What makes air move faster above an airplane wing?

[olivermsun]

Nonsense. Many wing sections (particularly those with high camber or flaps) s generate +ve lift at -ve AOA.

How are you defining AOA for a wing with high camber or flaps deployed? Nominal centerline?

 

[rcgldr]

Nonsense. Many wing sections (particularly those with high camber or flaps) s generate +ve lift at -ve AOA.

To make a airfoil more efficient, it helps to have it cambered and to have the leading edge extended downwards a bit so that it's oriented towards and closer to the flow separation point in front of the wing. For some cambered airfoils, this results in the leading edge being lower than the trailing edge but still generating downwash and producing lift.

Indeed, the faster moving air on top creates a lower pressure. The speed changes happen because the upper surface bulge squeezes the air like an inside-out venturi. Longer path and bulge are two descriptions of the same thing.

The reaction is not due to an inside-out venturi effect. Using the wing as a frame of reference, and assuming that the flow doesn't separate significantly, the flow follows that curved bulge on the upper surface, which requires centripetal like acceleration in order to follow that curved bulge (the flow curves from an upwash flow in front of the bulge to a downwash flow behind the bulge), otherwise a void would be created. Since the air has momentum, it can't quite follow that curve, so the pressure is reduced, such that the pressure gradient is sufficient to cause the air to follow that curved bulge without significant separation. Although the centripetal component of acceleration doesn't affect the speed of the air, the reduced pressure zone also produces an net acceleration of the air in the direction of the flow. Once sufficiently past the bulge, the pressure-gradient becomes adverse, decelerating the flow, although the pressure continues to remain below ambient until near the trailing edge where the flows from above and below a wing merge.

 

[rcgldr]

For a steady flow there is no net change in mechanical energy of the flow.

Using the wing as a frame of refrence, assume there is no net change in the mechanical energy of the diverted flow. The energy of the flow as it approaches the leading edge of the wing is the same as the energy of the diverted flow as it leaves the trailing edge of the wing. During the transition, the flow is experiences centripetal acceleration, resulting in pressure differentials that also accelerate and/or decelerate flow in the direction of flow. This is due to mechanical interaction with the wing as the surface curves into the flow below the wing, or away from the flow above the wing. So during the transition of diversion, energy changes could be occurring, violating Bernoulli principle during the transition, with the energy level of the diverted flow returning back to its original energy level only as or after the dirverted flows above and below a wing converge to form a single diverted flow.

There's still the issue for a real wing that if using the wing as a frame of reference, the speed of the outgoing diverted flow is less than the speed of the incoming flow, so energy is decreased, but if using the air as a frame of reference, then the speed of the affected air goes from zero to non-zero, and energy is increased. It would seem that an explanation for how a wing generates lift should apply in either a wing or air based frame of reference (so perhaps avoid using energy based explanations). An explanation would be different than a formula used to calculate lift since it's likely that the formula would depend on the frame of reference.

 

[Quickbobo]

OK, here's the thing. You can make a brick fly given sufficient thrust, also boards, flat disks, you grandma's pasta sauce stirring spoon, a child hand out a car window, etc. I'm not all that sure about the spinning propeller disk analogy, and if they took into account the "relative" wind for each half of the disc. When the aircraft is straight and level, each half of the disk produces equal thrust , but when the propeller is at an angle to the relative wind, such as on climb maneuvers, the downward moving half of the disk produces more thrust, that the upward, due to the differing angles of attack, to the relative wind." P- Factor" I believe. There are many wing design specs available providing lift factors, where you plug in the span, the chord depth, and you get the lift available for that configuration, from the ol' trusty Clark Y foil on a Cub, to the more "laminar flow" design of say a Lear (and you get to experience what's called the "drag bucket" in laminar designs) While the Clark'll give you more lift at a lower speed, (lower take off, and landing speeds)the drag of the design won't give much speed due to the drag it develops.More modern design laminar flow wings, while they provide less lift, develops less drag, and why they MUST have one of the 3 commonly used wing trailing edge flap designs(called lift enhancement devices). or leading edge devices such as that used on the ME 2629 (it had both, leading and trailing) in order to land at some sane speed, and not turn all that work into a smoking hole in the ground as it overran the runway. Now then craft, used in most propeller drive aerobatic displays,( take a REAL close look, frame by frame) and you'll notice the pilot must move the elevators DOWN (push forward on the stick)while inverted, diving up slightly, because his wing does NOT develop the same amount of lift upright or inverted. Lifting body designs have been around for AGES, from the "Flying Heel' to the "Flying Flapjack". I believe Bernoulli's in the lead sub sonic, and as air comprehensibility comes into play the other may have an effect.

 

[Quickbobo]

OOPS ! that should have said air compressablity. sorry spell check bugger'd it.