Loads on Wheels and Shock Absorbers
In its elementary representation, the undercarriage system acts as a spring-mass system, shown in Figure 7.12. Shock absorption is accomplished by its main spring and, to a smaller extent, by the tire pneumatics. Both spring and tire deflect under load. The oleo system acts as a damper; that is, it dissipates kinetic energy of vertical velocity. The strut can act as a spring for the lateral load of the ground friction.
The length of the strut is influenced by the extent that its shock absorber is compressed to the maximum. The minimum strut length is when both tire and shock
Figure 7.12. Undercarriage as a spring-mass system
absorber collapse simultaneously, yet provide sufficient ground clearance for flaps fully extended (see Figure 7.8). The most critical situation for flap clearance is when the main wheel has collapsed and the nose wheel is at the fully extended position. (In a practical situation, the nose wheel tire would also remain deflected under load, but the margin of the fully extended position is safer.) The flap trailing edge is at its lowest at aircraft rotation for liftoff. A simultaneous failure of the tire and shock absorber after decision speed V1 (see Chapter 13) would force the pilot to continue with the aircraft rotation and liftoff.
During landing, as lift is depleting with speed reduction, more aircraft weight is reacting at the ground contact, which increases the spring load of the strut. The energy is stored in the spring. On brake application, the kinetic energy of the aircraft is absorbed by the brake pads, increasing temperature. If the limits are crossed with rapid deceleration, a fire hazard exists.