881,431. Grounded aircraft trainers. GENERAL PRECISION Inc. Oct. 29, 1957 [Nov. 7, 1956], No. 33735/57. Class 4. [Also in Group XXXVIII] Apparatus for providing "feel" simulation in a ground aircraft trainer comprises a personallyoperable member, e.g. a simulated control stick or a member positioned by simulated rudder pedals, a power device for exerting on the member a mechanical force equal to the reaction force that would be exerted upon the corresponding member of a real aircraft during operation, a transducer that produces a first electrical signal representative of the force existing on the simulated member, computer means for producing, in response to, inter alia, the position of the member, a second electrical signal representative of the said reaction force, and means for comparing the first and second signals and for operating the power device so as to transmit to the member said mechanical force. Simulated elevator control stick system. Fig. 1. A simulated control stick 100, pivoted about an axis 102, is mechanically linked to a piston 133 disposed within a cylinder 134 which is connected on both sides of the piston, through an electrically operated servo-valve 137, to an hydraulic pressure source 140. A stiff elastic ring 114 is connected in the mechanical linkage, and to the inner periphery of the ring are fixed the core 115 and coils 120, 121, 122 of a differential transformer 116, the output of which is used to develop a signal, proportional to the force applied to the stick, across a potentiometer R100. A second differential transformer comprising a core 118 fixed to the linkage and movable with respect to coils 123, 124, 125, develops an output across a potentiometer B101 commensurate with the position of the stick 100. The signal from potentiometer B101 is amplified and demodulated and fed to a summing amplifier U101 wherein it is added to a potential representing simulated elevator trim tab position from a potentiometer R103, the slider of which may be positioned by a motor (not shown) operated by a button 101A on stick 100. The output of U101 is fed to a flight simulator computer 11 which produces a signal proportional to the force to be applied to the stick by the hydraulic system, which signal is fed via a resistor R120 to an amplifier U102 wherein it is compared with the "existing stick force" signal which is amplified, demodulated, and routed to U102 via a resistor R110 and a lag network comprising resistors R108 and R111 and a capacitor C106. The output of U102 is passed through a resistor R107, the coil of an overload relay K101, and a switch S100 to the coils 141, 142 of the servo valve 137, to vary the force applied to the stick by the hydraulic pressure source 140. The gain of amplifier U102 may be selected by adjusting a potentiometer R115 in the feedback circuit, and the L.F. response is increased by two lead networks each comprising a resistor and a capacitor in series. A second overload relay K100 is connected in the "existing force" signal circuit. A small 400 c.p.s. "dither" voltage is fed from potentiometer R106 to the servo valve windings to reduce the effects of friction, inertia, hysteresis, and backlash in the hydraulic system. To compensate for "stretch" in the linkage, a potential proportional to "existing force" may be added to the input of amplifier U101, allowance being made for any "stretch" in the real aircraft system. Flight simulator computer. Fig. 3. The output of amplifier U101 is fed, via a lag network 302 which simulates delay between stick and elevator movements in the real aircraft, to a buffer amplifier U301 wherein it may be modified by a potential, representing simulated coefficient of lift or angle of attack, applied at terminal 317. The output of amplifier U301, which represents simulated elevator deflection, is supplied to a potentiometer R305, the slider of which is positioned by the trainer dynamic pressure servo 14 to pick off a first simulated stick-force potential which is fed through a scaling resistor R331 to a summing amplifier U303 wherein it is combined with potentials representing the following simulated modifying factors. (a) Vertical acceleration which in a real aircraft acts upon a bobweight fixed to a bell-crank lever movable with the control stick. This potential is applied to terminal 304. (b) Pitching acceleration which also affects the bob-weight. This potential, which is applied to terminal 305, may be modified in accordance with the simulated centre of gravity of the aircraft. (c) Spring -dashpot effect. In the real aircraft a spring is connected between the control stick and the piston of a fluid dashpot, the force (F s pr) applied to the stick by this combination being given by : F spr = K1,##es- K2, #Fe, where # es = rate of movement of the stick, and F e = rate of change of stick force. In the computer, the potential representative of stick position applied at terminal 301 is fed to a differentiating network comprising capacitor C302 and earthed resistor R316 and applied to an amplifier U302 ; a potential representing instantaneous stick force is applied at terminal 306, differentiated, and fed to an amplifier U305. The outputs from U302 and U305 are combined in an amplifier U306 to produce a potential representing F spr which is fed to amplifier U303 via a resistor R321. (d) Dynamic friction. The # es potential from amplifier U302 is also fed to a high-gain amplifier U307 and applied to two earthed diodes, X303, X304, which limit the potential to a substantially constant value representing the dynamic friction of the linkage. This potential is scaled by resistor R326 to provide an input to amplifier U303 commensurate with the difference between the friction of the linkage systems of the real aircraft and the simulator ; if that of the simulator is deliberately increased, the potential may operate to oppose it. (e) Static friction. The # es potential is amplified at U308 and fed to amplifier U303, a neon lamp NE1 or a relay being provided to short to earth potentials due to all but extremely low stick velocities. (f) Elevator deflection limiting forces. In real aircraft the maximum elevator deflection capability of the servo system depends on conditions which in the computer are simulated by combining potentials representing Mach number, produced at the sliders of potentiometers R303, R304, and those representing altitude produced at potentiometers R322, R332, and comparing the combined potentials with the elevator deflection potential from network 302 in amplifiers U304, U306, the outputs of which are passed to earth via diodes X301, X302, so long as the elevator deflection potential is within the limit set by the combined potentials. If this limit is exceeded the polarity of the output from U304 or U306, according to the direction of elevator deflection, is changed, and the resulting potential applied through a resistor R329 or R330 to amplifier U303 sharply increases the "force required" output of the computer. (g) Inertia. The output of an accelerometer (not shown) may be rectified, amplified, and fed to amplifier U303 through a scaling resistor or weights may be attached to the simulator linkage to reproduce the inertia of the real aircraft system. Modifications. The potential applied at terminal 317 may contain components representing α/Vp and q<SP>1</SP>/Vp where α = angle of attack, q, = pitching rate, and Vp = airspeed, and a "propwash" effect potential may be added. The potential from potentiometer B305 may be modified by a shaped potentiometer having its slider positioned in accordance with Mach number. Friction forces may be neglected, or the simulator linkage designed to provide the same friction as that of the aircraft. If the aircraft has a rigid mechanical linkage between stick and elevator, lag network 302 may be omitted. Simulated rudder control system. Fig. 4. A potential representing the force applied to simulated rudder pedals 410 is produced by a transducer (not shown) e.g. differential transformer, strain gauge, &c., associated with an elastic ring 414, and a second transducer 420 produces a potential representing pedal displacement which is combined in amplifier U401 with a rudder trim potential from potentiometer R403, with pedal force potential, via resistor R404, to take account of "stretch", and with a yaw damper potential from amplifier U403. The yaw damper potential is derived from a potentiometer R413 supplied, at terminal 404, with a potential commensurate with simulated rate of turn or rate of change of side acceleration according to the yaw damper or autopilot used in the real aircraft, the wiper of potentiometer R413 being positioned by the Mach number servo to pick off an output which is routed through a switch S401, a buffer amplifier U404, and a lag network comprising resistors R416, R417, and earthed capacitor C401 to amplifier U403. The output from U401 representing rudder position is combined in amplifier U402 with a potential commensurate with sideslip angle applied to terminal 403, the output of U402 being modified by potentiometers B411, R412, whose wipers are positioned in accordance with Mach number and altitude respectively. The resultant potential is combined in U405 with a pedal displacement potential, to represent the force required to overcome a centring spring acting on the aircraft rudder pedals ; alternatively, a similar spring may be provided on the simulator pedals. The output of U405 represents the force required on the pedals and is compared at 425 with the pedal force signal, the difference actuating servo-valve 426 to control hydraulic power source 415. Friction and inertia potentials may be introduced into this system, A potential representing r + #/Vp where r = rate of turn and # = sideslip angle, and a potential representing "prop-wash", may be added to amplifier U402. Simulated aileron cont