The Physics of the ‘Stall’
The proposition that more the angle of attack, more the lift does not hold at all angles.
At about 14 degrees, something weird happens and the aircraft instead of soaring the skies starts to plummet to the ground.
When this happens it is known as a stall.
The main thing to know is that a difference in pressure across the wing–low pressure over the top and higher pressure below–creates the net upward force we call lift.
Upon reaching a certain velocity, the aircraft’s lift is more than its weight and as a result, the aircraft takes off .
The Concept of a Boundary Layer (BL)
There is a high chance that you might have heard this word even in a casual conversation about wings and that’s because its an important concept in the context of aerodynamics and associated fields.
To understand the physics of a stall, lets consider the interaction of a moving air on a flat plate.
The nature of airflow over a wing/plate is the result of stickiness or viscosity of air.
The first layer sticks to the wing/plate not moving at all.
The second layer in frictional contact with the first moves slowly over it.
And the third layer moves somewhat faster than the second
Thus layer by later the flow builds up to the free stream velocity or airspeed. These layers of flow are known as boundary layers.
What happens to the BL during a stall?
During a stall, these successive tiers of air that form the boundary layer lose their gripping on the surface and break away into turbulence.
( what i mean by turbulence is the chaotic wiggling of the test leads attached to the wing in the animation )
It takes a pressure difference between the top and bottom parts of the wing in order to produce lift. But when the flow of air becomes turbulent ( i.e during a stall ), this pressure difference is no longer established.
As a result of which, the lift drastically decreases and the aircraft starts dropping to the ground.
How to get out of a stall ?
Stalls can cause problems only when the pilot is not aware that the aircraft is stalling. ( Unlikely but has caused accidents in yester times )
As the airplane loses altitude, its nose dips down and airspeed picks up quickly. This restores the lift and the pilot would be able to regain control and bring the aero-plane into level flight.
How are stalls detected ?
On light aircraft there is a reed (much like used on a musical wind instrument) mounted on one wing root, which is angled such that at the Angle of Attack which would cause a stall, the reed “plays” which can be heard in the cockpit.
Here is a view of where this system is mounted on a Cessna
On some aircrafts, it is a similar principal, however instead of a reed, it uses a fin which at critical AoA pushes a micro-switch which activates a buzzer/horn inside the cockpit.
Here is the assembly on a Beech 18
Large commercial aircraft typically rely on either Angle of Attack (AoA) Vanes or Differential Pitot Tubes to supply input to flight computers for the purpose of calculating AoA.
A lot of important stuff regarding aerodynamics in this post. Here’s a summary of the post:
Boundary Layer concept — > Why do aircrafts stall ? — > How to get out of one — > How are stalls detected ?
Hope you enjoyed today’s post and learnt something new.
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