Physics behind the Coanda effect

[aerofreak]

Hi guys,

I was interested in the Coanda effect (what makes an airplane fly) and i was wondering if anyone, in somple terms, could explain the physics behind this to me.

Thanks in advance!

 

[rcgldr]

Coanda effect for aircraft combines two types of effects. One is related to skin friction and a concave surface, but that only affects flow in the boundary layer, and that flow is affected depending on if the flow is laminar or turbulent. The other effect is what I call void effect, or more appropriately void abhorence effect. After the peak of a cambered surface flows through a volume of air, the surface recedes from the air, and the air has to fill in what would otherwise be a void. Wiki's article on wings also explains this aspect of lift:

In that case a low pressure region is generated on the upper surface of the wing which draws the air above the wing downwards towards what would otherwise be a void after the wing had passed.

http://en.wikipedia.org/wiki/Wing#Science_of_wings

Not covered in the Wiki articleis how the air fills in that "void". Most of the time the path of least "resistance" (accceleration) is for the air to accelerate mostly downwards and somewhat forwards, so you get lift with minimal drag. The other flow pattern that occurs when the angle of attack is excessive, is for the air to form one or more vortices, sort of a giant and turbulent boundary layer, with a much lower amount of lift.

Void effect is clearly understood when streamlining high speed land vehicles. Long tapered tails are used to gradually introduce a void into the air, which allows the air to accelerate at a relatively slow inwards rate, as opposed to accelerating forwards at a much faster rate which occurs if the aft surface is essentially a vertical wall, like a bus.

 

[aerofreak]

Hey rcgldr, thanks for the info. What you explained makes sense.
Would you however also state that the Coanda effect is the reason that airplanes fly? The conservation of momentum, will cause the air to flow of the trailing edge of the wing, forcing air down, which in its turn, forces the plane up (this also explain how experimental flying saucers can fly). Or would you say that the effects you outlined above have more to do with Bernoulli´s theorum?

 

[rcgldr]

Would you however also state that the Coanda effect is the reason that airplanes fly?

It's the reason pressure is lowered above the wing. Simple deflection is the reason pressure is increased below the wing. Both effect contribute to lift, although for a conventional wing, most of the lift (more than 1/2) is due to lowering pressure above a wing.

The conservation of momentum, will cause the air to flow of the trailing edge of the wing, forcing air down, which in its turn, forces the plane up (this also explain how experimental flying saucers can fly). Or would you say that the effects you outlined above have more to do with Bernoulli´s theorum?

Downwards acceleration and flow of air starts to occur above a wing, not behind it. It's a dynamic situation where air flows downwoards towards the wing, which move forwards before that flow reaches the surface of a wing. Bernoulli explains how air reacts to pressure differentials within the air (ignoring issues like turbulence), but doesn't explain how those pressure differentials are created by interaction between a wing and the air. This website has some good but simple explanations:

http://www.avweb.com/news/airman/183261-1.html