5 ms·

Funnily enough, there's another web page on the NASA domain that outlines the "longer path" or "equal transit" theories as the correct cause of lift on an aircr

Funnily enough, there's another web page on the NASA domain that outlines the "longer path" or "equal transit" theories as the correct cause of lift on an aircraft wing: https://www.grc.nasa.gov/www/k-12/UEET/StudentSite/dynamicsofflight.html https://www.grc.nasa.gov/www/k-12/UEET/StudentSite/dynamicso... When I was studying for my PPL, they taught us using the equal transit theory, though the instructor couldn't help himself from telling us that it was a simplification. This video from Cambridge University illustrates what happens to the airflow over an aerofoil: https://www.youtube.com/watch?v=UqBmdZ-BNig https://www.youtube.com/watch?v=UqBmdZ-BNig

nabla9 5y ago
Euler equations explain the lift, but it can be still hard to grok. Newton’s Laws of Motion gives the clearest correct explanation: If the wing stays in the air, there must be a force that is equal but opposite to the gravity. If you divide the air around the wing into equal mass chunks of air (not equal volume) and visualize the velocity and acceleration of each chunk, you get very informative picture of where and when of the lift. If the wing stays in the air, air around the wing must be accelerating downwards.
- VBprogrammer 5y ago
  Expanding on that, the downwash theory is the easiest to understand in my opinion. If you think about the flow of water over the back of a spoon then imagine increasing the amount of the water flow and the size of the spoon until you could eventually feel the force pulling your hand opposite the direction the water is deflected.
  - bbojan 5y ago
    You don't have to imagine, simply put a spoon sideways against a flow of water from the faucet and you can instantly feel a pull.
- wbeaty 5y ago
  In 2D airfoil diagrams, first the parcels approach horizontally. Then they are mysteriously accelerated upwards. Then the airfoil accelerates them downwards. Then behind the airfoil, they're mysteriously accelerated upwards again, so they depart horizontally, for zero net acceleration applied to each parcel. As a whole, air around the wing ISN'T accelerating downwards. As long as we ignore the "mysterious" part, or try to sweep it under the rug, our explanations won't satisfy. A complete explanation must include the ground surface ...and then we discover that we're actually explaining a kind of "venturi effect," where the ground is a part of the system, and those mysterious accelerations before and behind the airfoil are caused by interaction with the ground. This is a "big AHA!" situation: a 2D airfoil can only explain ground-effect flight, where the Newtonian force-pair is located between ground and the airfoil, so no net downward acceleration of air exists. (In other words, if we erase the ground from our diagram, we've just violated Newton's 3rd law.) A 2D airfoil diagram without the ground surface ...that's a sort of "inertial drive," where the upward force on the airfoil has no corresponding down-force (it does not give a net downward acceleration to the air as a whole.) Easy to fix. Just add the ground surface back in. And then confront the fact that this is a venturi, and not an explanation of flight.
wbeaty 5y ago
I don't see it. "Longer path" is an incorrect explanation for why the air above the airfoil moves faster. Yet the air above the airfoil really does move faster, as that webpage correctly describes. (That webpage doesn't say why the air moves faster! Thus they avoid the "equal transit fallacy.") - "Airplane wings are shaped to make air move faster over the top of the wing." True. Giving the airfoil some camber will change the upper/lower velocities, making it produce lift, even at zero attack. No mention of "equal transit-time."

nabla9 5y ago

Euler equations explain the lift, but it can be still hard to grok. Newton’s Laws of Motion gives the clearest correct explanation: If the wing stays in the air, there must be a force that is equal but opposite to the gravity. If you divide the air around the wing into equal mass chunks of air (not equal volume) and visualize the velocity and acceleration of each chunk, you get very informative picture of where and when of the lift. If the wing stays in the air, air around the wing must be accelerating downwards.

VBprogrammer 5y ago
Expanding on that, the downwash theory is the easiest to understand in my opinion. If you think about the flow of water over the back of a spoon then imagine increasing the amount of the water flow and the size of the spoon until you could eventually feel the force pulling your hand opposite the direction the water is deflected.
- bbojan 5y ago
  You don't have to imagine, simply put a spoon sideways against a flow of water from the faucet and you can instantly feel a pull.
wbeaty 5y ago
In 2D airfoil diagrams, first the parcels approach horizontally. Then they are mysteriously accelerated upwards. Then the airfoil accelerates them downwards. Then behind the airfoil, they're mysteriously accelerated upwards again, so they depart horizontally, for zero net acceleration applied to each parcel. As a whole, air around the wing ISN'T accelerating downwards. As long as we ignore the "mysterious" part, or try to sweep it under the rug, our explanations won't satisfy. A complete explanation must include the ground surface ...and then we discover that we're actually explaining a kind of "venturi effect," where the ground is a part of the system, and those mysterious accelerations before and behind the airfoil are caused by interaction with the ground. This is a "big AHA!" situation: a 2D airfoil can only explain ground-effect flight, where the Newtonian force-pair is located between ground and the airfoil, so no net downward acceleration of air exists. (In other words, if we erase the ground from our diagram, we've just violated Newton's 3rd law.) A 2D airfoil diagram without the ground surface ...that's a sort of "inertial drive," where the upward force on the airfoil has no corresponding down-force (it does not give a net downward acceleration to the air as a whole.) Easy to fix. Just add the ground surface back in. And then confront the fact that this is a venturi, and not an explanation of flight.

wbeaty 5y ago

I don't see it. "Longer path" is an incorrect explanation for why the air above the airfoil moves faster. Yet the air above the airfoil really does move faster, as that webpage correctly describes. (That webpage doesn't say why the air moves faster! Thus they avoid the "equal transit fallacy.") - "Airplane wings are shaped to make air move faster over the top of the wing." True. Giving the airfoil some camber will change the upper/lower velocities, making it produce lift, even at zero attack. No mention of "equal transit-time."