this post was submitted on 09 Nov 2024
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That lift explanation is innacurate/incomplete. While there may be some equal and opposite forces depending on the angle of attack, the primary reason for lift is due to Bernoulli's Principle. Airfoils have a rounded upper surface with a longer path for air to take, relative to the underside. This requires air to move at a higher velocity over the top, and since A1 x V1 x P1 = A2 x V2 x P2, pressure over the airfoil decreases. It is this pressure differential that creates lift.
In regards to aircraft, Newton's third law of motion applies to thrust from a propeller or jet engine.
Bernoulli's explanation and Newton's explanation are the same explanation made from different frames of reference. They're equal, I don't understand why people insist that one or the other is incomplete or that they somehow both have different contributions to an airplane's flight. They're the same. The airplane flies because the air pushes it up turning some of the energy from its substantial forward movement through said air into enough upward acceleration to counteract gravity. That happens both due to pressure differential AND the sum of the deflection of air in exactly the same measure, they are directly linked and have to be equal. Bernoulli's explanation is one particularly nuanced and clever way of looking at and understanding the exact mechanics of how that happens and if you plug the resulting values into Newton's math it matches perfectly. The zero "angle of attack" for a cambered airfoil shape is actually measured this way not by measuring the angles of the physical surfaces or anything like that. The Newtonian explanation is just another way of looking at it. Either way it requires intense computations to come to exact numbers, but the numbers are the same either way. The pressure differential of the air IS the mechanical force of the air, happening as an equal and opposite direction to the deflection of the volume of air the plane is flying through, either of which is what we call lift. They're all the same thing, happening at the same time and yes you can look at them from different perspectives but that doesn't mean one perspective is wrong and the other is right. They're all accurately describing the same thing. It is useful to know both, but not necessary and it does not make either of them incorrect.
This discussion always reminds me of the "airplane on a treadmill" argument where both sides read the premise differently and scream at each other that only their way of interpreting the question is right.
From a "pressure make plane go up" point of view, yeah I guess it's all the same, but I think the methods are distinct enough. Bernoulli's works by reducing pressure above the airfoil. Action reaction increases pressure below it.
I don't follow. Unless you're trying to account for the total amount of lift with each separately and the math lines up. I guess that wouldn't surprise me because bernoulli's principle is derived from newtonian physics, specifically the second law F=MA, and both contribute to the total production of lift, but not in equal amounts. Symmetrical airfoils for example rely more on angle of attack to generate lift, while an asymmetrical one can maintain level flight at a negative aoa at high enough airspeed.
Yah, that's correct. I was trying to give the most simplified version, without getting into conservation of energy in a fluid.
Now I'm annoyed with myself for not explaining it further. You did a great job tho.
You don't need rounded wings to fly though and how are (some) planes able to fly upside down if that is the main explanation of lift?
It's the main production of lift for asymmetrical airfoils, symmetrical airfoils rely on angle of attack. Basically they point the wings at an upward angle to push air down. This can work upside down.