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RKoch

How an airfoil works...maybe we've been wrong?

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As a kid (our car didn't have A/C back then) and sticking my hand out the window was pretty obvious how lift worked. I never understood why some people try to make things more complicated.

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Something I read recently, there are 4 theory's why wings work, therefore no one actually knows why aircraft fly. Scary but cool at the same time.

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https://en.wikipedia.org/wiki/Kutta_condition




The vorticity in the starting vortex is matched by the vorticity in the bound vortex in the airfoil, in accordance with Kelvin's circulation theorem.[1]:§ 2.14 As the vorticity in the starting vortex progressively increases the vorticity in the bound vortex also progressively increases and causes the flow over the topside of the airfoil to increase in speed. The starting vortex is soon cast off the airfoil and is left behind, spinning in the air where the airfoil left it. The stagnation point on the topside of the airfoil then moves until it reaches the trailing edge.[1]:§§ 6.2, 6.3 The starting vortex eventually dissipates due to viscous forces.

As the airfoil continues on its way, there is a stagnation point at the trailing edge. The flow over the topside conforms to the upper surface of the airfoil. The flow over both the topside and the underside join up at the trailing edge and leave the airfoil travelling parallel to one another. This is known as the Kutta condition.[5]:§ 4.8

When an airfoil is moving with a positive angle of attack, the starting vortex has been cast off and the Kutta condition has become established, there is a finite circulation of the air around the airfoil. The airfoil is generating lift, and the magnitude of the lift is given by the KuttaJoukowski theorem.[5]:§ 4.5


TL;DR lift force from airfoils is the caused by the downward deflection of air (fluid) resulting from circulation. There are other ways to create lift.

 

... or it's invisible gremlins. At some level either is a useful explanation. Neither is really a complete answer. At some level "nobody really knows" is the answer to all human understanding. Isn't it? I don't really know. Damn. Back to work.

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"Any sufficiently advanced technology is indistinguishable from magic" ;)

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As every Naval Aviator knows, you need Bernoullis to fly. The ones on the top of the wing "pull" the aircraft up, the ones on the bottom of the wing "push" the aircraft up. If the Bernoullis become detached, the plane enters the out of control flight regime.

 

So keep your Bernoullis happy and attached, and life is good!

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Old old news at least for pilots. BTW, I used to rent an airplane with a symmetrical wing that flew as well inverted as upright :)

 

Did it fly at the same relative angle of attack inverted and upright?

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Cambridge scientists have been wasting their time - this has been known for many decades. Bernoulli's principle does work, but is a weak force. A cambered airfoil produces some lift at zero angle of attack, but very little. A board produces nearly as much lift as a modern airfoil, the airfoil's main advantage is in lower drag.

 

The various theories of lift are simply mathematical models attempting to describe - not explain - the physical phenomenon. Ultimately it is just Newton's Third Law - air is pushed one way and the wing the other.

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As a kid (our car didn't have A/C back then) and sticking my hand out the window was pretty obvious how lift worked. I never understood why some people try to make things more complicated.

 

They like A/C ??

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A sail works because it changed the direction of the wind. It bends its path. Air enters the front edge at maybe 30 degrees and leaves at 180 degrees (or 0 degrees relative to its initial direction). That change in direction comes from the force of the sail and the reaction to that force moves the boat. It is much like the hand out the window. But if you try and move the air too fast, at too steep an angle, you get turbulence instead. Shape is very important.

 

If the "textbook" story about wings having the curved surface on top creating lift were correct, planes could not fly upside down, which they can. But upside down they are not as efficient and can stall quicker. It is the shape and turbulence of the airflow. But the lift is like the hand out the window.

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Old old news at least for pilots. BTW, I used to rent an airplane with a symmetrical wing that flew as well inverted as upright :)

 

Did it fly at the same relative angle of attack inverted and upright?

 

I can't recall. The trim was way off upside-down though and it gave me a headache. This also depends on the angle of the wing chord line relative to the fuselage, so it would not be easy to tell one effect from the other just looking out the windshield.

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https://en.wikipedia.org/wiki/Kutta_condition

 

The vorticity in the starting vortex is matched by the vorticity in the bound vortex in the airfoil, in accordance with Kelvin's circulation theorem.[1]:§ 2.14 As the vorticity in the starting vortex progressively increases the vorticity in the bound vortex also progressively increases and causes the flow over the topside of the airfoil to increase in speed. The starting vortex is soon cast off the airfoil and is left behind, spinning in the air where the airfoil left it. The stagnation point on the topside of the airfoil then moves until it reaches the trailing edge.[1]:§§ 6.2, 6.3 The starting vortex eventually dissipates due to viscous forces.

As the airfoil continues on its way, there is a stagnation point at the trailing edge. The flow over the topside conforms to the upper surface of the airfoil. The flow over both the topside and the underside join up at the trailing edge and leave the airfoil travelling parallel to one another. This is known as the Kutta condition.[5]:§ 4.8

When an airfoil is moving with a positive angle of attack, the starting vortex has been cast off and the Kutta condition has become established, there is a finite circulation of the air around the airfoil. The airfoil is generating lift, and the magnitude of the lift is given by the KuttaJoukowski theorem.[5]:§ 4.5

 

... There are other ways to create lift.

 

...

Like a gentle rub

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The only myth they're seeking to be destroy is the one that says that two molecules of air separated at the leading edge of the wing somehow magically meet again at the trailing edge. But to a large extent I think its more of a "lie-to-children" than anything else, helping to provide a simplistic explanation of why flow over the top of the wing is important.

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Old old news at least for pilots. BTW, I used to rent an airplane with a symmetrical wing that flew as well inverted as upright :)

Did it fly at the same relative angle of attack inverted and upright?

I can't recall. The trim was way off upside-down though and it gave me a headache. This also depends on the angle of the wing chord line relative to the fuselage, so it would not be easy to tell one effect from the other just looking out the windshield.

 

Most competition acrobats are 0 degrees incidence. Add in a thrust line on the same axis and you don't need no stinking trim wheels....

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