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  1. why cl on 2D is better than 3D. and why cl 2D stall faster than 3D?

  2. what happens when CP continues to move forward on the wing. and what happens when CP continues to move forward on the elevator.

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  • $\begingroup$ Related: aviation.stackexchange.com/questions/14508/… $\endgroup$ – Peter Kämpf Oct 4 '17 at 19:20
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    $\begingroup$ Ade, please be aware that asking multiple questions in the same posts is generally frowned upon. In this case they are quite close to each other, so an answer should be able to cover all your questions, but please try to avoid repeating it. $\endgroup$ – Federico Oct 5 '17 at 20:40
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The 3D-wing allows for air to flow over the wingtip into the low pressure zone over the wing, thus reducing the overall pressure differential and reducing overall lift.

This explains the stall at higher AoA because the additional airflow over the wing surface "energises" the air. Flow seperation happens when the kinetic energy is equal to the pressure differential that the airstream flows into, so if you can increase the kinetic energy, you can decrease the tendency of the airstream to separate.

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  • $\begingroup$ Moving my answer to a comment: It may be due to initial assumptions -- I would think that a 2D model assume infinite length of the airfoil, whereas a 3D model would necessarily have a finite length. $\endgroup$ – AaronJPung Oct 5 '17 at 20:41
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You could phrase the question differently: why does a wing with low aspect ratio need a higher Angle of Attack to provide the same lift, and why is maximum lift lower? Because the pressure can escape over the wing tip. The larger the wing tip, the larger the effect.

Yes a low AR wing can reach a higher AoA than a high AR wing, but only because it produces less pressure differential, therefore less lift.

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Stall happens because the flow cannot follow the airfoil. If you have a 3d wing, the pressure difference between the pressure side (below the wing) and the suction side (above the wing) can be reduced due to wing tip vortices —> less lift —> smaller $c_L$. They are generated because of the pressure difference. This will reduce the adverse pressure gradient above the airfoil and hence reduce flow separation. These vortices additionally inject energy into the boundary layer (pushing the air toward the airfoil) such that the flow can better follow the air foil despite the adverse pressure gradient. Hence, stall is later for 3d wings.

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