88 MODEL AVIATION
Beware of lean needle settings—especially on four-stroke engines!
Also included in this column:
• PAW is still in business!
• New Super Cyclone replica
• New “old” engines and their
boxes
• Spark-ignition parts are available
The Engine Shop Joe Wagner | [email protected]
Well known for friendly, reliable small diesels, PAW makes equally
good big ones such as this .60. (Fly provided for scale reference.)
Tuning by ear isn’t accurate enough for best four-stroke performance. A digital
tachometer’s help makes it easy to adjust this Magnum .52 engine’s needle.
THIS BEGINS my 11th year with this column and my seventh decade
of foozling about with model engines. In all that time I’ve owned,
operated, and flown with more than a thousand of them.
In doing all that I’ve probably made almost every mistake possible.
That’s how I’ve become so experienced! (It’s well known that
experience is what you receive soon after the urgent need for it occurs.)
One thing I’ve learned above all else is to beware of lean needle
settings! They cause more model-engine problems than almost
anything else. Patrick LeRay (Shreveport, Louisiana) recently Emailed
me about his troubles with a four-stroker.
“Is prop spitting just something one has to live with, with fourstroke
engines?” he wrote. “I’ve just had this happen as I was benchrunning
one, when I leaned it too much by mistake ... ”
He went on with details of how, in spite of firmly tightened prop
nuts—the main nut and the “jam nut” ahead of that—the entire drive
assembly flew off soon after he started the engine.
That happened from a “backfire.” The fuel/oil/air charge fired while
the piston was still on its way up. The sudden stop of crankshaft
rotation caused the spinning propeller’s inertia to unscrew the retaining
nuts and set the whole propeller-drive assembly free.
So what caused the backfire? How could a lean needle setting do
that?
Theoretically a “lean” fuel-to-air mixture ought to offer maximum
engine efficiency. It allows almost all the fuel to burn, converting most
of its energy content into heat and expansion.
And there’s the problem! As heat builds up in the engine, it
effectively raises the compression, and that
can cause too-early firing of the mixture.
In model diesels we notice this
phenomenon as it happens and back off the
compression to compensate. But there’s no
way to do that with a glow engine. That’s why
it’s best to set your needle a little on the “rich”
side. The excess fuel entering the combustion
chamber acts as additional cooling.
I asked my friend Zach Allerton (New
Castle, Pennsylvania) about his “tuning
techniques” for four-strokers. He has been
flying RC with those almost exclusively since
the 1980s. Zach told me that once he gets one
of his engines broken in and properly
adjusted, he “ ... never has to touch the needle
valve one notch either way.”
This is the way it should be. As I’ve
written in the past, the four-stroke engines are
the least troublesome to operate as long as you
take care of them and treat them with respect.
Use plenty of oil, in the fuel and when you’re
finished flying for the day. I like a fuel with at
least 20% oil and no less.
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August 2007 89
New tooling has brought back the best pre-World War II spark
.60 engines. This replica even includes the box and “paperwork”!
The replica Super Cyclone (L) differs just enough externally from a
prewar original to avoid being passed off as a high-priced antique.
Larry Davidson sells quality spark-ignition parts for old-timer
use. New coil and transistorized module eliminate past
problems with sparkers.
A box replica—engine and coil not included. This pre-World
War II Ohlsson .23 box looks precisely like the original.
Zach wrote:
“If the stock fuel has less, then I add good old castor oil right off the
drug store or supermarket shelves. A 4-ounce bottle added to a gallon
of fuel seems like enough to give me the blue smoke trail I like to see.
“Plus, I never fly with the engine peaked out. I always run it rich. I
don’t trust my ears alone for adjusting the needle.
“Using a tachometer, I’ll lean the needle valve out for max rpm;
then back off about 200 rpm. My O.S. engines seem very happy
running about 9,000 to 9,200 [rpm] turning the propeller. I’ve found
that makes the best combo for airplane and engine.
“If you run it lean on the ground, then when in the air the engine
will more than likely backfire and throw the prop nut, washer, and
propeller off. Hopefully the drive washer and key will remain on. If
not, you’ll have an expensive flight to record. Also, the four-stroke
engine seems to like a weighty propeller. That provides a good
flywheel effect.
“I’ve flown with Enya, Saito, and Magnum four-strokers, but
haven’t been involved with the YS engines (yet). They are all good
engines—if you treat them right!”
There’s a large amount of model-engine information available on the
Internet, but some of it is erroneous. For instance, a recent report that
the British Progress Aero Works (PAW) has gone out of business is
bogus. That company—the only one in the world that offers a complete
line of model diesel engines—is still actively in production.
PAW is perhaps best known for its small diesels. However, the
company also makes big ones. A couple that greatly impressed me at a
recent trade show were the PAW .49 and .60. They are externally
identical except for the color of their cooling fins: pale green for the .60
and purple for the .49.
PAW’s US importer, Eric Clutton, tells me that it’s hard to make a
choice between the two. He says that the .60 is a “brute powerhouse”
and can happily swing 16- and even 20-inch propellers. The .49 is a
“nice, friendly engine,” according to Eric.
Both have PAW’s latest carburetor design, which I’ve gotten some
experience with on a PAW .40. Its action is remarkably linear.
The unquestioned classic pre-World War II spark-ignition engine was
the Super Cyclone. Far more rugged and powerful than its several
08sig3.QXD 6/22/07 10:50 AM Page 89
competitors in “Class C,” the Super “Cyke” was
reissued after the war. Not just once, either! One
after another, at least four companies acquired
the original tooling for the engine and put it back
in limited production.
The Super Cyclone is here again—but with
new tooling and today’s state-of-the-art
manufacturing techniques. Even the original
box, owner’s manual, and hobby-shop windowdisplay
poster have been meticulously
duplicated! The new maker is Walt Huhn (665
Chaparro Rd., Covina CA 91722; E-mail:
[email protected]).
I still have my old faithful, original Super
Cyclone. It’s in excellent condition, even after
six decades. Placed side by side, Walt’s new
version looks almost identical to it, although he
has lengthened the exhaust stack, substituted
socket-head screws, and made a few other
changes in detail to prevent his engine from
being mistaken for one of the early models.
On the topic of old-time engine replication, a
fair amount of that has been going on recently.
Shilen Aerosports (205 Metro Park Blvd., Ennis
TX 75120; E-mail: [email protected]) makes
exquisite replicas of the early spark-ignition
K&B Torpedo that come in three sizes: .24, .29,
and .32.
The company also carries a neat .19 sparker
that is not a replica but uses old-time design
characteristics with modern manufacturing
methods. I gotta buy one of those for myself one
of these days.
Woody Bartelt (3706 N. 33rd St., Galesburg
MI 49053; E-mail: [email protected])
has a wide variety of old-time engine replicas
available, from Syncro Bees to McCoy racing
.60s. And to top that, George Milano (1650 E.
Rowland Ave., West Covina CA 91791; E-mai