NBAA Convention News

Snecma sets aside $100M for Silvercrest research

 - September 20, 2007, 10:56 AM

Despite having lost out to Rolls-Royce in the competition to power Dassault’s still-under-wraps super-midsize Falcon, Snecma is moving ahead with development its Silvercrest engine for business jets. The French manufacturer (Booth No. 824) is here exhibiting a full-size mockup of the 10,000-pound-thrust-class turbofan.

The first Silvercrest core parts have been produced and engineers are now building compressor components. Simultaneously, they are putting together nonmoving subassemblies, according to a Snecma spokes-man, in anticipation of the full-scale program starting by the middle of next year. Between now and then, a demonstrator core engine will be evaluated under a $100 million technology program. The actual core engine should run soon–in the fourth quarter of the year by the latest, the spokesman said. Engine certification is tentatively set for early 2011, depending on the schedule of would-be OEM customers.

With its Silvercrest engine, the Safran subsidiary is offering business jet makers thrust levels between 9,500 and 12,000 pounds. Temperature margins are claimed to fulfill higher reliability and maintenance cost standards. The typical application is said to be a large-cabin, long-range business jet with an Mtow between 45,000 and 60,000 pounds.

The engine has a one-stage fan, no booster, a five-stage high-pressure (HP) compressor, a direct-flow combustor, a one-stage HP turbine and a three-stage low-pressure (LP) turbine. Snecma claims it will have give business jets the benefits long available to commercial aviation, hence the titanium wide-chord swept fan blades that use the same 3-D aerodynamics as Snecma’s bigger CFM56 engines. Fan diameter will be 40 inches and the bypass ratio will be 4.5.

The final stage of the compressor will be centrifugal, which is possible only because of the relatively low thrust. The maximum thrust for a turbofan to accommodate a centrifugal compressor is probably somewhere between 12,000 and 16,000 pounds, according to Snecma. Above this limit, the stress that parts are subjected to makes centrifugal compressors unsuitable. But under this threshold, advantages are significant, the engine maker said. “The pressure ratio with this single stage is very good,” said Jean-Pierre Cojan, Snecma’s executive vice president for commercial engines. “In our case, it translates into a pressure ratio greater than 17 on the entire five-stage compressor.” The engine’s overall pressure ratio is 27.

Also, clearance at blade tip is an issue on the final stages of an axial compressor. Clearance between blade tips and the internal side of the casing cannot be reduced below a given value. This value becomes relatively great as the blades decrease in size. Thanks to its different shape, a centrifugal compressor solves the problem.
The biggest challenge is the high temperature at compressor exhaust. The answer should lie in cooling and high-quality hardware, Cojan said.

Turbomeca, the helicopter turboshaft company in the Safran group, helped design the centrifugal compressor. This part of the engine is a core skill of a helicopter engine maker. One centrifugal stage can replace three or four axial stages, according to Turbomeca.

In terms of noise, Snecma is targeting 15 to 20 dB below the current ICAO Chapter 4 standard (that equates to 25 to 30 dB under the Chapter 3 standard, with which operators are more familiar). Emissions should be 50 percent lower than current CAEP6 standards. Specific fuel consumption is expected to be 15 percent better than those of existing engines in the class. Thrust at climb and cruise should improve by 25 percent over those engines.