gimbal
To move a reaction engine about on a gimbal so as to obtain pitching and yawing correction moments.
gimbal: to pivot a thruster to steer the aircraft
To gimbal is to tilt or rotate a reaction engine on its mounting points in order to change the direction of its thrust vector. This allows a pilot or control system to generate pitch and yaw moments without requiring separate control surfaces or secondary thrusters. The engine itself becomes the steering mechanism.
Aircraft with gimballed engines typically mount the engine on a pair of perpendicular pivots, called a gimbal mount. These pivots allow movement in two axes: one for pitch correction (nose up and down) and one for yaw correction (nose left and right). Actuators, either hydraulic or electric, receive signals from the flight control system and move the engine through these pivots. The engine's exhaust nozzle follows, and the thrust vector tilts accordingly.
Gimballing is standard on vertical takeoff and landing aircraft, particularly military types where hovering or near-hovering flight demands independent control of pitch and yaw without requiring airflow over control surfaces. It is also used on some high-performance fighter jets to improve manoeuvrability at extreme angles of attack. The Harrier GR9, for instance, gimbals its Rolls-Royce Pegasus engine nozzles through four nozzles in two axes to control flight in hover.
Limits and failure modes
Gimbal range is always restricted, typically to between 20 and 40 degrees from the engine centreline, depending on airframe design. Exceed this range and the control authority is lost. Hydraulic failure, electrical signal loss, or a jammed gimbal actuator will degrade or eliminate pitch or yaw control, making the aircraft difficult or impossible to control, particularly in hover. Some aircraft are designed so that engine gimballing is backed up by other control systems to prevent loss of all control authority in a single-channel failure.
The term gimbal derives from the medieval word for a ring or hoop, which is the fundamental shape of the pivoting support structure. In aviation, the verb came naturally from the noun as engineers began to command these mounts to move under control.