Even during the busiest times of my life I have endeavored to maintain some
form of model building activity. This site has been created to help me chronicle
my journey through a lifelong involvement in model aviation, which
all began in Mayo, MD
cleaning up (air and fuel flow passages), freeing up (sliding friction),
lightening and balancing, and breaking in are all part of the
effort necessary to create winning engines for model racing events.
This 1962 American Modeler article predates Schnuerle porting (in
model engines), ABC cylinder liners, and modern metal alloys, but
still the concepts are applicable to today's engines. Wankel loves
will appreciate the homemade engine shown.
Larrv Conover and Warren Kurth join forces to bring you this vital
series; Part 2 next issue!
This has become one of the traditions of annual air-model National
championships: motor repair service operated by K&B. John
Brodbeck (left) and Bill Wisniewski try to figure how contestants
can beat 'em up so.
This report on speed and engine re-work is designed to give a comprehensive
insight into the methods and general approach used by experts in
our various gas powered events. This includes both control-line
and free-flight power.
Perhaps the most important information
we can pass on is the use of a scientific method for improving the
performance of your models. We will refer to the measurement and
recording of rpm readings throughout the text. The results you derive
from "re-working" depends on your ability to accurately measure changes
in engine HP. The best way is with the use of a good tachometer.
So tachs are covered in this article.
Yes, you may be wondering
why the engine manufacturer, with all his professional know-how,
can't put an engine on the market with ultimate performance built
into it. First of all, if it is to sell to the mass market it must
operate easily and efficiently over a variety of speeds and loads.
By contrast., the hopped up speed engine is tuned to operate at
one specific high rpm while drinking large quantities of expensive
highly nitrated fuels. The modifications may also make it harder
starting and critical to operate. If it is run at a lower rpm its
power will drop off rapidly.
Second reason is the limiting
factor of economical production. The manufacturer can not afford
extensive hand work, or special materials (example: precision ball
bearings) if he is to keep the cost of his product within reach
of the average modeler.
Engine rework can be classified
under three main headings: RE-TIMING, CLEANING-UP, and FREEING-UP.
The last two terms are overworked catch-alls, but they do include
some basic methods and operations, so we explain them in general.
RE-TIMING: The trend is to engines which
turn faster on smaller props, and develop peak HP at high rpm. Many
endinges can have their rotary valve timing modified to give better
performance in the 20,000 to 25,000 rpm range. Usually, opening
the. rotary valve port so that the intake closes later in the cycle
will help. In fewer instances, opening the valve port to let the
intake open earlier may help, too. But remember, the exact timing
to be used is dependent upon several factors: Peculiarities of the
engine, its compression ratio, rpm at which it is expected to run,
diameter and length of venturi, bore-stroke ratio. All these factors
are interdependent, a change in one will nave an effect on the rest.
For example: Generally the rotary valve is still open for
40 to 60 degrees after the piston has started its downward motion,
building up crankcase pressure, and tending to force the fresh charge
of fuel vapor back out the venturi, However, the momentum of the
incoming fuel charge, due to its velocity build-up in the venturi,
will continue to carry fuel into the engine despite back pressure
being developed by the descending piston, As rpm increase, .this
fuel charge velocity and momentum builds up, therefore we can change
the timing and leave the valve open a wee bit longer.
Another thing we can do if we are going to run only at high rpm
is to change the cylinder port timing. Top edge of intake port may
be filed ,upward to let the port open earlier. Again, depending
on the particular engine, the exhaust port may be filed upward a
greater or lesser amount than the intake port.
of piston baffle or dome will have some effect on timing. Filing
down the top of the baffle, or leveling it off on the intake side
helps with some engines. An interesting extreme is the Italian Super
Tigre 15 which has no baffle on top of a flat piston, while the
intake and exhaust ports in the cylinder open almost simultaneously.
"CLEANING-UP": This is a term used
to define all the modifications which make for easier passage of
the fuel mixture from the venturi opening to the combustion chamber.
Most obvious suggestion is to polish all surfaces over which the
fuel mixture must flow, and to radius and streamline all edges over
which the mixture passes. Polishing thus includes the connecting
rod, crankshaft counterbalance, crankcase, bypass, and the inside
of the crankshaft - if the engine has a front rotary valve. The
lower outside edge of the cylinder intake port can be streamlined.
Also the lower end of the bypass, the edges of the connecting rod
if it has a rectangular cross-section, and - if a front rotary valve
- the rear opening of the fuel passage. On some front rotary engines,
the venturi opening and crankshaft port opening may be lengthened
and squared out to match each other. Rear edge of shaft port can
be radiused (details later) and if shaft-port has been put in by
milling flat across shaft, excess material may be ground away. But
be careful throughout all these operations that you don't accidentally
grind past the edge of the port and change the timing.
engineering feat 2 years in development: revising Wankel "pistonless"
internal combustion engine for 2-cycle running. Floyd Nell,
St. Louis, did it: .50 c.i.d.; 10,000-rpm; all steel; ball &
needle bearings; 20-oz.
On some engines it helps to open out the throat of the venturi,
but opening it too much can give negative results. The optimum venturi
size will vary with different engine set-ups (rpm, timing, engine
layout) so here again, investigate before you operate.
If your engine has a front rotary valve and uses a rear crankcase
cover, round off the cover so that it does not obstruct the lower
end of the cylinder bypass.
In general, the foregoing
operations which comprise "cleaning-up" an engine will produce the
least gain per time expended in re-working and are of lesser importance
than the re-timing of "freeing" modifications.
other point worth mentioning here is compression ratio - both cylinder
compression and base compression. The cylinder compression ratio
may be varied for better performance, but the governing factor will
be the fuel you will be using and to some extent the glow plug and
cylinder timing. Most of the racing engines now have compression
ratios pretty well on the high side as they come from the factory.
An old trick, especially with the various McCoy engines, was to
increase the base compression or "pack the crankcase" by various
means such as thinning down the connecting rod and shortening the
crankpin, which allowed facing off the crankcase to reduce Volume.
The lightening holes in the rotor disc were plugged with balsa also
to reduce the crankcase Volume. Such methods which worked well at
lower speeds generally are not satisfactory at today's high rpms.
"FREEING-UP": This includes all the
methods used to decrease mechanical friction in the engine so that
the greatest percentage possible of the energy released in the combustion
chamber can be transformed into useful horsepower at the propeller.
First step is cleanliness. This is extremely important.
.Any re-working of parts should be finished off with crocus and
oil (or jeweler's rouge and oil) to remove any trace of roughness.
Then after any polishing or lapping, the parts involved must be
thoroughly scrubbed with a small stiff brush in hot soapy water.
It is imperative that every trace of foreign matter be dislodged
from the pores of the metal or the engine will soon grind itself
to a worthless mess. Many a good engine has been ruined at this
point by a too-hurried cleaning.
Some engines as they
come from the factory have the piston-cylinder fit set up just a
little too tight for high speed running. It is advantageous to lap
these out a little with jeweler's rouge or a fine grade of rubbing
compound. It will take some experience, or the advice of an expert
on that particular engine to know how much to lap. The engine must
not be lapped out so much that it loses its compression. Generally,
as our operating rpm goes higher, we can stand a little more compression
loss for the sake of a free fit. Also, some engines benefit from
a fine polishing of all the wearing surfaces before they are even
diametral clearance between the crankshaft and its bearing, on a
plain bearing engine, is important for free running at high rpm.
Usually .001" for a 1/2A and .002" for larger engines is the minimum
acceptable - although in some cases, double this amount of clearance
will improve performance.
The bearing of a ball bearing
engine should feel absolutely free of any tight spots or roughness.
So be careful to always keep bearings free from dirt, and so not
touch the ball races themselves. Perspiration from the fingers will
ruin them. A few drops of light oil after each day's running will
help keep moisture from collecting on the bearings.
modelers replace the ball bearings in their engines with high speed
precision bearings, but there is a catch here. The running clearance
of a high speed bearing is just as important, if not more so, than
its precision for this clearance determines the actual sliding friction
and the loads imposed on the balls when the engine is running. A
precision bearing with the wrong internal clearance may do more
harm than good. Along this same line, the shaft fit in the bearing
is very important. So it's best to consult the bearing manufacturer
for your particular application.
Port hooking is another
robber of power. This is caused by the side loads which result in
slight tipping of the piston in the cylinder. What happens is that
the top lip of the piston catches on the top of the cylinder ports
as it travels past. A very slight radius or chamfer put on the piston
lip and on the inside edges of the cylinder ports will alleviate
this. Some engines have this already done when they come from the
factory. Go very easy when doing any work inside the cylinder.
Any re-working which reduces the the strength of the part
involved should be held to a minimum. The more we reduce the strength
of any part, the more it distorts under load, causing binds and
friction. And the net result may be a power loss instead of a gain.
This is a very important point to weigh when considering any reworking.
Turning down of cylinder fins is generally best avoided.
Certain internal stresses are set up in the metal of the cylinder
when it is machined. When the outside fins are turned down after
the manufacturer has honed the cylinder to shape, some stresses
are released, which may disturb the balance and warp the cylinder.
When all the re-working has been done, the engine should
be assembled carefully, tightening all screws slowly and evenly,
while checking for any binds in the engine. If any binds are felt,
the cause must be found and corrected before the engine is run.
Here's how a dedicated researcher records his data on engine
performance; accurate measuring devices are absolute "must."
BREAKING-IN: This should be accomplished in a series
of short one minute runs at high speed, using a speed fuel that
has plenty of oil in it. The reason for the short runs is twofold
- to prevent heating and to let the engine break in quicker on less
running time. The number of heating up and cooling off cycles is
a factor in break-in. It is best to break-in an engine at approximately
the same speed that it will be run in the air. The wear pattern
on the moving and bearing surfaces changes with rpm because the
inertia forces of the reciprocating parts increase and the effective
pressure in the cylinder decreases with increasing rpm. By holding
to one approximate rpm throughout the life of the engine we have
a better chance of keeping a nice smooth wear pattern. Take for
instance a plain crankshaft bearing which tends to become oval as
it wears. If we run it at various speeds it will, instead, assume
a rough clover leaf shape which produces extra friction and vibration.
Also at higher rpms there is less chance for shellac deposits
to form on the cylinder walls. This deposit, often referred to as
engine varnish, usually occurs with a new engine which is tight.
A combination of tight fits, damp weather, and fuel ingredients
cause it to form. You spot it first when the rpm starts to sag and
the engine heats up abnormally. The engine loses it free feeling
at the top of the stroke (plug out). You can see it on the cylinder
looking through the exhaust port ... a sort of brownish film. It
may be a little hard to recognize the first time, but when you clean
it off you will see the difference. There is no choice. You have
to remove it. Remove the cylinder and clean it with fine grade steel
Molybdenum disulphide is an additive which should
be used during break-in, either in the form of a sub-micron powder
which can be mixed directly in the break-in fuel, or as an oil base
suspension. Molyfilm, an oil suspension, has become the most popular
form of molybdenum disulfide with the speed fans. It is classified
as a solid lubricant, a slippery powder that has an affinity for
soaking into the pores of the metal and makes a thin film as little
as one-millionth of an inch thick. This prevents metal-to-metal
contact and reduces the tendency for galling and seizure. The result
is a reduction in running time needed for parts to mate properly,
and a reduction in engine wear during break-in.
for a free fit, take out the glow plug and loosen the rear crankcase
cover of your engine. Oil it with a light oil and turn the shaft
till the piston is at the top of the cylinder. If your engine has
been "freed" properly, the piston will return to bottom dead center
of its own weight. (Ringed piston engines excepted, of course.)
Assuming that your engine has the basic layout and timing
needed for speed work, this business of "freeing-up" will have more
to do with your engine output than any other factor.
LIGHTENING AND BALANCING: We might consider briefly
this additional type of modification. So far very little has been
accomplished in this area, and then only via extensive experimenting.
By removing some weight from the reciprocating parts-piston, connecting
rod, and wrist pin - we reduce the inertia forces which the motor
must overcome when it changes direction twice every revolution.
The net result is faster acceleration of the piston during each
stroke, and lighter loads on the bearings. However, there is usually
not a great deal for us to do here, since we encounter problems
of distortion and resultant binding when too much metal is removed.
Also, the manufacturers have improved this feature steadily over
Balancing is another problem. Since the inertia
forces caused by the reciprocating parts vary with different rpms,
it is impossible for an engine to be perfectly balanced at all speeds.
Therefore, it becomes a problem of balancing the system for the
specific rpm you wish to operate at. Unless you have a bad vibration
problem it is best to leave this balance set up as designed. If
you do have excessive vibration you should first check for unbalanced
props, poor motor mounting, or a bent shaft.
So there you
have the basic ingredients of a hop-up job. If you would like more
detailed recipes for your particular engine, go to contests and
talk with some of the experts. There have been some helpful hop-up
,articles in the past.
Dynamic Models Inc., 13755 Saticoy
Street, Van Nuys, Calif., has published some very good information
on this subject in their "Tech Tips" ... their issue # 6 deals specifically
with reworking engines. Obtain same by writing to the company.