Revisiting Rotaries Peter Garrison When I wrote to an old friend, who had built a beautiful Falco himself, that my homebuilt was at last flying, he replied, "I at the same time envy you the airplane and think there-but-for-the-grace." I knew exactly what he meant. Building an airplane is endless trouble, and once that ends, maintaining it is endless trouble too. But it's sweet trouble. Time spent taking things apart and putting them together, machining, drilling, shaping, laminating, is, at least for some people, time well spent. People who don't understand that wonder how you can stand to spend years alone in a garage or hangar puttering with pieces of metal and fiberglass. But then they don't understand how a yogi can spend so much time sitting crosslegged in the snow, either. The population of amateur builders has changed over the past couple of decades. The majority used to scrounge materials and build from plans --sometimes, as in the case of the once popular Wittman Tailwind, very sketchy plans indeed. Today they order hardware and materials over the Internet from huge suppliers and assemble airplanes from largely prefabricated kits. Like any old goat who sees the rest of the flock taking a different route than he, I grouchily disapprove. I fancy the design-it-yourself, build-it-from-scratch types are the loftier sort. But when I arrange amateur builders in a hierarchy that -- quite accidentally -- places my category near the top, I still see others higher up. For however bold may be the enterprise of designing a new airframe and building it from scratch, it ranks below that of another class of homebuilders: the ones who build their own engines and test them in the air. Two factors motivate amateur builders to seek alternative engines. One is cost. Certified engines, especially ones in the 200-and-up horsepower class, are prohibitively expensive for ordinary folk, as are their parts and maintenance. The other is a sense, often quite intuitive and unfocused, that modern technology must have something better to offer than engines whose basic plan was laid down more than half a century ago. The natural place to look for an alternative engine is under the hood of a car in a wrecking yard. Auto engines are cheap and readily available, and they come in all shapes and sizes. They are known to be reliable, though that reliability assumes a duty cycle -- 20 percent of power most of the time rather than 70 percent -- quite different from the aeronautical one. Auto engines are much lighter today than they were when their blocks and heads were made of cast iron, and despite the added weight and complexity of liquid cooling, one can easily convince oneself that it offers possibilities of drag reduction and superior temperature control. A basic problem for aviation applications of auto engines is that they are without exception designed to deliver their peak power and torque at nearly twice the rpm at which conventional propellers want to run. The typical rev limit of around 2,700 rpm was chosen by the ancients to allow propellers of reasonable diameter to maintain subsonic tip speeds without reduction gearing. A direct-drive engine needs a large displacement -- six to nine liters -- to achieve its characteristic power-to-weight ratio of 1.5 pounds per horsepower at low rpm. Auto engines yield the same power with less than half the displacement, but at twice the crankshaft speed. Gearing an engine down sounds like a simple matter, but it turns out not to be. The engine delivers its power in a series of impulses as cylinders fire one after another. The propeller reacts like a big spring, its blades flexing in response to the power pulses. The flywheels and torque converters that are used in cars to smooth out this turbulent relationship would add unacceptable weight to an airplane, and so the reduction gears (or chains or belts) must instead be more robust and carefully manufactured than you would at first suppose. Nevertheless, people do put auto engines in planes and they do work. Aluminum-block V8s and V6s have been popular, as have liquid-cooled Subaru flat fours. In my opinion, however, the most promising engine for aviation use is the Mazda rotary. There are fundamental reasons to prefer the rotary, of which its famous smoothness is the least important. Compactness is more important; the fact that it prefers to run on auto gasoline helps; but most important of all is its simplicity. Rotaries consist of a three-sided rotor spinning inside an oval, slightly eight-shaped case. The three edges of the rotor slide along the inner surface of the case, forming three separate combustion chambers whose volumes increase and decrease in turn, like those of conventional cylinders. In order to ensure complete combustion throughout the wide, flat combustion chamber, two plugs per cylinder are standard. Breathing is similar to that of a two-stroke engine, through openings in the casing that are covered and uncovered by the moving rotor. There are no valves, no connecting rods, no camshafts, no crankshaft. What makes the Wankel an appealing engine for aviation is that long list of parts that it doesn't have. The very few moving parts that it does have, furthermore, are extremely unlikely to break. The only thing in the core engine that can fail is one or another of the apex seals at the corners of the rotors, which are analogous to piston rings; but even if these break or stick the engine continues to run. The rotary engine combines, at least potentially, turbine-like reliability with the low cost of a simple mass-produced engine that does not require any high-temperature alloys. Mazda engines come in two- and three-rotor configurations of 1.3 and 2.0-liter displacement, with outputs in the 200- and 300-hp range respectively. (In auto racing, where they have been very successful, they achieve more than twice those power levels.) They have been produced by the millions, and cores are readily available from discarded cars. They are inexpensive to buy and to overhaul. Naturally, it's not that simple. Most of the automotive accessories -- fuel and ignition system, manifolds, exhaust headers, radiators and so on -- are unsuitable for aviation use. Then there is the matter of the reduction gearing or PSRU (Propeller Speed Reduction Unit). These are available from several sources, but are suitable for use only with wood or fixed-pitch propellers, not with conventional oil-controlled aluminum constant-speeds. Estimates of the cost of rotary installations for homebuilt aircraft are all over the map. George Graham of Bradenton, Florida, reports spending a mere $2,000 on his engine, but doubts that he could repeat the accomplishment today. His side-by-side two-seat canard does better than 150 knots at 7.5 gph; it has logged more than 320 hours in the air, on top of an initial 40 on the ground. Ed Andersen of Matthews, North Carolina, who has logged more than 180 hours in a rotary-powered RV-6A, estimates that powerplant costs might run $5,500 to $10,000, depending on the builder's choices of accessory types and vendors. Some improvisation may be needed. "We're still not quite to the buy it and hook it up stage," he comments, but adds that trying different approaches is what true experimenting is all about. "Once you standardize it, the experimenting is over." Oregonian Perry Mick dispensed with the reduction unit altogether, installing a direct-drive ducted fan turning at 5,700 rpm on the back of a Long-EZ. He gets 137 kts at 8 gph -- not great, but then again the entry to the fan is largely blocked by the airframe. Another extremely active promoter of the rotary -- the spider, so to speak, at the center of the web -- is Paul Lamar, who maintains a frenetically busy Internet newsgroup at http://home.earthlink.net/~rotaryeng/. Given all this amateur activity, and if the rotary is so great, it's natural to wonder why there are no commercial rotaries for aviation. To be sure, new aircraft engines are few and far between. Toyota certified an aviation version of the Lexus V8 a few years ago, but has made no attempt as yet to commercialize it. Honda is reported to be developing an aviation piston engine (and a turbine, too). Continental developed a four-cylinder diesel under a NASA contract, but then shelved it. France's Renault has also introduced a four-cylinder aviation turbo diesel which has a takeoff rating of 230 hp. It weighs about 100 pounds more than a comparable Continental or Lycoming, consumes about one to two fewer gallons per hour. It reportedly costs somewhat more than comparable gasoline engines. I find it hard to understand why these big companies are interested in getting into the aircraft engine business at all, given the small size of the market, the great difficulty of getting the few existing airframe manufacturers to try anything new, and the rich potential for generating product liability suits. They must hire very unreliable consultants. It is perhaps revealing that these tentative moves all come from huge companies with virtually unlimited funds. Mazda, which belongs to Ford, is the only company possessing both the capital necessary to involve itself in developing an aviation rotary engine and the technological competence to do so; but, if it ever considered the possibility, it evidently decided to steer clear. It would be a mistake, however, to apply Darwinian principles to the aircraft engine business and to conclude that if an aviation rotary does not exist, it must be that one does not deserve to. The same argument could have been made, a million years ago, about Man. But, now that I think about it... A twenty-year-old NASA study found a 340-hp "stratified charge omnivorous rotary engine" superior to both a diesel and a small turbine as a future engine for general aviation. Nevertheless, despite participation, at one time or another, of Curtiss-Wright, Lycoming, John Deere, and General Motors in rotary development, the job of making the rotary into a practical aviation engine has now fallen into the hands of a bunch of homebuilders. I hope that the combined efforts and ingenuity of these amateurs will settle, once and for all, the question of the rotary's suitability for airplanes. I suspect that they will find in its favor. In the meantime they remain the noblest and most daring of homebuilders, hairy-chested heroes for whom the evidence that their latest idea was not a good one may well come in the form of an engine failure just after takeoff. They continue undaunted, and their numbers increase. The world owes them a debt of gratitude.