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Key Facts 1 (Completed)
Correct Statements
Stalling involves loss of height and loss of control.
A pilot must be able to clearly and unmistakably identify a stall.
A stall is caused by airflow separation.
Separation can occur when either the boundary layer has insufficient kinetic energy or the adverse pressure gradient becomes too great.
Adverse pressure gradient increases with increase in angle of attack.
Alternative names for the angle of attack at which stall occurs are the stall angle and the critical angle of attack.
The coefficient of lift at which a stall occurs is CLMAX.
A stall can occur at any airspeed or flight attitude.
A typical stalling angle is approximately 16°.
To recover from a stall the angle of attack must be decreased. Maximum power is applied during stall recovery to minimize height loss.
On small aircraft, the rudder should be used to prevent wing drop at the stall.
On swept wing aircraft the ailerons should be used to prevent wing drop at the stall. Recover height lost during stall recovery with moderate back pressure on the elevator control.
The first indications of a stall may be unresponsive flight controls, stall warning device or aerodynamic buffet.
At speeds close to the stall, ailerons must be used with caution to lift a dropping wing. Acceptable indications of a stall are:
(1)a nose-down pitch that can not be readily arrested.
(2)severe buffeting.
(3)pitch control reaching aft stop and no further increase in pitch attitude occurs. Reference stall speed (VSR ) is a CAS defined by the aircraft manufacturer.
VSR may not be less than a 1g stall speed.
When a device that abruptly pushes the nose down at a selected angle of attack is installed, VSR may not be less than 2 knots or 2 %, whichever is greater, above the speed at which the device operates.
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Stall warning with sufficient margin to prevent inadvertent stalling must be clear and distinctive to the pilot in straight and turning flight.
Acceptable stall warning may consist of the inherent aerodynamic qualities of the aeroplane or by a device that will give clearly distinguishable indications under expected conditions of flight.
Stall warning must begin at a speed exceeding the stall speed by not less than 5 knots or 5 % CAS, whichever is the greater.
Artificial stall warning on a small aircraft is usually given by a horn or buzzer.
Artificial stall warning on a large aircraft is usually given by a stick shaker, in conjunction with lights and a noisemaker.
An artificial stall warning device can be activated by a flapper switch, an angle of attack vane or an angle of attack probe.
Most angle of attack sensors compute the rate of change of angle of attack to give earlier warning in the case of accelerated rates of stall approach.
EASA required stall characteristics, up to the time the aeroplane is stalled, are:
a.It must be possible to produce and correct yaw by unreversed use of the ailerons and rudder.
b.No abnormal nose-up pitching may occur.
c.Longitudinal control force must be positive.
d.It must be possible to promptly prevent stalling and recover from a stall by normal use of the controls.
e.There should be no excessive roll between the stall and completion of recovery.
f.For turning flight stalls, the action of the aeroplane after the stall may not be so violent or extreme as to make it difficult, with normal piloting skill, to effect prompt recovery and to regain control of the aeroplane.
An aerofoil section with a small leading edge radius will stall at a smaller angle of attack and the stall will be more sudden.
An aerofoil section with a large thickness-chord ratio will stall at a higher angle of attack and will stall more gently.
An aerofoil section with camber near the leading edge will stall at a higher angle of attack.
A rectangular wing planform will tend to stall at the root first.
A rectangular wing planform usually has ideal stall characteristics; these are:
a.aileron effectiveness at the stall.
b.nose drop at the stall.
c.aerodynamic buffet at the stall.
d.absence of violent wing drop at the stall.
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To give a wing with a tapered planform the desired stall characteristics, the following devices can be included in the design:
a.washout (decreasing incidence from root to tip).
b.an aerofoil section with greater thickness and camber at the tip.
c.leading edge slots at the tip.
d.stall strips fitted to the wing inboard leading edge.
e.vortex generators which re-energize the boundary layer at the tip.
A swept-back wing has an increased tendency to tip stall due to the spanwise flow of boundary layer from root to tip on the wing top surface. Methods of delaying tip stall on a swept wing planform are:
a.wing fences, thin metal fences which generally extend from the leading edge to the trailing edge on the wing top surface.
b.vortilons, also thin metal fences, but smaller and are situated on the underside of the wing leading edge.
c.saw tooth leading edge, generates vortices over wing top surface at high angles of attack.
d.engine pylons of pod mounted wing engines also act as vortilons.
e.vortex generators are also used to delay tip stall on a swept wing.
Tip stall on a swept wing planform gives a tendency for the aircraft to pitch-up at the stall. This is due to the CP moving forwards when the wing tips stall first.
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Key Facts 2 (Completed)
Correct Statements
The swept-back wing is the major contributory factor to super stall.
An aircraft design with super stall tendencies must be fitted with a stick pusher. Factors which can affect VSR are:
a.changes in weight.
b.manoeuvring the aircraft (increasing the load factor).
c.configuration changes (changes in CLMAX and pitching moment).
d.engine thrust and propeller slipstream.
e.Mach number.
f.wing contamination.
g.heavy rain.
In straight and level flight the load factor is one.
At a higher weight, the stall speed of an aircraft will be higher.
If the weight is decreased by 50%, the stall speed will decrease by approximately 25%. Load factor varies with bank angle.
The increase in stall speed in a turn is proportional to the square root of the load factor. High lift devices will decrease the stall speed because CLMAX is increased.
Forward CG movement will increase stall speed due to the increased tail down load. Lowering the landing gear will increase stall speed due to the increased tail down load.
Increased engine power will decrease stall speed due to propeller slipstream and/or the upwards inclination of thrust.
The effect of increasing Mach number on stall speed begin at M 0.4.
The effects of compressibility increases stall speed by decreasing CLMAX.
The formation of ice on the leading edge of the wing can increase stall speed by 30%. Frost formation on the wing can increase stall speed by 15%.
An aircraft must be free of all snow, frost and ice immediately before flight.
Airframe contamination increases stall speed by reducing CLMAX, increasing the adverse pressure gradient and/or reducing the kinetic energy of the boundary layer.
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Indications of an icing-induced stall can be loss of aircraft performance, roll oscillations or wing drop and high rate of descent. Artificial stall warning will be absent, but aerodynamic buffet may assist in identifying the onset of wing stall.
Very heavy rain can increase the stall speed due to the film of water altering the aerodynamic contour of the wing.
A stall must occur before a spin can take place.
In a steady spin, both wings are stalled, one more than the other.
A spin may also develop if forces on the aircraft are unbalanced in other ways, for example, from yaw forces due to an engine failure on a multi-engine aircraft, or if the CG is laterally displaced by an unbalanced fuel load.
The following is a general recovery procedure for erect spins:
1.move the throttle or throttles to idle.
2.neutralize the ailerons.
3.apply full rudder against the spin.
4.move the elevator control briskly to approximately the neutral position.
5.hold the recommended control positions until rotation stops.
6.as rotation stops, neutralize the rudder.
7.recover from the resulting dive with gradual back pressure on the pitch control.
A crossed-control stall can be avoided by maintaining the ball of the slip indicator in the middle.
A stall can occur at any speed or flight attitude if the critical angle of attack is exceeded.
A secondary stall can be triggered either by not decreasing the angle of attack enough at stall warning or by not allowing sufficient time for the aircraft to begin flying again before attempting to regain lost altitude.
An added complication during an accidental stall and recovery of a single-engine propeller aircraft is due to the rolling and yawing forces generated by the propeller. It is essential to maintain balanced, co-ordinated flight, particularly at low airspeed, high angles of attack.
In whatever configuration, attitude, or power setting a stall warning occurs, the correct pilot action is to decrease the angle of attack below the stall angle to un-stall the wing, apply maximum allowable power to minimize altitude loss and prevent any yaw from developing to minimize the possibility of spinning. “Keep the ball in the middle”.
If a large shock wave forms on the wing, due to an inadvertent overspeed, the locally increased adverse pressure gradient will cause the boundary layer to separate immediately behind the shock wave. This is called ‘shock stall’.
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