31st October 2005 - 05:02 PM
QUOTE (Justavian+Oct 31 2005, 04:33 PM)
The emphasis always gets placed on the region of low pressure above the wing that makes the wing work. But it seems to me that the more important parameter is the lift.
The low pressure above the wing IS lift. However, you're right that a wing experiences other forces that contribute to the lift.
Here is an article on How Airplanes Work
that will give you a pretty good explanation of lift (without all the math).
Thanks for that Link. Evidently, according to that link my approach is Newtonian rather than Bernoulian. That's a very helpful distinction for me. I'll examine this a bit more closely via the How Stuff Works link.
31st October 2005 - 05:03 PM
An increase of the flows of air over volume and density of that foil, produces lift, within that chamber.
The internal chamber of an airfoil, is more stead state, so the relationship is counted as volume and density.
This equals, to a part lift.
31st October 2005 - 05:39 PM
You can explain lift totally using Newtonian physics. Of course it is this Newtonian physics which support Bernoulli's principle, so that doesn't really answer the question.
What you can't do is explain lift via the idea that air travels faster over the longer curved surface, thus reducing pressure above the wing, i.e. "the equal transit theory". In your typical wing, the faster airflow above the wing more than accounts for the difference in length and if you put a wing in a air tunnel with smoke to track the different flows, you quickly realize that the air above the wing beats its "counterpart" below the wing to the trailing edge, what is also apparent is the direction of flow behind the wing is downward. It is this change in DIRECTION of the airflow which is normally not seen in your typical diagram of how wings work.
A wing "bends" the local airflow, it MUST displace an equal weight of air downward for a plane to stay aloft. This again is basic Newtonian physics.
HOW it actually does so is actually quite difficult to describe as there are a number of different elements to how a wing produces lift. (boundary layers, wing curvature, angle of attack, air flow at wing tips etc)
While a longer top curvature of a wing may increase the efficiency of a wing, it is not necessary for a wing to produce lift. A flat wing will, at the proper angle of attack, produce both displacement from the bottom surface and bending of the airflow, just like a curved surface wing.
What may help to visualize what is happening is that the airflow above the wing (for a distance of ~ 1/2 of the wingspan) is bent and accelerated downward. Above stall speed this accelerated flow is essentially vertical and since the wing is moving laterally the flow just misses the trailing edge of the wing. The greater the wing curvature (or thickness, or angle of attack) the greater the bending force, which is why there is a definite relationship between these and chord length. Below stall speed the wing doesn't "get out of the way" fast enough to generate sufficient lift to fly. Above a critical angle of attack and the bending becomes so extreme as to create turbulence, thus breaking up the smooth flow downward, thus the wing "stalls".
1st November 2005 - 03:08 AM
Bernoulli's is over a partial recess.
All I said, is volume of air, that is steady state, not Bernoulli's.
2nd November 2005 - 07:42 AM
Ummm, it's really much simpler than that.
You see, as the air rushes over the top surface faster, there's less time for the atoms in that air to collide with the wing. Those collisions are what creates pressure, so that means it exerts less pressure. That's what they mean by the "region of low pressure above the wing." The air rushes faster over the wing than under it, so the pressure is less on top than on the bottom.
But what they often forget to say is that the pressure on the bottom of the wing presses it upward because of the reduced pressure above it. Just like if you boil water inside a can that has a lid that seals, then when the water is boiling good, you cap the can and turn off the heat. As the steam condenses back into water inside the can, its pressure stops holding the can up against the pressure of the atmosphere- and the can is crushed. If you have a can that seals well, try it for yourself.
The pressure of air is 14 pounds per square inch, and if you think about it, you'll see that if there are a hundred forty-four square inches in a square foot, then that's 2016 pounds- a ton!- per square foot. No wonder the can is crushed! A small fraction of that pressure is more than enough to lift a wing that is hundreds of square feet in area, and the aircraft attached to it, as long as the wing doesn't break off.
Yes, it really does work exactly the way they say it does.
2nd November 2005 - 06:58 PM
Your explanation is fine given that air does move faster over the top of the wing, however what is normally taught incorrectly is WHY the air is moving faster over the top surface of the wing.
The most common, but incorrect explanation is known as the "equal transit theory", which is essentially that the air molecules that are split from each other at the leading edge of the wing have to "meet up" again with their counterparts at the trailing edge of the wing. Transit times for these two are presumed to be the same. Since the upper surface is curved the air molecules on the top surface have a longer distance to go, hence they have to go faster. Thus Bernoulli, thus lift.
Looks good on your basic HS science text book even if it is not correct.
Finally, 14 psi is at sea level but planes fly over 50,000 ft up.
Air pressure is not a fixed value but is dependent on altitude. Which is why lift is proportional to air speed at a given altitude. For a given aircraft (and angle of attack) the higher you fly the faster you have to go.
16th November 2006 - 05:53 PM
Finally someone gets it right!..
I was taught this early in my real flying training, in simple terms:
Lift is produced by down wash, dynamic pressure against the bottom surface of the wing (angle of attack versus airspeed).. The low static pressure above the wing is a bi-product of this lift - it's not the cause of it.
I haven't met a pilot schooled in Bernoulli theory who has been able to explain to me how a symmetrical wing works according to Bernoulli, much less how a basic aerobatic trainer, with it's traditional cambered wings, manages inverted flight..
Yet I wonder why this crap is still deemed acceptable theory in textbooks and documentaries...