1. CGS Ground School Principles Of Flight Stalling © Crown Copyright 2012
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The views expressed in this presentation do not necessarily reflect the views or policy of the MOD.

2. Stalling
In flight a wing generates a Total Reaction, and therefore lift.
As the angle of attack increases so the total reaction gets larger until a critical angle (15° for a typical wing) is reached.
If the angle of attack is increased beyond the critical angle the airflow over the top of the wing becomes turbulent and the lift is dramatically reduced.
This is known as the stall.
16°
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12° 8° 4°

3. Signs of the approaching stall
There are 4 signs of an approaching stall:
a. Nose high attitude.
b. Reducing airspeed.
c. Reducing airflow noise.
d. Reducing control effectiveness.
The stall can be prevented at this stage by taking the incipient recovery action, that is:
Lowering the nose to select an appropriate attitude and regaining normal flying speed.
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4. Symptoms of the stall There are 4 symptoms of the stall: a. Buffet.
b. Nose may pitch down - even though control
column is held back.
c. Increased sink.
d. Possible wing drop.
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5. Standard stall recovery
Standard stall recovery is:
a. Move the control column centrally forward
to adopt the recovery attitude.
b. Wait for flying speed to be regained (50 kts).
c. Roll level (if necessary).
d. Return to an appropriate attitude.
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6. Lift = CL (½ ). ρ V² S Stalling speed Consider the lift equation:
If, in level light, an aircraft slows down (V reduced) then lift will reduce.
To maintain level flight either the coefficient of lift, air density or wing area (CL , ρ or S) must increase.
Air density and wing area are fixed therefore only the coefficient of lift can be increased.
The coefficient of lift can be increased by increasing the angle of attack.
Eventually the stalling angle will be reached and the aircraft will stall.
The speed at which this occurs is the aircraft's stalling speed.
Lift = CL
(½ ). ρ V² S
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7. Stalling speed
The angle of attack at which a particular aerofoil stalls never changes (unless the aerofoil shape is changed by flaps or slats).
The speed at which the stalling angle of attack is reached (stalling speed) does change and is affected by:
a. Aircraft weight.
b. 'g' loading.
c. Wing configuration (airbrakes, flaps etc).
d. Engine - power setting.
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8. Stalling speed – effect of weight
Consider an aircraft in level flight, flying at just above the stalling speed:
Therefore increasing an aircraft's weight will increase its stalling speed.
In level flight lift equals weight, so if the aircraft's weight was increased it would need more lift to maintain level flight.
From the lift equation
Lift = CL (½ ρV²S), it can be seen that one of the factors CL, ρ, V or S must be increased.
ρ and S are fixed and increasing the angle of attack to increase CL would make the aircraft stall. The only way to increase the lift is to increase V (TAS).
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9. Increasing an aircraft's 'g' loading will increase its stalling speed.
Stalling speed – effect of ‘g’ loading
Weight =
Mass x
Acceleration
If we increase the acceleration ( 'g' loading) we increase the aircraft's weight.
Therefore by the same argument we have just used:
Increasing an aircraft's 'g' loading will increase its stalling speed.
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10. Lowering flaps reduces the stalling speed of an aircraft.
Stalling speed – effect of configuration
Flaps, slats and airbrakes change the shape of the aerofoil and therefore alter the lift.
By doing this they also alter the stalling speed of the aircraft.
LIFT
Flaps increase the camber of the aerofoil, which increases the coefficient of lift.
Using the lift equation
it follows that an increase in CL allows a reduction in V, therefore the stalling speed is reduced.
Lift = CL
(½ ). ρ V² S
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Lowering flaps reduces the stalling speed of an aircraft.

11. Stalling speed – effect of configuration
Flaps, slats and airbrakes change the shape of the aerofoil and therefore alter the lift.
By doing this they also alter the stalling speed of the aircraft.
Fowler flaps increase the camber of the aerofoil and the wing area, both of which effect the lift equation.
It therefore follows that an increase in CL and S allows a further reduction in V, therefore the stalling speed is reduced.
LIFT
Lift = CL
(½ ). ρ V² S
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Lowering fowler flaps reduces the stalling speed of an aircraft even further.

12. Extending slats reduces the stalling speed of an aircraft.
Stalling speed – effect of configuration
Flaps, slats and airbrakes change the shape of the aerofoil and therefore alter the lift.
By doing this they also alter the stalling speed of the aircraft.
Slats smooth the airflow over the top of the wing, increasing the angle of attack at which the wing stalls.
Using the lift equation:
it follows that an increase in angle of attack (increase in CL) allows a reduction in V, therefore the stalling speed is reduced.
Lift = CL
(½ ). ρ V² S
16°
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Extending slats reduces the stalling speed of an aircraft.

13. Opening the airbrakes increases the stalling speed of an aircraft.
Stalling speed – effect of configuration
Flaps, slats and airbrakes change the shape of the aerofoil and therefore alter the lift.
By doing this they also alter the stalling speed of the aircraft.
Airbrakes destroy the lift over part of the wing. This effectively reduces the wing area of an aircraft.
Using the lift equation:
it follows that, other factors remaining unchanged, a higher speed (V) is required. Therefore the stalling speed is increased.
Lift = CL
(½ ). ρ V² S
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Opening the airbrakes increases the stalling speed of an aircraft.

14. An increase in power reduces the stalling speed of an aircraft.
Stalling speed – effect of configuration
Flaps, slats and airbrakes change the shape of the aerofoil and therefore alter the lift.
By doing this they also alter the stalling speed of the aircraft.
With power applied another force has to be taken into account – thrust.
In a nose high attitude, the thrust has a horizontal and vertical component. This vertical component is lift and increases the CL.
Using the lift equation:
it follows that, with power applied the stalling speed is reduced.
The increase in power also provides an increased slipstream over the mainplanes. This increases the V in the lift equation.
LIFT
Lift =
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(½ ). ρ V² S
An increase in power reduces the stalling speed of an aircraft.

15. Wing tip stalling
Due to slight differences in surface finish or aileron setting, the aircraft being out of balance or in turbulence, it is rare for both wings to stall simultaneously.
This would cause problems if the wing tip stalled before the wing root.
Aircraft are therefore designed to ensure that the wing roots stall before the wing tips.
Preventing the wing tip from stalling first will:
a. Prevent a large wing drop at the stall should one wing stall before the other.
b. Ensure that the ailerons remain effective up to the point of the stall.
c. Ensure that buffet symptoms over the tailplane are felt earlier.
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16. Wing tip stalling Approaching the stall. If the wing tip stalls first:
The turbulent airflow does not affect the tailplane and buffet is not felt.
The aileron is in the stalled part of the wing and therefore is ineffective.
A large wing drop occurs because the loss of lift is a long way from the C of G.
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17. Wing tip stalling Approaching the stall.
If the wing root stalls first:
The turbulent airflow causes tailplane buffet.
The ailerons are still effective.
A smaller wing drop occurs because the loss of lift is closer to the C of G.
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18. Wing tip stalling
The following design features can be used to help prevent wing tip stalls:
a. Washout.
b. Changing the aerofoil section towards the wingtip.
c. Root spoilers.
d. Outer wing slats.
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19. Wing tip stalling - washout
Washout is the term used to describe a reduction in the angle of incidence (the angle at which the wing is set in relation to the aircraft) as the wing tip is approached.
As the angle of attack is increased, the inboard section of the wing reaches its critical angle before the outboard section.
The inboard section therefore stalls first.
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20. Wing tip stalling – aerofoil section
This technique progressively changes the aerofoil cross section, to one that stalls more gradually, as the wing tip is approached.
This is usually done by increasing the camber towards the wing tip.
As the angle of attack is increased, the less cambered section of the wing reaches its critical angle before the more highly cambered section and therefore stalls first.
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21. Wing tip stalling – root spoilers
Spoilers attached to the leading edge of the wing, near the root, alter the contour of the leading edge.
By making the leading edge sharper, the airflow has difficulty following the contour of the wing, and an earlier stall is induced.
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22. Wing tip stalling – outer wing slats
Slats on the outer section of the wing can be used to increase the stalling angle of attack for that section of the wing.
This ensures that the wing root stalls first.
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23. Spinning
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24. Auto-rotation
A wing drop at the stall is one of the causes for entry to the spin.
Consider an aircraft flying close to the stall.
If one wing stalls first, then a roll will develop.
This roll causes a change in the relative airflow over each wing and therefore a change in angle of attack.
The result is an increase in angle of attack for the down-going wing and a reduced angle of attack for the up-going wing which aggravates the stall.
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25. Auto-rotation
The up-going wing therefore produces even more lift, further enhancing the rolling motion.
The cycle continues with the aircraft rolling and yawing towards the down-going wing.
This cycle is known as auto-rotation and is the precursor of the spin.
When a wing stalls its drag increases dramatically.
The down-going wing therefore has increased drag.
The up-going wing has a reduced angle of attack and therefore reduced drag.
This sets up a yawing motion towards the down-going wing.
The yawing motion causes the up-going wing to travel faster than the down going wing.
Auto-rotation can be stopped at any stage by reducing the angle of attack.
This unstalls the down going wing, which breaks the cycle of events.
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26. Spinning
If, however, the auto-rotation is allowed to continue, then the aircraft will enter a spin.
The spin is a complicated manoeuvre, which takes place about all 3 axes simultaneously, and involves a significant loss of height.
The characteristics of the spin are dependent on a number of variables including: aircraft type, position of centre of gravity, aircraft mass, control position and entry attitude.
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27. Spinning The deliberate spinning of the Vigilant is prohibited.
The spin consists of:
a rolling motion,
a yawing motion,
a pitching motion,
all occurring simultaneously.
The deliberate spinning of the Vigilant is prohibited.
At VGS, the deliberate spinning of the Viking is prohibited.
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28. Spinning
However all pilots and instructors should be aware of the standard spin recovery for their aircraft.
Spin recovery for the Vigilant (similar for the Viking) is:
a. Close throttle.
b. Determine direction of yaw (given by turn indicator needle).
c. Apply full rudder to oppose the yaw.
d. With ailerons neutral, move the control column slowly forward.
e. When the rotation ceases, centralise the controls, level the wings and ease out of the dive.
f. Take care that airspeed and RPM limits are not exceeded.
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29. THE END
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