Retreating blade stall

August 9, 2011 by Tim McAdams

Flying a rotor system edgewise through the air creates a problem known as dissymmetry of lift. One side of the disc advances into the wind (headwind) while the other side is retreating (tailwind). For a fixed angle of attack, the lift on the advancing side is greater creating a lift imbalance that increases with airspeed. The rotor system equalizes lift by flapping.

How flapping works is by changing the angle of attack in response to the varying airspeeds the blade encounters as it moves around the rotor disc. When the advancing blade experiences a higher airspeed, the lift on that blade increases forcing it to move up. This upward movement changes the direction of the blade’s relative wind reducing its angle of attack. On the retreating side just the opposite happens. The reduced airspeed causes a decrease in lift causing the blade to move down, increasing its angle of attack. You can only increase an airfoil’s angle of attack so much before it stalls. As the helicopter continues to fly faster the retreating side must continue to increase its angle of attack to compensate. At some airspeed, the retreating blade begins to stall.

From the pilot’s perspective, when this happens an abnormal vibration will be felt, the nose can pitch up, and the helicopter can have a tendency to roll in the direction of the stalled side. The amount and severity of pitch and roll will vary depending on the rotor system design.

The tendency for the nose to pitch up is because the spinning rotor system acts like a gyroscope and therefore experiences gyroscopic precession (a physical property that states when an external force is applied to a rotating body the effect will happen approximately 90 degrees later in the direction of rotation). As such, when the retreating blade stalls and stops producing lift, the effect of this happens toward the rear of the rotor disc. This causes the disc to tilt back, and the nose to pitch up. The pilot should lower the collective pitch first and then reduce forward airspeed to recover.

Conditions like high density altitude, steep or abrupt turns, high blade loading (caused by high gross weight), turbulent air and low rotor rpm will increase the likelihood of encountering retreating blade stall when operating close to a helicopter’s Vne (never exceed speed).

14 Responses to “Retreating blade stall”

  1. pdxpilot Says:

    Good refresher!

  2. Mike Henry Says:

    I once had a quarter horse named Blade whose stall became infested with ticks. I called Orkin to come treat the stall, which they did. After about a week I noticed there were still some ticks in Blade’s stall, so I called Orkin again and they came back out. When I drove by the barn, there they were, retreating blade’s stall.

  3. Tim Says:

    Mike….you’re a strange bird.

  4. towser Says:

    That’s why having twin counter rotating blades would do two things. Eliminate the blade stall problem, and the torque created by the single rotor. Is it the expense or the complicated engineering required that you don’t see it more often?

  5. Aero Guy Says:

    The twin counter rotating blades would not solve the blade stall problem. If the blade stalls, you are less lift then you were before. If you were previously flying straight and level, and now you have less lift, you are either going to slow or drop altitude, or more likely both. You won’t start pitching up though, as the stalls pitching moment would be cancelled out by each other. Also – stalled blades produce much more drag requiring more power which may not be available.

    The same design also does eliminate torque, but so does the tail rotor on a conventional helo, and for a given amount of power, the two provide approximately the same lift. If there was a huge benefit of using twin counter rotating rotors over tail rotor designs in terms of power required for a given weight, then the industry would have shifted that way quite a few years ago.

  6. grumpy Says:

    Intermeshing counter-rotating twin rotors, as in Anton Flettners’ wotld war II “Kolibri” helicopter, should aleviate the problem since the advancing blades are outboard of the fusalage. Comments?z2zf

  7. Mike Says:

    Twin, counter rotating rotors, intermeshed or not, won’t eliminate retreating blade stall. You still have blades that are retreating….

    In either case, the retreating blade still loses lift and flaps down increasing the angle of attack. Eventually the critical angle of attact will be exceeded and the blade will stall.

  8. Jim Borger Says:

    The only solution of going comparable airplane speeds in a helicopter is to slow the rotor. Check out Carter Copters. Their aircraft, a rotorcraft but not a helicopter, is currently flying as a gyrocopter but will later add wings and be a gyroplane capable of speeds up to 300 with a recip and 400 with a turbine with coast to coast non-stop range. One can only dream, for now.

  9. jjsiewertsen Says:

    To my knowledge the best solution to the problem of undesired stall is the use of an computer to control flaps to adjust the angle of attack and also the blade tip being equiped with boomerang-shaped vortex reducers, ciao, joja*

  10. jjsiewertsen Says:

    PS… servo-flap by KAMAN, vortex reducer by WESTLAND, mini tip jet engine by VORTECH, computer by YOU-NAME-IT
    hollow mean-axle by me, (ray-input). Beam me up, Scotty!!!

  11. Michael Says:

    Nice Product, I Would Like to Have it,

    Thanks For Information
    See my Blade Helicopter MCX2 too

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