I’m not quite sure why I did this, as I’m way to busy to be wasting time
like this but…
I thought the ‘Revisiting Rotaries’ article was worth preserving as a
text file so I took the time and re-typed the entire article word-for-word (I
even resisted the temptation to edit out the PL reference). This will make it
easier for the old eyes in the group to read J. It’s pasted in its entirety below as well as attached in a
Word file. It is tempting to add a few footnotes to bring the article up to
date to reflect some of the advancements made in the last 5 years since it was
written. Things such as Mistral’s commercial project, Marcotte’s PSRU with
hydraulic output for CS props, Tracy’s ever expanding line of products tailored
for the rotary, and the continued success of members of this list.
Todd
Bartrim (it should be against the law to turn barley into ethanol)
Revisiting Rotaries
By Peter Garrison June
2003
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 cross-legged 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% of power most of the time rather than 70% - 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
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 2700
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 ration of 1.5 pounds per horsepower at low rpm. Auto engines
yield the same power 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 it’s power in a series of impulses as cylinders fire one after
another. The propeller reacts like a big springs, it’s blades flexing in
response. The flywheels and torque converters that are used in cars to smooth
out the power pulses 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 airplanes 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 likes 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 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, the
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 litre
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 in 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 (prop
speed reduction unit). These are readily available from several sources, but
are suitable for use only with wood or composite 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 having spent a mere $2000 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 Anderson of Matthews, North Carolina, who has flown more than 180 hours in a
rotary powered RV-6A, estimates that powerplant costs might run $5,500 to
$10,000, depending on the builders 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 5700 rpm on the back of a Long-EZ. He gets
137 kts at eight gph – not great, but then again the entry to the fan is
largely blocked by the airframe.
Anderson
estimates that 25 or 30 rotary-powered airplanes are now flying, many of them
Van Grunsven RVs (perhaps because RVs account for so many homebuilts overall).
The current high time installation on a fixed-wing airplane belongs to Tracy
Crook of Bell, Florida, whose RV-4 has logged over 1,400 hours and serves as a
stalking horse for the rotary-engine community. Crook, who has developed and
tested a full line of accessories for Mazda engines including a PSRU and
computer controlled ignition and fuel-injection systems, has an excellent
website, http://www.rotaryaviation.com. It provides, among other things, candid
discussion of the successes and failures of various accessory designs, and of
the present state of the Mazda-modifying art. Rotary users have encountered a
wide range of difficulties, but they have invariably involved accessories, not
the core engine. 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 and
consumes about one to two fewer gallons per hour. It reportedly costs much 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 manufactures to try
anything new and the rich potential for generating product liability lawsuits.
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 the 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
20-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 the 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.