The Engine Shop - 2004/12

66 MODEL AVIATION
Joe Wagner
T h e E n g i n e S h o p
212 S. Pine Ave., Ozark AL 36360
The AX-40 purrs smoothly on a PSP all-metal test mount. Its
swept-back needle allows easy, safe adjustment while running.
Norvel’s new AX-40’s flat-topped look is deceiving. The engine’s
unusual design provides ample head-cooling area.
The PAW 40 TBR being broken in. The fuel tank’s new springand-
loop hold-down won’t fail the way rubber bands do!
PAW’s .40 needs no battery for starting. Adjustable compression
lets it run well on an amazing range of propeller sizes.
I RECENTLY TESTED a pair of newly issued and innovative .40
RC engines. These turned out to be the smoothest-running model
engines I’ve ever handled! One’s a glow-plug type; the other is a
diesel. Both start extremely easily by hand. Since I’ve broken them
in, a finger choke and one or two flips of the propeller are all it takes
to get either of them going.
Norvel’s AX-40 features the same Revlite all-aluminum piston
and cylinder technology (well proven in the earlier, smaller Norvel
engines)—with hard-anodized cylinder surfaces that are harder and
more durable than chrome. Besides the anodizing, the new AX-40
includes a low-friction coating on its piston and cylinder called
“Sliktek.”
According to Norvel, these features, plus the lightness of the
aluminum piston, make its AX-40 the most economical-to-run .40-
size glow engine on the market. My tests confirmed that.
Norvel’s data sheet claims a fuel consumption of 30 ounces per
hour for the AX-40—but it doesn’t say at what rpm that was
measured. On my test stand, with an 11 x 6 APC propeller turning
just less than 12,000 rpm at full throttle, I varied the AX-40’s rpm to
simulate a typical RC flight. Six ounces of Sig’s 10%-nitromethane
fuel (with castor added to bring the total oil content up to 21%) kept
the AX-40 running for 20 minutes and 12 seconds.
Complete with muffler, Norvel’s AX-40 weighs 131⁄2 ounces.
That’s light! Another unique feature of this engine is its propeller
stud, with one end threaded “short” and the other “long.” It can be
screwed into the end of the crankshaft either way, to accommodate
12sig3.QXD 10/25/04 11:05 am Page 66
December 2004 67
Spring-and-cord tank retainers in use on Wayne Gladden test
mount. Adding four small screws was the only other change.
The small nut that retains a muffler can vibrate loose and cause
sudden in-the-air engine failure.
thick- or thin-hub propellers without the need to alter the shaft length
itself.
To retain the propeller, the AX-40 uses a longer-than-usual hex
nut equipped with a “captive” washer. (This nut has a 10mm hex,
which is just a trace too big to fit into the typical American four-way
“prop wrench.”)
The AX-40’s carburetor is a dual-needle, cam-actuated type, with
a swept-back needle for maximum behind-the-propeller clearance.
The needle taper is long and smooth, providing a nicely nonsensitive
adjustment. There’s even an O-ring seal on the needle shaft to stop
any air leakage.
Further technical details of this engine—and all other Norvel
products—can be found on the company’s Web site at
www.norvel.com. (I rather enjoyed the Russian-flavored English in
the AX-40’s descriptive text, such as “ ... reducing oil output onto
club fields.”)
The other exceptionally smooth-running RC .40 is the Britishmade
Progress Aero Works (PAW) 40 TBR diesel. (“TBR” stands
for Twin Ball Race.)
PAW seems to be the only model-engine company in the world
that still specializes in model diesel power plants. It is family owned
and has been making high-quality model diesels continuously since
1946. The present engine sizes range from .03 cu. in. to .60 cu. in.—
every one a sturdy, dependable, long-lasting, and powerful machine.
The PAW 40 TBR RC weighs just a half ounce more than the
Norvel, in spite of using a steel sleeve and a cast-iron piston. One
reason for the PAW’s light weight is its small muffler. That’s a
standard feature of PAW engines that I greatly appreciate. For one
thing, it doesn’t block access to any of the mounting screws.
For another, the muffler’s outlet shape allows easy, reliable
attachment of an exhaust extension. (I use vinyl aquarium tubing
to make the connection, with K&S thin-wall aluminum tube for
the extension itself. The tube can be quite long and doesn’t need
to be large in diameter because PAW diesels aren’t affected much
by exhaust back pressure.)
The variable-compression feature of model diesel engines
provides exceptional versatility in propeller selection. My 40 TBR
hand-starts and runs equally well on propeller sizes from 9 x 6 to
16 x 6. Just to see what would happen, I put an ancient wooden
Tornado 24 x 4 on the engine. No problem! It started right up for
me and perked steadily along at almost 5,000 rpm, throttle wide
open.
One innovation in PAW’s new .40 RC diesel is its carburetor
design. Earlier PAW RC diesels use a starkly simple air-bleed
type of barrel throttle. Those work dependably enough, although
their response isn’t particularly linear. The new carburetor—PAW
calls it the “Varijet”—has a cam-actuated lateral sliding action
and a needle that doesn’t rotate with the barrel (as those on the
earlier PAW carburetors do).
The linearity of the Varijet’s throttle response seems far better
than that of the original PAW carburetors. And with a 12 x 6
Graupner propeller, I managed to get reliable idling as slow as
1,800 rpm.
All PAW RC diesels are factory-assembled with their
carburetor needles oriented “straight across.” I’ve found that
PAW carburetors can easily be repositioned to angle the needle
back as much as 15°. That makes for safer adjustment and has no
adverse effect that I’ve ever noticed on throttle actuation.
Complete information about PAW’s full line of products—
with all the technical details, engine instruction sheets, and even
fuel formulae—can be found at PAW’s Web site:
www.progress.charitydays.co.uk/index.htm. The US importer is
Eric Clutton, 913 Cedar Ln., Tullahoma TN 37388; Tel.: (931)
455-2256. He also has a Web site: www.cafes.net/doctordiesel.
Eric hand-starts every engine he sells before shipping it. He
also provides an individual “settings sheet” with the optimum
needle and compression settings for that engine—plus a list of
suggested propeller sizes. (For my particular PAW 40 TBR, Eric
recommended 11 x 5, 11 x 6, 11 x 8, 12 x 5, 12 x 6, 13 x 5, 13 x
6, and 14 x 5 propellers.)
While bench-testing the two “vibrationless .40s,” I finally got
utterly fed up with the oil-slimed, prone-to-break rubber bands
used for holding down the fuel tanks on my PSP and Wayne
Gladden Custom model-engine test mounts.
I replaced the bands with small steel tension springs hooked to
macramé-cord loops (for the Gladden tank) and a loop made from
a pair of linked nylon cable ties (for the PSP). Now I’m certain
that a tank won’t annoyingly drop off its platform again during a
test run! Yet the springs allow removal of the tanks for cleaning
and adjustment as easily as rubber bands ever did.
Jerry Nelson (Saint Paul MN) wrote to me about mysterious inthe-
air stoppages of his Magnum .40 RC. This engine started
readily every time for Jerry and seemed to behave normally.
However, after a few minutes of flying (the exact time varied), it
would quit abruptly. Jerry tried all of the “usual fixes” (with no
results) before asking for my advice.
I suggested checking several of the more obscure and hard-todetect
causes of sudden model-engine failure, such as a “razor
cut” at the point where a silicone fuel line attaches to its brass
fuel-tank tube and a “clunk” whose end comes too close to the
12sig3.QXD 10/25/04 11:06 am Page 67
back of the tank. None of my suggestions
helped, but Jerry serendipitously found the
solution himself.
He wrote:
“I was checking my muffler pressure
line. That was okay; but when I put the
airplane down I heard something rattle. It
turned out to be the end cap of the muffler.
Its retaining nut had loosened from vibration.
That’s what made the engine quit! Muffler
pressure to the tank dropped way off when
the end cap joint cracked open far enough.”
Jerry tightened the nut, and that
eliminated the problem—but for only part of
the next flight. During that, the muffler nut
vibrated loose again. For a permanent fix,
Jerry added a lock washer under the nut. He
recommends an “external star” lock washer,
and I concur. Spring-type lock washers have
only one “antirotation point” on each side;
star lock washers have several.
(Loctite wouldn’t work for preventing
this sort of difficulty because the muffler
gets mighty hot in operation—and heat will
reliquify Loctite.)
In a later E-mail, Jerry wrote that he’d
found out that this kind of model-engine
problem isn’t as unusual as he thought.
“I went to the field with a flying buddy.
His airplane was in the air when his engine
(an O.S.) quit all of a sudden. He made a
good landing, though. When we checked his
engine, the rear part of the muffler was gone!
(Lost forever.) Here’s a case where the nut
vibrated all the way off the muffler. That
allowed the back part of the muffler to slide
off its screw, and go on its merry way.”
Later Jerry asked around among other
Saint Paul RC fliers and learned that some of
them had also experienced this problem. The
usual fix was to use a “double nut.”
For that to be truly effective, though, it
requires “double wrenching”: tightening the
bottom nut, and then holding it with a thin
open-end wrench while using another
wrench to tighten the second nut firmly
against the first one.
But as Jerry remarked, “Star lock
washers are so much easier.” MA
12sig3.QXD 10/25/04 11:06 am Page 68