Turbines, although easier to operate and more efficient than pistons, have traditionally been too expensive for the grass-roots pilot. But not for much longer

NASA administrator Dan Goldin has a vision of "Hertz Renta-Jet" counters at airports all over the USA and, later, around the world. Freeing wide tracts of airspace to unfettered public use, safely and efficiently, is a commonly shared dream of many in industry and government.

Realising the dream depends on several breakthroughs, the most fundamental of which is the development of simple, low-cost engines. The current focus is on two main powerplant types - a rugged turbofan with turboprop and turboshaft derivatives, and a diesel engine burning Jet-A fuel.

Turbines, although easier to operate and more efficient than reciprocating engines, have traditionally been too expensive for the grass-roots pilot. Airframe applications have come down in size, as turbine engines have penetrated lower-cost territory, but the price of these engines has remained prohibitive for most general aviation (GA) manufacturers to contemplate, even in new single-engine designs. The same cost sensitivity, in turn, has prevented engine makers from building smaller turbine engines for GA.

In the USA, NASA has taken on the task of breaking out of this positive feedback loop through its General Aviation Propulsion (GAP) programme. This is focused on the demonstration of "affordable, revolutionary propulsion systems for GA aircraft", and includes studies of new turbine and "intermittent combustion" (IC) concepts. The results will feed into NASA's concept of the small aircraft transportation system (SATS). Using GAP-type powerplants as a basis, SATS has a 20-25 year focus and embraces a vision of private vertical and short take-off and landing aircraft.

Williams International is tackling the turbine GAP with a goal of creating a turbofan that will cost well under $100,000. Led by Dr Sam Williams, champion of the "cheap and cheerful" turbofan, the company successfully completed the first run of its FJX-2 GAP engine in December 1998. Although shrouded in considerable secrecy, the 700lb-thrust (3kN) FJX-2 takes its lead from Williams' FJ33 and FJ44 small turbofans. In service on Cessna's entry-level CitationJet and powering several new small business jets such as the Raytheon Premier I, the FJ series traces its lineage to tiny turbofans like the F112, built in its thousands by Williams to power US cruise missiles.

"Our role, we believe, has been to make smaller engines that perform like the very large engines as far as specific fuel rate and thrust per pound is concerned, and we've been quite successful at that," says Williams. "We have about 10% of the parts count of the large engines, and we maintain the fuel rates of the bigger turbofans." Thrust ratings have been reduced from 1,900lb [the lowest rated FJ44], to between 900lb and 1,500lb for the FJ33, and towards 700lb for the FJX-2.

"Soon, aircraft will be developed around that size of powerplant," says Williams, referring to his GAP engine. "I see the future of GA as being very bright because these engines will bring improvements in safety, life, operating cost and speed - all the things you are after in aviation. It is all keyed around propulsion systems," he adds.

Two-pronged approach

Williams has a two-pronged approach to making successful small engines. Innovative design and miniaturisation form one element, while manufacturing efficiency forms the other. "We have developed manufacturing processes at the same time as we have designed the engine, so by the time we get to production, we know exactly how to build it and how much it will cost," he says. Concurrent engineering is "a major reason why our economy is flourishing. Everybody from automobile makers to vacuum cleaner manufacturers are with it. We are really pursuing that philosophy".

Flight tests are planned this year in a Burt Rutan-designed, Scaled Composites-built testbed aircraft called the V-Jet II. "The FJX is going to cause a revolution in the light aircraft industry," says NASA GAP programme manager Leo Burkardt. "It will give you the option of having a high-performance aircraft at very affordable prices. It will be a good product for small businesses, as it will give them something with the same convenience and comfort as a current corporate jet, but for something costing considerably less than $1 million. The cost driver is even greater than on the pistons, and we are talking about orders of magnitude reductions in price."

TSX-1 turboshaft and TSX-2 turboprop versions are being developed at Williams, though only the turbofan version is set for flight tests. The gearbox of the TSX-2, which will have two stages of reduction to reduce the output driveshaft speed to around 2,000rpm, has just been completed. The TSX-1, with a single stage of reduction to 5,000-6,000rpm, is in parallel development. "They are designed to be modular, so all you basically have to do is take off the fan and put a gearbox in its place," says Burkardt, who says "they are designed to have as few differences as possible. All of them are very light and small".

While Williams presses on with turbine developments, Teledyne Continental is busy with an IC development called "the GAP engine", which NASA plans to demonstrate this year in a Cirrus SR20. The powerplant is a four-cylinder, horizontally opposed, two-stroke diesel engine designed to run on JET-A fuel. "One of the big targets was to reduce the cost of piston engines to half that of current engines." says Burkardt. Tests so far show specific fuel consumption (SFC) is down to around 0.36 versus 0.42 for the "better" piston engines at optimum cruise settings, he says. "With air-cooled engines that run rich at high power settings, you get worse case scenarios of SFC levels of 0.6 to 0.7, and sometimes 0.5 in the cruise. With this engine, we get 0.36 at all stages of flight.

"We are shooting for a time between overhauls of 3,000h, almost double today's best of around 1,800h." A large part of the enhanced reliability and ruggedness will come from novel construction. Here, NASA has faced a dilemma. The legendary reliability of marine and truck diesels stems largely from the sort of solid construction that would be impossible in weight-critical aircraft applications. Low-weight piston designs, together with aggressive use of aluminium and magnesium in the engine block, have helped make the goal achievable. The engine block is cast in two halves that bolt together, the tie-bolts spanning the entire width of the powerplant and carrying stresses that would otherwise be taken by the casing. This means more weight saving, and lower cost. "Our goal has been to match the installed weight of a 200hp [150kW]-class piston engine such as the IO-360. We're close to that," says Burkardt.

Source: Flight International