Extract of the comments from some test flight of 300ER pilot -
Boeing devised a semi-levered arrangement for the main gear. A hydraulic strut connects the forward part of the wheel truck to the top of the vertical gear strut. On rotation, the diagonal strut locks in place to keep the gear truck perpendicular to the vertical strut. Instead of rotating about the centre axle, the aircraft now rotates about the aft axle, increasing the apparent height of main gear by around 0.3m.
Tailstrike danger
As with the stretched Airbus A340-600, tailstrikes are possible, so the control laws of the fly-by-wire 777-300ER in the pitch axis incorporate a tailstrike protection feature. This is available in the normal mode of the fly-by-wire control system and uses up to 10° nose-down elevator to prevent a tailstrike. Flight-test results show the semi-levered gear and tailstrike protection, along with minor improvements to the brakes, have combined to reduce the -300ER's predicted take-off field length by 305m.
(primarily SLG improves takeoff performance & simultaneously provides added Tail strike protection)
Source unknown -
Boeing 777 semi-levered main gear.
Because the vast majority of the weight of the airplane is borne by the lift of the wings at the time of rotation, the semi-levered gear acts as if it were “pushing” down like a longer gear. This allows a higher pitch attitude for the same tail clearance or more clearance for the same pitch attitude. A hydraulic strut provides the energy to provide this increased takeoff performance. Although designed to increase takeoff capability, the system provides increased tail clearance for the same weight and thrust as nonequipped airplanes.
Timely elevator input can help avoid tail strikes on both takeoff and landing. The tail strike protection command (TSP CMD ) is summed with the pilot’s input to form a total elevator command. The TSP CMD is limited in size to 10 degrees, which allows the pilot to overcome its effects, if desired, by pulling the column farther aft. The size of the TSP CMD is controlled by excessive tailskid rate relative to a nominal threshold of tailskid rate, and by excessive nearness of the skid to the runway, relative to a nearness threshold. Different thresholds are used for takeoff and landing. The TSP CMD is limited to commanding nose down increments only. Tailskid height and rate are computed from radio altimeter signals, pitch attitude, pitch rate, vertical speed, and the length between the radio altimeter location and the tailskid location. A complementary filter is used to provide acceptably smooth rate and height signals. Provisions are included to account for the bending of the forward fuselage when the nose wheel gear lifts off the ground.
To sum it up about SLG - source from boeing -
(1) the ability to tilt the bogie to provide an effectively longer main landing gear during takeoff rotation and liftoff;
(2) the ability to reposition the bogie beam to an appropriate angle for stoWing the landing gear;
(3) the ability to position the bogie beam to an appropriate pitch-up angle in preparation for landing after landing gear deployment, and thereby facilitate an early air-ground sensing upon initial ground contact of the aft bogie Wheels;
(4) the ability to effectively decouple the auxiliary strut during static and ground-roll operations so as to facilitate equal loading of all main gear Wheel and, accordingly, optimum braking ability; (5) the ability to deactivate the functioning of the auxiliary strut that provides the semi-levered action When desired, such as during landing, so that the auxiliary strut acts as a damping device for partially absorbing touchdoWn loads such that the load transmitted to the aircraft is reduced.