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# The forces on an airplane

How do airplanes fly? It's not magic. Learn about the forces that help (and hinder) airplane flight. Created by MIT+K12.

## Want to join the conversation?

• What exactly causes lift?
• Lift comes from three things:
*Camber - The net curve of the air foil. The camber creates the downwash that results in an upward force as described in philipyu_sep's answer. Imagine a sail on a sailboat. It has net curve and it makes the air go "down" (sideways for the boat) and that makes lift - a forward force on the boat.

*Thickness - The thickness gives us Bernoulli effect. Even a symmetric airfoil that has an angle of attack ( in the video) has thickness in the flow. The Bernoulli effect gets a bad rap, but it is real, it just isn't the only thing at play. Think of that sail again. It is very close to zero thickness, yet it produces lift.

*The kite effect - This is what you feel when you stick your hand out the car window. You can make lift on your hand just by making some angle of attack. Think about that one... You could take a piece of plywood, almost no thickness - no Bernoulli and no camber - so no downwash, but you could make it fly if you moved it through the air fast enough.

Samuel Langley demonstrated the kite effect by putting a flat brass plate on a rotating arm and showed that it generated lift. This strategy needs a lot of power and a powerful enough engine didn't exist then (1903).
The Wright brothers, on the other hand, spent a lot of time testing airfoil shapes and came up with a cambered, nearly constant thickness airfoil that was more efficient and could be warped to control the aircraft and ultimately won the race to controlled, sustained flight.
• Is there lift in a vacuum?
• In a vacuum there is no air. Lift DEPENDS ON AIR (any fluid) to occur. No air means there is NO pressure, therefore there can be no pressure difference
[zero minus zero = zero] between the top and bottom of the wing and, therefore no lift.
• What does the v squared represent?
• v squared aka v^2 is the square of the velocity of the air relative to the wing.
In the equation it shows that the lift is proportional to the square of the speed of the aircraft. For example, if the speed doubled, the lift would quadruple.
• At when he talks about the angle of attack, he explains what the angle of attack is but he doesn't say why it is called the angle of attack. Could someone please explain?
• It is called the angle of attack because angle represent the angle at which he is flying at and attack kind of means going forward. So you could say angle at which he is going forward. I hope that helps.
• Is an airplane capable of travelling straight up? If so how high can it go? If not what do you need to do this? Thanks!
• Yes if it has big enough engines.

When the airplane is flying straight up it doesn't generates any lift. It only uses thrust to overcome gravity. This is how rockets work.
• what is the cause of stalling?
• STALL occurs when the Angle of Attack (AOA) is increased to the point where the flow over the top of the airfoil no longer follows it smoothly, but becomes turbulent. This is because the flow over the upper surface provides a majority of the lift. For many 'regular' wings, this is roughly at an AOA of 16 degrees. See this video:
Seeing a wing in stall using tufts:
• What is thrust?
(1 vote)
• Thrust is the force created by the engine to pull, or push the aircraft forward.
• When does an airplane actually use drag?
• On the airbrakes. Airbrakes are panels that flip up and cause drag, also causing the plane to slow down.
(1 vote)
• It seems that you missed the area S in the formulae of lift L and drag D?
(1 vote)
• He defines L and D as lift and drag per unit surface area. So the formula is actually correct. However, in aeronautical engineering you would typically include the surface area S and thus L and D are defined as lift and drag on the complete airplane.