PPRuNe Forums - View Single Post - Lift

Jetstream Rider · 26th June 2008, 18:41

I've been having a discussion on Wikipedia recently about an edit I made to a page concerning lift. We have come to an agreement, but I wanted to ask a couple of questions to clear something up in my mind.

I have an Aeronautical Engineering Masters from London and came across a bit on Wikipedia that talked about the equal transit time fallacy for air travelling above and below and wing surface. It was quite rightly saying this isn't true, but the said "lift is actually produced because air is deflected downward". I changed that bit and it resulted in a tit for tat reversion of what each other had written. Our discussion hinged over what caused lift and Newton's laws.

My edit said that lift is caused by the pressure field around the wing, caused by pressure changes in the air both above and below the wing caused in turn by the fact that air is "turned". I backed it up with references to Anderson's Fundamentals of Aerodynamics and some other excellent books such as Kermode and Aerodynamics for Naval Aviators.

The chap in question kept saying that lift could be calculated by working out the downwash, and that lift was equal to the downward momentum transferred to the air.

I'm quite happy with the fact that Bernoulli's equation is initially derived from Newton's Laws and that Newton's laws can be applied to a control volume to derive the Euler equations and the Navier-Stokes equations. These are of course approximations to actual flow. However, I don't think its as simple as downwash = lift.

Looking at lift on a spinning cylinder, the upwash and downwash are identical, so surely there cannot be any net downwash? Also, one can turn the trailing edge of a wing up and still produce lift, albeit not very efficiently, but without downwash.

So have I missed something? Is lift = downwash = integrated pressure around the whole body?

I'm aware that there are some books around, like Stick and Rudder, that say lift is entirely down to downwash and nothing to do with pressure distribution. I reckon this is wrong - but have I missed something?

Looking forward to a discussion.

26th June 2008, 18:41	#1 (permalink)
Jetstream Rider Join Date: Oct 2002 Location: Heathrow Posts: 289	Lift - what mechanism? I've been having a discussion on Wikipedia recently about an edit I made to a page concerning lift. We have come to an agreement, but I wanted to ask a couple of questions to clear something up in my mind. I have an Aeronautical Engineering Masters from London and came across a bit on Wikipedia that talked about the equal transit time fallacy for air travelling above and below and wing surface. It was quite rightly saying this isn't true, but the said "lift is actually produced because air is deflected downward". I changed that bit and it resulted in a tit for tat reversion of what each other had written. Our discussion hinged over what caused lift and Newton's laws. My edit said that lift is caused by the pressure field around the wing, caused by pressure changes in the air both above and below the wing caused in turn by the fact that air is "turned". I backed it up with references to Anderson's Fundamentals of Aerodynamics and some other excellent books such as Kermode and Aerodynamics for Naval Aviators. The chap in question kept saying that lift could be calculated by working out the downwash, and that lift was equal to the downward momentum transferred to the air. I'm quite happy with the fact that Bernoulli's equation is initially derived from Newton's Laws and that Newton's laws can be applied to a control volume to derive the Euler equations and the Navier-Stokes equations. These are of course approximations to actual flow. However, I don't think its as simple as downwash = lift. Looking at lift on a spinning cylinder, the upwash and downwash are identical, so surely there cannot be any net downwash? Also, one can turn the trailing edge of a wing up and still produce lift, albeit not very efficiently, but without downwash. So have I missed something? Is lift = downwash = integrated pressure around the whole body? I'm aware that there are some books around, like Stick and Rudder, that say lift is entirely down to downwash and nothing to do with pressure distribution. I reckon this is wrong - but have I missed something? Looking forward to a discussion.