May 2001 139
The SpRing flying season will be around the corner when
this issue goes out in the mail. Hopefully it’s been a good year
for all so far.
This column includes the second part of the engine work series:
break-in procedures for aluminum brass chrome (ABC) and
aluminum aluminum chrome (AAC) engines.
In the future I will cover reducing internal drag, balancing the
engine, port timing, adjusting fits, head design, and more. The series
will take more than a year to get out in print, and maybe even two
years. Be patient—we’ll get there.
new equipment: I received a wonderful package in the mail:
Alberto Dona of Hobby Club sent me several engines to review. I
love to check out engines, whether I can use them or not.
In the package were three PAS .21 engines that would be very
good for .21 Sport Speed and .21 Proto. Alberto sent a car, a buggy,
and a marine version.
These are similar to NovaRossi engines, with lower exhaust
timing and five transfer ports, which I think is preferable to the
NovaRossi eight- and nine-port setups. The engines have 13mm
cranks, and the marine version comes stock with a .395 bore.
Typical with adaptations for Speed, the ideal engine would be a
hybrid with the crankcase and piston-cylinder assembly from the car
engine mated with the crankshaft from the marine engine.
The majority of my correspondence is from the Free Flight (FF)
community. Having started there myself, I understand that group’s
engine needs.
As a result, I will occasionally include info about engines I cross
paths with that aren’t for Speed, but look particularly useful to
others in the aeromodeling community.
The Profi Combat .15 from Hobby Club has been specially
modified with a larger venturi for FF for Alberto. It is beautifully
made, as are most Russian competition engines.
The Profi has a very compact and light crankcase, and is very
light at 126 grams (43⁄8 ounces). The exhaust is offset to clear the
leading edge of the wing (or pylon).
CONTROL LINE SPEED
Scott Newkirk, 4840 N. Glendale, Bel Aire KS 67220
The PAS .21s (L-R): car, buggy, and boat engines from Hobby
Club can be used for .21 Sport Speed and .21 Proto applications.
The Profi Combat .15 with Free Flight venturi, The Profi Combat .15. Photo shows quick-release head clamp. from Hobby Club.
It looks to be a very good engine for FF and perhaps some of the
Control Line Racing events.
engine Series, Part 2—Engine Break-in Procedure: I’m going to
assume you are running an ABC or AAC piston-liner assembly, since all
current Speed engines are. I’m making no differentiation between nickel
and chrome in the liners, because it doesn’t affect the break-in procedure.
Read all the text before you start. You won’t just run the engine;
you will tear it down, inspect it, and adjust it during the break-in.
I’m giving rpm data based on a form .40 engine; you will have to
alter the numbers to match your specific engine and the application
for which you are going to use it.
The running info and flying are different for piped engines with
beryllium copper cylinders; they cool and run completely different.
The only engines manufactured with the beryllium copper liners are
140 M ODEL AVIATION
for F2A, and I’ll assume that anyone taking
on that challenge is sufficiently adept at
working around the peculiarities of the gear.
Engines may have been bench-run before
they are sent from the factory, but time
limitations usually prevent this. As I
mentioned in the last column, it is nearly
impossible to assemble an engine without
dirt. Whatever you do with the engine
before reassembly, clean, clean, clean.
Except for very high-nitro events, use the
same fuel for break-in that you use for flying.
For very high-nitro events I usually break the
engine in with 40/40/20 fuel, then switch to
the high-nitro fuel for flying.
For 1⁄2A engines, use at least 10% castor oil
in the fuel plus other oil for break-in. I use castor
all the time in 1⁄2A engines, but I know several
other very successful Speed fliers who don’t.
The test propeller has to simulate
conditions in flight. According to Rob
Metkemeyer, a “standard” flying prop
shortened to 86-88% of its original length
gives a bench-test prop that simulates the
engine load in flight.
If you know the rpm at which your
engine will be running, select a test prop
that will run 500-1,000 rpm more than you
will fly in the air.
For some reason, if you don’t run the
engine faster during break-in than you
anticipate flying, it is like putting a governor
on the engine and it won’t want to go faster.
The first run is the most important, and it is
important to make sure you start the engine
rich. The first seconds will clean the last
contaminants from the engine, and you want it
rich to flush out any remaining debris.
Lean the engine to near-full rpm with
plug lead on, at approximately 25,000-
27,000 rpm for the first run. The engine has
to run approximately three times, roughly
two minutes each time without a pipe.
Let the engine cool completely after
each run, and repeat after the first time at
near-peak rpm the entire run. At the end
of each run, bring the engine to a peak
for a couple seconds, take it back to rich,
then shut it off.
For a piped engine, add more twominute-
or-shorter runs with the pipe on
the engine. Do the first rich run with
the pipe on, four-stroking at 24,000-
26,000 rpm. Do a couple short runs
leaned out to just more than 30,000
rpm for three to five seconds, do it
richened up, then shut down.
After running on the pipe for seven to 10
seconds more, rpm will increase to
approximately 31,000 or above because of the
higher gas temperature in the pipe, but we
usually don’t do this.
I don’t like much ground-running at
high load when the engine is new; it is
very hard on the connecting rod, and it
warps the crankcase differently than it
will in the air.
I’m convinced that in a properly built
engine, rotating parts, such as the ball
bearings, big-end, connecting rod, and
backplate rotor, are run in after a few
seconds, and should be absolutely
noncritical. The difficult part is the piston
in the cylinder.
When new, the piston gets a thermal load
on the surface by mechanical friction and from
the reflected (hot) pulse wave of the pipe. The
mechanical friction has to do with the fit of the
piston in the cylinder as it was made.
The piston has to “wear” itself onto the
cylinder surface a bit without getting a
too-high surface temperature. If the
temperature is too high, the piston will
show “shiny spots” at certain points—a
May 2001 141
sign of a local change of material structure.
To prevent high piston temperatures during
running in, the following helps.
Run the engine rich; fuel is cooling and gives
lubrication. On a piped engine, you can also run
high rpm to make sure the backpressure from
the pipe is “late.”
This reduces the filling of the cylinder
(less energy and pressure in the combustion
chamber), and the hot pressure pulse will
not push gases back into the transfer ports of
the crankcase, which builds up a great deal
of heat in the engine very quickly.
When you start flying, use a relatively
small prop for the first five flights and keep
ground-running time to the absolute
minimum. Start the engine from rich and
turn the needle slowly in, easy into
resonance, but keep it relatively rich and
release quickly. Air running for the first
flights should be slightly four-stroking.
Take the cylinder off and check the piston.
If there are shining spots, take them away
using wet-or-dry 800- to 1,200-grit sandpaper.
Sand very lightly and carefully—especially at
the “ring,” approximately 2.5mm from the top
of the piston where it seals.
Small shining spots at the position of the
small bridges in the transfer ports may be
neglected. They seem irrelevant, and will
disappear after more running.
Clean the piston (see the last column). If
you disassembled the connecting rod from the
piston for cleaning, check to see which side of
the connecting rod has the lubrication holes
and be sure to reassemble it the same way.
There is usually a radius on the rod that
goes against the crank-web to clear the
radius between the crank-web and the
crankpin. If the wrist pin has one closed end,
check which way it came out and put it back
in the same way. If it is symmetric, you can
put it in either way.
Check which way the piston came out.
Some engines have ports in them. The ports
will be opposite of or beside the exhaust.
They will never be visible in the exhaust port.
Once the engine has been run, put the
piston back in the way it came out; it will not
wear evenly. The exhaust side of the piston
will run hotter than the rest of the piston.
If the piston shows scratches, check the
top of the ports for too-sharp edges. If this
seems to be the reason, use 800- to 1,200-grit
wet-or-dry paper around your finger to round
the ports coming in from the bottom side or a
lap. (This will be covered in a future column.)
Never touch the cylinder surface with the
sandpaper at more than .040 inch above the
exhaust port.
Cleaning the cylinder after this is very
important and critical (see the last column).
If there are signs of detonation or broken
plug wires, make the top of the piston
smooth again and possibly round the edges
very lightly with wet-or-dry 800-1,200
paper. Keep it clean!
Once a piston looks good and the sealing
ring is pretty well round, it should last for a
very long time. Be aware that wear coincides
with high temperatures, which don’t normally
happen in the air, but during ground-running
at peak power with big props.
So try to prevent this “test” running as
much as possible; use it only to check
whether or not the engine will come into
resonance. Never run the engine more than
three to five seconds at peak rpm.
Now you really start to use the engine.
Find a prop that runs between 24,500 and
26,500 rpm on the ground. Try to keep
ground-running to a minimum, to get your
setting and launch.
I prefer to use fuel systems with hard
tanks or pressure-regulated bladder tanks so
I don’t need to touch the needle before
launching. Put the engine in the starter, and
launch as quickly as is safely possible.
Start your flights a bit on the rich side,
and sneak up on a peaked setting. The
engine is supposed to be run-in at this point.
Check the piston after every 15 or 20
flights. If you see any shining spots on the
piston, clean them up and fly again. If you
had to work on the piston and liner fits,
check after 15-20 flights.
are you a North American Speed Society
(NASS) member? If not, you should be! MA
Sources:
Profi and PAS engines:
Hobby Club
Box 6004
San Clemente CA 92674
(949) 240-4626
Fax: (949) 240-5931
www.hobbyclub.com
NASS:
Box 82294
North Burnaby, BC
Canada, V5C 5P7
