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  #1  
Old 11-23-2015, 04:36 PM
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apkp777 apkp777 is offline
 
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Default Incorrect Theories of "Lift"

Just read a good article from NASA. Thought it might be a stimulating discussion here. For those of us who are often asked "what makes a plane fly?" Looks like we were right when we said "it's PFM".

https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html#
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  #2  
Old 11-23-2015, 06:31 PM
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Tony, did not read the link as I am hungry. That comes first.
But, I learned 4 decades ago.... lift is created by money.
Simple physics, but true.
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  #3  
Old 11-23-2015, 07:02 PM
Ron B. Ron B. is offline
 
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I should be working for NASA as I never believed that theory. What would make anyone thing that two molecules next to each other at the leading edge would exit the wing next to each other.
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  #4  
Old 11-23-2015, 07:50 PM
Sam Staton Sam Staton is offline
 
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Default FAA theory of lift

I have always had a problem with the way lift is explained. It generally starts with Bernoulli's theorem, and the example is a Venturi (such as the throat of a carburetor). Then we are shown a wing, with the curved surface on top and a (mostly) flat surface on the bottom. We are told that this is half of a Venturi, but not told how it works, since there is no barrier for the top edge of the airflow. That's when most of us just press the PFM button and accept it. My theory, and the way it makes the most sense to me, is that the barrier (or the other surface of the half Venturi) is formed by the column of air above the surface. We know it has weight (14.7 psi at sea level, assuming a standard day), so it exerts a force on the top of the wing, and acts as a barrier. This is my rationalization of what causes lift - the Venturi effect of the atmospheric pressure acting on the top of the wing constraining the flow over the upper surface of the wing, and thus causing a reduction in pressure on the top surface in relation to the bottom surface. This also explains (to my mind, anyway) why generated lift decreases with altitude, since the weight of the column of air above the top surface of the wing decreases as altitude increases.

Just my opinion, and worth everything you paid for it.
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Last edited by Sam Staton : 11-23-2015 at 07:53 PM.
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  #5  
Old 11-23-2015, 08:40 PM
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Default Airflow

The airflow speeding up over the top of the wing theory has always left me a little confused too, but mainly because any explanations I have read always seem to talk about the flow of air molecules and the differences in pressure created over and under the wing. But first a disclaimer: I am in no way an expert in this so what I am suggesting here is more in the form of a request for my own understanding. But be warned, any long scientific words or calculations will probably make my eyes glaze over and my forehead end up resting on the keyboard. So, here goes…

So why is it we are taught to think in terms of air flowing over a wing? The way I look at it the air is stationary (discounting wind effect etc) and it is the aircraft/wing that is moving, effectively forcing the air molecules to accelerate from zero, upward and to a degree slightly forward of their starting point, and then I’m guessing slightly backward as the wing passes by. This seems to me like a different kind of action that would be more in keeping with the idea of changing the speed (from zero) and direction (up and forward) of air molecules to generate a lifting force.

Is it effectively the same thing as air flowing over a stationary wing? What am I missing here?

Clive Whittfield
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Old 11-23-2015, 11:11 PM
aerhed aerhed is offline
 
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Same difference Clive. Just like doing 50 in a 50 headwind.
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Old 11-23-2015, 11:38 PM
Robin8er Robin8er is offline
 
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"stick and rudder" says planes fly by pushing air down, which is very true. Angle of attack and dynamic pressure is all a wing cares about.

The air going faster over the top and creating less pressure on top than the bottom is true, but not really that important. More of your lift comes from having an angle of attack and dynamic pressure.

Last edited by Robin8er : 11-23-2015 at 11:44 PM.
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  #8  
Old 11-23-2015, 11:57 PM
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To mess with your minds... A PhD student at my company showed me how our principals of magnetic induction could be explained by applying relativity to electrostatic attraction.... and getting the same result! Very cool and mind bending. Maxwell = Einstein

So, I think that various theories of lift are all equivalent in the end. You can apply Newton or Bernoulli and get the same results.
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  #9  
Old 11-24-2015, 12:24 AM
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N546RV N546RV is offline
 
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Quote:
Originally Posted by Robin8er View Post
"stick and rudder" says planes fly by pushing air down, which is very true. Angle of attack and dynamic pressure is all a wing cares about.

The air going faster over the top and creating less pressure on top than the bottom is true, but not really that important. More of your lift comes from having an angle of attack and dynamic pressure.
Any time someone asks me how wings make lift, I try to reframe the conversation. There's nothing magic about an airfoil that enables it to generate lift - anyone who's stuck their hand out the window of a moving car knows this. If you deflect air downward, you create lift.

The problem isn't "how to make lift," it's "how to make lift with a minimum of drag." So instead of being magic lift-generating devices, airfoils are just aerodynamically efficient air deflectors.

Disclaimer: Not an engineer. I bailed out of an ME degree after a couple years...
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  #10  
Old 11-24-2015, 01:41 AM
Bevan Bevan is offline
 
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Yes, the mass of air being forced down results in an equal and opposite reaction... the force pushing the plane up. The airfoil is configured to do this with minimum drag so that the engine can keep up with the adding of the required energy back into the "system" to overcome that lost by drag.

IOW, apply enough thrust to pretty much any shape and weight of an object and it too will fly.

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