Helicopter attitude and airspeed acquisition and retention system

A pitch axis autopilot system for a helicopter having a longitudinal cyclic pitch control stick, comprising:

• a trim actuator responsive to electrical signals applied thereto to position the longitudinal cyclic pitch control stick of the helicopter;
• attitude means for providing an attitude signal indicative of actual aircraft pitch axis attitude;
• airspeed means for providing an airspeed signal indicative of actual helicopter airspeed;
• position means responsive to said trim actuator for providing a stick trim position signal indicative of the longitudinal cyclic pitch stick trim position established by said trim actuator;
• and
• signal processing means, responsive to said airspeed signal, said attitude signal, and said stick trim position signal, for providing in response to said airspeed signal, a cruise speed transition signal whenever the airspeed indicated by said airspeed signal transitions from a predetermined threshold cruise speed to a sub-cruise speed or vice versa, for providing in response to the presence of said cruise speed transition signal, an airspeed reference signal equal to said airspeed signal, an attitude reference signal equal to said attitude signal, and a stick synch signal equal to said stick trim position signal, for providing an airspeed error signal as the difference between said airspeed reference signal and said airspeed signal, an attitude error signal as the difference between said attitude reference signal and said attitude signal, for providing in the absence of said cruise speed transition signal a pitch autopilot command signal as a function of said airspeed error signal if said airspeed signal indicates a cruise speed or as a function of said attitude error signal if said airspeed signal indicates a sub-cruise speed, and for providing a stick reference signal as a function of the difference between said pitch autopilot command signal and said stick synch signal and a stick command error signal applied to said trim actuator as a function of the difference between said stick reference signal and said stick trim position signal, whereby said stick command error signal can be zero during said cruise transition signal when said pitch autopilot command signal is zero valued, said airspeed reference signal is set equal to said airspeed signal so that said airspeed error signal consequently is zero valued and said attitude reference signal is set equal to said attitude signal so that said attitude error signal consequently is zero valued, while said stick trim position signal retains a finite value indicative of current stick trim position, whereby transient attitude disturbances, resulting from transitions between cruise and sub-cruise speeds with the pitch axis autopilot engaged, are mitigated.

An airspeed hold autopilot system for a helicopter having a longitudinal cyclic pitch control stick with a forward beeper switch and an aft beeper switch disposed thereon, comprising:

• a trim actuator responsive to electrical signals applied thereto to position the longitudinal pitch control stick of the helicopter;
• airspeed means for providing an airspeed signal indicative of actual helicopter airspeed;
• attitude means for providing an attitude signal indicative of actual helicopter attitude;
• beeper means responsive to said beeper switches for providing in response to operation of either of said beeper switches a .+-.beep signal, the polarity of which depends on which of said beeper switches is operated;
• position means responsive to said trim actuator for providing a stick trim position signal indicative of the longitudinal cyclic pitch stick trim position established by said trim actuator;
• and
• signal processing means, responsive to said airspeed signal, said attitude signal, said .+-.beep signal, and said stick trim position signal, for providing in response to initial presence of said .+-.beep signal, an airspeed reference signal equal to said airspeed signal, an attitude reference signal equal to said attitude signal, and a stick synch signal equal to said stick trim position signal, for thereafter providing in the presence of said .+-.beep signal said airspeed reference signal as a function of said .+-.beep signal if said airspeed signal indicates a cruise speed or said attitude reference signal as a function of said .+-.beep signal if said airspeed signal indicates a sub-cruise speed, and for providing an airspeed error signal as the difference between said airspeed reference signal and said airspeed signal, an attitude error signal as the difference between said attitude reference signal and said attitude signal, a pitch autopilot command signal as a function of said airspeed error signal if said airspeed signal indicates a cruise speed or as a function of said attitude error signal if said airspeed signal indicates a sub-cruise speed, a stick reference signal as a function of the difference between said pitch autopilot command signal and said stick synch signal, and a stick command error signal applied to said trim actuator as a function of the difference between said stick reference signal and said stick trim position signal, whereby said stick command error signal can be zero during initial presence of said .+-.beep signal when said pitch autopilot command signal is zero valued, said airspeed reference signal is set equal to said airspeed signal so that said airspeed error signal consequently is zero valued and said attitude reference signal is set equal to said attitude signal so that said attitude error signal consequently is zero valued, while said stick trim position signal retains a finite value indicative of current stick trim position, whereby transient attitude disturbances, resulting from beeping of the pitch axis autopilot trim point, are mitigated.

A pitch axis autopilot system for a helicopter, comprising:

• pitch control means including a longitudinal cyclic pitch control stick and a trim actuator responsive to electrical signals applied thereto to position said control stick to a stick trim position;
• airspeed means for providing an airspeed signal indicative of actual helicopter airspeed;
• override means responsive to said pitch control means for providing an override signal indicative of said control stick being moved a threshold amount away from said stick trim position;
• and
• signal processing means, responsive to said airspeed signal for providing an airspeed reference signal indicative of an airspeed which said autopilot is to hold and an airspeed error signal as the difference between said airspeed reference signal and said airspeed signal, for providing in the absence of said override signal a pitch autopilot integrator signal as a function of the time integral of said airspeed error signal if the magnitude of said airspeed error signal exceeds a predetermined airspeed error limit magnitude or as a function of the time integral of said airspeed limit magnitude if the magnitude of said airspeed error signal is less than said limit magnitude, for providing in the presence of said override signal said pitch autopilot integral signal equal to the pitch autopilot integral signal which existed at the moment said override signal appears, and for providing a stick command signal for application to said trim actuator as a function of said pitch autopilot integral signal.

Apparatus for providing an aircraft airspeed signal comprising:

• a Pitot static airspeed sensor for providing a raw airspeed signal indicative of the Pitot static indication of the airspeed of the aircraft;
• a longitudinal accelerometer for providing a longitudinal acceleration signal indicative of the acceleration of the aircraft along its longitudinal axis;
• and
• signal processing means for providing an integrated, filtered airspeed signal, for providing an airspeed difference signal as the difference between said raw airspeed signal and said filtered airspeed signal, for providing a proportional airspeed difference signal as a proportional function of said airspeed difference signal, for providing an integral airspeed difference signal as an integral function of said airspeed difference signal, for providing an aircraft acceleration signal as the summation of said proportional airspeed difference signal, said integral airspeed difference signal, and said longitudinal acceleration signal, and for providing said integrated, filtered airspeed signal as the integral of said acceleration signal.