An automatic flight control system, for an aircraft having a roll attitude retention outer loop actuator (29), a roll stability inner loop actuator (25) and a control stick (26) for positioning control surfaces of the aircraft to control its roll attitude, includes means (54, 55) to provide a roll error signal (56) indicative of the deviation in roll attitude from a desired roll attitude. The roll attitude retention outer loop actuator (29) is controlled by a proportional (61) and integral (62) function of the roll error when force is not applied to the stick, but only as a proportional function when force is applied to the stick (48, 49, 51, 62). The roll stability inner loop actuator is controlled by a washed out (72) proportional (64) function of the roll error signal to provide short-term roll retention at roll attitudes established by the control stick during turns against trim. Sensing force on the stick (51, 95, FIG. 1) and roll attitude of a predetermined threshold magnitude (107) provides adjustments in relative gain (113, 117, 70, 71, FIG. 1; 154, 157, 160-163, FIG. 4) during turns against trim.

The air speed is determined by solving the equation ##EQU1## in which P.sub.z is the pitch of the antitorque device, .gamma..sub.z, the vertical acceleration of the helicopter, k.sub.1 -k.sub.10, parameters depending on the type of helicopter, p.sub.x,y, the longitudinal or transverse cyclic pitch, p.sub.o the collective pitch of the main rotor, .theta..sub.x,y the angle of bank of the helicopter about the transverse or longitudinal axis of the helicopter. The system for determining the air speed may also include a conventional anemometer for hybridization with the computed speed. The parameters k.sub.1 -k.sub.10 are determined by self-calibration and by assuring that the variations of the ground speed and of the air speed are the same from one flight configuration to another one of a pair of flight configurations, among a series of pairs, for which the sensiting parameters of the helicopter are measured in the earth's reference by means of an airborne ground speed reference.

A transient free synchronizer stores an input trim signal value on command, and provides an output signal value which transitions in a smooth, continuous manner, when the synchronizer is commanded to the store the input signal. Synchronizers are typically used to capture desired aircraft attitude, position, or velocity trim points for use in aircraft autopilot and automatic flight control systems.

A control system is provided which prevents a powerful, propeller driven, jet training aircraft from requiring corrective action to throttling and gyroscopic effects when the student wishes to fly the aircraft in coordinated flight. The control system includes a digital memory having stored therein flight test generated data representative of the relationship between the amount of rudder trim required to maintain the aircraft in coordinated flight as a function of aircraft engine torque and airspeed. Sensors are provided to measure aircraft engine torque aircraft altitude aircraft speed, and pitch rate. The measured parameters are coupled to the digital computer. The digital computer combines the parameters provided by the sensors and data read from the memory to produce a composite rudder trim signal which automatically reduces rudder forces induced by throttling and gyroscopic effects produced in response to pilot commanded maneuvers to thereby maintain the aircraft in coordinated flight. Thus, once the student trims the aircraft, the control system maintains the aircraft approximately trimmed throughout the flight envelope regardless of flight condition changes or acrobatic maneuver.

Wind tunnel data for a three control surface aircraft is developed for lift, pitching moment, and drag coefficient characteristics. This data is then input into a Lagrange optimization program to determine a unique combination of canard, flap, and strake flap position that trimmed the pitching moment coefficient to zero and provided the minimum drag coefficient as a function of lift coefficient and/or angle of attack, Mach number, and altitude. This program is exercised over the entire Mach number, altitude, and angle of attack range of the aircraft. The output from the Lagrange optimization program are then tabulated and loaded into the memory of a digital flight control computer of an aircraft. As the aircraft flies, the angle of attack sensor, air data sensor and altimeter determine the angle of attack, Mach number and altitude of the aircraft. By means of the computer, the position of the control surfaces are changed to the predetermined settings of the look-up table for minimum drag.