling friction that the thrust cannot overcome it. Since the conveyor may operate at any speed, and can react instantaneously, it will continue to speed up until either the bearings fail, tires melt, or the rolling friction is too much for the thrust to overcome.
This has been an interesting topic to watch and I have alternately persuaded by both sides several times, but I've got the answer.
The text from the OP is... "The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in opposite direction)."
So the answer is MAYBE. What does "plane speed" mean?
If it is the speed of the planes tires as measured like a car speedometer, then NO. Actually, I think the moment the jets turned on, the wheels and conveyor would go to infinity and all would crash and burn.
If the speed of the plane means what would be seen from the tower with a radar gun or something, then YES. As Justavian mentioned above, the wheels would simply be going twice as fast as usual causing an insignificant amount of extra friction, but the airspeed would be the normal takeoff amount without too much effort.
The text from the OP is... "The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in opposite direction)."
So the answer is MAYBE. What does "plane speed" mean?
If it is the speed of the planes tires as measured like a car speedometer, then NO. Actually, I think the moment the jets turned on, the wheels and conveyor would go to infinity and all would crash and burn.
If the speed of the plane means what would be seen from the tower with a radar gun or something, then YES. As Justavian mentioned above, the wheels would simply be going twice as fast as usual causing an insignificant amount of extra friction, but the airspeed would be the normal takeoff amount without too much effort.
QUOTE (Inc\/\/orm+Aug 29 2005, 08:07 PM)
The variable here is friction. If we assume that there is no friction from the wheel bearings, then it makes no difference what speed the 'ground' [conveyor] is moving: the aircraft will advance and ultimately take off. IF the friction of the moving surface is enough to overcome the thrust of the engines, then the craft will not move and not take off. I don't think the friction is that great, but I agree with an earlier post that there would be some hellacious wheel spin!
Exactly!
QUOTE (Montec+Nov 2 2005, 07:30 AM)
The answer is simple. the plane will take off. Think of the conveyor that has a frictionless surface, it has no effect on anything on top of it no matter how fast it moves.
Right again!
Right again!
QUOTE (Guest_Steve+Nov 2 2005, 05:55 PM)
QUOTE (Montec+Nov 2 2005, 07:30 AM)
The answer is simple. the plane will take off. Think of the conveyor that has a frictionless surface, it has no effect on anything on top of it no matter how fast it moves.
Right again!
No, Steve/Montec, the conveyor is not frictionless, and it does affect what is on top. Go home and put a matchbox car on your treadmill and turn it on. I guarantee the car goes shooting off the back rather than just sitting there motionless with its wheels spinning like crazy.
Wrong. The airplane requires airflow over the wing to get it's lift, while your hovercraft does not. "THEY" is correct in her post. I agree with her, let it rest...
Try driving your car at 10,000 mph. The heat from the friction will not be negligible. I think i established earlier that the conveyor will continuously speed up if the plane moves forward relative to a fixed point. The conveyor, in order to reach its "goal" of keeping the plane in one place (which is the same as matching its speed) can theoretically reach any speed.
Try driving your car at 10,000 mph. The heat from the friction will not be negligible. I think i established earlier that the conveyor will continuously speed up if the plane moves forward relative to a fixed point. The conveyor, in order to reach its "goal" of keeping the plane in one place (which is the same as matching its speed) can theoretically reach any speed.
the question wasn't about the conveyerbelt moving at any speed necessary to keep the plane from taking off
Right again!
No, Steve/Montec, the conveyor is not frictionless, and it does affect what is on top. Go home and put a matchbox car on your treadmill and turn it on. I guarantee the car goes shooting off the back rather than just sitting there motionless with its wheels spinning like crazy.
Sorry to butt in folks, but I just can't believe this is still being debated after three months.
If the conveyor is heading in the opposite direction, the "ground speed" of the airplane will be the speed of the conveyor belt even with the plane at rest. The requirement for the plane to fly is "air speed" causing sufficient air to flow over the wings. A conveyor heading in the opposite direction does not increase the airspeed so the conveyor only makes the wheels turn faster. It won't make the plane takeoff. If the conveyor was in the same direction as the plane's intended takeoff path, that would help.
That is from a Boeing engineer, and if Boeing doesn't know how to make airplanes fly, then I don't exist. Can we please let this rest? If you still believe the plane will take off the ground, please test it before you argue it again.
If the conveyor is heading in the opposite direction, the "ground speed" of the airplane will be the speed of the conveyor belt even with the plane at rest. The requirement for the plane to fly is "air speed" causing sufficient air to flow over the wings. A conveyor heading in the opposite direction does not increase the airspeed so the conveyor only makes the wheels turn faster. It won't make the plane takeoff. If the conveyor was in the same direction as the plane's intended takeoff path, that would help.
That is from a Boeing engineer, and if Boeing doesn't know how to make airplanes fly, then I don't exist. Can we please let this rest? If you still believe the plane will take off the ground, please test it before you argue it again.
QUOTE ("THEY"+Nov 2 2005, 08:43 PM)
Sorry to butt in folks, but I just can't believe this is still being debated after three months.
If the conveyor is heading in the opposite direction, the "ground speed" of the airplane will be the speed of the conveyor belt even with the plane at rest. The requirement for the plane to fly is "air speed" causing sufficient air to flow over the wings. A conveyor heading in the opposite direction does not increase the airspeed so the conveyor only makes the wheels turn faster. It won't make the plane takeoff. If the conveyor was in the same direction as the plane's intended takeoff path, that would help.
That is from a Boeing engineer, and if Boeing doesn't know how to make airplanes fly, then I don't exist. Can we please let this rest? If you still believe the plane will take off the ground, please test it before you argue it again.
If you're saying the Boeing engineer says the plane won't take off, then they didn't understand the question..or maybe they aren't an aeronautical engineer.
There are no logical arguments in the stream for why the plane won't take off. There are plenty explaining why the plane will take off as per normal (negating the fact that the wheels will turn at exactly twice they're normal rate).
If you can understand that a hovercraft would move forward in relation to the control tower in the exact same experimental set up (I'll assume you easily can). And if you can argue that you can lean over the side of the hovercraft and touch a free spinning wheel on a spoked pole to the ground (conveyerbelt) and the wheel will turn (at double the speed of the hovercraft in relation to the control tower) but not stop the hovercraft. Then you can understand that the plane will likewise move forward (in relation to the control tower) and therefore generate lift.
Hell...why not lower 3 free spinning wheels from the hovercraft, then attach them, then put wings on it.
The only difference between an unmodified hovercraft and a plane for the sake of this experiment is that the plane uses wheels to keep it from dragging it's underside on the runway, and the hovercraft uses a cushion of air.
If the conveyor is heading in the opposite direction, the "ground speed" of the airplane will be the speed of the conveyor belt even with the plane at rest. The requirement for the plane to fly is "air speed" causing sufficient air to flow over the wings. A conveyor heading in the opposite direction does not increase the airspeed so the conveyor only makes the wheels turn faster. It won't make the plane takeoff. If the conveyor was in the same direction as the plane's intended takeoff path, that would help.
That is from a Boeing engineer, and if Boeing doesn't know how to make airplanes fly, then I don't exist. Can we please let this rest? If you still believe the plane will take off the ground, please test it before you argue it again.
If you're saying the Boeing engineer says the plane won't take off, then they didn't understand the question..or maybe they aren't an aeronautical engineer.
There are no logical arguments in the stream for why the plane won't take off. There are plenty explaining why the plane will take off as per normal (negating the fact that the wheels will turn at exactly twice they're normal rate).
If you can understand that a hovercraft would move forward in relation to the control tower in the exact same experimental set up (I'll assume you easily can). And if you can argue that you can lean over the side of the hovercraft and touch a free spinning wheel on a spoked pole to the ground (conveyerbelt) and the wheel will turn (at double the speed of the hovercraft in relation to the control tower) but not stop the hovercraft. Then you can understand that the plane will likewise move forward (in relation to the control tower) and therefore generate lift.
Hell...why not lower 3 free spinning wheels from the hovercraft, then attach them, then put wings on it.
The only difference between an unmodified hovercraft and a plane for the sake of this experiment is that the plane uses wheels to keep it from dragging it's underside on the runway, and the hovercraft uses a cushion of air.
QUOTE
The only difference between an unmodified hovercraft and a plane for the sake of this experiment is that the plane uses wheels to keep it from dragging it's underside on the runway, and the hovercraft uses a cushion of air.
Wrong. The airplane requires airflow over the wing to get it's lift, while your hovercraft does not. "THEY" is correct in her post. I agree with her, let it rest...
can you say the hovercraft will stay in one spot in relation to the control tower??
The hovercraft is experiencing negligible friction (or none, if the skirt is not even touching), and has lift without having to move forward through the air. So there's no normal force acting in that situation, and thus no friction. The plane DOES have a normal, since the weight of the plane (as experienced by the conveyor) could only decrease if it had lift - which it doesn't.
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ps that offer is good for anyone who wants to argue the plane will take off
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lets reuse the bike example.
If I sit on a treadmill and pedal the bike, then the treadmill matching my speed will keep me from going anywhere. If I get off the treadmill and push the bike from beside the treadmill, the wheels turn faster but the bike moves at whatever speed I push it.
For those saying that friction is the key, one must assume that the plane is trying to take off as normal. If there were that much friction normally present without breaks on that the engines at full power can't overcome the friction, then every plane would snap its undercarraige on landing from that much applied force.
And for the record, screwing around in my college physics class we did try this. We took a wood rod, put it through a wheel and ran on a treadmill. It rolled perfectly fine. We then took a wheel which would only move if you turned a crank (much like a car) and we had to crank it at a much higher speed then the treadmill. A convayer only works if the friction is high enough and a wheel trying to turn in the opposite direction of the convayer is about as high of friction as you can get.
If I sit on a treadmill and pedal the bike, then the treadmill matching my speed will keep me from going anywhere. If I get off the treadmill and push the bike from beside the treadmill, the wheels turn faster but the bike moves at whatever speed I push it.
For those saying that friction is the key, one must assume that the plane is trying to take off as normal. If there were that much friction normally present without breaks on that the engines at full power can't overcome the friction, then every plane would snap its undercarraige on landing from that much applied force.
And for the record, screwing around in my college physics class we did try this. We took a wood rod, put it through a wheel and ran on a treadmill. It rolled perfectly fine. We then took a wheel which would only move if you turned a crank (much like a car) and we had to crank it at a much higher speed then the treadmill. A convayer only works if the friction is high enough and a wheel trying to turn in the opposite direction of the convayer is about as high of friction as you can get.
QUOTE (Justavian+Nov 2 2005, 09:50 PM)
The hovercraft is experiencing negligible friction (or none, if the skirt is not even touching), and has lift without having to move forward through the air. So there's no normal force acting in that situation, and thus no friction. The plane DOES have a normal, since the weight of the plane (as experienced by the conveyor) could only decrease if it had lift - which it doesn't.
ok...look at it this way then.
same set up but the plane has a smooth boat hull underbody (no wheels) made for taking off in water and ice and snow, and the conveyerbelt is made of ice or some other equally low friction substance (assume for all intents and purposes it's friction is negligible). now the plane taking off from a frozen lake will experience some friction but can still generate plenty of air speed to take off. so what if it had to deal with twice the amount of (negligible)friction due to the take off surface moving backwards the same speed that the plane is moving forwards in relation to the control tower. it could still take off.
doubling the amount of friction on the hull of the plane or on the ball bearings does not stop it from moving forward.
if you equip the regular plane with a set of good wheels (say the mechanic gives the wheel a spin in the shop and it rotates 10 times) or a not so good set of wheels(gives it a spin and it rotates 5 times), the not so good wheels won't keep the plane in one spot on the run way even though they may have double the friction.
ok...look at it this way then.
same set up but the plane has a smooth boat hull underbody (no wheels) made for taking off in water and ice and snow, and the conveyerbelt is made of ice or some other equally low friction substance (assume for all intents and purposes it's friction is negligible). now the plane taking off from a frozen lake will experience some friction but can still generate plenty of air speed to take off. so what if it had to deal with twice the amount of (negligible)friction due to the take off surface moving backwards the same speed that the plane is moving forwards in relation to the control tower. it could still take off.
doubling the amount of friction on the hull of the plane or on the ball bearings does not stop it from moving forward.
if you equip the regular plane with a set of good wheels (say the mechanic gives the wheel a spin in the shop and it rotates 10 times) or a not so good set of wheels(gives it a spin and it rotates 5 times), the not so good wheels won't keep the plane in one spot on the run way even though they may have double the friction.
If this is a real plane, it won't take off. If it experiences zero rolling friction (the tires don't deform even microscopically), and zero friction in its bearings - then it will. If it experiences any friction at all, the conveyor will either cause the wheels to fail, or it will exert enough rearward force to counteract any forward movement - since the conveyor can move at any speed and can react instantaneously.
Actually, friction might not even matter - meaning that even if the tire does NOT deform, and even if the bearings are perfect, it still won't take off. Provided that there is enough friction to prevent the wheels from slipping.
Part of the thrust of the engines must necessarily go into adding to the angular momentum of the wheels. The conveyor must simply speed up until that force is too much for the engines. This might be a ridiculously high speed, but still within the bounds of the problem.
Part of the thrust of the engines must necessarily go into adding to the angular momentum of the wheels. The conveyor must simply speed up until that force is too much for the engines. This might be a ridiculously high speed, but still within the bounds of the problem.
QUOTE (Justavian+Nov 2 2005, 10:52 PM)
If this is a real plane, it won't take off. If it experiences zero rolling friction, and zero friction in its bearings - then it will.
but if real wheels were designed that poorly, with such substantial friction not being overcome by their design so that they could spin at x number of rotations/second but not 2x,( 100 rotations/second but not 200), then car racing would be all about wheel ball bearing design...not engine design. Wheels ball bearing assemblies are designed specifically to make friction for all intents and purposes...negligable
so doubling the amount of friction on the ball bearing assembly of the plane will not stop it from taking off.
but if real wheels were designed that poorly, with such substantial friction not being overcome by their design so that they could spin at x number of rotations/second but not 2x,( 100 rotations/second but not 200), then car racing would be all about wheel ball bearing design...not engine design. Wheels ball bearing assemblies are designed specifically to make friction for all intents and purposes...negligable
so doubling the amount of friction on the ball bearing assembly of the plane will not stop it from taking off.
QUOTE
Wheels ball bearing assemblies are designed specifically to make friction for all intents and purposes...negligable
Try driving your car at 10,000 mph. The heat from the friction will not be negligible. I think i established earlier that the conveyor will continuously speed up if the plane moves forward relative to a fixed point. The conveyor, in order to reach its "goal" of keeping the plane in one place (which is the same as matching its speed) can theoretically reach any speed.
QUOTE (Justavian+Nov 2 2005, 11:05 PM)
Actually, friction might not even matter - meaning that even if the tire does NOT deform, and even if the bearings are perfect, it still won't take off. Provided that there is enough friction to prevent the wheels from slipping.
Part of the thrust of the engines must necessarily go into adding to the angular momentum of the wheels. The conveyor must simply speed up until that force is too much for the engines. This might be a ridiculously high speed, but still within the bounds of the problem.
but now your saying the conveyerbelt is designed to move at any speed necessary to keep the plane from moving. that's a whole different question.
that would be dependent on the wheel, ball bearing, aircraft structure design...you could say then:
"if the conveyerbelt moved backwards at 500 mph could the plane take off"
but that wasn't the question.
the plane takes off in the original question
Part of the thrust of the engines must necessarily go into adding to the angular momentum of the wheels. The conveyor must simply speed up until that force is too much for the engines. This might be a ridiculously high speed, but still within the bounds of the problem.
but now your saying the conveyerbelt is designed to move at any speed necessary to keep the plane from moving. that's a whole different question.
that would be dependent on the wheel, ball bearing, aircraft structure design...you could say then:
"if the conveyerbelt moved backwards at 500 mph could the plane take off"
but that wasn't the question.
the plane takes off in the original question
QUOTE (Justavian+Nov 2 2005, 11:16 PM)
QUOTE
Wheels ball bearing assemblies are designed specifically to make friction for all intents and purposes...negligable
Try driving your car at 10,000 mph. The heat from the friction will not be negligible. I think i established earlier that the conveyor will continuously speed up if the plane moves forward relative to a fixed point. The conveyor, in order to reach its "goal" of keeping the plane in one place (which is the same as matching its speed) can theoretically reach any speed.
the question wasn't about the conveyerbelt moving at any speed necessary to keep the plane from taking off
QUOTE (dirak+Jul 19 2005, 09:53 AM)
A plane is standing on runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in opposite direction).
The question is:
Will the plane take off or not? Will it be able to run up and take off?
Therefore the wheels rotate at double the velocity they would otherwise.
If the plane moves forwards at 10 mph (relative to the control tower) and the belt move backwards at 10 mph (relative to the control tower), the wheels are turning at 20mph. The friction on the ball bearing assembly is double what it would normally be in this situation, but thankfully they're quite capable of handling it.
The plane takes off.
The belt is not moving at whatever speed is necessary to keep the plane in one spot (or until the wheel assembly seizes and falls off).
so the plane moves forward, but with it's wheels turning double what they would normally have to
The question is:
Will the plane take off or not? Will it be able to run up and take off?
Therefore the wheels rotate at double the velocity they would otherwise.
If the plane moves forwards at 10 mph (relative to the control tower) and the belt move backwards at 10 mph (relative to the control tower), the wheels are turning at 20mph. The friction on the ball bearing assembly is double what it would normally be in this situation, but thankfully they're quite capable of handling it.
The plane takes off.
The belt is not moving at whatever speed is necessary to keep the plane in one spot (or until the wheel assembly seizes and falls off).
so the plane moves forward, but with it's wheels turning double what they would normally have to
QUOTE (STAGGERBOT+Nov 2 2005, 11:49 PM)
The plane takes off.
Alan Mulally just phoned me and asked for me to ask you which planet this works on? He is interested in moving R&D to that planet, and then start a new manufacturing facility. Boeing stock will skyrocket!
Alan Mulally just phoned me and asked for me to ask you which planet this works on? He is interested in moving R&D to that planet, and then start a new manufacturing facility. Boeing stock will skyrocket!
QUOTE ("THEY"+Nov 3 2005, 12:21 AM)
QUOTE (STAGGERBOT+Nov 2 2005, 11:49 PM)
The plane takes off.
Alan Mulally just phoned me and asked for me to ask you which planet this works on? He is interested in moving R&D to that planet, and then start a new manufacturing facility. Boeing stock will skyrocket!
I went over this already. The speed of the plane must be measured by the simulated ground speed as measured by the tires. If the belt is matching the speed of the aircraft, it keeps it in one spot (provided that the wheels do not slip on the belt). If the plane moves at ALL relative to some fixed point, the wheels spin faster and conveyor registers a higher simulated ground speed. It speeds up to compensate.
Say a plane normally takes off at 140mph air speed. The plane gets up to 140mph ground speed, but there's a tail wind of 10mph. So now it needs to increase the ground speed to 150mph to take off.
If a conveyor were moving at a constant 10mph backwards, the plane would also have to have a ground speed of 150mph to take off (if there was no wind). But the conveyor can vary its speed, to match the forward movement felt by the wheels (aka the simulated ground speed). But everytime the conveyor speeds up, the tires speed up, and this forces the conveyor to speed up even more. No matter how fast the wheels turn, the conveyor moves such that the wheels cover exactly zero ground relative to an external fixed point. This is the definition of matching its speed. So after a minute or two of applying power, the conveyor may have sped up to 10,000 mph or more.
If the plane were some body that was losing no energy to the surroundings, and all the thrust was used to move the plane, it would take off. But the plane must expend energy to spin the wheels (albeit indirectly), and it loses energy in the form of friction of bearings, and the rolling friction that deforms the tires. The only question is whether the wheels and tires fail, or if the thrust from the engines is no longer to add angular momentum to the wheels - which it must do in order to increase its simulated ground speed.
Alan Mulally just phoned me and asked for me to ask you which planet this works on? He is interested in moving R&D to that planet, and then start a new manufacturing facility. Boeing stock will skyrocket!
QUOTE ("THEY"+Nov 2 2005, 08:43 PM)
Sorry to butt in folks, but I just can't believe this is still being debated after three months.
If the conveyor is heading in the opposite direction, the "ground speed" of the airplane will be the speed of the conveyor belt even with the plane at rest. The requirement for the plane to fly is "air speed" causing sufficient air to flow over the wings. A conveyor heading in the opposite direction does not increase the airspeed so the conveyor only makes the wheels turn faster. It won't make the plane takeoff. If the conveyor was in the same direction as the plane's intended takeoff path, that would help.
That is from a Boeing engineer, and if Boeing doesn't know how to make airplanes fly, then I don't exist. Can we please let this rest? If you still believe the plane will take off the ground, please test it before you argue it again.
If the conveyor is heading in the opposite direction, the "ground speed" of the airplane will be the speed of the conveyor belt even with the plane at rest. The requirement for the plane to fly is "air speed" causing sufficient air to flow over the wings. A conveyor heading in the opposite direction does not increase the airspeed so the conveyor only makes the wheels turn faster. It won't make the plane takeoff. If the conveyor was in the same direction as the plane's intended takeoff path, that would help.
That is from a Boeing engineer, and if Boeing doesn't know how to make airplanes fly, then I don't exist. Can we please let this rest? If you still believe the plane will take off the ground, please test it before you argue it again.
QUOTE ("THEY"+Nov 2 2005, 09:55 PM)
Hey Staggerbot, I have some gorgeous ocean waterfront property in Arizona for sale. It is a steal at $100,000 US dollars. Interested? The view is absolutely amazing, and you won't have neighbors for a mile on either side. I hate to sell the property, but I am trying to afford a college education for my ten year old daughter known here as "they"2.
ps that offer is good for anyone who wants to argue the plane will take off
ps that offer is good for anyone who wants to argue the plane will take off
QUOTE
The belt is not moving at whatever speed is necessary to keep the plane in one spot (or until the wheel assembly seizes and falls off).
I went over this already. The speed of the plane must be measured by the simulated ground speed as measured by the tires. If the belt is matching the speed of the aircraft, it keeps it in one spot (provided that the wheels do not slip on the belt). If the plane moves at ALL relative to some fixed point, the wheels spin faster and conveyor registers a higher simulated ground speed. It speeds up to compensate.
Say a plane normally takes off at 140mph air speed. The plane gets up to 140mph ground speed, but there's a tail wind of 10mph. So now it needs to increase the ground speed to 150mph to take off.
If a conveyor were moving at a constant 10mph backwards, the plane would also have to have a ground speed of 150mph to take off (if there was no wind). But the conveyor can vary its speed, to match the forward movement felt by the wheels (aka the simulated ground speed). But everytime the conveyor speeds up, the tires speed up, and this forces the conveyor to speed up even more. No matter how fast the wheels turn, the conveyor moves such that the wheels cover exactly zero ground relative to an external fixed point. This is the definition of matching its speed. So after a minute or two of applying power, the conveyor may have sped up to 10,000 mph or more.
If the plane were some body that was losing no energy to the surroundings, and all the thrust was used to move the plane, it would take off. But the plane must expend energy to spin the wheels (albeit indirectly), and it loses energy in the form of friction of bearings, and the rolling friction that deforms the tires. The only question is whether the wheels and tires fail, or if the thrust from the engines is no longer to add angular momentum to the wheels - which it must do in order to increase its simulated ground speed.
I can't believe people are still arguing this.
The problem with this question is a false assumption in the premise, namely that the conveyor belt will keep the plane stationary. If that were possible, then of course it woudln't take off. Unfortunately, the conveyor belt can't do that, so the plane will roll along and take off like normal, except that it's wheels will be rolling faster than normal. That's all there is to it.
The problem with this question is a false assumption in the premise, namely that the conveyor belt will keep the plane stationary. If that were possible, then of course it woudln't take off. Unfortunately, the conveyor belt can't do that, so the plane will roll along and take off like normal, except that it's wheels will be rolling faster than normal. That's all there is to it.
since thge engines push against the air and not propel by the wheels,it should take off.(theoretically)
OK, first, do we agree that the “speed of the plane” referenced in the orignal scenario is measured in relation to the ground (groundspeed)? Second, do we agree that that the speed of the conveyer is also measured in relation to the ground? If these are the correct assumptions, based on the original scenario, then consider this:
When the groundspeed of the aircraft is 0 mph, the conveyer will also be stationary (easy enough).
When the groundspeed of the airplane is 10 mph, the conveyor is moving 10 mph in the opposite direction and the wheels are rolling at 20 mph in relation to the conveyor (still following?).
When the groundspeed of the airplane is 100 mph, the conveyer is moving 100 mph in the opposite direction and the wheels are rolling at 200 mph in relation to the conveyer (duh).
When the groundspeed of the airplane is 200 mph, the conveyer is moving 200 mph in the opposite direction and the wheels are rolling at 400 mph in relation to the conveyer (duh). But, the airplane is still moving forward at 200 mph….and takes flight!
This is really not a physics problem at all (except for the obvious physics involved with flying, which really don’t matter if we all agree that the plane will become airborne at 200 mph groundspeed, assuming no head or tail wind). This is a logic problem!!
Think about it, if the conveyer speeds up to match the groundspeed of the aircraft in the opposite direction, and this is supposed to keep the airplane from moving forward at all, then the groundspeed of the airplane is 0 mph, so the speed of the conveyer (which matches the groundspeed of the airplane, as specified) is also 0 mph. By definition of the problem, if the airplane does not move in relation to the ground even when operating at full thrust (which is what the “non-flyers” seem to be saying) then the convyer is also stationary. This is a “Catch-22”. The very fact that the problem is defined as follows:
“ A plane is standing on runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).
The question is: Will the plane take off or not? Will it be able to run up and take off?”
and states that “the plane moves in one direction” tells you that the plane moves in one direction. If you hold that the plane remains stationary with respect to the ground, then you are negating the premise upon which the whole problem is based. READ THE SCENARIO!!!
When the groundspeed of the aircraft is 0 mph, the conveyer will also be stationary (easy enough).
When the groundspeed of the airplane is 10 mph, the conveyor is moving 10 mph in the opposite direction and the wheels are rolling at 20 mph in relation to the conveyor (still following?).
When the groundspeed of the airplane is 100 mph, the conveyer is moving 100 mph in the opposite direction and the wheels are rolling at 200 mph in relation to the conveyer (duh).
When the groundspeed of the airplane is 200 mph, the conveyer is moving 200 mph in the opposite direction and the wheels are rolling at 400 mph in relation to the conveyer (duh). But, the airplane is still moving forward at 200 mph….and takes flight!
This is really not a physics problem at all (except for the obvious physics involved with flying, which really don’t matter if we all agree that the plane will become airborne at 200 mph groundspeed, assuming no head or tail wind). This is a logic problem!!
Think about it, if the conveyer speeds up to match the groundspeed of the aircraft in the opposite direction, and this is supposed to keep the airplane from moving forward at all, then the groundspeed of the airplane is 0 mph, so the speed of the conveyer (which matches the groundspeed of the airplane, as specified) is also 0 mph. By definition of the problem, if the airplane does not move in relation to the ground even when operating at full thrust (which is what the “non-flyers” seem to be saying) then the convyer is also stationary. This is a “Catch-22”. The very fact that the problem is defined as follows:
“ A plane is standing on runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).
The question is: Will the plane take off or not? Will it be able to run up and take off?”
and states that “the plane moves in one direction” tells you that the plane moves in one direction. If you hold that the plane remains stationary with respect to the ground, then you are negating the premise upon which the whole problem is based. READ THE SCENARIO!!!
Also, Justavian's statement that "the speed of the plane must be measured by the simulated ground speed as measured by the tires." is incorrect. Again, read the scenario. It clearly states that "the plane moves in one direction" not that the wheels of the plane are turning at X rpm. How can anyone expect to solve a physics problem, or any problem for that matter, if you cannot read and comprehend the problem!!
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