11 A stall occurs when the smooth airflow over the unmanned airplane`s wing is disrupted, and the lift degenerates rapidly. This is caused when the wing
C. exceeds its critical angle of attack.
Page 5-25, Stalls
The stalling speed of a particular aircraft is not a fixed value for all flight situations, but a given aircraft always stalls at the same AOA (Angle of Attack) regardless of airspeed, weight, load factor, or density altitude. Each aircraft has a particular AOA where the airflow separates from the upper surface of the wing and the stall occurs. This critical AOA varies depending on the aircraft’s design. But each aircraft has only one specific AOA where the stall occurs.
There are three flight situations in which the critical AOA is most frequently exceeded with fixed-wing aircraft: low speed, high speed, and turning. Multirotor aircraft wings are the propellers. There are some conditions where a helicopter style aircraft can exceed the critical AOA: High speed and Vortex Ring State (VRS).
Helicopter propellers are an airfoil and as with any airfoil, if the angle of attack increases beyond a certain point, the blade will stall. This phenomenon is known as “retreating blade stall.”
At high speeds, the retreating blade ceases to produce any more lift, as would occur with any stalled wing, because it is moving slowly relative to the air mass. When this occurs the helicopter pitches nose up, then rolls to one side or the other, quite arbitrarily.
The speed of small UAS helicopters is highly unlikely to reach Vne (Velocity not to exceed). Multirotor aircraft are less subject to retreating blade stall as the counter-rotating propellers would offset each other and the aircraft would simply lose lift.
Why is this important to sUAS pilots?
All aircraft, including multi-rotor UAS can enter an aerodynamic stall when the critical AOA of the airfoil is exceeded. To avoid VRS, simply keep your multirotor aircraft in motion during a descent. This keeps the aircraft out of its own downwash.
FAA-H-8083-25, "Pilot’s Handbook of Aeronautical Knowledge "
This is correct.