Boss weights

March 21, 2013 by Tim McAdams

The tail rotor on Eurocopter’s AS350 AStar helicopter uses weights to generate a Centrifugal force to help balance the forces that exist when changing the blades’ pitch angle. Known as boss weights, exactly how they work is sometimes misunderstood.

Eurocopter uses composite technology in the AStar’s main and tail rotor systems. The helicopter’s two-blade tail rotor uses a single composite spar that runs through both blades. It is clamped in the middle at the hub and pitch changes are accomplished by twisting the composite material. The spar resists the twisting and tries to return to its natural state (it has a 10 degree pre-twist). This force is referred to as a zero-pitch-return-force and is fairly strong. Making the spar thick enough to have the necessary strength also makes it hard to twist. In normal operation with hydraulic boost, the tail rotor servo delivers enough force to overpower the zero-pitch-return-force and twists the spar as necessary changing the blades’ pitch angle. Thus, producing the amount of tail rotor thrust the pilot requires.

The boss weights assist by generating a centrifugal force that opposes the stronger zero-pitch-return-force. Essentially, they help hold twist in the spar reducing the workload on the tail rotor servo. During a hydraulic system failure the pilot must change tail rotor pitch by manually twisting the spar. The centrifugal force generated from the boss weights reduces the amount of pedal pressure required by the pilot to maintain yaw control. To further assist the pilot during hydraulic failures Eurocopter added a yaw load compensator to the tail rotor control linkage in the higher gross weight variants (B1, B2 and B3).


  • Avi Weiss


    Not sure if you read the comments on the blog anymore, but I do love these “devils in the details” aircraft/manufacturer-specific posts, as there are little if any magazines or such that cover them, and unless you are fully checked out in the particular aircraft, would likely not have heard of it.

    If you have more “eccentricities” (pun intended) about star-flex, or maybe the asymmetrical Fenestron blade-layout, would love to see them.

  • http://hoverpower Bill Evans

    Did NOTAR die when MD sold or is there an inherent defect? An article on it would be of interest.

  • Jason

    NOTAR: My local Sheriff’s office flies 2 similar MD choppers, one with NOTAR and another with tail rotor. He told me that since NOTAR is a blown air system (down the tail boom and out the side) it takes a second or two for that force to kick in. The turning force on the chopper with the tail rotor happens right now. He contrasted them as saying one is like driving a sports car (tail rotor) and the other is like driving a pickup truck (NOTAR). Only other advantage he found of NOTAR was increased user safety on the ground.

  • Tim McAdams


    Thanks for the suggestions. I will put some info together on those subjects. I am always interested in what topics readers find the most interesting.