Investigation of rotorcraft-pilot couplings under flight-path constraint below the minimum-power speed

Abstract

In some situations, closed loop control by the pilot can result in the
combined pilot-aircraft system becoming marginally stable or even
unstable. This can happen whether the pilot is controlling attitude or
flight path. In this paper, an investigation into helicopter stability
under flight-path constraint below the minimum-power speed is
reported. The work provides a theoretical basis for flight path
handling qualities criteria particularly for flight on the, so-called,
back-side of the power curve. The research uses the theory of weakly
coupled systems by partitioning the helicopter longitudinal dynamics
to investigate three interacting subsystems – classically the surge
mode, the phugoid mode and the heave mode. Under certain conditions,
strong control of flight path or vertical speed is shown to drive
the aircraft-pilot system unstable and a conflict is shown to exist
between feedback gain values to guarantee stability of both the surge
and the flight path motions. This conflict constitutes a potential
source of adverse rotorcraft-pilot couplings. The problems are exacerbated
in cases when the use of collective control is restricted. The
phenomenon is explored in both ground based simulation and flight
test to provide a verification of the theory