Business Aviation Turbine Technology

 - February 27, 2009, 8:02 AM

It seems unlikely that new engine architectures such as the geared turbofan or the open rotor will make it to business aviation in the near or even mid term. According to engine manufacturers, these concepts are not suited to the needs of business aircraft, which require a lot of thrust during almost the entire flight. In previous decades the direct-drive turbofan gained popularity over the geared turbofan and the open-rotor concepts, but the fuel burn advantage the latter offer has prompted renewed interest recently. Separately, biofuels will probably find their way on board business aircraft.

According to Andrew Tanner, v-p for business and regional aviation at Pratt & Whitney Canada (P&WC), a geared turbofan is good for takeoff thrust but not as strong for the high cruise speeds business jets must provide. The manufacturer is developing a geared turbofan for two regional jets–the Mitsubishi MRJ and the Bombardier C Series–and claims the engines will provide major improvements in noise and fuel burn.

Honeywell has long been producing both direct-drive and geared turbofans, as its HTF7000 and TFE731 series, respectively; Mike Bevans, manager for technical sales, sees both geometries as “viable going forward into the foreseeable future for business jets.”

Last year Alain Bellemare, then P&WC’s president, commented that the geared TFE731 probably brings benefits “in that you can maybe stretch the family a bit more because you’ve got one more set of variables in the engine.” Designers can play with the fan size and the gear ratio, he explained to AIN. But he had doubts about any “benefit or value to fuel burn, thrust, power-to-weight ratio and so forth.”
To reduce noise and fuel burn, a geared turbofan relies on a higher bypass ratio. The loftier cruise altitudes of business aircraft present a challenge because a huge fan is needed for a high-bypass-ratio engine to generate the required thrust in the thinner air. Instead, designers use the propulsive thrust coming from the engine core.
“That’s why our bypass ratios are smaller for business aircraft applications,” said Bellemare.

Open-rotor Architecture
As for the open-rotor concept, Rolls-Royce’s director of research and technology, Ric Parker, told AIN, “We will never see one on a business aircraft because speed is essential to business aviation.” To retain its fuel-efficiency advantage, an open rotor should fly at about Mach 0.75, slower than the Mach 0.8 to 0.9 speeds that are commonplace in business aviation. However, for business jets derived from airliners, open-rotor power might be the propulsion of the future.

At General Electric (GE), marketing leader for business and general aviation Shawn O’Day said he does not see open rotors finding an application in business aviation in the near term. He cited the cost equation and the diameter of such an engine as impediments to its adoption by business aviation.

GE’s initial wind-tunnel evaluations will use a contrarotating configuration with 10 blades in the first rotor and eight in the second rotor. As of late January, a component rig test to evaluate subscale fan systems was imminent at NASA’s Glenn Research Center, according to the engine maker.

While early open-rotor demonstrators were known to be fairly loud, modern designs are much quieter, according to Parker, who attributed the noise reduction to lower tip speeds and larger blades designed in three dimensions. Open rotors are also expected to cut fuel burn by 25 to 30 percent compared with current turbofans.
The open-rotor configuration “will struggle on a business jet,” Bevans told AIN. He cited acoustics, both for community noise and cabin noise. Moreover, as with the geared turbofan, higher altitudes would diminish performance. Ramp logistics could also be an issue, as open rotors would be nearer to ground personnel and equipment, Bevans noted.