Re: how does the shape of the wing on an airplane affect the airplane's flight?

Hi Joella,

Thanks for your question. I'm glad you have an interesting hobby like building airplanes. When I was a child I destroyed my Dad's enthusiasm for building 'planes by wrecking a plane he was just finishing made from balsawood and tissue paper. I learned more than just that the propeller worked when it flew off the table and nose-dived to the floor.

You asked how the shape of the airplane affects the flight. I assume you mean the wings themselves, the parts which provide the lift. I had answered another question sent to the Madsci website about the related subject of the Bernoulli Principle. This is often used to explain how airplane wings or aerofoils work According to the Bernoulli Principle, air which is moving faster exerts less pressure than slower moving air. Because of the shape of the aerofoil which usually is more curved on the top surface than the bottom surface, the air travelling over the top is thought to travel faster than the air going underneath because it has to go further before it meets up with the air it was split from by the leading edge of the wing. Because of this pressure difference there is a net force upwards which provides the necessary lift.

In my answer, (http://www.madsci.org/posts/archives/2000- 07/962892716.Ph.r.html) I mentioned that I wasn't too happy with using Bernoulli's principle because I still wanted to understand why the effect occurred. When I received your question, I did some searching on the internet and found out that I wasn�t alone and that this subject is the cause of lots of argument.

One person, William Beatty, (http://www.amasci.com/wing/airfoil.html) for instance makes a strong argument for keeping clear of Bernoulli's Principle when explaining how aerofoils work. Another site makes a similar case. (http://www.uni-frankfurt.de/~weltner/Mis6/mis6.html).

One reason they use is that the argument that the air has to move faster over the top of the wing is that it has to catch up with air which has gone underneath. It seems that in wind-tunnel experiments this is not the case. Also, if you look at the Wright Brothers' wings they were very thin and there is no significant difference in the length of the upper and lower surfaces. We know however that they did manage to fly!

The main reason I sympathise with thoughts in the websites I mentioned is that if something can be explained with more basic, fundamental ideas then we shouldn't try to use anything else. In this case, we must remember the Law of Conservation of Momentum ( Momentum cannot be created or destroyed ). In order to provide a lifting force on the airplane, something else, namely the air, has to be pushed down. The role of the wing therefore is to push air downwards.

Two main features of the wing enable this to happen. One is the shape of the wing. (This is the feature emphasised in the explanation based on Bernoulli.) . The diagram comes from an excellent site explaining flight. http://www.av8n.com/how/

Usually, a wing has a curved upper side and a relatively flat lower side, the difference being the amount of 'camber'. If the angle is such that the bottom surface is effectively horizontal, the long downward curving upper surface guides the flow above the wing downwards. This is called the 'Coander' effect.

The other feature is the angle the wing is tilted at, sometimes called the 'attack angle'. It is a little easier to see how the attack angle provides lift. Even a perfectly planar surface will do. As the wing moves forward, air is deflected downwards. In other words, pressure under the wing is increased while above it, the pressure is lowered so the wing is lifted. So why don't we just use flat surfaces. The answer is strength. A curved surface provides rigidity and prevents the wing buckling. Important!

I have not talked about the angle of the wing relative to the body of the 'plane. For low-speed 'planes, the wings are usually at right angles to the body. Only high-speed 'planes use swept or 'delta' wing shapes to help cut through the air more easily and your models probably won't go that fast.

I have only given you a simple picture of course. I would guess that if you make your wings thick enough to be strong enough and use a curved top surface you could experiment with the attack angle until you get the best compromise between lift and speed. Don�t forget to make them big enough. And finally, when you test your propellers, hang onto the plane!