For More than 60 years, one particular size of model engine
has been almost an orphan, at least in competition classifications:
the .10 displacement engine.
When the Academy of Model Aeronautics (AMA) established
“classes” of model-engine sizes in the 1930s, it set the upper limit of
Class A at .199 cubic inches. That was purely an arbitrary decision—
a “best guess” on someone’s part as to what would be a good
dividing line to separate “little engines” from the “midsize” Class B
engines (.200 through .299).
At the time, that probably didn’t seem important. Most gas model
flying—all of it Free Flight (FF) then—was done with Class C
engines (.300 and upward). Few engines on the model market were
much less than .36 displacement.
That began to change near the end of the 1930s, with the introduction of
small engines such as the 1939 Mighty Atom .097 and the 1940 Elf .097.
After that, model magazines regularly published airplane
designs for these miniature power plants, and they became popular
despite their lack of “competition status.”
After World War II, AMA briefly tried a different way of
categorizing model engines, for competition purposes. “Class I”
covered .000 to .099 cubic inches; “Class II” went from .100 to
.199; and so on up the scale, to “Class VI.”
The lovely little Arden .099 ruled in Class I while that category
lasted. However (as always with model airplane engines), nearly all
Arden .099s were bought for sport-flying rather than for use in
contests. (A little later, the Arden .099 became the first massproduced
glow-plug engine.)
AMA’s “Roman Numeral” model-engine classes didn’t last long, and
AMA returned to the earlier “letter” classification by the 1950s. Class A
resumed its .199 top limit. (The later addition of the .049-maximum
“1⁄2A” AMA Class should logically have been called “1⁄4A”.)
Yet the .099 engines survived, because they provide an almost ideal
Joe Wagner
T h e E n g i n e S h o p
927 Pine Ave., Ozark AL 36360
The authoritative reference for identifying and valuing older
model engines—the latest Anderson’s Blue Book.
Modern glow .10s: Enya at top; Thunder Tiger on left; O.S. Max
.10 FP on right. (Note the aft-mounted needle valve.)
A Cox Tee Dee .09 converted to RC use with Canadian-made
Tarno carb. Neither is made today, but functioned reliably.
Spark-ignition .09s: 1939 Mighty Atom, 1946 Arden used air
throttles instead of needle valves to regulate fuel/air mixture.
102 M ODEL AVIATION
power output for all types of sport flying: FF, Control Line (CL), and
Radio Control (RC).
Bigger and more powerful than the 1⁄2As, the “Tens” are less
finicky and more rugged, yet their models are still compact,
inexpensive, easy to build and transport, and generally lightweight
enough to minimize damage from “unplanned landings.”
There are several modern .10s on today’s hobby market. Although
the long-lived and well-loved Cox .099 Medallions and Tee Dees are no
longer made, O.S., Enya, and Thunder Tiger manufacture excellentquality
glow .10s. Imported diesels are readily obtainable by mail order.
This size engine deserves to be much more popular. It can power
CL models that have wheels large enough for takeoffs and landings on
grass—which few 1⁄2As can accomplish—yet are still small enough for
easy car-trunk transport.
Flying on 43- or 50-foot lines, .10-powered CL models are quiet
enough for in-town use without causing the complaints about excessive
noise that banished most .29-and-up CL flying from city parks long ago.
In RC airplanes, .10s can provide many benefits. The models
can be big enough to carry standard-size RC gear easily, yet still
sufficiently small to be built and stored in “minimal facilities.”
And .10s don’t use appreciably more fuel than .049s or .060s.
I’ve loved the .099 engines since the first Ardens came out in
1946. It recently occurred to me to make some comparisons of the
old originals (of which I still own a few) and the modern .10s.
An obvious difference is the weights of the engines; an Arden .099
weighs 2.7 ounces, compared to 6.2 ounces for the new O.S. Max .10
FP with the “remote needle” feature.
Yes, but the Arden has no muffler or RC carburetor, and its
delicate design is incapable of much abuse. Even a mild crackup
with an Arden-powered model can snap off its radial mounting lugs.
(A fix for that problem is to make a “backup plate” from 3⁄32 or
1⁄8 hard-tempered aluminum, to fit between the engine and the
firewall. The stiff metal plate resists bending, and prevents the
Arden’s lugs from being twisted off—even in a serious crash.)
The weights of today’s .10s don’t vary greatly. The Enya .09
weighs 5.3 ounces with its muffler installed, and the Thunder Tiger
weighs exactly six ounces. Those are more than one ounce heavier than
a Cox Tee Dee .09 equipped with a “Tarno carb” and a Cox muffler.
However, I’ve invariably had to add nose ballast to my Cox .09-
powered airplanes. It makes more “engineering sense” for that added
weight to be part of a rugged engine’s structure, instead of a mere dead
metal piece attached somewhere ahead of the firewall.
Most .10-size model diesels are in the same weight range as the
glow engines. The A-M .09 diesel is unusually light, at only four
ounces; the dual-ball-bearing PAW .09 weighs almost two ounces
more; and the meticulously made Russian MK-17 is an even five
ounces, without a muffler.
(Once I tried to add a “simulated muffler” to my MK-17, but
unlike most model diesels, it is sensitive to added back-pressure at
its exhaust ports, and it wouldn’t stay running.)
Instruction sheets for today’s glow .10s usually call for seveninch
propellers. However, I think an 8 x 4 works better for sporttype
performance.
On my new O.S. 10 FP R/C (not fully broken in yet), an 8 x 4
Graupner propeller turns up to 12,400 rpm at full throttle, and idles
reliably at approximately 2,700 rpm. I get comparable performance from
my PAW .09 diesel. Both engines run quietly—even at full speed.
Once I asked my next-door neighbors if the noise from breaking in
my PAW .09 bothered them as they worked in their garden (roughly 15
feet from my outdoor engine test bench). They said, “What noise?”
Speaking of the new O.S. FP-series engines, their instruction sheets
are considerably more informative than “the usual.” There’s a neatly
diagrammed and well-thought-out section on “Carburetor Air-Bleed
Adjustment,” and another entitled “Trouble Shooting When the
Engine Fails to Start.”
(O.S. is using a multipurpose sheet of instructions for its whole FPseries
line, covering the .10, .15, .20, .25, .35, .40, and .60 engines.)
One item in the instructions caught my eye: the recommendation to use
Diesel .10s: Russian-made MK-17 at the top; British PAW on
lower left; and A-M (with custom throttle) on the right.
20% castor oil fuel blends. As a longtime
advocate of extra-oily model engine fuel, this is a
major and necessary step forward—away from
the too-skimpy oil content of many of today’s
model fuels.
Approximately one year ago, a company sent
me four quarts of a new glow-fuel blend it was
introducing. Each bottle was plainly marked with
the nitromethane content, but there was no
mention of the oil percentage or type!
I immediately wrote back to the maker,
stating that I’d never risk one of my own model
engines on fuel with unknown lubrication
properties. And I’m far from being the only
model flier who feels that way!
Most glow engines for sport-model flying
perform adequately with anywhere from 5 to
15% nitromethane—or even none at all—in
their fuel. But they won’t last one run without
damage if there’s not enough oil.
The company that sent me the sample fuel
reluctantly admitted to a 16% lubrication
percentage—but firmly refused to state how
much of that was castor. “That’s proprietary
information,” it insisted. (I refused to take a
chance on using the fuel.)
Let’s look into this “oil percentage” topic
one more time.
A .29 CL Stunt engine runs at
approximately 11,000 rpm, and burns roughly
.55 fluid ounces of fuel per minute (during a
51⁄2-minute run on a three-ounce tank.)
Since glow fuel has a specific gravity near 0.8,
that means only .0004 (avoirdupois) ounces of
fuel passes through the engine per revolution!
Converting ounces to grams, only 1.13
milligrams of fuel are used in each rotation of
the .29’s crankshaft, and a mere 1⁄5 of that (for
fuel with 20% lubricant content) is oil.
Looking at it that way, it becomes
evident how sparse the lubrication of a twostroke
model engine really is.
Yes, the outside of a .29-powered CL
airplane can get mighty oily in the course of a
flight. However, each time its engine’s piston
goes up and down, a measly 1⁄4-milligram of
fresh oil keeps the moving parts from seizing
solid. (And that’s with 22% oil—my standard
for “minimum oil content.”)
I receive a couple letters each month from
readers who have an old-time engine of one
sort or another that they’d like to identify
and/or get an idea about its current value.
If the engine was made in America (or
Canada—but that’s in America too, right?), a
superb new reference for identifying and
appraising is Anderson’s Blue Book.
Now in its second edition, this is a wellillustrated
and complete listing of the various
makes and versions of American-made model
engines manufactured between 1911 and 1965.
There’s even a separate section that covers the
1⁄2A types made between 1948 and 1965.
This book is 81⁄2 x 11 inches, plastic-combbound,
and contains close to 100 pages of detailed
identification and pricing data on probably 1,000
model engines. All major types are wellillustrated
with photos and line drawings.
This book is available direct from its author,
old-time modeler (and past president of the
Model Aeronautics Association of Canada) Frank
Anderson, 753 Hunan St. NE, Palm Bay FL
32907. The price is $41 postpaid. MA
