This paper presents a concise review of the state of the art for vibration reduction in rotorcraft using active controls. The principal approaches to vibration reduction in helicopters described in the paper are 1) higher harmonic control, 2) individual blade control, 3) vibration reduction using an actively controlled flap located on the blade, and 4) active control of structural response. The special attributes of the coupled rotor/flexible fuselage vibration reduction problem are also briefly discussed to emphasize that vibration reduction at the hub is not equivalent to acceleration reduction at specific fuselage locations. Based on the comparison of the various approaches, it appears that the actively controlled flap has remarkable potential for vibration reduction.

Vibration reduction in rotorcraft using active control - A comparison of various approaches

The original version of this paper was presented and distributed as part of the 37th Israel Annual Conference on Aerospace Sciences Proceedings. Without attempting a thorough review of the burgeoning literature, the results of a representative sampling of recent papers dealing with smart materials and structures as actuators in aeronautical systems are summarized here. Their potential for improving performance, handling qualities in a stall, and increasing fatigue life is discussed briefly as requiring relatively slow-acting shape and shape-distribution changes. A similar review is made of applications for improving aeroelastic divergence, flutter instabilities, and tail buffeting on fixed-wing aircraft; and reducing vibrations, improving external acoustics, and providing flight controls for rotating-wing aircraft - all of which require a high-frequency response. The status of some of the most promising developments is noted and the remaining problems are touched on. Two approaches, which have not been given substantial attention elsewhere, are reviewed; these are: developing concentrated, namely nondistributed, piezoelectric actuators in helicoidal configurations, on the one hand, as a way to improve force - deflection output; and using control surfaces purposefully designed to be marginally unstable and stabilized by smart structures, on the other hand, as a means of reducing the force - deflection combinations required of smart-structure actuators.

https://iopscience.iop.org/article/10.1088/0964-1726/6/5/001/pdf

Recent developments in smart structures with aeronautical applications

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. Vibrations are problematic not only for pilot and passenger comfort, but also give rise to an increase in maintenance effort. The high noise level limits the acceptance of helicopters in the public, e.g. landing of helicopters on or close to hospitals during Emergency Medical Services 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 and hence low range, limited speed of flight, etc. To overcome these drawbacks, 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). In a previous paper motivation on active rotor control technology was recapitulated as well as achievements on HHC. This paper continues that work and gives a survey on IBC concepts and achievements. An outlook on the idea of the swashplateless helicopter concludes the paper.

Active rotor control for helicopters: individual blade control and swashplateless rotor designs

This paper is motivated by the problem of active control of vibrations in helicopter A129 of Agusta S.p.A. Taking into account the specific features of the A129 rotor, whose model is characterized by two nonminimum phase zeros, a new method for active control is proposed. Based on simple pole-placement considerations, the method presents a number of advantages. First, it enables harmonic cancellation without perturbing the low frequency dynamics. In other words, the control system does not affect the overall dynamics of the flight as decided by the pilot. Moreover, it supplies a stable controller. Second, the controller design computations are extremely simple. The performances achievable with such a controller are tested by a number of simulations on an A129 rotor model. >

Active control of vibrations in helicopters via pole assignment techniques

On the Zero Dynamics of Helicopter Rotor Loads

The novel active control system presented for helicopter rotor aerodynamic and aeroelastic problems involves the individual control of each blade in the rotating frame over a wide range of frequencies (up to the sixth harmonic of rotor speed). This Individual Blade Control (IBC) system controls blade pitch by means of broadband electrohydraulic actuators attached to the swash plate (in the case of three blades) or individually to each blade, using acceleratometer signals to furnish control commands to the actuators. Attention is given to IBC's application to blade lag, flapping, and bending dynamics. It is shown that gust alleviation, attitude stabilization, vibration alleviation, and air/ground resonance suppression, are all achievable with a conventional helicopter swash plate.

N. D. Ham

Papers

Helicopter gust alleviation, attitude stabilization, and vibration alleviation using individual-blade-control through a conventional swash plate