Why does air travel faster over a wing?

[kidsmoker]

Hi,
I was just wondering why the air going over a wing travels faster than the air going underneath? My teacher once said "because it has farther to travel" and I just accepted that at the time, but thinking about it now i don't really see why two adjacent air molecules that part company at the front of the wing should neccessarily meet up again on the opposite side. Is there a better explanation?

Thanks.

 

[mgb_phys]

Well done for questioning it !
That model for how a wing works is wrong - but is believed by a large number of scientists and engineers who should know better.

This has a good explanation of a numberof the models: http://science.howstuffworks.com/airplane5.htm

 

[rcgldr]

If you're still in contact with your teacher, show him these pictures and ask how these pre-shuttle lifting bodies with flat tops and curved bottoms fly?

m2-f2 glider version:
m2-f2.jpg

m2-f3 rocket powered version (max speed mach 1.6):
m2-f3.jpg

Wings produce lift by applying a downwards force on the air, which responds with an equal and upwards force on the wing, following Netwons 3rd law of motion. The downwards force results in a downwards acceleration of air, and the air's reaction to this downwards acceleration is an upwards force on the wing.

You're correct in that equal transit time is wrong. With a conventional wing, the air above ends up displaced further behind the wing than the air below, but note that most of the acceleration and motion of the air is downwards (corresponding to lift), with only a small amount forwards (corresponding to drag).

How wings accelerate air downwards is a combination of foward speed and effective angle of attack (air is "deflected" downwards). The final result is a significant increase in kinetic energy of the air (1/2 m v²), which would normally require a lot of work done on the air, but most of this increase in kinetic energy is offset with a decrease in pressure energy, in a Bernoulli like transition. Wing's aren't 100% efficient, so there is some work done on the air and not all of the transition is Bernoulli like.

 

[kidsmoker]

Cool thanks for those. I've read the explanation on howstuffworks and i understand it better now.