What causes the decrease in lift after passing the critical angle of attack?

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SUMMARY

The decrease in lift after passing the critical angle of attack is primarily due to boundary layer separation from the airfoil surface. When the boundary layer separates, it creates a region of "dead air" that leads to increased pressure over the upper surface of the airfoil, reducing the pressure difference between the upper and lower surfaces. This results in a significant decrease in lift, despite the upper surface pressure remaining lower than that of the lower surface. The relationship between drag and lift is also critical, as increased drag contributes to a reduction in speed, further impacting lift generation.

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bbillingsley
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Hi,
I'm a little confused as to why the lift acting on a body would decrease after the critical angle of attack has been passed. I understand the increase in Drag but i can't help but think that the top of the body would still have less pressure and lift would increase.

As i write this I'm starting to think that it would be because the speed would necessarily decrease from the drag, but I'm still not sure.

Any help would be appreciated.

Thanks
 
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The lift decreases because the boundary layer is no longer able to overcome the adverse pressure gradient and it separates from the surface of the airfoil. Once the boundary layer separates there is a large region of what is sometimes referred to as "dead air" over the surface of the airfoil. This "dead air" is a recirculating mass of air that is moving very slowly compared to the air that would be flowing over the surface had the boundary layer remained attached. This leads to increased pressure over the upper surface of the airfoil and a decrease in the pressure difference between the lower and upper surface. This results in less lift.

So you are correct in thinking that the pressure on the upper surface will still be less than the lower surface, however the difference is not as great (because the upper surface pressure has increased) so the lift is not as great.
 
Okay yeah that makes a lot of sense.
Thanks
 

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