Using Redundant Effectors to Trim a Compound Helicopter with Damaged Main Rotor Controls

Control of a helicopter’s main rotor is typically accomplished through use of three hydraulic actuators that act on the non-rotating swashplate and position it to achieve any combination of collective and cyclic blade pitch. Failure of one of these servos can happen through loss of hydraulic pressure or by impingement of the piston within the hydraulic cylinder. For a compound helicopter with an articulated main rotor, a range of failed servo states (by piston impingement) are simulated in RCAS at hover, 100 kts, and 200 kts, to show the extent to which the compound effectors can generate additional forces and moments to maintain trimmed flight. After the loss of control of any of the three main rotor actuators in hover, the compound helicopter is capable of trimming with little change to the state of the vehicle by replacing control of the locked servo with control of rotor speed. This strategy can be used to maintain trim over 6 – 12% of the total servo ranges. At forward flight speeds, each of the three actuators produces a different result on the trimmed aircraft due to the non-axisymmetry of the rotor. Reconfiguration is accomplished through use of the ailerons, stabilator pitch, wings, and wing-mounted propellers to trim rotor produced roll moments, pitch moments, lift, and drag, respectively. The additional effectors in forward flight increases the range of tolerable failures to 20 – 63% of the maximum actuation limits.


Reddinger, J.-P., and Gandhi, F., "Using Redundant Effectors to Trim a Compound Helicopter with Damaged Main Rotor Controls ,"

Proceedings of the 73rd American Helicopter Society Annual Forum, Fort Worth, Texas, May 9–11, 2017.