Turboshaft engine performance comparison between CVT and fixed ratio transmission for a variable speed rotor helicopter

In a usual turboshaft engine for helicopter applications, the free power turbine (FPT) rotational speed is normally governed ensuring that the rotor speed is as constant as possible for each type of flight maneuver. The main reason for choosing a constant rotational speed is linked to the supposed decrease in engine efficiency at part load (typical variations in speed do not exceed 15% [1]), as well as because variable speed drives trains of resonant frequencies into the airframe [2]. However, recent studies [3],[4] and real implementations bringing up composites into the airframe structure make it possible to successfully accommodate varying speed rotors without hitting resonant frequencies and with even better engine performance [3]. A recent example is the variable speed Pratt & Whitney Canada PW207D turboshaft, a single-stage shrouded power turbine which powers Boeing's A160 Hummingbird and Bell Helicopter's Eagle Eye UAV.