Geared Turbofan Is What's Needed for Single-Engine Personal Jets

Photo:  Honeywell Aerospace

Vern Raburn and I were chatting Tuesday in the wake of Piper's cancelling the Altaire single-engine jet program. "So what about the proposed next generation of single engine turbine aircraft from Diamond and Cirrus Design?" I asked him.

Both of these aircraft are intended to cruise no higher than 25,000 ft, not up at FL 350 as would have the PiperJet Altaire. So it's logical to assume that the new Cirrus and Diamond single engine turbine aircraft would be powered by PWC PT6A or Honeywell TPE331 turboprop because cruising in the mid-twenties is the strong suit of turboprops.

Not so. Raburn pointed out that a turbofan is considerably quieter and smoother than a turboprop, plus it doesn't have a low hanging, exposed propeller that can be damage by FOD. And, for many potential buyers, a propeller chops up their egos, thus they would only consider a fanjet. Advantage turbofan.

But what about efficiency? Current technology turbofan engines, built for today's business jets, operate most efficiently in the mid to high 40s, some 20,000 ft higher than the cruise altitudes of the Cirrus and Diamond jets. At high speed cruise, a typical fanjet consumes nearly twice the fuel at FL 250 as it does at FL 410. Conversely, at long range cruise it cruises 25 pct slower at FL 250 than at FL 410 on the same fuel flow.

Vern believes that Pratt & Whitney's cutting edge, geared turbofan [GTF] technology could provide a solution. The GTF may be new to P&W, but light geared turbofans have been built by Garrett, AlliedSignal and Honeywell for almost three decades. The venerable TFE731 indeed proves the concept.

But the 3,000 lb to 5,000 lb thrust 731 still is designed for high altitude cruise efficiency.

If Honeywell, or another firm, were to develop a 2,000 lb thrust class geared turbofan with a tiny gas generator core and a 6:1 to 10:1 bypass ratio, that might be a perfect powerplant for a single-engine turbofan aircraft designed to cruise at FL 250. That could couple turbofan quiet and smoothness with turboprop fuel efficiency.

What do you think of Vern's concept?

Tags: ba99 GTF Honeywell TFE731