Principles of Flight

Chapter 7: Stalling, Spinning, and Recovery

Technical General for Aviators — Capt. Pankaj Pahil

7.1 The Cause of the Stall

A stall is a loss of lift resulting from the separation of airflow from the wing's upper surface.
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 It is caused by exceeding the
critical angle of attack (AOA), which is typically around 16 degrees.
 A stall is an aerodynamic phenomenon related to AOA, not airspeed. An aircraft can be
stalled at
any speed, in any attitude, if the critical AOA is exceeded.

 The primary cause is the
adverse pressure gradient on the rear portion of the wing becoming too strong for the
boundary layer's kinetic energy, causing the flow to stop, reverse, and separate. 149149149149

As an aircraft approaches a stall, the pilot may observe:
 Flight controls becoming less effective ("mushy").
 Speed is low and decreasing for a given power setting.
 Nose attitude is higher than normal.
 Aerodynamic buffeting as the separated, turbulent air from the wing root strikes the tailplane.
 Activation of a stall warning device (e.g., horn or stick shaker).
At the stall itself, the aircraft may experience a wing drop, a sharp nose drop, and a loss of
altitude.

The only way to recover from a stall is to restore smooth airflow over the wing.
1. Reduce the Angle of Attack: The primary and most critical action is to move the control
column forward to reduce the AOA below the critical angle.
2. Apply Maximum Power: Simultaneously apply maximum authorized power to minimize
altitude loss and accelerate the aircraft back to a safe flying speed.
3. Level the Wings: Use rudder to prevent wing drop on straight-wing aircraft. On swept-
wing aircraft, ailerons may be recommended.
A
secondary stall can occur if the pilot pulls back on the control column too aggressively
before the aircraft has regained sufficient airspeed.

The indicated stall speed is affected by several factors:
 Weight: Stall speed increases with the square root of the change in weight. A 20% weight
reduction results in an approximate 10% reduction in stall speed. 163163163163
 Load Factor (g): In a turn, lift must be increased to maintain altitude, which increases the
load factor. The stall speed increases with the square root of the load factor. 164164164164For
example, in a 60° level turn, the load factor is 2g, and the stall speed increases by about 41%.
 Configuration (Flaps/Slats): High-lift devices increase the Cʟₘₐₓ, which decreases the stall
speed. 166166

 Center of Gravity (CG): A forward CG requires a greater tail-down force to maintain
balance, which means the main wing must produce more lift overall. This results in a higher
stall speed. An aft CG reduces the required tail-down force, resulting in a
lower stall speed.
 Power: Engine thrust has a vertical component that helps support the aircraft's weight, and
propeller slipstream over the wing increases lift. Therefore, the power-on stall speed is
lower than the power-off stall speed.
 Wing Contamination: Ice, frost, or even dirt on the leading edge disrupts the airflow,
reduces Cʟₘₐₓ, and increases the stall speed.
 Mach Number: At high altitudes and speeds, compressibility effects can cause flow
separation at a lower AOA, reducing Cʟₘₐₓ and increasing the stall speed. 170170170170This
leads to the phenomenon of "coffin corner," where the high-speed buffet (Mach buffet) and
low-speed buffet (stall) converge.

 A spin is an aggravated stall that results in
autorotation, a helical flight path where the aircraft is descending while rotating.
 It occurs when one wing is more stalled than the other. The more-stalled (down-going) wing
has both less lift and more drag, which causes the aircraft to roll and yaw into the spin.
 Spin Recovery: The generic recovery procedure is:
1. Power to IDLE.
2. Ailerons NEUTRAL.
3. Rudder FULL OPPOSITE to the direction of rotation.
4. Elevator PUSH FORWARD to break the stall.