This treatise on engine idling techniques is yet another example
of how extensive and detailed model aircraft magazine articles used
to be. Maybe refinement in design and production has, over time,
yielded engines that are easier to start and and adjust, and are
more reliable in general, but there are plenty of older engines
still in operation, whose owners could benefit handsomely from the
advice offered in this column. It has been my experience that even
the newer engines - particularly those typically purchased by those
of us on a limited modeling budget - still exhibit strange operation
at times, so unless you always buy the best engines on the market,
read on... you'll be glad you did.
Although aimed at radio control flyers this valuable
data can be of help to all modelers who own a glow plug powerplant...
Engine Idling Secrets
Part 1 (see
By Harvey Thomasian
Researcher Thomasian (far lt.) with Dr. W.A. Good. First clothes-reel
test rig (bwloe); final rig considerably more sophisticated.
The appearance of glow plugs in engines did much to popularize
the model airplane hobby by eliminating almost all the evils of
an ignition system. However, the glow plug rates low for one condition
that the ignition system did well: idle - glow plug engines do not
run happily at low R.P.M.
Hardly any information has been written on idling glow engines
because theoretical knowledge on the subject is limited. Many modelers
know how to idle such an engine properly, but not why.
In this report, we shall try to provide some insight as to the
whys and wherefores and present some information to enable modelers
to adjust their engines for an acceptable idle. We thought achieving
an idle would be an easy task, but in studying the subject seriously,
we became convinced that any success was due more to good luck than
This is not a complete how-to-do-it article. Our intent is to
explain why and how different factors affect idle, and we shall
endeavor to explain how to best adjust your engine to make it tick
Although engine manufacturers have done much good work to achieve
optimum low speed operation, many factors outside their control
make an occasional engine difficult to idle. Also, engine design
and materials have a marked effect. Some of these factors, or variables,
are compression ratio, base compression, glow plug design and material,
engine port timing, cylinder head design, heat balance, fuels, exhaust
dampers, intake throttles, temperature, humidity, altitude.
Nose of Thomasian low-winger shows aluminum mounting plate which
permitted different engines to be installed for flight testing.
Super Tigre 51 has K&B intake throttle and rotary exhaust
An S. T. 51 with fins cut into cylinder head.
Since these are just some of the factors, you can see it is extremely
difficult to make specific recommendations for all types of engines
running on all the glow plugs and fuels available, for all the different
areas in this country with their peculiar weather conditions. And
quite honestly, the biggest variable is the modeler himself. That's
right ... you!
What we wish to emphasize here is that there exists no specific
set of recommendations which can be followed to achieve the desired
slow speed. Also, any information provided shies away from making
major changes on engines since the manufacturers have done much
work to give their products a good balance between idle and maximum
Tests have been run on 31 multi-speed engines of 9 manufacturers.
These tests were conducted in the air with many models, as well
as on the ground.
At the beginning of our tests, we had to decide on a target R.P.M.
This was selected as 2500 RP.M. which roughly gives zero prop drag
at 14 m.p.h. with a 6" pitch prop and around 10 m.p.h, with a 4"
Our ground testing was done by fixing an E-Z Just engine mount
to the clothes line. (This type of clothesline is mounted on a central
pole and goes around and around). The engine angle was adjustable
about all three axes, could be mounted anywhere from the center
of the pole out to the end of one of the arms. This contraption
was set up to test the effects of centrifugal force, different engine
and tank angles, at various radii, and at all R.P.M.s, from idle
to wide open. Although this rig did not completely duplicate all
flight conditions, actual flight tests showed that results from
this whirlygig proved out. It was especially helpful in determining
if the engine would remain idling in a spin and especially taught
us a few things about fuel tanks.
The target R.P.M. figure is one where the engine will run the
whole tank at idle: 2 oz. for .15 disp.; 3 oz. for .19 disp.; 4
oz. for .29 disp.; 6 oz. for .35 disp. and 8 oz. for the .45 and
up sizes. Also, a severe shock such as a bad touch-and-go landing,
(a bouncy one) or violent maneuvers, will not kill the engine.
All of our trials included temperatures down to 20° F. with some
taking place below 0° F. In one instance, we spun a K&B 45 in
an old Astro, 23 turns, during a snow flurry where the ground temperature
was 0° F. No, it didn't quit - mind you that this spin started around
700 ft. - we just couldn't see the airplane at that point!
Anyway, to get to the meat of this thing - what follows sort
of rambles on with a minimum of continuity. We'll mix engine design,
a bit of theory and how-to-do-it into one big bowl.
Firstly, the engine should be in decent shape meaning good cylinder
compression, good crankcase compression, fairly clean of carbon
inside, no dirt on the outside either in the fins or elsewhere,
no leaks, either through cracks, gaskets or very sloppy crank bearings,
no binds or tight spots, and reasonably good fitting parts.
To illustrate: carbon in the combustion chamber not only increases
compression ratio, which alters timing, but also acts as an insulator
and hinders heat release to the air around the cylinder and head.
Carbon on the underside of the piston can make problems, too, especially
when combined with other shortcomings. Needless to say, poor base
compression is detrimental to high power output and slow speed because
2-cycle operation is all due to pressure differentials and if seriously
upset, gives you problems. Don't misunderstand us ... an engine
does not have to be new. As a matter of fact, some sloppy engines
work well. Remember the loose engine that screams at the top and
ticks over smoothly at the bottom? It may feel loose when flipped,
but it seals well when running.
The next few paragraphs will discuss how design factors in engines
can cause changes in power and idle in our power plants, and why
various factors alter their performance.
The ideal compression ratio for multi speed work, lies somewhere
between 6 and 8 to 1, depending on the make. Compression ratios
above 9 to 1 are detrimental to idle and lowering will definitely
improve it, but this can be overdone as reductions below 6 to 1,
while they do give a small additional gain, seriously reduce maximum
Crankcase compression ratio is satisfactory on current radio control
engines and is not particularly critical in the performance areas
around which our present engines are designed. However, if we suffer
poor base compression, due to leakage at low R.P.M., our idle suffers
as there is a loss of velocity at the throttle due to reduced suction,
and insufficient pressure to properly boost the mixture into the
cylinder. Also, in some cases, fluctuating low base compression
can cause uneven draw at the throttle during intake. An interesting
sidelight here is that modelers have experimented in speed control
by varying crankcase compression through a variable leak, but to
date, this has been unsuccessful.
Shaft intake and exhaust port timing can be altered to make an
engine idle beautifully, but a generous amount of top R.P.M. will
be sacrificed and this, of course, is not good. In the engines tested,
the intake timing ran anywhere from 185° to 210°, and in one instance,
220°. Almost all of them close at 45° after dead center. The closure
point has much more significance than the opening point. What you
look for here at low speed is that the shaft not be open for too
long a period after the piston starts down, because crankcase compression
will force mixture back out the intake. I'm certain most people
have observed this condition in a small way when an engine is idling.
Two of the many R/C models used by H.T.
Harv reports "final"
clothes-reel test rig was destroyed when a Veco 45 accidentally
jumped into wide open speed ("unbalanced reels at high rpm are
At real high speed, late closure timing increases power because
fuel and air have inertia, and will continue to pass into the crankcase
though there may be a small amount of counter-compression building
up in the case. This is another way of saying that the engine has
passed the peak in the troque curve resulting in a reduction of
Volumetric efficiency. For those of you who care to experiment,
try closing the shaft timing between 25° and 35° for a pleasant
surprise ... but watch the drop in maximum R.P.M.!
Heat dissipation or thermal balance is a hard nut to crack as
fuel, glow plug, cooling and basic materials all tie together. Best
success usually comes with the cylinder head operating at a maximum
of 400° F. after the engine has been running at top R.P.M. for at
least five minutes. This should be followed by two or three minutes
of idling after which the head temperature should stabilize at 220°
or so thereabouts. As you can imagine, this is a somewhat difficult
problem due to the fact that you never get the fuel, humidity and
temperature conditions ideal.
Fuels are a book in themselves, to discuss them without consideration
of the glow plug would be foolish, so we will try to tie them together
as we go along.
Mixtures containing between 5 % to 10% nitromethane with 25%
castor oil give us a good balance of power, smooth running and idle.
In some engines, castor oil content can be lowered to 20%, but I
would not recommend anything below that. Additional nitro does not
destroy idle as some are prone to believe. Hot fuel does not raise
the engine temperature at idle since nitro does not bum at a higher
temperature. At top speed nitro increases power by liberating more
oxygen, not increasing 'temperature. Your engine does run hotter
at higher speeds because it is developing more power (burning more
B.T.U.'s) and will not pass heat through the fins at a proportionally
higher rate than when running slow. Elimination of nitromethane
does not affect engine performance other than to reduce top speeds.
Carburetor drawing 1.
Carburetor drawing 2.
Carburetor drawing 3.
Carburetor drawing 4.
Carburetor drawing 5.
One thing to watch if you make a drastic change in the nitro
content ... check idle performance before flying. Once a carburetor
is tuned to a fuel with a specific amount of nitro, you should stick
to that fuel inasmuch as nitro needs three to four times as much
air as does methanol. Any drastic change in nitro means that the
filed notch or idle air bleed in the throttle should be altered...
that is to say, as more nitro is added, more air is needed. Just
watch the filing, because if too much is removed, throttle suction
is reduced to the point where idle is not dependable. A little side-light
on nitromethane: it is a double edged sword because while it lowers
the flash point, which in some instances helps idling a mite, it
also advances timing which can cause pre-ignition and detonation
when your mill is running flat out.
As stated previously, the glow plug is almost married to the
fuel. To make a quick, simple suggestion, we recommend a mild fuel
(0% to 10% nitro) with a hot plug. The plug should not be hot enough
to cause pre-ignition and/or detonation as a run in this condition
can do your engine harm. We stick to one fuel the year round (K &
B 100), and adjust for temperature with just 2 glow plugs, a cold
one for hot weather and a hot plug for cooler temperatures. This
combination takes care of us from 0° to 120° F. Changes in humidity
do not alter low speed characteristics seriously, but may show a
change at top R.P.M.
With regard to the relationship between so called hot fuels and
the heat ranges of glow plugs, there is currently no definition
of a hot fuel which is entirely acceptable to the glow plug manufacturers
and it is inadequate to simply categorize a glow plug as being hot
or cold. We normally think of a long plug as being hot and a short
reach plug, cold, but this assumption is rather general since plug
materials, coil shapes, coil diameter, wire diameter, wire size
and length, idle bar, all contribute to heat range determination.
Actually, a plug should be classified as to its ultimate effect
on the engine; We have proven to ourselves that an idle bar definitely
assists idle, especially in colder situations. Wire type and diameter,
as noted in relation to heat ranges, have a decided effect on determining
a good idling glow plug. As you may remember, the old A.C. plug
had a small cavity opening at the bottom which somewhat protected
the upper coils of the element from fuel spray, but the biggest
improvements in plugs are the cross bar or idle bar, and the longer
and heavier element.
The cross or idle bar helps retain the heat so necessary to keep
a plug operating - when it is being drowned out with a rich fuel
mixture. The idle bar's primary function is to retain heat in the
coil as the incoming mixture hits it. Secondly, it helps somewhat
to keep spray out of the plug. On this basis, it could be contended
that the larger the idle bar, the better. However, in going to these
larger diameter bars, the point could be reached where possibly
no fuel could touch the coils. Therefore, the engine probably would
not run since fuel has to bum to start the combustion chain.
While location of the glow plug in the cylinder has some effect
on idle, where to place it is the sixty-four dollar question. Hours
of experimentation revealed nothing conclusive and if we read correctly
between the lines in letters from manufacturers, they don't know
much more than the rest of us.
For those of you who want more basic information on plugs-the
whys, wherefores, design and operation - take a look at the September
1960 American Modeler where Bill Netzeband presented a comprehensive
run down on the glow plugs, their heat ranges, and some of the mysteries
associated with them. I don't completely concur with everything
Bill says; but on numerous points I am in accord. Actually, Netzeband's
article is a bit of a classic since no one previously has tied heat
range and fuels together. Considering the various charts and graphs
he made up, we suspect that he went much deeper into some of the
mysteries of plugs than even some manufacturers who have made them."
As far as preferences in multi-speed engines go, we hesitate
to specify any in particular since many reliable ones are available.
We do prefer those in which the crankcase and cylinder housing are
cast in one piece and has an inserted steel liner. Our experience
shows that ball bearing engines are not mandatory in R/C since a
properly fitted sleeve bearing has little more friction when running
fast than balls. Two reasons why ball bearings have a small advantage:
at idle, the ball bearings do have less friction, promoting smoother
operation, secondly, ball bearing engines with their greater mass,
due to bearings and larger castings, help damp out vibration.
Concerning throttles, this runs the gamut and we have tried choke
throttles, carburetor throttles, exhaust dampers, (damper, not baffle,
is the correct nomenclature) and crankcase bleeds. We hooked a nickel-cad
battery to the plug which helps in some cold situations, but under
normal conditions this is not necessary and merely indicates that
the trouble lies elsewhere.
Generally speaking, throttles fitted to production engines nowadays
are quite adequate for the job... but due to variables in individual
engines which come off the line, quite often a bit of doctoring
of one sort or another is required.
Our experiments indicated that throttle type can relate to engine
size... for which we can find no reason. But as an example, small
engines idle more reliably with exhaust dampers (no intake throttle)
than the big inchers. It indicates that the intake throttle becomes
more necessary as displacement goes up. While a choke-type intake
throttle works well on small engines (.15 disp.), larger engines
show an improvement at idle when this type is replaced with a carburetor
Curiously enough, exhaust throttle design has an effect on engine
idle performance, too. We found that the drum-type exhaust damper
worked a bit better than the sliding vane or railroad signal variety,
and this, incidentally, is the style used on the Veco-Lee 45. This
damper is a rod with its center portion cut away so that there remains
a thin web which shuts off the exhaust when vertical and opens when
the web is rotated horizontally. It seems the closer this device
is located to the cylinder sleeve, the better your engine will idle.
We can furnish no explanation for this and queries to several manufacturers
confirmed this - they could offer no reason why. In conclusion,
we recommend that for good idle, an engine be equipped with both
an intake throttle and exhaust damper.
If you own an engine whose idle is unsatisfactory, we will outline
the steps to take in determining what is wrong and how to rectify
As mentioned in the beginning, check your engine over closely
to see that it is in good shape. Next, select a good fuel that gives
adequate power and has inhibitors which retard formation of gum.
Assuming you have a good engine and fuel, we now have the glow plug,
intake throttle, and exhaust damper to work around. First, almost
all engines which come equipped with intake throttles have some
sort of idle stop or adjustment for the throttle barrel. For those
engines which do not, and those of you who want to adapt a carburetor
to an existing engine, Dwg. # 2 shows the installation of a stop
and a means of securing the parts on a Bramco throttle.
Our next step is to mount your engine on a test block with the
same type of tank to be used in the airplane, in the same position.
This part of the sequence can be conducted in the airplane, but
having the engine in the open makes working on it considerably easier,
especially if the throttle barrel has to be removed.
Fill your tank a little over half full and run the engine wide
open. Get her up to a screaming two-cycle and and then .back the
needle valve off some to run it slightly rich. This is your needle
valve setting to be used in adjusting the throttle so don't change
it. Now we select a glow plug. Start with a hot plug and run wide
open to determine if the engine crackles. If it crackles the slightest
bit, go to a cooler plug, because sure as shootin' if you leave
the hot plug in, the engine will crackle on that hot day when she
is under a load in the air. Our objective is to use as hot a plug
as possible with no sign of detonation - something that can make
a wreck of your engine eventually.
At this point, we start work on idle by altering the intake throttle
if necessary. Bleeding of additional air into the engine is necessary
at idle or else the engine would cut out very shortly due to a grossly
over-rich mixture. This is done in one of three fashions: (1) File
a notch on the top side of the throttle barrel as appears on K&B
and Veco engines - Dwg # 2; (2) Drill an idle air bleed hole in
the front of the carburetor body in the location shown, such as
done by Harold deBolt - Dwg. #3; (3) Make a screw adjustable idle
air bleed as used on O.S. Max and Merco engines - (Dwg. #4).
Once you have decided on one of these systems, or if it is already
provided, idling tests can commence. In the following, we will refer
to filing the V-notch and the operation is analogous to the other
two, drilling the hole larger or opening up the bleed screw. In
the deBolt system, start with a #55 drill and proceed one drill
size at a time.
Take off the exhaust damper or disconnect it and wire it wide
open. Start your engine and slowly retard the idle. If it begins
to rich en up and quit, remove the throttle barrel and increase
bleed area by filing the idle notch deeper and wider with Swiss
pattern files - take a few swipes at a time. Start the engine up
again and repeat the following procedure, filing a bit at a time
- do a neat job-file symmetrically and watch the file so that metal
is not removed on the opposite side of the barrel. Repeat the procedure
as many times as necessary until the engine idles fairly well at
between 2,800 and 3,300 R.P.M. Do not judge R.P.M. at this point
by ear as it is very deceiving with the throttle damper removed.
In our tests, we had one of the new General Radio Strobotacs which
a local company loaned us. Use a tachometer of some sort. If using
a reed type tachometer (Vibra-Tak) try to have it checked somewhere
first. Accuracy at lower ranges should be within 200 R.P.M. of a
Back to the battle. While the engine is idling fairly well at
the aforementioned speed, accelerate it and decelerate it. It is
not necessary to do this any faster than a servo does, not instantaneously
anyway. Chances are the engine will decelerate okay, but may quit
on acceleration while tossing off some smoke from the exhaust. This
is an indication that the engine is loading up on idle so make the
V-notch a bit larger and continue until the engine goes up and down
fairly well. During this operation, do not touch the needle valve
as set for full power on the rich side. Also do not make any attempt
to get lower than 2,500 R.P.M. or the plug will probably cool off.
Also, if you keep filing past the point where the engine idles well,
it will die out due to lack of fuel suction and this is remedied
by filing a very small V-notch (1/32" deep) 180° degrees from the
first one, on the opposite side, to increase suction at the throttle
and richen the mixture.
Bolt on or connect the exhaust damper and idle your engine again.
Generally, you will experience an additional 200 to 300 R.P.M. drop.
If the engine strangles to a stop, the damper is fitted too tight
and prevents some exhaust leakage which is remedied by opening the
damper a hair or otherwise fitting it looser. Conversely, if the
engine R.P.M. does rise, then tighten up on the exhaust damper to
cut down leakage.
"Idling Secrets" will be concluded in December issue of American
*The editors advise us that this valuable
report will be presented in updated form in the forthcoming American
Modeler ANNUAL for 1963 which goes on sale November 15.
Posted December 17, 2011