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.