The gearbox and propeller are two points that could rob all power if they should fail, so Lear Fan engineers have taken special pains to design in reliability and fault detection. Electronics measure pressure and temperature in the gearbox as well as search for loose chips of metal. More than two gear teeth are always in contact so the transmission could survive a loss of one or two. If both oil pumps failed or oil leaked out, the heat generated would melt a special wax in the gears and drive shaft, providing enough lubrication for up to five hours' engine operation at reduced power.
The prop is controlled by two governors. Should both fail, the blades lock at a midrange pitch and the crew can control power with throttle, as with a fixed-pitch prop. Of course, there is always the possibility that all of these safety backups could fail, leaving the airplane without power. But asymmetric-thrust accidents in conventional twins take their toll despite their completely separate power systems.
While the Lear Fan 2100 owes some of its spectacular projected performance to the inherent efficiency of the pusher design, the real secret is nothing new at all-it's simply the very light weight of the airplane.
Maximum gross takeoff weight is to be only 7,200 pounds with a projected empty weight of around 4,000 pounds. By comparison, the new Cessna Corsair, the lightest turboprop now built, has a maximum gross weight of 8,200 pounds; while the Piper Cheyenne I, another light turboprop, weighs in at 8,700 pounds. A Beech King Air C90, with a cabin similar in size to the Lear Fan, weighs 9,650 pounds for takeoff.
None of these conventional turboprops has more than 1,100 shp and the Corsair has only 900 shp, compared with 1,300 shp on the Lear Fan. It doesn't take an aeronautical genius to figure out where the Lear Fan's extraordinary performance comes from.
These super-low Lear Fan weights result from the graphite composites used to build the airframe. Lear Fan people say the graphite material is twice as strong as aluminum of equal weight. Despite its light weight, the Lear Fan design is stressed for plus six to minus four Gs.
While graphite composites have been used before for parts of airplanes, no airplane has ever been built entirely of the material. The Marine Corps' new vertical-takeoff Harrier II, with an all-graphite wing, comes closest.
Graphite composite is not fiberglass, and the comparison nearly drives the Lear Fan people to apoplexy. While fiberglass has proven an excellent material for boats and Corvette, it is to graphite as pot metal is to steel. The image of a plastic airplane disappears wheri you actually see and touch the Lear Fan. Formed and cured graphite composite looks and feels like a very hard metal.
The basic graphite composite material is actually a cloth woven from graphite threads impregnated with resin. Union Carbide makes the carbon fiber and Fiberite weaves the coarse cloth. The cloth is 13 thousandths of an inch thick, and the thinnest parts of the Lear Fan are made from at least four layers. Thicker parts, like the main wing spar, are made of up to 72 layers of graphite cloth.
To compensate for directional-strength properties found in woven materials, cloth layers in the Lear Fan are layed in precise formations. The correct number of cloth layers are cut and positioned in a mold, and then sealed in place with a vacuum. Laying up the cloth requires techniques more common to the garment industry than aircraft production.
The entire mold goes into an autoclave to be cured at high temperatures under 90 pounds of pressure. What comes out of the autoclave is a very smooth and very rigid component. Using this method, large components, such as an entire upper or lower wing skin, are built in a single mold.
Lear LEARAVIA LEAR FAN 2100
All performance figures, weights and specifications are based on design projections and windtunnel model testing. Certification and production airplanes are scheduled for the third quarter of 1982.
|Engines||P&W PTO-35, flat rated at 650 shp each|
|Prop||Hartzell, 4-blade, 90-in. dia.|
|Length||39 ft. 8 in.|
|Height||11 ft. 6 in.|
|Wingspan||39 ft. 4 in.|
|Wingarea||163 sq. ft.|
|Max ramp weight||7,250 lbs.|
|Max takeoff weight||7,200 lbs.|
|Standard empty weight||4,000 lbs.|
|Max useful load||3,250 lbs.|
|Zero-fuel weight||5,900 lbs.|
|Max landing weight||6,850 lbs.|
|Wing loading||44.2 lbs. per sq. ft.|
|Power loading||5.5 lbs. per hp|
|Max usable fuel||250 gals./I,675 lbs.|
|Certificated ceiling||41,000 ft.|
|Max pressurization differential||8.3 psi 8,000-ft. cabin altitude at 4 1,000 ft.|
|Max rate of climb||3,700 fpm|
|Single-engine rate of climb||1,400 fpm|
|Single-engine service ceiling||29,000 ft.|
|Max cruise speed||(31,000 ft.) 350 kts.|
|Economy cruise speed||(41,000 ft.) 304 kts.|