Definition
A condition in which a rotor blade or propeller blade exceeds its critical angle of attack and loses lift. On a helicopter, blade stall typically occurs on the retreating blade in forward flight when its angle of attack becomes too high to maintain smooth airflow. On a propeller, blade stall occurs when the angle of attack of the blade sections becomes too steep to produce thrust efficiently, usually at very low forward speed combined with high power.
Plain English
The blade is trying to bite the air at too steep an angle, so the airflow breaks away and the blade stops producing the lift or thrust it should.
Context Anchor
Seen in propeller and helicopter rotor discussions, especially when talking about high blade angle, high load, high speed, or abnormal vibration.
Derivation
Stall comes from an old sense of 'coming to a standstill.' In aerodynamics it describes the moment a wing or blade stops flying smoothly because the air can no longer follow its surface. A blade is just a rotating wing, so the same rule applies.
Why Pilots Care
Uncorrected blade stall produces strong vibrations, reduced control, and can lead to loss of the aircraft if airspeed or collective is not promptly reduced.
Grounding Statement
If a blade is forced through the air at too steep an angle, the smooth flow breaks away and the blade loses much of its effectiveness.
Intuition Check
Do not read “stall” here as “the engine stopped.” In blade stall, the engine may still be running; the problem is that airflow over the blade has broken down.
Example Sentence 1
As the helicopter approached its never-exceed speed, the instructor warned that pushing further could lead to retreating blade stall.
Example Sentence 2
Retreating blade stall first appears on the left side of the rotor disk in forward flight.