Thursday, March 1, 2007
Liberty XL2: Trainer With A Difference
Cross-country comfort and performance enter the two-seat, flight-training class
Two-seat general aviation airplanes have had a checkered career at best. For every Cessna 150/152 or Citabria that’s had a model run of 30 years, there have been a half-dozen other types that only lasted for three or five.
The Liberty XL2 is loosely based on the Europa design born in the U.K. in 1992. In this case, “loosely based” is an exercise in understatement. Park the two aircraft side by side, and you’d note a myriad of differences. Despite the Europa’s acknowledged innovations, the Liberty design is light years ahead in virtually every area.
Ivan Shaw, an Airbus engineer, designed the Europa, and his concept was to produce a “light touring aircraft.” The Europa was Shaw’s experimental, Rotax-powered kitplane, essentially a motorglider that balanced on a single wheel with outrigger wheels to keep the wingtips from dragging on the ground. Some 1,000 kits were sold in 32 countries during the 1990s. The Europa was a revelation for the time and won a number of awards in Europe.
The Liberty was launched at the turn of the century and developed over the last half-dozen years in search of its FAA certificate. The Feds issued that authorization last spring, making the XL2 the first two-seat piston aircraft certified in the United States since the Piper Tomahawk. (Before Diamond fans object, consider that the C1 was first certified in Austria, then, approved in America under reciprocal agreement.) The Liberty also has the distinction of being the first piston airplane fully approved for FADEC (full authority digital engine control) operation.
The XL2’s structure is about as 21st century as Liberty could make it. Its fuselage is pre-preg carbon fiber, and the 4130 tube-steel frame absorbs loads from the engine, nosegear, main gear and wing attach points. The XL2’s wing is also a little unusual. Designed by European aerospace engineer Don Dykins, who had a hand in choosing sections for the Concorde wing, the Liberty’s relatively small 112-square-foot airfoil provides a cruise of more like Mach .20 than Mach 2.0. (As partial compensation, the XL2 burns only about 6 gph, compared to about 8,000 gph on the Concorde.)
The XL2’s wing is a true natural laminar flow (NLF) airfoil, maintaining attached laminar flow far back on the chord. Dykins also fitted the XL2 with multiple sets of vortex generators on the outer wing to help preserve aileron response at high angles of attack, providing better roll control when approaching, and actually in, the stall.
Motive force on the Liberty is a four-cylinder, Continental IOF-240B engine driving a fixed-pitch Sensenich prop. You’ll notice the letter “I” at the beginning of the model number, designating fuel injection. That’s an unusual technology for such a small engine. Fuel injection is a relatively expensive feature, normally applied only to engines of 160 hp or more. The overriding benefit of fuel injection is that it allows very precise fuel distribution between cylinders, and that translates directly to reduced fuel burn. Injection in place of carburetion was necessary to accommodate the Continental’s PowerLink FADEC system.
True to its promised “full authority,” FADEC operates through a computerized electronic ignition system. The FADEC scans all aspects of engine operation several times a second, evaluating temperature, air pressure, CHT, EGT, fuel and manifold pressure, the phase of the moon and your astrological sign; then, it automatically adjusts mag timing and mixture for all stages of flight, from takeoff and climb to cruise, descent and landing.
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