MadSci Network: Physics
Query:

Re: If lift is caused by the shape of the wing, how do planes fly upside down?

Area: Physics
Posted By: Steve Czarnecki, senior technical staff member, systems engineering, Lockheed Martin
Date: Mon Jun 30 15:07:08 1997
Area of science: Physics
ID: 866073095.Ph
Message:

That's the kind of question a natural scientist asks, Mary! A good scientist takes the information she learns and wonders about its implications in new situations. This is what Einstein called doing a "thought experiment", so you're in good company.

A wings can indeed fly "upside down" even though its shape may have been chosen for best performance "rightside up". The short answer is that we can adjust the wing's "angle of attack" (explained below) and make the wing fly, even though it's upside down. To understand how this can be, first let's look at how a wing flies "rightside up".

I'll try to explain some key points below on why both "rightside up" and "upside down" wings can fly. I'd also recommend looking at the Take Off! Web page sponsored by NASA and the FAA that provides tons of explanations about the principles of flight plus some hands-on activities you can try at home illustrate these principles.

Any wing needs to produce an upwards force called lift to oppose the downward force of gravity. It does this by creating an area of low pressure above the wing; lift is the difference between the ordinary air pressure below the wing and the low pressure immediately above the wing.

To visualize how it does this, think of air blowing past the wing rather than the wing moving through the air. Now think of two particles of air that start off side by side, but one is destined to flow over the wing, while the other flows under the wing. Because they started side by side, they must end up side by side after passing by the wing. The physical laws governing this situation include Newton's 1st law of motion, conservation of momentum, and the continuity principle of fluid flow.

But for the two particles of air to wind up side by side after passing by the wing, the particle of air passing over the wing must speed up once it separates from its neighbor. The particle of air flowing over the wing takes a longer path than its neighbor that flowed under the wing, because the top of the wing is curved, and because the front edge of the wing is actually tilted up a bit (the leading and trailing edges of the wing are not perfectly level), making the path over the top of the wing "the long way around" compared to going under the wing. Because the air over the wing speeds up, its pressure decreases while over the wing. The fact that the air speeds up over the wing is an example of the Venturi effect, with the exact relationship between air pressure and speed governed by Bernoulli's Law. Often the effect and the speed/pressure relationship is simply called Bernoulli's Principle.

You can illustrate Bernoulli's principle by taking a piece of paper a few inches wide and perhaps a foot long, and blowing over the top surface as you hold one end to your lips. The paper will rise due to lift created by the difference in pressure above and below the paper; you created low pressure above the paper by blowing, and thereby moving, air over it.

The more that the upper surface of the wing is curved, the more lift is produced at a given speed (an aside: that's why wings on airliners extend flaps on the trailing edges at low speeds -- to create more curvature in the wing, and therefore, more lift at low speeds. Contrary to popular belief, they're not a sort of "speed brake" used to slow the airplane down). Just as increasing the curvature of a wing increases lift, increasing the angle of attack of the wing (the amount of tilt) increases the amount of lift at a given speed (this is also why airliners tend to tilt upwards in the last few minutes before landing as they slow down-- the pilot is increasing the angle of attack of the wing to generate more lift and compensate for the loss of speed). Both increasing the wing curvature and the angle of attack work to increase lift because their effect is to increase the path length for air flowing over the wing, compared to air flowing under the wing.

It turns out in practice that the amount of lift produced by a wing depends much more on the angle of attack than the exact shape of the wing -- so much so that we can take an ordinary airplane wing, turn it upside down, and still get upwards lift ***provided we tilt this "upside-down" wing upwards at the correct angle of attack***.

It won't fly very efficiently (that is, it will create much more drag and require much more engine power to fly this way) but it *will* fly! The reason this "upside down" wing flies is that air going over the wing takes a longer path and must speed up compared to wing flowing under the wing. This makes the air pressure above the wing lower than the air pressure below the wing. But that's the same thing the air is doing in our "rightside up" wing. So by properly adjusting the angle of attack, we can make the "upside down" wing fly. If we didn't properly adjust the angle of attack for our upside-down wing, the lift force would push our aircraft downwards instead of pulling it upwards.


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