Geoff Bye 2 Posted November 6, 2008 Share Posted November 6, 2008 Hi All.Just been reading the article in Novembers RCME about how aerofoils work and got me thinking on an old problem i have with it all. My problem is this. all the theory assumes air flowing over the wing. But this is not the case for a flying machine. the air is static ( no wind) and the wing is moving through the air. If the air is not flowing how can bernouli have an effect. The molecules of air can only be moving up and down with the profile of the wing as it passes by. so they have no rearwards velocity with which to speed up and slow down to change the pressure and thus give lift. the molecules of air have a relative velocity to the wing but the body of air is still not flowing. please someone put my mind at rest. and i've got me tin at on just in case. regards Geoff Quote Link to comment Share on other sites More sharing options...
Bruce Richards Posted November 6, 2008 Share Posted November 6, 2008 If I am flying my model from a moving car the air is flowing at a fair old lick. Its all relative. Thus what is important is whether the air is flowing with respect to the wing and not the ground. Quote Link to comment Share on other sites More sharing options...
Ernie Posted November 6, 2008 Share Posted November 6, 2008 I just cant see the difference between air flowing over a wing and the wing moving through the air, but then, I don't do scienceernie Quote Link to comment Share on other sites More sharing options...
Geoff Bye 2 Posted November 6, 2008 Author Share Posted November 6, 2008 My Personal theory is this. Bernouli has little to do with lift generation. as the air is not flowing.what force is applied to the air molecules that would cause them to speed up around the top surface, if they have further to go they should just arrive later than the below molecules. if you blow over the wing then bernouli does have an effect as the air has velocity and thus can be accelerated around the curvature, in the same way a venturi works in a carb. and you actually have something imparting a flow regime in the air such as your lungs putting energy into the air. there are 2 forces primarily at work. firstly the angle of attack ( AOA ) is effectively compressing the air under the wing generating some of the lift, hence why a flat plate angled up will generate lift. Secondly, as the wing passes through the air, the molecules of air are pushed up and out of the way, this is drag.as the molecules now try to re-attach themselves to the curved surface as it falls away, (coander effect) the wing is sucked up to meet the pulling down of the molecules ( Newton ).If the curvature is too great or the AOA is to steep the coander effect is not sufficient to maintain attachment to the surface and flow seperation occurs. RegardsGeoff ( anti bernouli league ) Quote Link to comment Share on other sites More sharing options...
Former Member Posted November 6, 2008 Share Posted November 6, 2008 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Geoff Bye 2 Posted November 6, 2008 Author Share Posted November 6, 2008 Think of how a cheap air brush works........blow fast moving air over an open pipe with one end dipped in paint..........it sucks the paint up the pipe........The faster moving air over the top of the aerofoil sucks the wing upward. this is where my problem comes in. With an air brush you have blown the air over the pipe.the compressor inputs energy to the airflow and accelerates it. in the sky the air is not moving backwards at all and what force accelerates the air as energy will be needed to do this. in order for the relative speed of the air to go up the wing will have to slow down as it is the only thing moving forwards. confused as ever. don't do physics but am a chartered engineer with own engineering business designing engines.Geoff Quote Link to comment Share on other sites More sharing options...
Former Member Posted November 6, 2008 Share Posted November 6, 2008 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Bruce Richards Posted November 6, 2008 Share Posted November 6, 2008 If you were a spider sitting on the top of the wing you could not tell the difference between air flow caused by wind and air flow caused by forward movement of the model. The air can't tell the difference either and it behaves in the same way regardless of what caused the air flow. I think you problem is the assumption that there is no air flow if the wind is not blowing. But as I and others have said the concept of air flow is relative to your viewing point. Quote Link to comment Share on other sites More sharing options...
Geoff Bye 2 Posted November 6, 2008 Author Share Posted November 6, 2008 i agree in terms of drag, but not in terms of lift generation. consider this.we have a flat plate angled down at 45 degrees. and a tennis ball aimed at the middle of the plate.firstly the tennis ball is fired at the plate which is static. the ball will be deflected up and over the plate and carry on behind it. now we have the ball suspended in space and move the plate at the ball. the ball will now move up and forwards away from the plate as we have now accelerated it forwards in the same way we hit a golf ball. so you see the relative motions are the same but the effects are different due to which part has the energy.Geoff Quote Link to comment Share on other sites More sharing options...
Bruce Richards Posted November 6, 2008 Share Posted November 6, 2008 How can you draw a difference between a wing moving through air and air moving over a wing. The only thing that matters is the relative motion. Quote Link to comment Share on other sites More sharing options...
Myron Beaumont Posted November 6, 2008 Share Posted November 6, 2008 Just thinking about a rudder or elevator for that matter at maybe 30 plus degrees from the central position .Never heard of or experienced stalling on these surfaces. Isn't it basically a question of deflection of the flow of air/like water/like any fluid ? Yes of course the aerofoil section matters on gliders etc that are basically powered by gravity versus rising air currents compared with topography of ground that alters airflow anyway ? Ever watched a bird ? Sorry ! Time for bed ! Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted November 7, 2008 Share Posted November 7, 2008 GeoffIn your example of a tennis ball and plate. Imagine you are on the tennis ball. All you would see was the plate coming towards you. You could not tell if it was you or the plate that were actually moving. On impact the result to you on the ball would feel exactly the same. In the same way it matters not to air molecules whether they are moving towards the airfoil or the airfoil is moving towards them, the result (on the molecules) is exactly the same so Bernoulli's principle is preserved. Quote Link to comment Share on other sites More sharing options...
Former Member Posted November 7, 2008 Share Posted November 7, 2008 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Former Member Posted November 7, 2008 Share Posted November 7, 2008 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted November 7, 2008 Share Posted November 7, 2008 Very good Phil Quote Link to comment Share on other sites More sharing options...
Geoff Bye 2 Posted November 7, 2008 Author Share Posted November 7, 2008 Thanks Phil, i'll accept it doesn't matter which bit is moving. But going back to my original question.The ball is moved up and out of the way in a vertical direction.so how does the ball suddenly get some energy to accelerate backwards.it must accelerate along the surface of the wing for Bernoulli to be valid. as the velocity has to increase for the pressure to drop.or is it just that due to the curvature of the wing the molecules are accelerated up and down more and therefore the their vertical velocity increases, reducing pressure. regards Geoff Quote Link to comment Share on other sites More sharing options...
Bruce Richards Posted November 7, 2008 Share Posted November 7, 2008 Well done Phil I was planning on writing a very similar post but you saved me the trouble and you have done it much better than I would have managed. Quote Link to comment Share on other sites More sharing options...
Former Member Posted November 7, 2008 Share Posted November 7, 2008 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted November 7, 2008 Share Posted November 7, 2008 here ya go Phil.... Quote Link to comment Share on other sites More sharing options...
Former Member Posted November 7, 2008 Share Posted November 7, 2008 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Bruce Richards Posted November 7, 2008 Share Posted November 7, 2008 This is freaky I drafted an post very similar to yours Phil but scrapped it because it go too confusing. You have simplified it brilliantly. Quote Link to comment Share on other sites More sharing options...
Mike Hawkins Posted November 7, 2008 Share Posted November 7, 2008 And the air molecules travelling over the top at the higher velocity will reduce in mass as according to special relativity, so the air on top is lighter than on the bottom......just. Mv = Mo(1 - v^2 / c^2)^1/2 Actually what I have learnt in school physics this year applies well here. If you are in deep space with one other space ship at different velocity, no other reference points in an inertial frame (non accelerating) it is impossible to tell if you are moving or at rest, and you could say the other ship is approaching you and it takes x seconds to catch up and they could say you are approaching them and it took y seconds and both be right, it depends on your reference and is all relative. School science sure helps in life. Quote Link to comment Share on other sites More sharing options...
Former Member Posted November 7, 2008 Share Posted November 7, 2008 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Former Member Posted November 7, 2008 Share Posted November 7, 2008 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Geoff Bye 2 Posted November 7, 2008 Author Share Posted November 7, 2008 If the don't arrive at the same time there will be a shortage of air above the aerofoil and that means a vacuum. This is my whole point.the velocity of ball 1 and 2 is the same in the direction and relative to the arrow and therefore they arrive at the TE at the same timeball 1 only moves up and down relative to ball 2.as ball 1 is pushed up the slope the pressure increases, as it goes back down the slope it is pulling the wing up to meet it.like a camshaft pushing down a valve Quote Link to comment Share on other sites More sharing options...
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