AN ENGINE FOR HOMEBUILDERS

As of 2009 Amateur aircraft builders are largely limited to various
Volkswagen conversions. While such conversions may be as large as
140cid (2332cc) when using a Type I crankcase -- and up to 170cid
(2884cc) if you begin with the heavier Type IV -- the SUSTAINABLE
output of these engines is thermally limited by the design of their
heads, which were designed for a 40hp engine. But the root problem is
much worse than many imagine.

In a recent thread in this Group it was pointed out that in some
locales the aircooled Volkswagen engines have become rare and the
price of a suitable conversion. This makes any discussion of the most
appropriate VW conversion moot since we are running out of VW's to
convert.

This lead to a general discussion regarding engines in general which
evolved into several specific messages discussing the conversion of
other, water-cooled engines.

Using history as my guide it appears that the ideal engine for home-
builders has already come and gone. It was the Pobjoy radial, first
built in 1926 and abandoned at the close of World War II. The rights
to the 7-cylinder, 130 lb, 75hp engine is presently held by the same
people who manufacture the Rotax, who have no plans for its re-
introduction, although its STC remains valid. This geared (double-
herringbone) radial was rated at 75hp @ 3000 rpm, 85hp @ 3300, with a
TBO between 1500 and 2000 hours. The exceptional TBO was largely due
to the incorporation of a centrifugal oil filter, a feature not seen
on other engines until the mid-1950's. With equal-length intake
runners and a heated intake manifold, the engine was remarkably
efficient, having a specific fuel consumption which rivaled that of
many large radials of the future (SFC 0.485 to 0.504). It managed to
do all that whilst burning 70 octane fuel using a CR of 6.5:1. This
outstanding thermal efficiency was largely due to the elimination of
ALL plain bearings, which are ball, needle or roller through-out.

Despite its sophistication his engine has a number of features that
make it a near perfect match for most of today's homebuilts.
Paramount among them is the high percentage of identical parts, such
as the cylinders, intake manifold(s) and valve train which make the
engine an excellent candidate for 'kitting.' It's weight of only 130
pounds is partly reflected by its small size. The cylinders are 75mm
bore (same as the early VW) by 87mm stroke, a classic high-torque
'under-square' design. Fully mantled and installed, the engine is
less than 24" in diameter. The 75mm pistons are fitted to flanged,
cast-iron barrels which are threaded to the dual-plug, cast aluminum
heads in what was to become an industry-wide technique.

At the time of its inception its designer understood that it could not
compete for price with the mono-bloc 4-cyl in-line engines being
produced by de Havilland but felt there was still a market for an
engine that got its power and fuel economy from a design having
inherently greater efficiency. This proved correct and for the next
twenty years Pobjoy engines went on to power an impressive number of
winning racers as well as setting many long-distance records (ie,
England to Australia; London to Cape Town, etc.)

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As for least-cost, as mentioned above, the key factors were recognized
shortly after World War I, when the useful life of an engine was
measured in tens of hours. This lead to a family of strudy in-line
engine that remain in production today, an by doing so offer the
amateur builder of airplanes a well marked path to follow.

The least expensive engine will always be the one which is
manufactured in the largest numbers. In countries having few
petroleum resources -- where the price of fuel will always be a
determining factor -- the engine most commonly available will tend to
be quite small, typically 1300cc or less. In order to gain sufficient
torque to power a vehicle, these small engines will usually be fitted
with a cam that puts the power-curve at or above 3000rpm. These
engines are of little use for driving the propeller directly since the
propeller needs an rpm of 1500 to 2500 for best efficiency under
Standard Day conditions. It is possible to re-grind the cam so as to
move the torque-peak into the range most suitable for propellers but
the odds are, with a displacement of 1300cc or less, the amount of
thrust from a directly-driven propeller will not be enough to fly the
typical amateur-built design.

If a suitable Propeller Speed Reduction Unit (PSRU) is available, it
will have a radical effect on the equation. But it will also have a
remarkable effect on the WEIGHT, in that a PSRU of suitable
durability.

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In much of the world the most practical engine for conversion will be
an in-line, liquid-cooled 4-cylinder engine having a displacement
between 1500 and 2000 cc. The propeller will be mounted directly to
the clutch-end of the crankshaft using an aluminum spool to extend the
propeller beyond the engine's transmission flange. Ideally, the
manufacturer of the engine will offer a range of cams. By rpm by
application is typically Marine Engine = highest rpm/torque curve,
Automobile Engine = high rpm/torque curve, Truck Engine = medium rpm/
torque curve, and Industrial Engine = lowest rpm/torque curve.

With the exception of the cam, which should be swapped to give maximum
torque at the lowest rpm, such an engine may be used WITHOUT further
conversion. However, it's high weight will limit its use to Single-
Place designs having a wing area typically of 120 square feet or more.
While the engine may be installed in aircraft having less wing area,
wing-loading will result in a stall and landing speed that may be
unacceptably high.

The most successful of several weight reduction efforts will be to
convert the engine to Composite Cooling, in which the heads are liquid-
cooled, the cylinder barrels and sump are air-cooled. This usually
involves the fabrication of a deeply finned aluminum sump. The
cylinder are cut off of the original engine casting and replaced with
after-market air-cooled barrels. The head is cut off from the
original engine's barrels and modified to mate with the replacement
air-cooled barrels. The head is modified so as to allow liquid
cooling and suitable arrangements are made for driving a water pump.
This assume that the person doing the conversion has access to a fully
equipped welding and machine shop. If that assumption is not valid,
or if the cost of the conversion is too high, then you will have to
fall back on the use of an un-converted engine, perhaps adjusting your
wing area to bring the stalling speed into an acceptable range.

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Modifying an airframe so as to increase it's wing area is NOT a
trivial chore. But it is doable. Be sure to keep in mind that the
existing wing and tail is a SET. If the wing area is increased then
the moment between the wing's center of lift and the horizontal
stabilizers center of lift must be increased proportionally. In the
same vein, the Volume of the horizontal stabilizer and the vertical
stabilizer must retain the same RATIO with the new wing. As for
structural strength, you may use the dimensions of the existing spars
or struts, compared to the existing wing, and increase them according
to the RATIO of old vs new. This is NOT the correct way to do it but
since the standard practice is to provide for more than the required
strength in the original design, you will be reasonable correct so
long as you limit the load of any accelerated maneuvers to 3.3g or
less.

-R.S.Hoover