Kolasinac138
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What exactly causes the stall of an aircraft and why does this prevent lift being created?

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uberteknik
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(Original post by Kolasinac138)
What exactly causes the stall of an aircraft and why does this prevent lift being created?

Thanks.
There are two types of stall, engine stall and lifting surface stall.

At the onset of aerodynamic stall, the air flow over the lifting surface changes from laminar to turbulent with a corresponding separation of the air flow over the top of the lifting surface.

This changes the pressure differential between the lower and upper surfaces and hence the net up-thrust acting on the lifting surface is destroyed causing it to fall under the force of gravity.

A stall can be induced by:

a) loss of aircraft forward momentum and consequential lowering of the speed of the airflow over the lifting surface. Hence an engine stall will reduce net thrust and forward airspeed will drop thus potentially inducing an aerodynamic stall.

b) lifting surface angle-of-attack to the incident airflow surpasses a critical angle and drag increases significantly causing the airflow over the top of the lifti9ng surface to break away with a loss of lift and forward momentum thus inducing a stall.

c) banked turns require greater lift to keep the aircraft in the turn. Exceeding the critical bank angle for the rate of turn will cause one of the wings to stall and the aircraft will enter a spiral dive.

d) aircraft flies into wind-shear caused by different atmospheric conditions where the forward wind speed and direction over the lifting surface changes dramatically and without warning. (Air turbulence flying through storm clouds and on approach to land pose the greatest threat).
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Kolasinac138
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(Original post by uberteknik)
There are two types of stall, engine stall and lifting surface stall.

At the onset of aerodynamic stall, the air flow over the lifting surface changes from laminar to turbulent with a corresponding separation of the air flow over the top of the lifting surface.

This changes the pressure differential between the lower and upper surfaces and hence the net up-thrust acting on the lifting surface is destroyed causing it to fall under the force of gravity.

A stall can be induced by:

a) loss of aircraft forward momentum and consequential lowering of the speed of the airflow over the lifting surface. Hence an engine stall will reduce net thrust and forward airspeed will drop thus potentially inducing an aerodynamic stall.

b) lifting surface angle-of-attack to the airflow surpasses a critical angle and drag increases significantly causing the airflow over the top of the lifti9ng surface to break away with a loss of lift and forward momentum thus inducing a stall.

c) banked turns require greater lift to keep the aircraft in the turn. Exceeding the critical bank angle for the turn will cause one of the wings to stall and the aircraft will enter a spiral dive.

d) aircraft flies into wind-shear caused by different atmospheric conditions where the forward wind speed and direction over the lifting surface changes dramatically and without warning. (Air turbulence flying through storm clouds and on approach to land pose the greatest threat).
How does it change exactly, and how does this prevent the lift caused by the standard laminar high-low pressure differential?
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Kolasinac138
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(Original post by uberteknik)
There are two types of stall, engine stall and lifting surface stall.

At the onset of aerodynamic stall, the air flow over the lifting surface changes from laminar to turbulent with a corresponding separation of the air flow over the top of the lifting surface.

This changes the pressure differential between the lower and upper surfaces and hence the net up-thrust acting on the lifting surface is destroyed causing it to fall under the force of gravity.

A stall can be induced by:

a) loss of aircraft forward momentum and consequential lowering of the speed of the airflow over the lifting surface. Hence an engine stall will reduce net thrust and forward airspeed will drop thus potentially inducing an aerodynamic stall.

b) lifting surface angle-of-attack to the airflow surpasses a critical angle and drag increases significantly causing the airflow over the top of the lifti9ng surface to break away with a loss of lift and forward momentum thus inducing a stall.

c) banked turns require greater lift to keep the aircraft in the turn. Exceeding the critical bank angle for the turn will cause one of the wings to stall and the aircraft will enter a spiral dive.

d) aircraft flies into wind-shear caused by different atmospheric conditions where the forward wind speed and direction over the lifting surface changes dramatically and without warning. (Air turbulence flying through storm clouds and on approach to land pose the greatest threat).
Specifcally this. Could you explain why drag increases so much pass this angle?
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uberteknik
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(Original post by Kolasinac138)
How does it change exactly, and how does this prevent the lift caused by the standard laminar high-low pressure differential?
You need to think about what causes the laminar flow and skin effect over the upper wing camber:

Think about why there will be a laminar flow? You will need to consider both the reduction in pressure over the upper camber caused by an increase in pressure ahead of the wing leading edge and lower camber, together with the pressure field extending beyond the wing in all directions and pressure gradient at any point in the field surrounding the wing.

Then look at why an increased angle of attack will inevitably create a turbulent wake (randomised vortices) and what that means in terms of average pressure across the upper (leeward) and lower (windward) surfaces and also the change in the centre of pressure acting along the wing chord.

It may help to think in terms of individual air molecules behaviour at different points around the wing and resultant force vectors acting on the molecules.


Colour represents the pressure at any point in the space surrounding the wing. Dark = high relative pressure, light = lower relative pressure.
Streamlines indicate flow of individual air molecules within the bulk air mass:

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