Active rotor control for helicopters: motivation and survey on higher harmonic control

Abstract

Since the early helicopter developments, these aircraft have made a tremendous progress in performance, handling qualities, comfort and efficiency. However, modern helicopters still suffer from many problems that hinder a further increase in their efficiency, acceptance and hence their market share. The high level of vibrations and the noise generated by the rotor are the most important reasons for this. While vibrations are a concern of pilot and passenger comfort, they also give rise to an increase in maintenance efforts and costs. The high noise level limits the acceptance of helicopters in the public, e.g. landing of helicopters on or close to hospitals during EMS missions. High noise levels also lead to an early aural detection during military missions. Further drawbacks of helicopters are the high fuel consumption in high speed forward flight due to the excessive power required, the limited speed of flight, the low range for the same reason, low lead-lag damping, etc. To alleviate these drawbacks of helicopters, active rotor control technologies have been investigated for a long time. Many different approaches have been investigated and most of them are not being followed any more. First investigations started with so-called Higher Harmonic Control (HHC) which has been replaced by Individual Blade Control (IBC). The paper gives a survey of the typical problems and explains the vibration and noise issues in more detail. Since active means have to compete with passive ones, such methods are also briefly addressed. Next, the paper gives a review on important HHC achievements. Due to space constraints, the paper mainly focuses on wind tunnel and flight test results. A second paper reviews IBC and gives an outlook on the idea of the swashplateless helicopter.