Active Rotor Control Systems for Helicopters

Kurzfassung

In 2007, the helicopter community celebrated 100 years of helicopter flight. Although no photos or film evidence is available it is widely assumed that Paul Cornu experienced the first man-carrying powered helicopter flight. Even more, a recent analysis revealed that a permanent hovering flight has not been possible. Since then, helicopters experienced a tremendous improvement in performance, safety, controllability and handling qualities. Yet still a niche product, they concurred their market and can not be replaced by any other aircraft. The ability to take-off and land vertically, to hover and the excellent low speed flight performances and handling qualities in comparison to other VTOL aircraft enable and consolidate this success. On the other hand, helicopters still suffer from many problems that hinder a further increase in 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, but give also rise to an increase in maintenance effort and costs, the high levels of noise limit 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. Nobody, however, realized that in 2002 active rotor control celebrated its 50th anniversary. In 1952 first theoretical studies started to address the principle of Higher Harmonic Control (HHC) to alleviate typical helicopter problems. In 1965 a first flight with a HHC system on a Bell 212 has been done. HHC is based on actuators located under the swashplate, thus limiting mechanically the applicable control frequencies in the rotating frame for rotors with more than 3 blades. Although HHC demonstrated its capabilities to reduce vibrations and noise caused by blade-vortex-interaction (BVI), other active control means were investigated more and more in the 1980s. The main drawback of HHC is the limitation to certain applications due to reduced control frequencies, and the fact, that noise and vibrations could often not be reduced at the same time. The most promising alternative to HHC is Individual Blade Control (IBC). IBC is based on actuators in the rotating frame and hence gives the engineer the opportunity to overcome the limits inherent to HHC. Many IBC concepts have been designed and tested, both in wind tunnel as well as in flight. Early concepts focussed on blade root actuation where hydraulic actuators replace the control rods that connect the swashplate with the pitch horns. Advanced designs address the principle of smart actuators that drive a trailing edge flap. Even more advanced applications of smart actuation integrate distributed actuators into the blade (blade spar or skin) to generate active twist along the rotor blade radius. Further concepts are nose droop or leading edge flaps, Gurney flaps or soft trailing edges, multi-swashplate systems and so on. Despite more than 50 years on R&D on rotor active control technology, no serial production helicopter makes use of such a powerful system. This sad fact is a tribute to the challenging requirements on minimum system complexity, high reliability and of course effectiveness with respect to the mentioned problems, minimum weight, costs, and last but not least the high loads acting on the rotor and the blades. 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 addressed briefly. Next, the paper gives a historic review on important HHC and IBC achievements. Due to the limited space, the paper mainly focuses on wind tunnel and flight test results. Recent results gathered with IBC are shown before an overview on some DLR activities is given. An outlook on the idea of the swashplateless helicopter concludes the paper.