Current Issue
May 2014 magazine cover
Subscribe

Save 47% off the cover price!

Cessna’s T-37 was dubbed “Tweety Bird” for its shrill Teledyne CAE J-69s. (USAF)

The Little Engine That Couldn't

The new Eclipse 500 lightjet will no doubt make a lot of customers happy

To be commercially viable, a small jet engine had to be fuel-efficient. That meant it had to be a turbofan. While Pratt & Whitney and Rolls-Royce began pushing ahead with turbofan technology in large engines in the 1960s, it was left to a young Purdue graduate and former Chrysler engineer named Sam Williams to create a small, fuel-efficient turbofan.

Williams left Chrysler in 1954 to start his own company. His first jet engine, prosaically named Jet No. 1, made its first run in 1957 at a meager 60 pounds of thrust. It weighed just 23 pounds; an old Williams publicity photo showed a smiling June Cleaver lookalike holding it in one hand. An improved version, the WR2, ran in 1962. Hewing closely to Frank Whittle’s 1930 turbojet configuration, the WR2 had a single-stage centrifugal compressor and a single-stage turbine. The reference book Jane’s All the World’s Aircraft described the engine as “simple in design, almost to the point of appearing crude.” In 1964, a more powerful version of the WR2 became the first Williams jet to fly, powering the Canadair CL-89 reconnaisance drone. The follow-on WR24 series, despite horrendous fuel consumption, was Williams’ first big commercial success, eventually powering more than 6,000 short-range Northrop target drones.

In 1967, Williams completed its breakthrough engine. The WR19, a turbofan based on the WR2 core, produced 430 pounds of thrust, weighed only 67 pounds, and was nearly twice as fuel-efficient as the WR2. It powered two short-lived 1970s contraptions: the Bell Jet Flying Belt, a Buzz Lightyear-style jet backpack; and the WASP II flying platform, a sort of aerial Segway Human Transporter.

The WR19 also caught the eye of military planners studying the concept of a long-range cruise missile. Williams’ timing was perfect; the WR19 was the only small engine with the fuel efficiency the cruise missile mission demanded. An up-rated version of the WR19, the 600-pound-thrust F107, eventually became the prime mover for the Navy Tomahawk and Air Force Air-Launched Cruise Missile, with production of more than 6,500 engines over 30 years. For creating the F107, Williams was awarded aviation’s highest honor, the Collier Trophy, in 1979.

Williams had begun tinkering with a small civilian turbofan based on his cruise missile technology as far back as 1971. But it would be a huge step to take a specialized Tomahawk powerplant, which only had to start once and run for three or four hours, and adapt the technology to produce a commercially viable engine.

Small size itself creates many design problems. Turbine blades can be made smaller, but air molecules can’t; as a result, skin friction and boundary layer effects are proportionally greater. (In engineering argot, a small engine is inherently less efficient because it operates at a low Reynolds number, an aerodynamic coefficient that relates component size to the air’s inertial and viscosity effects.) Compressor and turbine blade tip clearances are proportionally greater, resulting in greater tip losses. To maintain the most efficient turbine and compressor blade tip speeds, small engines must spin faster. Small turbine blades are also harder to cool. Oil passages become narrower, making lubrication tricky. Manufacturing tolerances shrink to watchmaker scale.

In 1978 Williams signed a deal to develop the WR44, an engine with 850 pounds of thrust for the five-passenger Foxjet 600, an aircraft eerily similar to the Eclipse but doomed to mockup status. A subsequent flirtation with the ill-fated American Jet Industries Hustler likewise went nowhere, and it wasn’t until 1988 that a Williams engine finally took wing with a human aboard. A pair of 1,800-pound-thrust FJ44s powered Burt Rutan’s Triumph, a proof-of-concept prototype for a Beech light business jet.

It was Cessna that jumped on the light-jet concept, however, and in 1992 the Cessna CitationJet, with a pair of FAA-certified FJ-44-1As, rated at 1,900 pounds of thrust and weighing 450 pounds, became the first production aircraft with Williams engines. At a bargain $3.2 million, it quickly became the best selling bizjet in history. Once again, Williams had jump-started a whole new class of aircraft, and once again he had the niche to himself.

But the elusive Foxjet category still beckoned. In the early 1990s, Williams began developing a fanjet in the 700-pound-thrust class. The new engine would be a clean break from the philosophy of gradual evolution and refinement that had guided the 35-year progression from Jet No. 1 to the FJ44. Developing this new technology would be expensive, but again Williams’ timing was impeccable. The General Aviation Propulsion (GAP) initiative, a pet program of NASA Administrator Dan Goldin, promised to revitalize the moribund lightplane industry with innovative engine technology. In 1996 Williams teamed up with NASA for a four-year, $100 million effort to “reduce the cost of small turbine engines by a factor of ten and revolutionize the concept of personal air transportation,” as a NASA press release put it.

When NASA engineers first saw Williams’ radical new GAP design, the FJX-2, they were skeptical. “We weren't sure if they could really do this,” recalls Leo Burkardt, the GAP program manager. “Their projected performance, weight, and cost were so much better than the other proposals that even if they only got halfway there, it would still be better than anybody else.”

Comment on this Story

comments powered by Disqus