Prediction and Simulator Verification of Roll/Lateral Adverse Aeroservoelastic Rotorcraft–Pilot Couplings

Abstract

The involuntary interaction of a pilot with an aircraft can be described as pilot-assisted oscillations. Such phenomena are usually only addressed late in the design process when they manifest themselves during ground/flight testing. Methods to be able to predict such phenomena as early as possible are therefore useful. This work describes a technique to predict the adverse aeroservoelastic rotorcraft-pilot couplings, specifically between a rotorcraft's roll motion and the resultant involuntary pilot lateral cyclic motion. By coupling linear vehicle aeroservoelastic models and experimentally identified pilot biodynamic models, pilot-assisted oscillations and no-pilot-assisted oscillation conditions have been numerically predicted for a soft-in-plane hingeless helicopter with a lightly damped regressive lead-lag mode that strongly interacts with the roll mode at a frequency within the biodynamic band of the pilots. These predictions have then been verified using real-time flight-simulation experiments. The absence of any similar adverse couplings experienced while using only rigid-body models in the flight simulator verified that the observed phenomena were indeed aeroelastic in nature. The excellent agreement between the numerical predictions and the observed experimental results indicates that the techniques developed in this paper can be used to highlight the proneness of new or existing designs to pilot-assisted oscillations.

Understanding pilot biodynamical feedthrough coupling in helicopter adverse roll axis instability via lateral cyclic feedback control

“…Looking at this figure, one can see that a stiffer pilot destabilizes the regressing lag mode. This was also concluded by [12]. However, from this figure it appears that the 'Stiffer' pilot is destabilizing not only the regressing lag mode but also the high-frequency advancing lag mode -although this mode's frequency is higher (6.91 Hz) than pilot biodynamics (1.1 Hz and 2.3 Hz).…”

“…input processing by a digital flight control system, insufficient bandwidth, saturation of control system actuators). In helicopters, roll axis instability is mainly attributed to the regressing lead-lag mode as this mode eigenfrequency is close to pilot biodynamics [12]. The literature of specialty explains mainly the pure mechanism through which the flap-lag rotor motions can couple to lateral/roll via FCS feedback, however without the involvement of pilot biodynamics [13,14].…”

“…The literature of specialty explains mainly the pure mechanism through which the flap-lag rotor motions can couple to lateral/roll via FCS feedback, however without the involvement of pilot biodynamics [13,14]. When involving pilot biodynamics, it is recognized that "predictions suggest that the roll/lateral PAO phenomena are more likely to occur on helicopters with soft in-plane rotors that have lightly damped in-plane rotor modes, more sensitive to time delay than gearing ratio with respect to the lateral cyclic control, more dangerous when the flight speed increases and more likely to occur with pilots that are characterized by a natural frequency of the biodynamic poles that is close to the lightly damped in-plane rotor mode" [12]. The goal of the present paper is to give a thorough understanding of the mechanism through which the pilot can destabilize the air resonance mode and induce roll axis instability via cyclic control stick feedback.…”

Aeroelastic Rotorcraft–Pilot Couplings in Tiltrotor Aircraft

“…[40]. Time delays, eventually justified by the digital acquisition and filtering of control device motion or by signal processing before feeding inputs to the actuators [38], can be introduced in the LTF. The pilot's BDFT is described by the TF reported in Eq.…”

“…The structural properties identified for the three test pilots in Ref. [38] are reported in shows the highest frequency, 2.95 Hz. The static gain, µ p , of pilot 1 is appreciably higher than that of the other pilots.…”