Just as the stall of an airplane wing limits the low speed possibilities of the airplane, the stall of a rotor blade limits the high speed potential of a helicopter.

A phenomenon of helicopter aerodynamics, one which limits the maximum speed of helicopters and must be taken into account in helicopter and rotor blade design, to say nothing of the pilot who has to fly it. It only affects a helicopter when it is moving forwards, backwards or sideways, but to all intents and purposes it occurs in forward flight when the helicopter flies fastest.

A bit of background here: A helicopter has one or more sets of rotor blades, each one of which has an aerofoil cross section. When an aerofoil moves forwards (blunt end first) through the air it shapes the flow of the air over it due to the Coanda Effect, which states that fluid substances tend to follow gently curved surfaces they come into contact with. The aerofoil's shape tends to force air flowing over it downwards. The act of forcing air downwards exerts an equal and opposite force on the aerofoil and it rises: lift is created. Thus, aeroplanes fly, helicopters fly, and the sysop for the universe keeps their job.

A helicopter spins its rotor blades fast enough that they create sufficient lift to lift themselves and the helicopter off the ground. They also make the helicopter fuselage want to spin wildly in the opposite direction, but that's a whole other node. Fair enough so far, but there's always something.

Now, it's not too difficult to surmise that when the helicopter is moving in some horizontal direction - say forwards or backwards - that as the rotor blades turn, their speed relative to the ground and air will vary. To analogise, if you threw a ball out of a moving car, the ball would fly a lot faster over the ground if you threw it in the direction the car was moving, than it would do if you threw it in the opposite direction with the same amount of force. When a helicopter blade is moving the same direction as the helicopter is travelling, it moves faster through the air than it does when moving in the opposite direction.

The faster a rotor blade is moving, the more lift it will generate. Nominally speaking, the rotors will generate much more lift when they are moving in the same direction as the helicopter is moving in, than they will when they are moving in the opposite direction. Take the following fictional helicopter:

     Blade Rotation <--- \
                _____            \
               /     \            \
               |     |             |
              /       \            |
 _____________|__ _ __|_____________
              |       |
 |            \       /
 |             \     /
  \             \   /
   \             | |
     \           | |
        \        | |
            \--> | |  ||
                 | | _||  
                 | ||_| 
                 |_|  ||

This helicopter's rotors turn counter-clockwise, as do those of most modern helos. If it were to fly forwards through still air, each rotor blade would generate most lift when in the 3 o'clock position because that's when they would be moving the fastest. However due to gyroscopic precession (a principle which states that when a force is applied to a rotating object, that force will take effect 90° 'later' in the direction of rotation) this lift would actually affect the part of the rotor disc that was pointing forwards, causing the helicopter nose to pitch upwards. The least lift is generated in the 9 o'clock position as the rotor blade speed relative to the ground is lowest.

This is compensated for in one of two ways, both of which effectively alter the angle of attack of each rotor blade as it rotates (so that the angle of attack is low when the rotor is moving the fastest, and high when it is moving the slowest). This equalises the overall lift generated by the rotor disc. These systems are usually automatic.

To continue this phenomenon to its logical conclusion, as a helicopter accelerates in a lateral direction it will eventually reach a point where its airspeed is such that the angle of attack of the retreating rotor blades will be too high for them to generate lift. The root of each retreating blade passes through an area called the Reverse Flow Area (where, because the root of the blade is travelling more slowly than the tip, and because the blade is retreating, the air actually flows over it backwards from sharp to blunt end) which advances further along the rotor blade as the helicopter's speed increases, so lift decreases further. Just as an aircraft will stall if its wings are at too high an angle of attack at too low an airspeed, the retreating rotor blades of a helicopter will stall once it has reached a certain airspeed.

The reason this is aggravated by increasing the helicopter's airspeed is because of the relationship between cyclic (pitch control applied to individual rotor blades) and collective (pitch control applied equally to all rotor blades at once). When cyclic control is applied, the rotor blades create 'lateral lift' which detracts from the vertical lift they generate (since the overall amount of lift generated by the rotor disc does not change), so the collective pitch of the rotor blades must be increased to keep the helicopter aloft. When a helicopter moves in a horizontal direction, it will lose altitude if the collective is not increased to compensate for the 'loss' of lift.

Rotor systems are designed so that as the airspeed of an individual blade reduces, its angle of attack will increase so it generates the same amount of lift. It is not difficult to see that the higher the collective is set at, the easier it is for retreating rotor blades to stall as their airspeed reduces and their pitch increases further.

Retreating blade stall is most likely to occur when a helicopter is travelling and/or manoeuvring at high speed, carrying equipment on the outside that creates extra drag (like a loudspeaker, a floodlight or floats), or flying in high winds or turbulence. A sudden, strong application of cyclic or collective control under one or more of these conditions is likely to create blade stall. The first hint that it is occurring is a vibration or shuddering of the helicopter airframe and/or similar feedback in the cyclic control stick.

Without corrective action, further effects may be sudden pitching up or down (depending which direction the main rotor turns) and/or rolling in the direction of the retreating rotors. The more severe the blade stall, the more violent these effects may be. Combined, these factors may become serious threats to helicopter stability and structural integrity.

Retreating blade stall is best avoided by not manoeuvring strongly at high speed or at all, if carrying equipment that is heavy or creates significant drag. If encountered, it is best combated by reducing cyclic control and collective, avoiding sudden manoeuvres and/or increasing the RPM of the rotor blades if possible. If you don't succeed, I hope you memorised this.

See also:
  • Dynamic Flight Inc; "Retreating Blade Stall"; <http://www.dynamicflight.com/aerodynamics/retreating/>
  • Stanchfield, Mott F; "Flight Dynamics: Controlling Dissymmetry of Lift"; <http://www.aviationtoday.com/reports/rotorwing/previous/0702/0702flightdynam.htm>
  • National Air and Space Museum; "McDonnell XV-1 Convertiplane"; <http://www.nasm.si.edu/nasm/aero/aircraft/mcdonnel_xv1.htm>
  • Meager, Chuck; "Helicoptorial"; <http://www.aero.com/publications/helicoptorial/9510/9510.htm>
  • Federal Aviation Administration; "Basic Helicopter Handbook - SOME HAZARDS OF HELICOPTER FLIGHT - Retreating Blade Stall"; printed word, via <http://www.geocities.com/flyingmouse1/Chapter_9.html>