The year of the One Engine Jet.
Single engine turbo-props have been around for years; most offering impressive performance numbers, good economy and good safety records. And in spite of the fact most turbo-props can carry bigger loads out of shorter runways and with greater range, they are still prop driven and just don't have the panache that comes with owning a "jet".
Piper, Cirrus, Epic, Diamond and Eclipse all have new SEJ's in various stages of development and all were being promoted at Oshkosh. What accounts for the emergence of the one-holers? The answer is quite simple, economics.
In the early days of commercial jet aviation, 4-engine jets such as the 707, 720, DC-8, and 880 reigned supreme. Then along came the 3-holers, the 727, DC-10 and 1011 all of which offered better economy and replaced their predecessors for many routes. Eventually, nearly all of these aircraft were sent to the bone-yard by more economic twin engine transports, the 737, 757, 767, 777 and now the 787.
Here are some reasons why fewer is better:
Ceteris paribis - one large engine producing twice the thrust as a small engine will cost less than a pair...pylons/nacelles for a big engine along with systems for one large engine will cost less than the same for two half-size engines.
Again ceteris paribus - bigger engines have better fuel specifics than smaller engines and the cost to overhaul one big engine is far less than the cost to overhaul two engines half the size.
Likewise, one large engine will weigh less than two half-size engines.
Yet nothing is free, and the price is safety; fly a single engine airplane with an engine out and you better figure out where you are going to land - quick. But obviously a significant number of buyers discount the risk and willingly fly everyday in pistons and turboprop singles, so why not a SEJ? At least five companies are betting there is a market.
What is interesting in the five offerings is the manner in which the designers have chosen to deal with the engine installation. While the thrust needs to be on centerline, the choice is pretty much limited to on top of the tailcone or buried in the tailcone.
Each configuration has its advantages and disadvantages. Bury it in the fuselage, then worry about ducting and getting sufficient airflow. Mount it on top of the tailcone, then deal with the pitch down moment with the application of thrust, and manipulating the tail feathers so that they are not in the jet wash. There is no perfect solution.
Piper's configuration elicits the most questions. Not only does it have the highest thrust line, it is going to take some pretty heavy and expensive machined beams to carry the vertical tail loads around the engine. The situation will repeat itself at the attachment of the stub vertical tail to the fuselage since this will also be carrying engine loads into the fuselage. Though the arrangement looks good, my view is they are paying an unnecessary aerodynamic and weight penalty with the engine located at the present location.
Epic and Diamond have elected to bury the engine in fuselage. This gives them the option of doing whatever they want with the horizontal/vertical tail configuration. Their challenge, ducting the inlet air.
Both Cirrus and Eclipse decided to locate the engine on top of the tailcone. The Cirrus engine/nacelle combination is semi-buried in the fuselage and more conformal to the fuselage contours. Cirrus will also need to provide ducting for the inlet air.
Eclipse opted for a nacelle on a pylon. To keep the thrust line low, they severely necked down the fuselage aft of the cabin section. Beauty is in the eye of the beholder, but what Eclipse came up with IMHO is not real pretty.
The high centerline thrust on both the Eclipse and Cirrus designs drives the need for a V-tail. In theory, a V-tail can result in less drag and less weight than that of a conventional tail. In the case of the Eclipse, the tail surfaces are quite large given the short coupling of the wing to tail location. There is nothing wrong with this compromise...better too large than too small.
Here is another area to watch with the single engine jets, a couple of quirks in the FAR's. Unless the FAA changes the rules, single engine airplanes need to stall at 61 kts (or less) or face higher crash worthiness standards for the seats (FAR 23.49, FAR 23.565(d)). Also, single engine airplanes are required to be tested for spin recovery (FAR 23.221).
A clinical definition of a non-recoverable spin is when the inertia forces in a spin can not be overcome by aerodynamic recovery forces. With full fuel in the wings, all of these designs will develop quite a bit of inertia in a spin and it is problematic whether any could recover using conventional aero controls. While I am not 100% sure, I seem to recall that the FAA concluded that in the case of Cirrus, the ballistic parachute provided an equivalent level of safety in the event of a spin which eliminated the spin testing requirements.
So perhaps all of these designs may end up with parachutes.
On a more general view of Oshkosh 2007, attendance appeared to be down and a pilot friend remarked he did not think there were near as many airplanes on the infield as previous years. I suspect high fuel prices are beginning to bite which tends to make the case for a single engine jet.