The Little Engine That Couldn't
The new Eclipse 500 lightjet will no doubt make a lot of customers happy
- By David Noland
- Air & Space magazine, November 2005
(Page 4 of 6)
With $60 million in investors’ money, a board of directors studded with high-tech corporate heavyweights, and an exclusive deal with Williams for the EJ22, Raburn launched Eclipse Aviation in March 2000. Williams, citing the Eclipse deal, persuaded NASA to skip the FJX-2 flights in the V-Jet II. This enabled Williams to get its final GAP payment sooner and turn immediately to the task of transforming its test-cell tour de force into a viable FAA-certified engine.
Exactly how did such a little engine achieve such extraordinary performance? Officially, nobody’s saying. The Williams company, privately held and with a long history of military projects, is secretive about technical details. NASA and Eclipse people who worked on the project, bound by confidentiality agreements imposed by Williams, are likewise mum.
“I think I can tell you that the main reason for the engine’s light weight is the architecture,” says NASA’s general aviation champion, Bruce Holmes, referring to the configuration of a jet engine’s fan, compressors, combustor, and turbines. “But I’d go to jail if I told you what that architecture was.”
Holmes can rest easy. I managed to ferret out the FJX-2’s architectural secret anyway: Instead of the usual two compressors, it had three, each spinning independently at its optimum rotation speed on one of three concentric shafts and driven by its own turbine. Designers call this unusual configuration a three-shaft, or three-spool, engine (see “Spools,” above).
The giveaway is on the instrument panel of the original Eclipse 500. Most jets have two readouts: N1 for the low-pressure (LP) compressor/fan, and N2 for the downstream high-pressure (HP) compressor. The Eclipse had an N3 gauge, which points to the presence of a third, intermediate-pressure (IP) compressor. Ed Lays, a retired Williams engineer not bound by any secrecy agreements, confirms that the FJX-2 was a three-shaft design.
A three-spool engine can be very efficient. “It gives you a lot of flexibility in matching compressors and turbines,” says Burkardt (“Not that I’m saying the FXJ-2 was or was not a three-spooler,” he adds dutifully). However, a three-shaft engine is mechanically complex, with “bearings and seals out the ying-yang,” in the words of Teledyne CAE veteran designer Gerry Merrill. Only two three-spool engines have ever been certified for commercial use: the Rolls-Royce RB.211 family of airliner engines first certified in the ’70s, and the Garrett ATF3, a fearsomely complex and troublesome bizjet engine that flopped in the marketplace 10 years later.
The decision to abandon the simple, well-proven two-shaft configuration of all previous Williams fanjets set off controversy within the company. “Some of the guys who’d worked on the FJ44 didn’t have much confidence in the EJ22,” says Lays, who explains that one impetus for the three-shaft design came from Sam Williams’ son Gregg, then a Williams VP and now company president, who’d spent two years working with Rolls-Royce on the RB.211. “Gregg was hooked on three-spool engines back then,” Lays recalls.
The axial high-pressure compressor showcased at Oshkosh was also a departure for Williams, which had used centrifugal compressors in all its previous engines (see “Compressors,” p. 23). Other rumored design features-—compact in-line combustors, tiny integral accessories mounted directly on the main shaft—will not be revealed until next year, when a five-year NASA embargo on the release of FJX-2 technical publications expires.