Learjet 75

By Fred George fred_george@aviationweek.com
Source: Business & Commercial Aviation

The aircraft also has several cabin upgrades inspired by the Learjet 85, a transcontinental U.S. range business jet being developed by Bombardier. Double club seating is the only configuration offered. Each chair pivots, moves laterally and has a retractable, aisle-side armrest. The center four seats have optional fore/aft tracking, providing additional room for four passengers if the other half of the cabin is unoccupied.

The forward galley has 27% more catering storage volume and a third more trash container capacity. There are separate clean- and dirty-ice containers, considerably better space utilization in the various compartments and even an optional, galley rest seat next to the entry door.

Lufthansa Technik supplies its nice HD IFE system for the cabin. Standard kit includes an LHT niceview moving map and flight data system. Options include a Blu-ray player, XM satellite radio receiver and video-on-demand file server, along with iPod interface, single 12.1-in. monitor on the left forward cabin bulkhead and individual, 7-in., pop-up passenger seat monitors. Wi-Fi Internet access is available through optional Aircell ATG and Inmarsat SwiftBroadband transceivers. Also optional is a 12.1-in., slide-out monitor for the aft cabin bulkhead, ahead of the lavatory.

Retaining Proven Design Features

The Learjet 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 modified NASA supercritical airfoil designed by Bombardier 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 the Cessna Citation CJ4.

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