Busting an aerodynamic myth -- Planes don't fly due to Bernoulli Effect

[winkosmosis]

I still don't understand how the predicted lift from air speed difference and redirection of the flow can both be correct.

What about the example given in the NASA article about an airfoil with a more curved underside?

[fidalgoman]

It gets a little complicated. The Bernoulli Effect is real and is what is being portrayed in the lift distribution models on airfoils. This effect, airfoil shape, angle of attack and flow velocity all work together to accelerate the air mass downward at a sufficient rate to maintain the required lift to maintain the aircraft. As the aircraft gets close to the ground (ground effect) as in landing the trapped airflow pushes against the asphalt which doesn't give and creates more effective lift. Many a car has suffered troubles with more air-pressure under the car than over the top. That's why they have front air dams.

It has always been known that air displacement is the working effect. By the time you get done explaining Bernoulli effect most peoples eyes glaze over, so most books and instructors go for the simplist explanation.

[phord]

Here is how I understand how does ( subsonic) airplane flies ( eah its a wallet) seriously I have some charts that show how much the top surface and the bottom surface contribute to lift and this ratio changes when angle of attack changes . The angle of attack is measured not from the chord but from angle that produces zero lift (absolute angle of attack) flat airfoil is more draggy than conventional airfoil so the airfoll portion reduces drag next wing beats air down and finally Bernuilli principle.

[dcb]

presumably this is a low drag v/s lift arrangement (not relying on large angle of attack and extra throttle to keep flying)



You can see that if you want to make lift efficiently, you have to finesse the air a bit, i.e. rely mostly on bernoulli, not on angle of attack.


Here the lift/drag ratio peaks at ~0 degrees angle of attack (which cannot be the zero lift angle in this example then).

Also the coefficient of drag increases exponentially with angle of attack

Max lift coefficient is at about 18 degrees, but the lift/drag ratio is about 8:1 there vs about 22:1 at zero degrees. To create the same amount of lift with the 16 degree angle of attack would take almost 3 times as much power as the 0 degree.

[winkosmosis]

I don't get the top diagram. Air hitting the top leading edge pulls the wing forward, but air hitting the bottom leading edge pushes it rearward? How does the air know the difference?

[ik04]

Originally Posted by winkosmosis

I don't get the top diagram. Air hitting the top leading edge pulls the wing forward, but air hitting the bottom leading edge pushes it rearward? How does the air know the difference?

Those little lines are vectors and do not represent the total aerodynamic force. The vector diagram below the airfoil section represents a sum of the forces acting on the wing. It does not show the relative wind, which is the angle of the arrow at the leading edge of the wing.

Air is a fluid and does not compress below supersonic flow, so the effort required to pull that fluid along the upper surface of the wing becomes greater by the square of the velocity of the relative wind. That means it takes four times as much thrust to maintain level flight at twice the speed increase. If the air is forced to follow an airfoil at too high an angle of attack (the angle opposite the arrow), the flow will pull away from the wing and just tumble, that is called stalling. The airflow is no longer producing useful lift, but mostly drag. Because the drag also increases by the square of the velocity of the relative wind, there is not enough power to overcome the stall.

So the increasing angle of attack does not produce more lift because of Newton "pushing" the air downward. That would cause so much drag that in order to keep the wing from stalling, the aircraft would be forced to descend, regardless of power setting... That is how flaps and other drag-producing wing modifications help slow an airplane down for landing without stalling the wing.

A glider has no engine, but produces thrust by keeping the local flow of air over the wing as it descends. They stay aloft by carefully taking advantage of updrafts and rising warmer air. There is still Lift, weight, drag and thrust acting on the aircraft, thus the term "total aerodynamic force." Keep that force vector pointing forward and the airplane flies (regardless of whether it is climbing or descending).

Hope that didn't confuse things more...

[gone-ot]

...OK, then de-bunk how VENTURI's in CARBURETORs can't really work (wink,wink).