Airframe and Powerplant

Engines Emerging

November 1, 1993

Will these be our next powerplants?

Amid the fanfare surrounding the granting of the Quicksilver GT-500's type certificate this year at Oshkosh was an unusual sight. Rolled to the edge of a circle of aircraft were two engines — the Rotax 912 and the Zoche diesel radial — as different from one another as can be expected from internal-combustion powerplants. The purpose of the roundup was to show off nine airplanes whose makers had already or were about to apply for certification under one of the newly introduced compliance paths.

Along with the new types and models of airplanes this revised set of certification rules will usher in, there will be new engines. Some of these will be familiar, at least to those with knowledge of the homebuilt- aircraft world, while others will be complete unknowns. We thought it would be worthwhile to take a closer look at the two engines poised at the edge of the certification circle, and to consider some other likely candidates.

First off, understand that the days of Lycomings and Continentals on the noses of new designs are not over. Significantly, Zenith Aircraft's CH2000, an ambitious replacement for our existing trainers, uses a Lycoming O-235. And Continental recently introduced the IO-240, a relatively sophisticated, 125-horsepower four-cylinder aimed at tomorrow's trainers. Also, a great many kit-built designs employ standard aircraft engines as a matter of course — and we have seen kit airplanes as the potential source for new production airplanes. What has changed is the action at the light end of the scale, particularly in engines of about 100 hp and under — areas in which the Big Two have few offerings.

The first two new engines off the blocks are the Rotax and the Zoche. There will be at least two other companies ready to tackle the certification standards very soon, says the Small Aircraft Manufacturers Association.

Two main methods can be used to get these engines from drawing board to U.S.-certified installations. One is to have the engine certified all by its lonesome, either through the appropriate Federal Aviation Regulations or via the equivalent Joint Airworthiness Regulations. The other is, as has been exploited in the Quicksilver GT-500, to certify the engine as part of the whole airplane within the Sportplane category. This final avenue opens up many possibilities, but it also places the burden of proof on the airframe maker and marries that engine's approval to the specific airframe.

Rotax's 912 is fast becoming a familiar face in the homebuilt world — and it is used by the Avid Flyer, Dimona Katana, SkyStar (Kitfox) Vixen, among other models hoping for certification. (The Dimona has been teetering on the edge of certification approval since before Oshkosh this summer, and approval of the Flyer or Vixen is still a ways off.)

Rotax is a subsidiary of Bombardier, a Canadian mega-glomerate, and its model 912 sports conventional engineering applied in unconventional ways. Just like familiar aircraft engines, the 74-cubic- inch 912 is a horizontally opposed four-cylinder, with pushrod-activated valves and a single, central camshaft. In current form, it produces 80 hp at 5,600 engine rpm and provides excellent economy, burning about 4 gallons per hour at 75-percent power. An integral gearbox — with a variety of ratios available — brings the propeller speeds down to more conventional numbers.

Liquid provides the bulk of the cooling medium, with a glycol- water mixture surrounding the combustion chambers. The cylinders, unlike those of conventional aircraft engines, are bolted up from separate head and barrel sections. Small fins encircle the cylinder sleeves to provide additional air cooling. Rotax has demonstrated that the engine can maintain cruise power without any coolant; it will reportedly continue on indefinitely without immolating itself.

Electrical needs are provided by an integral 24-amp generator, but the aft end of the crankshaft is accessible, so it's not too much of a stretch to believe that an external alternator could be fitted if more watts are needed. That generator shares room with a self-contained, fully redundant electronic ignition with variable timing; dual spark plugs are fitted. Also, all current 912s have a drilled and tapped accessory pad behind the gear reduction unit; this has been fitted with a dry vacuum pump in some applications. Also, a conventional prop governor has been fitted at that pad, as in the new Dimona Katana trainer.

It's no news that the JAR-certified 912A differs from the kit- market model — but you might be surprised to learn how: mainly price. Expect the overall cost to be 40 percent greater than the standard experimental-class 912, which currently goes for about $8,000 new. The manufacturers of experimental-category kits, by the way, appreciate Rotax's pricing philosophy because their customers don't have to pay for paperwork they don't need.

Are there hardware changes? None...zip...nada. The additional cost goes toward the effort needed to keep the production paperwork in line and to perform additional quality-assurance testing at the factory. The initial time between overhauls is pegged at 600 hours but is expected to increase; the experimental engine carries a 1,000-hour TBO, which will likely be extended to 1,200 hours soon. Rotax says 1,200 hours will be the TBO cap.

Further work continues on the 914, a turbocharged variant of the 912. Two are currently flying in the United States, with several more about to enter testing. Rotax has been developing this powerplant for more than two years. The project encountered early setbacks when the Japanese turbocharger supplier got cold feet at the idea of its product powering an airplane. So Rotax engineers contacted Garrett and began anew. The engine produces 100 hp (115 hp for takeoff) up to a density altitude of 16,000 feet. The wastegate is electronically controlled.

Rumors also persist of a growth version of the 912, possibly a normally aspirated 125- to 160-hp powerplant. The basic architecture — cylinder bores and crankshaft stroke — of the 912 does indeed allow room for growth, and more horsepower. About 500 of the 912s are now flying in European motorgliders and American kit-builts combined.

From the fairly conventional Rotax we move to the highly unconventional Zoche. Two models are proposed, a four-cylinder, 150-hp turbocharged and supercharged single-row two-stroke diesel radial, and a 300-hp, eight-cylinder, twin-row variant. Among the more unusual features are the connecting-rod big ends that ride on the crankshaft main journal, rather than being bolted around it. This eliminates the master and slave rods found in conventional radials and simplifies construction.

Its two-cycle combustion process means there are no valves, and the supercharger helps maintain good cylinder filling and consistent scavenging of exhaust gases from the cylinders. These are areas in which traditional two-strokes are weak, shortcomings that produce narrow power bands and high fuel consumption. The Zoche engines are said to have excellent fuel specifics, better than conventional aircraft powerplants by 15 to 20 percent. The engines will burn diesel or Jet-A fuel.

According to Zoche, the eight-cylinder O2A will experience its first flight in a Westinghouse Airship in 1994; certification under JAR-E is expected in 1995. The company also expects to fly the twin-row model on an Extra 300.

While it's too early to tell if the Zoche will make its mark on new models, the Rotax seems well on its way. It has some service history on it, as well as considerable corporate resources behind it. But where Rotax has boldly stepped in, don't expect it to be the only company. Smaller manufacturers are taking a wait-and-see attitude, trying to determine where the market leads. Don't discount the Canadian Airmotive CAM 100, a highly revised Honda Civic engine mated to a belt reduction drive; the company says that provisions in the Sportplane category make jumping in at that level attractive. Also, you might see one of the Mosler and Limbach engines (heavily reworked Volkswagen flat-fours) appear on a new, production airplane soon. And there's also plenty of talk about converted Mazda rotary engines for aircraft, though no company has yet submitted one for certification trials.

In any event, we are, thanks to new certification rules and increasing competition in the light trainer field, seeing significant activity — which is always a good thing.