Design and flight testing of flight control laws integrating incremental nonlinear dynamic inversion and servo current control

Abstract

Traditionally, aircraft actuator control systems are commanded by means of control surface position or rate references from the flight control laws. This is natural for hydraulic actuators, but less so for electro-mechanical actuators (EMAs). EMAs apply forces or moments that are proportional to their current inputs. For this reason, it has been investigated if this principle can be used within an aircraft flight control system. This has several distinct advantages, including prevention of force fighting between parallel actuators, natural alleviation of structural and actuator loads in the face of atmospheric disturbances, and the possibility to restore reversible control behavior for irreversible systems. A complete design and a flight test campaign were performed using a Cessna Citation II with experimental fly-by-wire system. As a first step, current control loops were designed and tested for the electric aircraft servos. In a second step, an existing set of flight control laws based on incremental nonlinear dynamic inversion was adapted to command current instead of control surface angle commands. After implementation on the aircraft, the integrated system was intensively tested in flight. Direct comparisons with open-loop control were made. The tests were highly successful and very encouraging to further investigate this integrated approach for future applications.