Landing Gear
Skid and wheeled gear, oleo struts, and ground resonance and its prevention (lead-lag dampers, oleo/tyre servicing).
Landing gear, summary notes
- 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.
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.
Landing gear concept map
Landing Gear
Landing gear quiz
Landing Gear, quiz
1. Light helicopters commonly use
Skid landing gearRetractable tricycle gearFloats only2. Skid gear absorbs landing loads by
Flexing the cross-tubes (and deforming in a hard landing)Oleo struts at each corner, metering fluid through an orifice to damp the landing loadsCompressed gas held in a sealed chamber, which is progressively squeezed as the aircraft settles3. Ground resonance is a
Self-excited oscillation from blade lead-lag coupling with the gearNormal start-up vibration, felt as the rotor passes through its own natural frequency on run-upAn electrical fault in the rotor speed sensing, producing a false vibration indication on the ground4. Ground resonance is prevented by correctly serviced
Lead-lag (drag) dampers and oleo struts/tyresFuel pumpsSlip rings5. Wheeled helicopter gear uses
Oleo-pneumatic struts (oil damping + gas spring)Solid unsprung legs, relying on tyre deflection alone to absorb the landing loadsSkids only, with the wheels fitted purely as ground handling aids around the hangar6. In the oleo strut the gas acts as the
SpringDamperLubricant7. Retractable helicopter gear is normally actuated by
Hydraulics, with up/down locksThe batteryBleed air only8. A hard vertical landing on skid gear may cause
Permanent skid spread/deformationNothingTyre burst