On the benefit of an active horizontal tailplane to the control of the single main and tailrotor helicopter
Possible helicopter flight mechanics benefits associated with the use of an actively controlled horizontal tailplane are identified, influencing the areas of agility and manoeuvrability. In both cases, control strategies are postulated and implemented by means of control laws. They are then used with mathematical descriptions of the helicopter in digital computer simulations of manoeuvres to quantitfy the benefits. In the field of helicopter agility, use of a relatively small horizontal tailplane is shown to enhance agility, relative to the helicopter with a fixed tailplane. Popup maneouvres to SOm can be flown up to 7% faster with the active tailplane; alternatively, geometrically tighter manoeuvres can be flown to the extent of reducing manoeuvre distance by up to 10%. The control law moves the tailplane proportionally with the contributions of the three rotor controls and helicopter pitch rate to the longtitudinal component of hub moment. It is however suggested that a tailplane control law based on functions of pitch attitude would be applicable to a wider range of manoeuvres than the popups simulated. Helicopter manoeuvrability is enhanced by using the tailplane to decouple the pitch attitude from the flight path. The benefits are demonstrated by simulation of the acquisition and tracking of an airborne target. For a helicopter with the conventional pattern of control, significant changes in flight path result when the target is tracked with fuselage pointing; by comparison, the helicopter with a decoupled flight path and attitude controller changes flight path and speed by a negligible amount. It is suggested that this mode of control may be more generally applicable to control of the helicopter in that it mitigates the speed/flight path/attitude compromise the pilot faces in flying his aircraft, or the possibly large hub moments when accelerating or decellerating. The philosophy behind the use of the active tailplane differs from that of contemporary applications of moveable tailplanes in that it is an integrated element of the flight control system endowing (in its own right) control capabilities on the helicopter that are otherwise precluded by configuration. The addition of this extra control demands active control technology for several reasons: the applications require full control authority; the control laws are multivariable and change with speed; and the cockpit control setup would have to be simplified to the extent of the radical changes facilitated by active control technology.