Retaining Proven Design Features
The 75 retains all the best original design features of the Learjet 45. It was certified in 1997 as an all-new FAR Part 25 transport category aircraft, through Amendment 77. This means it has damage-tolerant structure, 2-sec. engine failure recognition time built into takeoff performance computations, more robust flutter margins and jammed flight control protection, plus separated, redundant control linkages, 16-g passenger seats, enhanced ice protection and improved cabin fire protection, among other improvements. The only part it has in common with previous Learjets is the nosewheel.
The primary airframe is time-proven aluminum alloy, with a three-spar wing having single piece upper and lower skins, along with a semi-monocoque fuselage and empennage. Composites are used for secondary structures. The windshield outer ply is glass for durability.
The wing has a modifiedsupercritical airfoil designed by using advanced computer tools. It has a far higher lift-to-drag ratio and it is much larger in area than the 1940's-vintage 64000 series NACA airfoil used for all previous Learjets.
Each leading edge has two “sawtooth” or offset breaks, plus four vortilons and other boundary layer control devices to enhance low-speed handling qualities. We know of no other business jet with a “hard” leading edge wing that has more docile stall characteristics.
Systems are simple but amply redun–dant. The primary flight controls are manually actuated, with hydraulically powered spoilerons adding roll control authority. A DC-powered rudder boost system reduces pedal effort below 180 KIAS. A computer-controlled flaps/spoilers/stab interconnect neutralizes pitching moments with configuration changes. There is a three-axis electric trim system. Trimming the stabilizer nose down with increasing speed also increases pressure on the up/down elevator spring force system to prevent pitch-over control at high speeds.
The 28-VDC split-bus electrical system is supplied by left and right starter-generators, plus dual 24-volt, 38-amp-hour batteries in the tail and an emergency battery in the nose. The architecture includes automatic load shedding, starter to generator switching and bus tying. AC alternators supply power for windshield anti-icing and defogging. But the alternators can't serve as backup power sources for other systems as they do aboard theCitation CJ4.
Fuel is stored in left and right wet-wing tanks, plus an aft fuselage tank, in keeping with long-standing Learjet design protocol. A single-point pressure refueling receptacle ports fuel into the fuselage tank, which then flows by gravity into the wing tanks. DC boost pumps supply fuel for engine start, APU operation and cross flow. After start, left and right jet pumps, using motive flow from the high-pressure engine-driven fuel pumps, transfer fuel to the feeders and supply the engines.
Bleed air is used for wing, engine inlet and horizontal tail anti-ice protection, plus air-conditioning and pressurization. As noted, the APU supplies bleed air on the ground for heating and air-conditioning. DC electric heaters are fitted to the probes, sensors, windshields, cockpit foot warmers and the baggage compartment.
A single, 3,000-psi hydraulic system using conventional mineral MIL-H-5606 red fluid, powered by left and right engine-driven pumps, supplies actuators for landing gear, spoilerons, flaps and thrust reversers, plus the wheel brakes. A DC aux pump provides power for the brakes prior to engine start, plus the landing gear and flaps if the engine-driven pumps fail in flight.
The Learjet 45/75 are the first Learjets to be fitted with trailing-link landing gear. Rolling stock, including carbon disc/rotor brake heat packs, is considerably larger than in any previous Learjets, providing excellent stopping power.