Airfoil vs. Flat Wings?

I built a model airplane which is rubber band powered. At first I built the wing of the shape of an airfoil and I can assure you that the shape was perfect. When I tested the plane, it just kept on going and going straight with very high speed but never took off. I guessed that the weight was more. So, after removing the weight it still didn't take off.

I then used just a flat piece of cardboard for the wing with a slight tilt, so that it would deflect the air downward and produce the upward thrust. I then tested the plane, and even though the wing was heavier than the airfoil one and the plane moved on the ground slower than before maybe due to increased drag and weight.....the plane took off just beautifully just after a few seconds of launch!!!

I don't know what was the reason, the airfoil shape was perfect, the surface area of the both wings was same. I guess that flat wings produce more lift.....I doubt that the airfoil produced any lift at all, or maybe they require significantly higher airflow over them than flat tilted wings to produce a fair amount of lift...can you tell me what could be the reason?

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The Bernoulli theory accords for only one third of the lift an aerofoil provides, the rest is through downdraft. With a small airplane such as yours, an aerofoil is basically useless, although a curved section might be.

Leighten2 years ago

If you adjust the back of the wing down you should be able to achieve the same result.

Jaycub4 years ago
It is about creating high and low pressure zones, and redirecting the airflow. Where the wing curves out of the incoming air flow a low pressure zone is created, where the wing curves into the incoming air flow a high pressure zone is created.

Check out the page on my website about aerodynamic propulsion: http://www.ourbadscience.com/#!aerodynamic/c9w4
Aerodynamic propulsion 5.jpg
kelseymh4 years ago
Did you tilt the airfoil, or did you mount it flat? Any airfoil needs a non-zero angle of attack (tilt) for takeoff and climb. Of course, if the angle of attack is too high, the airflow over the top surface will detach and the will will stall.

Real aircraft have control surfaces (flaps, leading-edge slats, etc.) so that the airfoil shape and angle of attack can both be varied to match flight conditions. For your model, you need to choose some intermediate configuration which lets you do everything without control surfaces.
Wisaam (author)  kelseymh4 years ago
No, the angle of attack was zero for the airfoil, because I was taught that even then it produces lift, you know according to the bernouli's explanation. So, if even then you have to have a non-zero angle of attack then why use an airfoil shape, you can achieve the same thing with tilted flat wing?
This is a common misconception.
Look at it this way - An aerofoil with a flat bottom will produce lift according to Bernoulli's principles.

If you turn that aerofoil over so the flat is on the top you might expect the aircraft to fall down. It doesn't - Flat aerofoils fly almost as well as curved ones.

In the same way a lot of high performance aircraft have symmetrical aerofoils - These surly wouldn't fly at all if we simply relied on Bernoulli's principle. Yet they do.

A large part of the lift is created by the angle of incidence. That is the positive angle the wing makes with the air it meets as the aircraft flies. This positive angle forces air downwards creating an upwards reaction.

It can be shown that with a totally flat wing the air "shadow" over the top of the wing caused by the angle of attack creates a sort of false aerofoil shape giving more lift through Bernoulli's principles.

http://en.wikipedia.org/wiki/Angle_of_attack

gives you the technical terminology and maths.

http://flitetest.com/articles/FT_Flyer_Scratch_build

These guys will show you how to et about building successful aircraft at a very low cost.
Wisaam (author)  rickharris4 years ago
Yes I very well know about this misconception but this is what is taught everywhere. So, am i right? I don't really have to use airfoil shaped wings, I can make them flat with a non-zero incidence angle and they'll do the same thing, at least for my small scale models.
Both flat and airfoil need SOME positive angle of attack.
Yes a flat wing with the appropriate angle of attack will fly - produce lift. This is particularly right in models.

Your trying to get a rubber powered model to rise off the ground, this can be done but is a waste of power - You should hand launch a rubber powered model so all the power is used to make a longer flight.

Watch the Your tube videos i linked to before.