The Cirrus SF50 Vision single-engine jet development program had been on hold since last fall while the company’s new Chinese based owner, CAIGA, analyzed and reviewed the program. But the new owners are apparently satisfied with the prospects for the small jet and Cirrus says development work on the program is now going ahead full speed.
A few weeks ago the Diamond D-Jet got a similar shot in the arm in the form of new company ownership and financing from investors fromDubai. The D-Jet program has been in the works for years and three prototypes have been flying. But like the Cirrus Vision, the D-Jet program had been on the shelf for months awaiting new funding commitments.
In contrast, Piper spent lavishly on a large redesign of its PiperJet over the past couple of years. Piper created a much larger fuselage for what became the Altaire single engine jet, and predicted a higher cruise altitude, longer range, and higher speed. Piper went so far as to recruit engineers for the program in Wichita, an effort complete with billboards in the Air Capital.
But suddenly, after a large presence at the NBAA show complete with full scale mockup of the Altaire cabin, Piper pulled the plug and canceled the program. The move was so abrupt that advertisements for the jet were still appearing in aviation magazines after the program had been suspended.
The owners of all three single-engine jet programs are now located outside the U.S. with Piper’s parent being based in Brunei, Cirrus in China, and Diamond in the Middle East. That certainly means there is a global view at work when it comes to single-engine jets. But how can three companies look at similar programs and come to opposite conclusions on how to proceed?
While I can’t possibly know how the discussions evolved at each of the three companies, and what factors were considered, I do know what the biggest challenges are for any single-engine jet design. It really boils down to the inefficiency of jet engines – particularly small engines with a modest bypass ratio – when flying below 37,000 feet or even higher, and the 61-knot stall speed limit on single-engine airplanes.
Cirrus and Diamond expect to certify their jets with a maximum ceiling of 25,000 to 28,000 feet. It is extremely doubtful the FAA would ever approve an operating ceiling for a single above 30,000 feet. The reason is that cabin pressurization could be suddenly lost if the engine were to fail. Or even more likely, pressurization would be lost by the failure of any element of the single string pressurization system, including pipes, couplings, valves, and so on. Multiengine airplanes have at least two of everything needed to maintain cabin pressure.
Being restricted to a low maximum cruise altitude means the single-engine jet will have higher fuel flows than a single-engine turboprop, or piston. The extra speed of the jet will help make up for the higher fuel flows, but, still, a single-engine jet will need to carry more fuel for the same trip than a piston or turboprop single.
The weight of the extra fuel increases wing loading, of course, and a higher wing loading leads to a higher stall speed. To meet the 61-knot stall maximum in landing configuration the single-engine jet needs a bigger wing than is desirable for efficient cruise. A very effective wing flap can help reduce the stall speed, but in the end, weight will win out and the single-engine jet won’t be able to carry as much fuel as most of us want to make the nonstop trips we want to fly.
There is also the one turn spin recovery rule that applies to all singles and that can be complicated in any airplane. It’s most likely that a successful single-engine jet will employ a stall barrier “stick pusher” system that actually prevents an aerodynamic stall and thus prevents a possible spin.Pilatus uses such a system on its PC-12, and the big majority of larger jets all have stick pushers.
A stick pusher system adds weight and cost because everything including sensors and the actual stick pushers must be dual. But an even larger penalty for single-engine airplanes is that the pusher must activate at a higher speed than the actual stall so that margin eats into the 61-knot stall maximum at least a little.
Then there are all of the typical developmental issues that any airplane faces such as performance goals, flying qualities, dealing with icing, system design, and so on.
Can a single-engine jet be designed and certified? Of course it can. Will such an airplane be accepted by pilots and be a sales success great enough to return the development cost? Nobody knows. But three investor groups have voted with their dollars in the past few months and by two to one, the bet is that a single-engine jet can succeed.
But we can’t hold our breath waiting for the outcome. Diamond has set no firm delivery schedule that I am aware of, and Cirrus recently inferred that first delivery of an SF50 Vision is about three years away so there is plenty of time for more twists and turns before the first single-engine jet is delivered.