The Engine Shop - 2004/03

March 2004 93
I HAVE GOOD news for fans of highpowered
Radio Control (RC) engines: Rossis
are back! These superb Italian-made power
plants were off the market for a while, but
they’ve been available again since last
September, in a new Matrix model.
Several US dealers will probably be
carrying Rossis by the time you read this. One
that I recently visited (Hobby House, Inc.,
17721 Vanowen St., Reseda CA 91335; Email:
[email protected]) stocks all
four Matrix sizes: the .40 (rated at 1.9
horsepower!), the .45 (2.1 horsepower), the .53
(2.3 horsepower), and the .60 (2.6
horsepower). However, as I’ve stated before at
least twice in previous columns, horsepower
ratings for model engines are less important
than propeller selection—much less important!
For example (a good example), let’s look
at the Good brothers’ famous pre-World War
II RC airplane. This model—the Big Guff,
now on display at the Smithsonian—won the
RC event at the AMA Nationals in 1938,
1939, and 1940.
The Big Guff weighed 8 pounds—2
pounds of that was the RC system!—and it
had an 8-foot span with 1,350 square inches of
wing. It was powered by a Brown Junior .60
spark-ignition engine rated at a mere 1⁄5
horsepower. That turned a 14 x 4 wooden
propeller at approximately 6,500 rpm.
For comparison, one of today’s most potent
competition Free Flight engines is the Cyclon
.061. It develops roughly 0.8 horsepower, with
a 5-inch carbon-composite propeller spinning
at more than 30,000 rpm. The Cyclon’s
horsepower rating is four times that of the
ancient Brown sparker that powered the Big
Guff, even though it has only one-tenth the
Brown’s displacement.
The Cyclon pulls one of today’s Fédération
Aéronautique Internationale Free Flight
models upstairs in a 90+ mph vertical climb,
Joe Wagner
T h e E n g i n e S h o p
212 S. Pine Ave., Ozark AL 36360
In foreground is the new Rossi Matrix Radio Control engine. This one is the .45—a
potent power plant, indeed—and it has a list price of approximately $150.
Propellers are 9 x 4s but don’t perform alike! Graupner (top) works well at moderate
rpm, APC (bottom) is best at higher speeds, wood one is inferior to both.
Hughes RC’s CHARGE+ 1C4S automatic recharging system for glow-plug igniter
batteries provides foolproof peak charging at home or on the flying field.
With custom-made carbon-composite
folding propeller, Cyclon .061 is worldclass
competition Free Flight engine.
94 MODEL AVIATION
but can you believe that the shrieking little
Cyclon .061 could haul the Good brothers’ 8-
foot, 8-pound RC model off of the ground
and into the air? I can’t.
Horsepower rating alone doesn’t provide
performance. This holds equally true in fullscale
aviation. Early-model P-47
Thunderbolts couldn’t outclimb Spitfires or
Focke-Wulfs, but changing the P-47’s
propeller to a different, “paddle-blade” type
fixed that! The engine’s horsepower remained
the same, but with the new propeller the P-
47’s rate of climb improved tremendously.
As P-47 ace Robert Johnson wrote (in
Thunderbolt, published by Ballantine, New
York, 1958):
“Never again did a Focke-Wulf 190 or a
Messerschmitt 109 out climb me in the
Thunderbolt. The new prop was worth 1000
horsepower more—and then some. Later I
had the opportunity to mix it up with a
Spitfire 9B, the same model that had flashed
past me in a climb. This time the tables were
reversed: I was astonished as we both poured
the coal to our fighters, and the Thunderbolt
just ran away from the Spit.”
Correct propeller choice is vital for
optimum performance in model airplanes too.
Merely specifying the diameter and pitch is
far from sufficient. Propellers of the same
nominal size but of different makes can vary
enormously in the thrust they produce. This
shows up distinctly in Control Line (CL)
models.
With those, comparative takeoff ability,
flight speed, and ease of maneuvering can
readily be judged. In RC models, differences
in propeller effectiveness are harder to
distinguish. But those are just as important to
performance there as they are in CL airplanes.
There is yet another aspect of propeller
selection. My good friend Zach Allerton
(Volant PA) once built a big, lightweight Old-
Timer model—a Scientific Red Zephyr—for
relaxed RC flying. He powered it with a fourstroke
O.S. engine, and it flew beautifully.
However, there was one problem: the O.S.
The heat gun in the background made it easy for the author to
hand-start this tightly fitted O.S. .10 engine.
The 1935 Brown .60—only rated at 1⁄5 horsepower—could haul
big, heavy RC models aloft well with 14 x 4 propeller.
March 2004 97
couldn’t idle slowly enough to let Zach’s Red
Zephyr land under power. I suggested to Zach
that he try a flatter-pitch propeller on the
model. That solved the problem!
A classic engineering rule of thumb
explains why. The speed (in miles per hour)
at which any propeller wants to move its
vehicle (airplane or boat) equals the propeller
revolutions per minute in thousands,
multiplied by its pitch in inches.
For instance, a 10 x 5 propeller spinning at
11,000 revolutions per minute tries to pull its
model at 11 (thousand revolutions per
minute) multiplied by 5 (inches pitch) = 55
miles per hour. Of course, no propeller is
100% efficient, and air drag holds model
airplanes back in flight.
Still, the rule of thumb I stated in the
preceding gives us a useful relationship. In
the example given, according to the rule,
changing the propeller to a 10 x 4 (of the
same make) ought to reduce the model’s
flight speed by roughly 20%.
However, as with almost everything else
in model airplanes things aren’t quite that
simple. Reducing the propeller’s pitch will
usually let the engine revolutions per minute
increase—maybe even enough so that the
model’s top speed stays approximately the
same.
In fact, if the original pitch was so high
that the propeller’s rotating blades were
forced to work at an excessive angle of attack,
reducing the propeller pitch will make the
model fly faster—not slower.
But altering propeller pitch doesn’t affect
idling revolutions per minute much. The
throttle setting is the major determinant there.
Thus changing the propeller on Zach’s Red
Zephyr from a 6-inch pitch to a 4-inch pitch
reduced the model’s flying speed at idle by
something like 33%. And that’s all it took to
permit touch-and-gos and power-on landings.
Bob Rode (Sauk Village IL) provided a
useful tip for initial starts of a tightly fitted
new aluminum-brass-chrome (or similar
taper-bore design) model engine. Some of
those have pistons that fit their sleeves so
squeaky tight at the top of the stroke that they
can hardly be turned over. Bob gets around
that difficulty by lightly heating the head and
cylinder top “just until the piston will go over
top center without binding or a ‘squeak.’”
Bob uses a propane torch to do the
heating. I tried his method but used a
MonoKote heat gun instead. That’s safer to
use around model fuel, and it worked fine for
me. I tried Bob’s head-heating technique to
start an especially tight O.S. .10 that had
stubbornly resisted every previous attempt of
mine to hand-start it. (The little O.S.’s shaft is
too short to accept a spinner for electric
starting.)
My heat gun probably took longer than a
propane torch would have to get the O.S. top
end hot enough for easy hand-starting, but it
did work—and after five one-minute fastrevving
runs, the piston-cylinder fit of my
O.S. .10 has freed up noticeably. Now the
engine seems ready for me to fly an RC
airplane with it.
By the way, have you noticed how few
.10 model engines are available anymore in
the USA? Looking through the latest mailorder
catalogs recently, I found hardly
anything listed under .15 displacement. The
.10s are still popular in Europe and the Orient,
and since 1946 I’ve greatly enjoyed flying
with that size engine. I wonder why they’re
becoming so rare in the USA.
In a previous column I discussed the
problem of ruining Hot Shot-type glow-plug
igniters by longtime overcharging. Hughes
RC (1733 Campus Plaza Ct. Suite #17,
Bowling Green KY 42101; Web site:
www.hughesrc.com) has come up with a cure
for that.
Its CHARGE+ model 1C4S provides
foolproof automatic peak charging of glow
igniters—the Hot Shot clip-on type powered
by integral C-size Ni-Cds and larger
“improvised” glow igniters such as the 2000
mAh one I’ve been using so successfully for
more than a decade.
The complete 1C4S system contains
everything one could possibly want for
recharging glow igniters. The basic charging
unit itself requires 12-volt power. That’s
provided for with a 12-volt heavy-duty pluginto-
the-wall unit.
Another option Hughes includes is a long
(and fused) cord for connecting the
CHARGE+ to a car’s cigarette-lighter socket.
A third alternative Hughes adds is a pair of
insulated alligator clips, to allow powering
the CHARGE+ unit from a field-box 12-volt
gel cell. The complete CHARGE+ 1C4S
system costs approximately $60 and is
available direct from Hughes RC. MA
A Message to All AMA Members
Help in maintaining and acquiring flying sites–it’s
the heartbeat of aeromodeling.
The Flying Site Assistance Program consists of
volunteers who serve as the eyes and ears of the
Academy of Model Aeronautics and its members.
These volunteers are tasked with being the best
informed members in their local areas with
regard to any public or private activity that could
impact an existing or future flying site.
They accomplish this by reading the newspaper,
watching television, listening to the radio, etc.
The accumulation, dissemination, and
distribution of such information could prove
vitally important in maintaining an AMA
Chartered Club in any given area.
If you, as an AMA member, would be interested
in volunteering for such an important and
challenging endeavor, please contact:
Districts I-VI
Joe Beshar
198 Merritt Drive
Oradell NJ 07649
Phone: (201) 261-1281
Fax: (201) 261-0223
[email protected]
Districts VII-XI
Wes De Cou
202 W. Desert Flower Lane
Phoenix AZ 85045
Phone: (480) 460-9466
Fax: (480) 460-9434
[email protected]