With a crop of very light jets (VLJs) in development it’s interesting to look back at another would be revolution in airplane design, the Lear Fan. In the late 1970s inventor and promoter Bill Lear conceived a turboprop airplane that would have twin engines driving a single propeller mounted on the tail. The airplane was made entirely from carbon graphite material which was expected to give it an unprecedented light empty weight. Maximum cruise speed was projected to be 350 knots, faster than Cessna’s new Mustang light jet.
The Lear Fan garnered a pot full of orders, several prototypes flew many hours in flight test, but in the end unsolvable problems with the gearbox that combined the output of the two turbine engines, and other issues with weight and aerodynamics, doomed the project. One difference between the Lear Fan and some of today’s proposed small jets is the price which was $1.6 million in 1981. That was a lot of money, but then the Lear Fan promised to do things never before possible. This story was written not long after the first prototype Lear Fan flew. FLYING 1981 Article: AIRCRAFT DESIGN: LEAR FAN BITES INTO THE BUSINESS FLEET By J. Mac McClellan
AVIATION’S NEWEST AIRPLANE is reaching out to stir up our oldest feelings. The Lear Fan 2100 has rekindled emotions not generated by big companies with engineering groups carrying out the direction of large corporate management staffs. The Lear Fan takes us back to the days when individuals put their names on airplanes and then went out to see how well they would fly and if anyone would buy them.
LearAvia headquarters in Reno, Nevada is filled with pictures of and sayings by the late William P. Lear, the inventordesigner-promoter who shaped the Lear Fan. A favorite saying, and one that best describes the Lear Fan program is, “Don’t take a nibble, take the big bite.” That is exactly what the people at Lear Fan have done.
The “big bite” is an attempt to build the first commercially successful twin-engine, singleprop pusher airplane, and if that isn’t challenge enough, the airplane will be built entirely from composite materials. Either the pusher design or the nonmetal structure would be enough to label the Lear Fan as revolutionary; together they can only be called radical.
Lear’s wife, Moya, has directed the Lear Fan project since her husband’s death in 1978. Among his final instructions, it is reported, he told her, “Finish it, Mommy, finish it.” Customers have plunked down hard cash to reserve 180 delivery spots and the prototype is flying. The British Government has supplied $50 million in loans and grants to assure production of the Lear Fan in Northern Ireland. All told, $100 million is committed to the project, which puts it in the big leagues. But the Lear Fan, both by design and circumstance, will always be credited to one man.
Bill Lear first advocated a twin-engine, single-propeller pusher airplane in a story in nowdefunct Skyways Magazine in 1954. Lear contended that such a design would, because of reduced weight and drag, perform better and more safely, without the possibility of asymmetric thrust if an engine failed.
Lear kept his pusher design concept on the back burner as he presided over advances in avionics, car radios and the first business jet. During the 1970s, he directed his aviation design talents to a new business jet he called the Learstar 600. By 1977, Canadair had purchased the production rights to the airplane; it enlarged the cabin and began production of the airplane, now called the Challenger.
With the Learstar gone to Canada, Lear decided the time was right for his pusher. Applying his “big bite” theory, he concluded that the airplane could be transformed from revolutionary to spectacular through use of lightweight composite materials that were now being used for parts of various aircraft. The Lear Fan 2100 began to take shape in the inventor’s mind and on paper.
By 1978, Lear had preliminary performance figures, production schedules and was taking orders for his all-graphite epoxy airplane. Later that year he died, leaving his wife and a small team in Reno with the command to “finish it.”
Early designs of the Lear Fan included an inverted V tail and Lycoming LTS-101 engines. As engineering and testing progressed after Lear’s death, the inverted V tail was replaced by a Yshaped tail, the vertical stabilizer and rudder pointing straight down. The LTS-101 engines were traded in favor of Pratt & Whitney of Canada PT6B free-shaft turbines, and the engine intake-exhaust ports were moved down to the sides of the rear fuselage. But the design team never flinched from a total composite airframe. As design questions were answered, so were the enormous financial problems facing the company. In just 10 days, 200 limited partnerships at $150,000 each were sold, raising $30 million in capital. The British Government, searching for production projects to supply jobs in Northern Ireland, promised $50 million if the airplane would fly in 1980, and deposits on orders for 180 Lear Fans kicked in a reported $20 million more. If the airplane could just make its first flight in 1980 to get the British bucks, the money problems would be over.
Working round the clock, more than 300 employees at LearAvia wrapped up the thousands of details, pushing for a December 30 first flight. Too-tight brake pads heated up and blew a tire during taxi tests that day. Then an accidentally discharged fire extinguisher ended hopes of flying on the last day of 1980. However, British officials in Reno to see the first flight declared New Year’s Day to be December 32, 1980. The airplane flew and at least part of Bill Lear’s dying wish was fulfilled.
Hank Beaird, who flew the first Lear Jet test flight in 1963, commanded the first flight of the Lear Fan 2100. The 17-minute hop went so smoothly that Beaird and copilot Dennis Newton traded seats so Newton could make the first landing from the left side.
That first flight, pressured by the British funding deadline, was not a normal exercise. The landing gear remained down because no gear retraction mechanism was installed. Leaks in the wing fuel tanks had forced use of a makeshift fuselage tank. The normal high-resolution airdata boom was not even installed. But the airplane flew: hundreds of people watched and cheered, and national TV news covered the final achievement of a great inventor. Sixty cases of champagne enhanced the celebration afterward.
The theoretical advantages of the pusher design have Graphite fabric for a wing panel is laid out in preparation for a high pressure and temperature treatment in the autoclave. A prop working behind the fuselage forces its wash into free air, increasing efficiency because flow over the fuselage is disturbed less, thus lowering drag. And the Lear Fan twin-engine arrangement, with two engines sharing the same prop, enhances single-engine performance.
The very first airplanes were pushers, but no recent pusher designs, except some seaplanes, have been commercial successes. The Cessna 337 Skymaster addressed the benefits of a centerline-thrust twin, but for many reasons never lived up to its promise. At one point, Beech considered building a single-engine turboprop pusher, but the idea never reached prototype stage.
Two of the disadvantages of a pusher design are its center-of-gravity considerations and increased mechanical requirements. To maintain a reasonable CG range, the engine cannot be mounted too near the tail. A forward-mounted engine requires a drive shaft and its associated bearings and supports, increasing weight and complexity.
However, by using two turbine engines with a single propeller, Lear Fan engineers actually save weight when that installation is compared to a conventional twin turboprop. The Lear Fan design eliminates the extra weight of independent nacelles, uses lighter reduction gearboxes on the engines and saves the weight of a second propeller.
The PT6B-35 engines used in the Lear Fan are variations of the world’s most popular turboprop engine. The engines have been flat rated from 850 to 650 shaft horsepower, which means they can develop full power to an altitude near 17,000 feet. The PT6 family of engines has an excellent reliability record, and carries a recommended time between overhaul of 3,500 hours. Differences between the engines used by the Lear Fan and those that power conventional turboprops are exhaust ports on a single side for the Lear Fan and a different reduction gearbox. Pratt & Whitney has also designed the engines for top performance at the 41,000 foot cruise ceiling of the Lear Fan.
Two large drive shafts transmit power from the engines to a Western Gear transmission mounted inside the tail of the Lear Fan. One-way sprag clutches connect the drive shafts to the gearbox, so when an engine is not delivering power it is automatically uncoupled. The design is similar to the free: wheeling overdrive transmissions used by many automobiles in the 1950s. If an engine fails, the sprag clutch automatically takes it off line and there is nothing for the pilot to do except, when convenient, turn the fuel control off.
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.|
The actual structural design of the Lear Fan is conventional, with parts made from graphite composite instead of metal. The wing has the usual spars and ribs, and inside the fuselage are the normal stringers and longerons. Everything is black instead of teen zinc chromate primer color that adorns the aluminum inside traditional airplanes. The Lear Fan is held together by adhesives, so no rivets mar the perfectly smooth surfaces.
Controls and systems in the Lear Fan are conventional, with purely mechanical flight control actuation. Although the horizontal tail is shaped like a V. its control surfaces are not ruddervators, as found on the Bonanza, but simply elevators.
Cabin measurements of the Lear Fan are about the same as those of a 90 Series King Air, although the Lear Fan baggage area is not as convenient to reach. Standard seating is for six passengers and two crewmembers. There is to be a private lavatory and refreshment center.
Projected Lear Fan performance is no less than spectacular, with a maximum cruise speed of 350 knots expected. In an engine-out situation, without asymmetric thrust or the drag of a dead engine and the displaced rudder needed to fly a straight line, Lear Fan engineers expect a 1,400fpm single-engine rate of climb, and a single-engine service ceiling of 29,000 feet.
A Lear Fan’s fuel efficiency should be phenomenal, too, if weight goals are met. At maximum gross weight the airplane is projected to cruise at 300 knots for 1,800 nm with reserves, obtaining about eight nm per gallon. No turbine-powered airplane can come close in fuel efficiency. None of this performance comes cheap. Its present $1.6 million price tag will be escalated with inflation between order and delivery; that base price is expected to climb again soon. Initial purchasers got their orders at $850,000 with no inflation escalator.
For all its promise and accomplishments so far, however, the Lear Fan is only beginning. The entire FAA certification process is ahead, and, given the current climate within the FAA, it will be tough for the Lear Fan to stay on schedule for fall 1982 deliveries. The second preproduction aircraft won’t be completed before this summer. Two complete aircraft must be built and then tested to their structural limits to demonstrate the strength of the airframe.
The Lear Fan will be certificated under the same FAR Part 23 rules that govern any piston or turboprop airplane weighing less than 12,500 pounds. – No airplane that relies heavily on graphite composites has yet been certificated, so the task of demonstrating the strength and durability of the material falls totally on the Lear Fan. The FAA attitude, LearAvia people say, is positive, but there are many questions. The graphite composite material is impervious to corrosion, but -what are the effects of sun, moisture and lightning strikes?
Graphite composite does not flex, so fatigue is not a problem. But are there other wearing and aging factors? Nobody knows for sure. Conventional radio antennas will not work on a nonmetallic surface and the effects of precipitation static on a nonconductive airframe- are not fully understood. And certificating a turboprop to Flight Level 410 will be a new experience for everyone. –
It’s easy to look at the Lear Fan now and imagine all the hurdles ahead. But for Moya Lear, J.S. “Torch” Lewis, the vice president, and the others who have been at LearAvia from the start, it is much easier to look back and see how far they have come: to look at the half-full glass rather than the one that’s half empty. That’s the way Bill Lear would have looked at it, anyway.