The damping coefficient and spring constant of shock absorbing struts for commercial jet aircraft landing gears are selected to allow optimum absorption of forces on the landing gears at touchdown for an aircraft that has been descending at the maximum permissible "sinkrate" (descent rate immediately prior to touchdown). When so selected, the shock absorbing struts are unable to effectively dampen vertical oscillations of the aircraft body ensuing from a downward plunging of the weight of the aircraft due to the deployment of lift spoilers immediately after touchdown. To rapidly attenuate these post-touchdown vertical oscillations (for more efficient braking and ground handling), while still affording the proper spring constant and damping coefficient necessary for absorbing touchdown impact forces at maximum "sinkrate", the damping of each shock strut is substantially increased after the first cycle of strut compression and re-extension by automatically sensing the occurrence of touchdown, and after a predetermined time delay therefrom, actuating a damping control within each shock strut, to change the damping coefficient to an increased level.

An aircraft undercarriage suspension includes a liquid filled telescopic suspension strut and a gas spring. Liquid displaced by the strut into the gas spring is selectively directed through alternative flow restrictors, one of which is matched for optimum damping during aircraft landing and the other of which is matched for optimum damping during taxing. A ride control valve operates on the nose wheel undercarriage to control aircraft pitching, the ride control hydraulic system being separated from the damping oil by a floating piston.

A vibration damper of viscous damping type for damping low-frequency (10 to 20 Hz) vibrations has at least one subpassageway oblique to the axis of a main passageway. Since vortexes are generated around the main passageway as damping fluid passes through the subpassageway, the fluid can effectively reciprocate through the main passageway in response to pressure gradients induced by the vortexes, thus increasing the damping force generated as the fluid flows through the main passageway. Further, since the frequency at which the maximum flow rate, that is, the maximum damping factor can be obtained is determined on the basis of the dimensions of a passageway, the dimensions of the main passageway and the subpassageway are selected so as to match the maximum-flow-rate frequency of the subpassageway with that of the main passageway.

A shock absorber comprising a strut (1), a sliding rod (3) mounted to slide inside the strut, at least one gas chamber (13, 17) under pressure, an oil chamber (6) associated with the gas chamber, and an expansion chamber (5) connected to the oil chamber via expansion throttling valve (8), the volume of the expansion chamber (5) varying as a function of the extent to which the sliding rod is received in the strut. The shock absorber also includes an expansion short circuit passage (18) between the oil chamber (6) and the expansion chamber (5), the expansion short circuit passage having a cross-section which is greater than the flow section of throttling valve (8), and the shock absorber includes controlled closure partition (32) for closing expansion short circuit passage (18).

A fluid dynamic restrictor limits the speed at which the bed of a dump truck can be lowered by discharging fluid from a lift cylinder. The fluid dynamic restrictor defines a fluid vortex that provides a pressure drop proportional to the square of the fluid flow rate. Therefore the maximum rate at which the bed can be lowered is much the same regardless of whether the bed is empty or full. In contrast to a restrictor in the form of a throttled valve or wire mesh, the fluid dynamic restrictor is not easily plugged by debris. The fluid dynamic restrictor also offers little resistance in its reverse direction, which facilitates rapid lifting of the bed.