How to Select the Proper Glow Plug ... and Why
1963 Annual Edition American Modeler
with most aspects of every type of hobby, there is a plethora of
different types of glow plugs available for your selection. The
same goes for engines in which to use them. The quandary that haunts
many modelers is which glow plug is the best for a certain engine,
fuel, ambient temperature, altitude, etc. In the April 2012 edition
Model Aviation, Bob Aberle addressed just that topic and provided
a website for an extensive treatise on
glow plug specifications and usages. In it James McCarty, Brian
Cooper, and Brian Gardner list the major glow plug manufacturers
and present voltages, heat ranges, applications, intended fuel nitro
content, short versus long, idle bar, etc.
Fifty years earlier, William Netzeband published an extensive
article in the 1963 Annual Edition of American Modeler that employs
a very methodical and scientific approach that resulted in extensive
graphs and tables that are still largely applicable to today's equipment.
If you are still one of the decreasing percentage of internal combustion
engine flyers, then this treatise will be well worth your time.
How to Select the Proper Glow Plug ... and Why
By William Netzeband
Since our original glow plug report
appeared in the August 1960 issue of American Modeler most of the
plugs tested have been replaced or removed from manufacture. The
original charts have been revised to include the latest group as
of early in 1962 ... and will be updated from time-to-time.
It should be noted, that the report shows how to get the best life
from a plug, as well as safely operating an engine. Maximum contest
performance does not follow this safe pattern, so when you are trying
for the last ditch REV, use as hot a fuel and as hot a plug as you
can get by with. This holds even if you burn out during the flight,
as in speed or F/F gas. In fact, if your plug is still good after
one of these flights, you ain't getting all the soup available.
You sport types, read and heed the info. It has been revised slightly
based on increased learning and will help your happy flying.
Electrical Characteristics of Glow Plugs and Batteries
Do you really understand your glow plug? How come there are
so many different types available? As a nosey type I set about to
research the glow plug situation. After burning much fuel, many
plugs and considerable midnight oil I found that there are several
things which should interest you. Included in this report are data
on all plugs available at the time of writing, complete information
on batteries, a useful field test gadget and some startling hitherto
unpublished facts about glow engines. None of you can afford to
miss out ... so let's go.
A glow plug is machined
from steel bar stock and assembled per the cutaway drawing. Several
were sawed in half and polished to get this info. They come with
three thread lengths; 7/32", 3/16", and 5/32"-long reach, medium,
and short reach respectively. The medium reach can be used for either
long or short. In general most engines use long reach with a few
smaller types needing the short type. Since the threads and hex
are standard the variety comes in the coil, the insulation and seal
material, the plating, and the size of the cavity. Before continuing
into the depths let us establish the background for the discussions.
The glow plug provides heat to fire fuel about like
a spark plug, except it remains hot continuously. To brush over
a lot of territory let me state flatly that as glow heat is raised,
engine timing advances followed by RPM increases. Nitro methane
is blended with alcohol to provide oxygen for more power, but it
also lowers the flash point of the fuel. So more nitro also advances
the timing. The trick with plugs and the reason for this report
is to use as much nitro as necessary to do the job and use the PLUG
to control the timing.
Many of us have long operated
under the mistaken impression that timing is at the mercy of nitro.
The proper use of plugs as a control item opens new horizons of
engine operation. And it is much simpler than brewing fuel. The
proper use of plugs also increases their life. It has been the custom
to feel that a plug is expendable, but I have two personal pets
which have been in continuous operation so long, I don't really
know if they will unscrew anymore. There is a plug available now
which will even hold up in speed engines. Interesting?
The glow plug coil is heated initially by passing an electrical
current through it. The amount of current is determined by the coil
resistance and the battery voltage applied. More on this later.
When the battery is removed, however, no current flows (oh sure,
thermal emf, but we don't worry about that). Coil heat must be absorbed
from the fire during the power stroke, and be retained by the coil
during the intake and compression stroke. Platinum also gets heat
by catalysm[sic] in the presence of nitromethane. During this time
the coil gets pushed around by the pressure of expanding fuel and
then blasted with cold fuel, so it receives an unmerciful beating.
Consider also that this cycle occurs from 150 to 300 times per second.
The actual design of a glow plug is a ticklish business.
Coil material must have proper electrical resistance to heat enough
to start an engine, using a wire whose length and diameter were
picked to have the right area to operate the engine properly. It
must still be tough enough to stay together during that pounding.
Three materials are in common use: platinum and platinum-iridium
(90%-10%) alloy. Tungsten is alloyed into some wire materials in
Platinum melts at 32000 F with an electrical
resistance of 6.16 times copper. It is resistant to decomposition
from oxygen and heat (which we have lots of), and it retains good
physical strength and hardness at high temperatures. Tungsten and
iridium toughen the platinum alloys and are used as a control of
Suppose we kick around this
Hot plug-Cold plug idea for a while. Use of the word HOT causes
some confusion due to modem slang usage. Mention Hot and most of
you think of high performance (Jet's stick to machinery, Clyde).
Being a literal type my Hot plug is one which allows larger amounts
of fuel to reach the coil surface. The term is relative in that
each plug is compared to every other plug. I considered using Hotter
and Colder, but gave it up. There are several measurable features
of a plug which determine its heat rating:
diameter and length of wire in the coil.
(2) The diameter
of the coil and the diameter of the cavity as related to it.
(3) The physical properties of the seal and insulation material
(s) and the size of the stem.
(4) The location of
the coil in the cavity.
(1) and (2) determine the surface area exposed to the fuel, which
is the most important factor. Simply thought of it is comparable
to a leetle bitty match and a big torch. With a larger surface area,
more of the fuel is heated and the initial flame is more efficient.
Also the larger mass of wire retains more heat, hence is hotter
for the next power stroke. If, however, the coil is too close to
the sides of the cavity, some heat will be lost thru radiation to
the relatively cool surface. Nickel and gold plating serve to reflect
some of this, but plating is more useful to retard corrosion, lower
electrical resistance and pretty up the product.
has been found that black surfaces make better plugs in that the
dark color reflects less heat to the coil.
has little effect on heat compared to other items and is more important
to the plug's resistance to high pressure and high temperature.
Item (4) is interesting, however, and deserves discussion. Its effect
shows up after the plug has been used and the coil has moved about.
Generally the coil is pushed up or over to one side making the plug
colder. Some weird cases have come up where the coil was moved out
of the cavity and into the firing chamber. Heat went way up and
detonation set in. In fact on several occasions a somewhat cool
run has turned into a screaming high RPM type. Examination of the
plug showed no coil except a 1/16" to 1/8" piece sticking straight
out So who needs a coil? Remember this position feature, because
in emergencies you can adjust a given plug by pulling or pushing
the coil. A short reach plug will run cold in a long reach engine
and vice versa.
Someone said, "So what!" There's a
definite use for all this information. By knowing the relative heat
of all the plugs it is possible to use one fuel over a wide range
of temperature and humidity. General practice has been to change
fuel with weather causing heavy tool kits and much frustration.
The plug change is not considered adequate for flat-out speed operation,
but for all other events where a few RPM won't show. For instance
during our test program, which covered a year of Mid-western weather,
our test engines were operated in temperatures from 5° below zero
to 100° in the shade, with wide humidity variations. By varying
only the plug and needle valve it was always possible to get a good,
solid, peaked RPM run. Speeds couldn't be proved different by a
Now all you need is one fuel can and a
set of plugs. After a while you can settle on about 3 plugs which
will run your engine nicely. It should be stated firmly and unequivocally
that there is no "BEST" plug on the market, per se. There is a "best"
plug for your engine and your fuel on the day you run it, and there
are enough "you's," days and engines to keep them
all selling. Some plugs have more substantial insulation and seals,
but you know them already. The weak ones blowout before the coil
folds up. This report has to do with the not so obvious characteristics.
Table 1 -
Glow Plug Data & Heat Range Position (Long Reach Plugs)
To arrange the Heat Range Table, tests were made as follows
(1) Current thru the coil and volts across the
plug were measured at various applied voltages. This curve was
plotted as Voltage versus Current. Data points were averaged from
tests on two plugs of each type with calibrated lab instruments.
(2) Tests of common voltage sources, i.e. dry cells and wet
cells, were run to obtain terminal voltage characteristics under
various loads. They appear as sloped straight lines on the sample
(3) The actual resistance of a standard set
of battery leads, including contact resistance of a clip-on device,
was measured on a laboratory Wheatstone bridge. This value was used
to determine the voltage drop thru the leads and hence the voltage
across the plug, after deducting voltage drop thru the leads.
(4) By reading the intersections of the voltage versus
current curve and the "volts across the plug" curve the current
drain on the battery was determined. We also wanted to find out
the effect of battery aging on starting qualities. Most plugs allow
a 40% safety factor when using 2 dry cells in parallel.
(5) Relative heat characteristics were obtained as follows ... (a)
A well broken in Fox 35 Stunt engine was used with a fuel consisting
of 30% Castor oil and 70% Alcohol. This low-compression (head was
shimmed up) cold fuel combo was operated with a constant head tank,
on the bench, to determine the following - starting, rich run, lean
run, and exhaust throttle characteristics. A variable voltage power
supply was used to apply rated current for starts. Plugs were grouped
tentatively from these data. (b) A Fox '58 Combat Special with shaved
head, modified for better breathing and pressurized for constant
fuel feed was used with Missile Mist and Hi Nitro fuels for the
hi compression-hot fuel runs. Test runs identical to Cold combo
were made, excluding throttle runs. This information was used to
determine final grouping. Since this hot combo is notoriously hard
on plugs, we observed durability also. (c) Plugs in the center of
the table (medium heat) could not be decisively separated from each
other in operating tests, so they are grouped by physical characteristics.
Table 2 -
Glow Plug Data & Heat Range Position (Short Reach Plugs)
Of some interest is the fact that most decisive separation showed
during lo-comp cold fuel runs. The souped mill blew out most plugs
before any observations were made. Also received a mangled cranking
hand with the hotter plugs. Whop!!!
Before covering some
points on use of batteries let's mull over several mismatched combinations,
their symptoms and cures. These will be exaggerated conditions.
Take a lo-compression (6 to 1) ratio engine; a cold (low nitro say
5%) fuel and a cold (maybe 35°) day. Your plug happens to be one
at the cold end of our table. Starting will be slow and sloppy.
You may find that you cannot get it to 2 cycle peak or perhaps it
will quit when the battery is removed. If you get it off, it will
soon get so rich and sloppy it will probably quit. All of these
symptoms point to insufficient heat. Solution is to use a hotter
plug. Simple, what? %) ) fuel and a cold (maybe 35°) day. Your plug
happens to be one at the cold end of our table. Starting will be
slow and sloppy. You may find that you cannot get it to 2 cycle
peak or perhaps it will quit when the battery is removed. If you
get it off, it will soon get so rich and sloppy it will probably
quit. All of these symptoms point to insufficient heat. Solution
is to use a hotter plug. Simple, what?
we take your combat or speed engine on a warm day or even a cool
day. Compression ratio around 12 to 1 and you're using Blast fuel
(over 20% nitro). Pick a plug from the hot end of the range (we
dare you). Right off you know you're wrong cause you get kicked.
Go ahead and crank it up, it's only blood. Peaked out okay, so throw
After a few laps she starts to sag. Impossible; we got
off rich enough. Sure, but the plug is heating too much and the
effect accumulates in a vicious circle of advanced timing, more
heat, more advance, detonation and boom. If you're lucky the coil
will melt and blow out the stack. Otherwise the poop out continues
as long as any wire is in thar. Solution - Colder plug. It is left
to you as an intelligent individual to sort out the symptoms of
too much or too little heat. The hot plugs will make rich stunt
type runs on zero days, while the cold plugs will operate souped
mills on hot days. The range is quite adequate for whatever you
need. We have more dope for you anyhow.
Fig 2. - Glow Plug Field Test Circuit
Fig. 3 - Parallel Connections of Two Batteries for 1.5 Volts
The matter of battery care and operation seems to be little
understood so let's square it away. First some basic electricity.
When a wire is carrying current, some energy is used up in heat.
Heat is proportional to current and resistance - that is the larger
they are, the more heat is lost. Also, as the heat increases, the
resistance increases, until, if we apply a fixed voltage, the system
stabilizes. Since our battery internal resistance is fixed, and
our lead resistance is fixed, our coil will heat up until its resistance
absorbs the rest of the voltage. If the resistance is too high,
not enough current will flow to generate heat. Conversely if resistance
is too low, too much current will overheat the wire and melt it.
The fact that the wire in getting hot raises the resistance
to some required level is important. Most plugs heat to around 1800°
F. for a starting while drawing current shown in the chart. Since
plugs need around 3 amps to heat properly, total circuit resistance
is kept low. Coil on the order of .5 ohms. Lead resistance about
.04 ohms. Considering the two types in common use, only, we'll point
up a few things to give you better starts, fewer blowouts and longer
The "dry cell" is most popular and most of
the plugs are designed for it. Open circuit voltage is 1.55 volts.
Construction-wise it consists of a carbon rod (plus terminal); a
mixture of ground carbon and Manganese Dioxide moistened with a
Sal Ammoniac and zinc chloride solution; and a zinc case (minus
electrode). These are the important parts. By the way, a dry cell
is not "dry." It has a moist paste which won't slosh. Current is
generated by chemical action, which also causes hydrogen gas to
form on the carbon rod. This decreases the cell's ability to generate
current and the terminal voltage drops. When not generating, however,
the manganese dioxide removes the hydrogen and the cell "heals."
We are demanding large current and must keep in mind that the cell
is not designed to put out for long periods at this rate. The dry
cell curves on the graph reflect this. Even little short circuits
Do not test these batteries with an ammeter!
Look at the date of manufacture instead. They will last a year and
a half on the shelf. Buy only the freshest ones. By wiring two cells
together in parallel (see sketch) we can generate more current and
increase battery life about 4 times. On a 3 amp load each cell must
supply only 1.5 amps. Thus it works more easily and maintains a
higher voltage. Remember 4 (four) times the life for twice the price!
Keep your clips clean and check connections often.
cell or 2 volt battery is a lead-acid battery. An automobile battery
is a batch of these cells connected in series to boost voltage.
Terminal voltage is 2.2. Chemical action occurs between a lead
dioxide plate and a lead plate with sulfuric acid as the electrolyte.
The plates are stacked and connected in parallel inside the cell.
This cell is capable of tremendous current output compared to a
dry cell and can be recharged. Attractive features? HOWEVER, tread
very carefully. Some plugs will not stand 2 volts and some that
will stand up get so hot on starting that they give out with kick-backs
and reverse rotation. This occurs when standard leads are used.
Solution is simple. Use leads four times as long (10'),
or use # 20 wire three times as long (90"). This will drop the volts
across the plug closer to the range of a dry cell. My personal solution
was to add a resistor (approx. 0.3 ohm) in the circuit with a switch,
so I could use either 2 or 1 1/2 volt plugs. In addition I threw
in an ammeter for quick field checks. A light type checker is good
(both Ohlsson and Sullivan checkers will take 2 volts), but they
don't show a shorted coil. After losing a few rat races and other
aggravating experiences thru partially shorted coils, I got busy
and whipped in the ammeter circuit. My unit has a 3 position switch
with a momentary Test position, an OFF position and an ON position,
and a phone jack so different leads can be used.
if you want to use this circuit on dry cells leave off the phone
jack and get a good toggle switch. The jack and switch can build
up resistance too much. The two testers mentioned earlier are a
good investment and are highly recommended. To get a .3 ohm resistor
I used a 1 ohm 25 watt wire wound variable resistor available at
some larger radio supply houses. A piece of iron wrapping wire can
be used just as well. Length of wire determines its resistance,
so proceed this way. Wire the ammeter, glow plug, battery, your
standard battery leads, and a long (2 foot) length of iron wire
in series. Consult the chart for 1.55 volt plug current and slide
connection down the iron wire to shorten the length of wire included
in the circuit. When the ammeter shows proper current, the plug
Glow Head Current Drain
Try several other plugs for a cross check and adjust for best average
length. The length of wire may then be placed in the system permanently.
Wrap the wire around a doped wood dowel making certain no coils
touch each other. Dope over it and solder wires to resistor. Tie
the connections down to make a good physical installation. Finally,
on dry cells, if you have to delay starts for any reason, disconnect
This research was undertaken to find out as
much about glow plugs as possible without treading on trade secrets.
One manufacturer asked that coil data not be revealed, so naturally
none of them can be. The fact that both length and diameter of coil
wire determine heat range and current drain led to some bad guesses.
Looking at wire diameter is not enough to suppose how hot it will
be. The length must be checked. One of the cold plugs uses wire
identical in diameter to that used in the hotter plugs, but the
length is shorter. See? There are several things worth mentioning.
In the focused head, pressurized Combat Special, using
a 50% nitro fuel we beat the plugs to death to see which would stay
together. The KB-IL held up for 30 minutes under this. No other
plug lasted for more than 15 minutes. The plugs on the cold end
of the chart work quite well under these high heat conditions, and
should be used in order to get reasonable needle settings and to
eliminate the tendency toward overheating, which has become so common.
For throttle operation, use the K&B Idle Bar plug, or the Johnson
GL-1B or GL-SB. The shields used in these plugs keep the cold fuel
from flooding the coil and cooling it. They tend to be a bit hot
on the high RPM phase, but respond well on the low RPM in both low
and high compression engines.
Incidentally, those of you
still hoarding Champion plugs might as well use them. They are out
of date. Most of the manufacturers are now using the best insulating
and sealing materials available and the problem of blowing out the
seal is practically non-existent. Most of them use two materials,
one an asbestos compound for insulation, the other a high temperature
sealing compound ... and bonded glass or ceramic is in use.
It is difficult to state whether welding or crimping is superior
for fixing coil ends and my list shows that the methods are used
about equally. Welding gives a better electrical connection, but
good crimping gives a solid mechanical joint. So - no contest. All
plugs are well made and if used properly are more durable than most
of you think. The manufacturers deserve a bouquet for the high degree
of quality in our plugs. As stated earlier the glow plug is basically
a nasty device and there must have been much skull-cracking associated
with the design of each plug. I -was happily surprised to find the
spread of characteristics available, because when. I started I wasn't
sure but what they would all be about the same.
forget to say thanks to those manufacturers who so generously provided
plugs I couldn't buy. (Incidentally fellows, I burned them up just
like I promised.) The data shown for the special "plug-in-the-head"
types is more academic than startling. Since the coil and construction
is specially matched to its own engine, and since heads are not
interchangeable, not much else matters. So we listed the battery
drain for your convenience.
Just remembered the kid
who came out to fly a 1/2A. He pulled out a 2 foot iron stake and
a big hammer, proceeding to pound the stake in the dirt. Then he
pumped the plane up with fuel and attached one lead of the battery
to the top of the plug. Then he "grounded" the other battery lead
to the stake and started his futile cranking. Manufacturers please
note! "CONNECT" beats "ground." Or the fellow who attached his battery
clips together "so the juice wouldn't run out." P.S. - IT DID!
In review then, a hot engine and fuel generally need a cold
plug. A cold engine and cold fuel generally need a hot plug. Running
hi nitro fuel in a low compression (cold) engine is generally a
waste of talent. Likewise there is little use for cold fuel in a
hot (high comp.) engine. These facts are based on the "use" requirements.
This field is the source of another batch of research. The various
charts will show you general characteristics which should give you
a quick-look check on plugs for different operations.
Posted June 2, 2012
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. There
is a lot of good information and there are lot
of pictures throughout the website that you will probably find useful, and might
even bring back some old memories from your own days of yore. The website began life around
1996 as an EarthLink screen name of ModelAirplanes, and quickly grew to where more server
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