Landing Gear

Skid and wheeled gear, oleo struts, and ground resonance and its prevention (lead-lag dampers, oleo/tyre servicing).

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Main ideas

Helicopter landing gear is either SKIDS (simple, light, fixed — common on light helicopters) or WHEELS (retractable or fixed, with oleo struts, brakes and steering/castoring nose or tail wheels on larger types).
Skid gear absorbs landing loads by flexing the cross-tubes and, in a hard landing, by permanent deformation; it must be checked for spread and cracking.
Wheeled gear uses oleo-pneumatic struts (oil damping + gas spring) like fixed-wing, with wheel brakes and, where retractable, hydraulic actuation, up/down locks and a warning system.
GROUND RESONANCE is a dangerous self-excited oscillation that can occur on the ground when rotor blade lead-lag motion couples with the gear/oleo dynamics — it is controlled by properly serviced lead-lag dampers and correct oleo/tyre servicing.
Ground-handling wheels may be fitted to move skid-equipped helicopters.
⚠ Exam trap: GROUND RESONANCE (a self-exciting oscillation from blade lead-lag coupling with the landing gear) is prevented by correctly serviced lead-lag dampers and correct oleo strut/tyre servicing; light helicopters commonly use skid gear.

Solved examples

What is ground resonance, and which components' servicing is critical to preventing it?
Ground resonance is a violent, self-amplifying oscillation that can build up while the helicopter is on the ground and the rotor is turning, caused by the blades' lead-lag movements getting into step with the natural bounce of the landing gear/oleos. It can destroy the aircraft in seconds. Prevention depends on correctly serviced blade lead-lag (drag) dampers and properly serviced oleo struts and tyres, which detune and damp the coupling.

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