The HondaJet, whose prototype first flew in 2003, is the only business jet that does not look exactly like all other business jets. Its sagging chin, bulging forehead and overwing engines — all carefully optimized aerodynamic refinements — are unique among current types. They are products of the purist strain in Japanese aircraft design that also brought us the MU-2, whose full-span double-slotted flaps and 50-psf wing loading were similarly unique among its fellow twin turboprops.
The HondaJet was preceded by another Honda prototype, the odd and now forgotten MH02, which was built and tested for Honda by the Raspet Lab at Mississippi State University two decades ago. The MH02 had a forward-swept high wing, big double-slotted Fowler flaps, a T-tail on an extremely tall and narrow fin, and two JT15s perched atop the wing roots. It looked like an Aero Commander wearing a jetpack.
Though a good deal more conventional than its precursor, the HondaJet is still a pretty bold effort, not only because it departs, however modestly, from the accepted formula for a business jet but also because it combines a new airframe with a new engine, thereby violating one of the most fundamental aviation rules of thumb, as basic as Vizzini’s “never get involved in a land war in Asia”: Never combine an experimental airframe with an experimental engine. The program suffered a setback last spring because of a bolt in the engine’s accessory case that has delayed deliveries, though Honda reports it is already closing in on certification.
Jet designs today are remarkably uniform. They form two classes. Modern airliners invariably mount their engines on short pylons under the wings, while business jets, with the sole exception of the Honda, place theirs on the aft fuselage. It was not always so, but that is how things have shaken out. Aerodynamically, there is probably little to choose between the two arrangements. The only significantly different approach would be to bury or embed the engines in the fuselage, but external nacelles have practical advantages that trump everything else.
That business jets don’t have underslung engines is due to the impossibility of squeezing engines into the space between the wing and the ground — although McDonnell Aircraft once tried it with its ill-starred four-engine Model 119 — but is, say, a high wing business jet with underslung engines, a sort of micro-C5, an impossible idea? I doubt we will ever find out; what would be the advantage of attempting a design that was novel in appearance but perhaps no more than merely equivalent in performance?
Has convergent evolution brought innovation in civil aircraft almost to a standstill?
Every part of an airplane can take many forms, and to the extent that comparative testing yields rankings along one scale or another, there appears to be a best engine, a best airfoil, a best placement of wing or empennage, and so on. Young professional designers, the keen edge of their ambition as yet undulled by friction with management or with practical realities, and amateurs, who are more like artists than engineers, instinctively long to create an airplane that unites all the best features. To this impulse add another: the desire to recapture a certain look that somehow or other got embedded in the designer’s psyche when he or she was 12 or so. And so you get things like ducted fans, mid-engine pushers, flying wings and what have you — in short, all the stuff that still makes sketching on napkins fun. But none of that finds its way into manufactured products.
Take, as a single example, the pusher configuration. It is a huge favorite among doodlers. Bill Lear believed in it. Kelly Johnson believed in it. When the later-to-be-deified Johnson set down some design ideas for future fighter aircraft back in 1941, all of them were pushers. I cannot help suspecting that his purpose may have been to throw foreign designers off the track, but in any case belief in the superiority of pusher propellers, which sometimes borders on fanaticism, has always been widespread.
What makes people believe in pusher props is the experience of standing behind an airplane while it is running up. The force of the propwash is enormous, and it’s natural to suppose that when the prop is on the nose, that powerful wind is pushing the airplane backward.
The reality is different. At a standstill, a propeller absorbs power by imparting a very large acceleration to a very small amount of air. In cruise, however, the amount of air encountered by the propeller is much greater — a 78-inch propeller moving at 180 knots plows through about 10,000 cubic feet of air per second — and the acceleration of the slipstream required to absorb the power of the engine is just a few knots. The propeller does add some friction drag to the parts of the airframe in its wake, but far less than you might think.
Still, look at the Cessna Skymaster, which climbs better single-engine on the rear engine than on the front one. And what about the pusher Piaggio Avanti, which is way faster than all of its competitors? Don’t they prove the point?
The trouble with this sort of evidence is that so many different factors are involved that it is impossible to say what the pusher configuration itself is responsible for. Besides, there are numerous examples of pusher airplanes that perform no better than similar tractor ones. Large samples of comparable types — RV-4s vs. Long-EZs, for example — show big differences among individuals but no overall advantage for either class.
Suppose, however, that pushers and tractors were equivalent as far as thrust and efficiency were concerned. Would we not expect to see pusher and tractor designs in equal numbers? Or perhaps more pushers, since it appears as though putting the propeller behind the people at least ought to cut down on perceived noise?
But pushers have their own problems. One, which applies to recips and not to turbines, is cooling.
A tractor propeller blows on its engine; a pusher doesn’t, and pushers are hard to cool on the ground. Prop clearance can be an issue during rotation. Another problem is foreign object damage. A prop on the tail is in the path of stuff kicked up by the nosewheel and of anything that might come adrift in the engine compartment.
And then, about that noise ...
As a kid, I once watched a loose formation of B-36s pass overhead — gigantic Korean War era bombers, each powered by six 3,000 hp 28-cylinder radials. They were high up, but their peculiar and attention-getting sound made them seem much closer. That sound was characteristic of the B-36, whose 19-foot pusher propellers spun in the wake of a wing that was seven feet thick at its root. Each blade would slice alternately through fast-moving undisturbed air and the wing’s slow-moving boundary layer. B-36s are long gone, but the snarl of rapidly fluctuating propeller aerodynamics is still with us; both the Avanti and the Skymaster are noisier, in a special waspish way, than other airplanes.
If you want to sell an airplane in Europe, noise is going to be a big issue. If you want to sell it in Africa or Arizona, ground cooling and gravel damage will. Though the thought is repellent to idealists, design decisions in the airplane business are usually made by tossing a salad of conflicting considerations with a dressing of habit and sending what comes out to the marketing department for a taste test. Whatever the theoretical attractions of some innovation may be, it’s likely to get shunted aside by the tried-and-true — if not forever, then at least until next time. The rare deviations from this rule do not always end well. The Beech Starship, offspring of an unlikely dalliance between traditionally stodgy Beech management and radical visionary Burt Rutan, is a case in point.
On the other hand, at Costco I saw a remote-controlled twin-rotor toy helicopter with a built-in spherical cage so that it could collide harmlessly with walls and ceilings. Twenty bucks. Now that’s innovation!
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