Since irreversible power controls isolate the pilot from aerodynamic hinge moments, artificial restoration of the hinge moments, or “artificial feel,” is required.
Longitudinal artificial feel systems range in complexity from simple springs, weights, and stick dampers to computer-generated reactive forces applied to the control column by servos.
A particularly simple artificial feel system element is the bobweight. The bobweight introduces mass unbalance into the control circuit, in addition to the unbalances inherent in the basic design. That is, even mass-balanced mechanical control circuits have inertia that tends to keep the control sticks, cables, and brackets fixed while the airplane accelerates around them. Bobweights are designed to add the unbalance, creating artificial pilot forces proportional to airplane linear and angular accelerations. They also have been used on airplanes without irreversible power controls, such as the Spitfire and P-51D.
The most common bobweight form is a simple weight attached to a bracket in front of the control stick. Positive normal acceleration, as in a pullup, requires pilot pull force to overcome the moment about the stick pivot of increased downward force acting on the bobweight. There is an additional pilot pull force required during pullup initiation, while the airplane experiences pitching acceleration. The additional pull force arises from pitching acceleration times the arm from the center of gravity to the bobweight. Without the pitching acceleration component, the pilot could get excessive back-stick motions before the normal acceleration builds up and tends to pull the stick forward.
In the case of the McDonnell Douglas A-4 airplane’s bobweight installation, an increased pitching acceleration component is needed to overcome overcontrol tendencies at high airspeeds and low altitudes. A second, reversed bobweight is installed at the rear of the airplane. The reversed bobweight reduces the normal acceleration component of stick force but increases the pitching acceleration component.
Another interesting artificial feel system element is the q-spring. As applied to the Boeing XB-47 rudder (White, 1950) the q-spring provides pedal forces proportional to both pedal deflection and airplane dynamic pressure, or q. Total pressure (dynamic plus static) is put into a sealed container having a bellows at one end. The bellows is equilibrated by static pressure external to the sealed container and by tension in a cable, producing a cable force proportional to the pressure difference, or q. Pilot control motion moves an attachment point of that cable laterally, providing a restoring moment proportional to control motion and to dynamic pressure.
It appears that a q-spring artificial feel system was first used on the Northop XB-35 and B-49 flying wing elevons, combined with a bobweight. Q-spring artificial feel system versions have survived to be used on modern aircraft, such as the elevators of the Boeing 727, 747, and 767; the English Electric Lightning; and the McDonnell Douglas DC-10. Hydraulic rather than pneumatic springs are used, with hydraulic pressure made proportional to dynamic pressure by a regulator valve. In many transport airplanes the force gradient is further modulated by trim stabilizer angle. Stabilizer angle modulation, acting through a cam, provides a rough correction for the center of gravity position, reducing the spring force gradient at forward center of gravity positions. Other modulations can be introduced.
Advanced artificial feel systems are able to modify stick spring and damper characteristics in accordance with a computer program, or even to apply forces to the stick with computer-controlled servos.