The Engine Shop - 2009/08

O.S.’S LATEST “Alpha”-series four-stroke
engine—the 72FS-a, available from Great
Planes—has all the advanced features of the
earlier 56FS-a. Those include a compact
die-cast muffler, shortened pushrod tubes,
an advanced carburetor assembly, and the
absence of a crankcase breather tube.
Instead of a breather tube, the O.S Alpha
four-stroke engines contain a novel internal
system for lubricating the shaft, ball
bearings, gears, and camshaft/followers
assembly. For their lower-end lubrication,
other four-strokes depend on the small
amount of oil that combustion-chamber
pressure forces through the tiny clearance
gap between the piston and cylinder sleeve.
The Alpha design uses a better system.
A small hole through the head alongside the
intake valve allows crankcase vacuum on
the intake stroke to draw a small amount of
fuel-and-oil mixture downward and into the
space surrounding the shaft and camshaft
assembly.
From there, a hole in the shaft and a
crossport between
the bearings
delivers oil to
those—but not a lot;
Alphas don’t spit
oil from behind
their propeller
drivers. Enough oil
is forced into the
front bearing to
provide the
lubrication it needs
for long service life.
The carburetor
assembly has a 3/4-inch-long intake
extension with a smoothly radiused “bellmouth”
entry. And instead of the “reversed
layout” of the earlier 56FS-a, the 72FS-a
carburetor has its needle on the opposite
side from the muffler. That design feature
is a more convenient arrangement.
O.S.’s owner’s manual for the 72FS-a
advises using only
an electric starter
with this engine,
because it lacks
convenient
provisions for
manual choking.
However,
especially with a
new engine, I much
prefer hand-starting
(with a reinforced
leather glove on in
case of kickback). I
can get a better feel
for what’s
happening during startups that way.
The 72FS-a wasn’t easy to hand-start,
mostly because of its upright mounting in
my test stand. The highly intermittent
airflow up the 23/4-inch vertical passage
between the carburetor inlet and the head
made it difficult for me to prime the engine
“wet” enough. Side or inverted mounting
would eliminate that difficulty.
For break-in I used a 13 x 8 Master
Airscrew K-Series propeller. After
following O.S.’s recommended “Running-
In” procedure—I always read and follow
the manufacturer’s instructions—I obtained
9,800 rpm at full throttle, burning 15%
nitro fuel; more about the fuel later in this
column. The engine idled reliably at 1,800
rpm after only four tanks of fuel.
The O.S. .72 Alpha runs smoothly
indeed. Its short stroke—only 76% of the
cylinder bore—and lightweight ringedaluminum
piston contribute greatly to that.
I’m sure that the engine’s sturdy, compact
design helps too.
Tom Dixon of Marietta, Georgia, kindly
sent me two of his .60-size Double Star (DS)
CL engines with which to experiment.
Made with high precision in Moldova (an
eastern European nation between Romania
and Ukraine), these power plants were
designed and manufactured purely for use
in CL Precision Aerobatics (Stunt) flying.
The D-S engines feature aluminum
pistons and cylinders, and they use
bushing-type main bearings.
“Ball bearings add nothing to an 8,500
rpm Stunt engine’s performance and can be
a source of front bearing air leaks, plus
corrosion problems,” said Tom.
He also stated that the D-Ss require no
test-bench running. They can be taken
directly out of the box, put in a model, and
flown.
One reason why this can be done safely
is that these engines are intended to be run
in an almost steady “four-cycle” setting in
flight. Stunt fliers don’t need or want high
speed, and the only time a D-S is expected
to break into two-cycle running is when a
sudden change of flight direction occurs—
such as when starting into a Wingover.
And with the rich fuel-mixture setting
that four-cycling requires, D-S engines run
extremely cool. Tom wrote to me that
excessive oil content in its fuel will
overcool a D-S and cause erratic running or
flameouts. D-S power plants run so cool
that Tom found that they need an RC-type
glow plug.
He supplies all D-S engines with Sig
plugs (item SIGGP003) installed. This is an
“idle-bar” plug that many four-stroke fliers
prefer for its heat-holding ability at idling
speeds.
The rear-exhaust D-S 61 RE’s muffler
extends roughly 5 inches behind the
cylinder. That makes it resemble a tuned
pipe, but it’s not. Tuned pipes have been
tried in CL Stunt aircraft, but they don’t do
enough good to justify their extra bulk and
cost.
The D-S 61 RE is designed for use in
inverted installations, with its muffler
extending underneath the fuel tank and the
exhaust exiting the bottom of the fuselage.
D-S engines run best with Uniflow fuel
tanks. Muffler pressure is unnecessary, so
the D-S mufflers have no pressure fittings.
Now for Fuel: A new supplier—Maxy’s
Fuel in Miami, Florida—makes a glow-fuel
blend that’s ideal for both the O.S. Alphas
and the D-Ss. The instructions for those
engines call for 20% oil content, preferably
half castor and half synthetic, and up to 15%
nitro. I test-ran the engines with Maxy’s
Fuel, meeting precisely those specifications.
As I expected, I had no problems
whatever with the fuel. But I was pleasantly
surprised by its exhaust aroma.
Back in the “Good Old Days,” the
popular glow-fuel blends that Francisco
Laboratories (Los Angeles, California) made
had a distinctive and pleasant exhaust odor.
My nose says that Maxy’s Fuel smells the
same; it’s nostalgic for me.
Glow-fuel manufacture is the most
competitive area in the model and hobby
industry. All of the major fuel blenders use
the best-quality ingredients they can get, and
most of them have fully computerized
blending and bottling facilities.
I asked Stan Jackson, Maxy’s
representative for model airplane and
helicopter fuels, what special qualities to
expect from this new glow fuel. He provided
some background.
“Maxy Velazco, president of Maxy’s
Fuel, is an avid RC car racer, and using fuels
that he developed for that sport, holds many
state and national RC car racing records. His
fuels are widely known in the RC Car world,
both nationally and internationally.
“Maxy’s is the premier fuel at many RC
car racing events; and late last year he
decided to expand his product line into the
model airplane and helicopter fields.”
Maxy’s will gladly custom-blend glow
fuel to any desired “formula.” The fuel that I
obtained for my recent testing was a new
blend that I had requested. The highest nitrocontent
model aircraft fuel that Maxy’s
provided previously was 10%. The company
has an online “Club Price” list for its stock
fuel blends; “Special Recipe” prices are
comparable.
Earlier in this column, I mentioned the
“reinforced leather glove” I use for handstarting
big model airplane engines. I’d used
a plain leather glove for years, but the sharp
kickbacks that .60 and larger engines
sometimes deliver while I’m sorting out
their starting settings can be painful through
that relatively thin protective layer.
I tried “chicken sticks” for a while.
Those work okay but don’t deliver the same
feel that I get through traditional handstarting.
Then I thought, “I only use the
right-hand glove of the pair.”
I cut off the first two fingers of the lefthand
glove and forced those onto the index
and middle fingers of the right glove. Now I
have plenty of protective padding on my
flipping fingers and no longer dread handstarting
engines for 1/4-scale RC models. MA
Sources:
Great Planes Model Distributors
(800) 637-6050
www.osengines.com
Tom Dixon
(770) 592-3279
http://members.tripod.com/~TomDixon/engi
nes.htm
Sig Manufacturing
(641) 623-5154
www.sigmfg.com
Maxy’s Fuel
(305) 299-3578
www.maxysfuel.com