70 MODEL AVIATION
The elements of glow-engine adjustment
Dean Pappas | DeanF3AF2B@If It Flies ... pappasfamily.net
HI, GANG. The weather is a never-ending
source of inspiration. This time it was a
string of warm flying days that reminded
me of airframe and engine setup
adjustments that come with the hot days of
summer. I made few adjustments to the
airplane itself as weather changes, and most
of them were part of the fine trimming
needed for Aerobatics competition.
Since I haven’t competed in years, I
don’t have much use for that sort of detail
fussing, but changes in weather still require
adjustments to keep the engine running
powerfully and reliably. The need for
reliable horsepower never changes.
Glow and, to a lesser extent, gas/ignition
engines respond dramatically to substantial
weather changes. As I mentioned, this
subject came to mind with the onset of
warm but tolerable flying weather, but this
week has seen unrelenting 100° days that
make you wait patiently for that little bit of
relief just before sunset.
Despite the heat, it is the nature of the
magazine business that this will be
published in an issue labeled “October,”
that many of you will read in September.
That is good, because many of the changes
we make for hot weather have to be undone
when summer ends. I might get ahead of
the curve.
Language is a funny thing. The names
we give things sometimes describe them
very well, and other times they confuse you
or hide what is actually happening. Take
the term “Figure Skating,” for instance.
You might have seen it on television.
Did you know that the event took its
name from the first round of the
competition, in which skaters had to
perform precise figures such as circles,
figure eights, intersections, and
combinations of them? Then the judges
would walk out and closely inspect the
skate marks in the ice to assign scores.
The entire “figures” round was dropped
from the event maybe 40 years ago. It was
mind-numbingly boring to watch. The
“figures” have been gone since before half
of the world’s population was born, yet we
still call the event by that name.
I guess it’s what you’d call tradition. As
far as I can tell, the real question is whether
or not it should be called “artistic skating”
or “athletic skating.”
“Glow ignition” is a similarly
misleading term. Thermal or
thermocatalytic ignition is more
descriptive, and in some languages that’s
exactly what glow ignition translates as.
Yes, I’ve had enough of the vocabulary
lesson too!
A properly controlled spark-ignition
system is superior to glow ignition in all
regards but one: weight. When it comes to
flying, weight is the enemy. It always has
been. For that reason alone, the
combination of alcohol fuel and a hot
catalytic metal glow plug has been
dominant in aeromodeling starting nearly
60 years ago.
So much has changed since the advent
of the glow plug. Gasoline engines with
lightweight, reliable, electronically timed,
capacitive-discharge systems are available,
and even electric power plants threaten to
topple the glow plug from its lofty perch.
But still, more glow engines are used at
the flying field every day than anything
else. Their care and feeding is an acquired
skill, and it perplexes some of us
aeromodeling “lifers” that some good pilots
out there haven’t learned it.
The glow plug is a deceptively simple
device. Simplicity does not mean that all
glow plugs are the same; one size does not
fit all, as the old Frank Zappa song title
suggests.
Picking the right glow plug makes an
engine run better, but several variables
must be juggled in a good-running
combination. By paying proper attention to
managing those factors, we can avoid
unnecessary frustration.
This juggling act is far from difficult.
You actually have a fair bit of flexibility in
the variables, which are:
1. Compression ratio
2. Fuel and nitro content
3. Air density
4. Glow plug heat range
5. Fuel/air mixture or needle-valve setting
Of those, probably the most powerful is
the needle valve. Any setup can be ruined
by mishandling that dainty little thing. How
can such a small item be so important?
In a glow-ignition engine, the “fire” is
started when the fuel/air mixture is heated
by compression of the fuel/air mixture. The
heating of a gas that is suddenly
compressed is described as “adiabatic.”
That means that there is little or no heat
lost, at least in the short term.
The resulting sudden temperature rise is
then enough to start the combustion
process, with help from the catalytic action
of the glow plug element. A “catalyst” is a
substance (often a metal) whose surface
takes part in a chemical reaction in a way
that promotes the chemistry without being
changed in the process. (More about this in
a minute.)
The catalyst, in this case the hot surface
of a platinum-alloy coil, lowers the
activation-energy “hump,” so that the
Only 10 years ago, the idea that a meaningful percentage of model aviation enthusiasts
would not have owned a glow-ignition engine would have occurred to only a handful of
visionaries. The YS .61 shown used to power the author’s sport workhorse Tiger 60. The
old standby K&B 1-L glow plug was the ticket in the summer heat; when temps dropped
below 70°, a hotter plug such as the Enya #3 improved idle reliability.
10sig3.QXD_00MSTRPG.QXD 8/20/10 12:45 PM Page 70
sudden compression heating is able to light
the alcohol and nitro. This combustion
process does not take place all at once; it is
not an explosion in that sense of the word.
How many times have you heard the
workings of an internal-combustion engine
described as an “explosion”? Yet again, the
words we use to describe things can confuse
rather than inform. Combustion rate
depends on the peak compression-induced
temperature, type of fuel, and strength of the
fuel/air mixture. Higher compression ratios
tend to start the combustion process sooner
and more quickly because of the greater
compression heating.
That is especially useful when an engine
is running at high rpm, because if it takes
too long to light the fire, the engine won’t
run well at high rpm. Conversely, good lowrpm
operation tolerates a wider range of
compression ratios.
Nitro content has an interesting effect.
For one thing, adding more nitro to your
fuel behaves similar to adding compression;
ignition occurs earlier, but the difference is
that nitro prolongs the burn.
That is because nitro releases oxygen as
it burns, allowing the remaining alcohol and
nitro to burn hotter. This makes more
horsepower, provided—and this is a big
“if”—the fuel/air mixture is rich enough. If
it is lean, combustion is quick and
extraordinarily hot.
Combine a too-high compression ratio
with too much nitro and a lean needle-valve
setting, and you get the model airplane
engine version of a thermonuclear
meltdown. Nothing hurts my ears the way a
death wail of tortured metal does.
The air density is a contributor too. Low
barometer readings mean that there is less
air to compress in the cylinder. Leaning the
needle valve a little bit restores the proper
fuel/air mix and the leaner mixture burns
more quickly, so that the combustion still
happens at approximately the right time.
Still, this means less torque, even when the
ignition is just right.
When higher-density air returns, failure
to fatten the needle will make for more
trouble. Low-barometer days don’t have to
mean lost performance. More about that
later.
Glow plugs come in a variety of heat
ranges. That mostly has to do with the plug
construction. If there is a longer, skinnier
path of metal between the platinum element
and the relatively cool metal of the cylinder
head, the plug’s element will run hotter,
helping promote the start of combustion.
The same is true when thinner-gauge
wire is used for the element itself, especially
in conjunction with a larger number of coils.
The center of the wire runs hotter than the
ends because of the heat-sinking action of
the glow plug body.
If you hook up your glow driver to a
plug and look at it in the sunlight, you’ll see
that only the middle glows hot enough to be
seen until you create some shade with your
hand. Cooler plugs generally have thicker
elements with fewer coils.
If the catalytic element runs too cool, it
does its job poorly. If it gets too hot, the
surface gets corrupted. Tiny bits of metal
from inside the engine bond to the surface
of that expensive platinum alloy, and all of a
sudden it isn’t a good catalyst anymore.
As it turns out, that happens to all glow
plugs with running time. And even before a
plug burns out completely, you might lose
the reliability of the idle or the needle valve
will become inconsistent.
The telltale sign of a bad plug is the
surface of the platinum wire having a lumpy
grayish-white appearance rather than a
shine. (Even a soft shine is okay.) This is
tough to describe adequately, but I spent
most of the evening a few days ago trying to
take pictures of a worn-out glow plug with a
magnifying glass and bright light. I needed a
photomicroscope similar to the one I had at
an old job for taking pictures of tiny
microcircuits.
Eventually I gave up. I couldn’t produce
a picture that would help you tell the
difference between merely used and used
up.
In the case of extreme heat, and maybe
slight engine vibration, the element will
self-destruct. Sometimes it leaves the engine
without much fuss, but just as often it makes
it only halfway out of the exhaust port
before the piston comes up and tries to chop
it in half. The piston, ring, and cylinder liner
usually loses this battle, and the resulting
scratch kills the compression. Replacement
parts aren’t cheap.
If you blow glow plugs every 10 or 20
flights, it’s time to change to the nextcooler-
range plug or set the needle valve
richer.
The needle valve is the 900-pound
gorilla in all of this. If you set it a bit rich,
the combustion process starts late and
progresses extra slowly because the excess
fuel has to be heated to get it to burn. I
already described the consequences of an
overly lean setting: even the glow plug
element gets too hot to do its job properly.
What happens when the weather changes
considerably, such as when comfortable
spring days give way to the heat of summer
and then back to fall? For one thing,
summer heat means less dense air, both for
the wings and engine with which to work.
This means lost performance.
The first temptation is to regain that
performance by leaning the needle valve a
bit more. But higher temperatures already
mean that the combustion process starts
earlier and gets hotter, so even if you don’t
set the engine too lean, you will wear out
glow plugs prematurely.
This happened to me recently. Two
Saturdays ago I flew maybe a half-dozen
flights with the old Tiger and its normally
dead-reliable O.S. .91 four-stroke. On the
first flight the next day, the engine quit as I
did a spin.
Okay, I thought, I’ll speed up the idle
100 rpm, and that problem will go away.
But when it quit in the same place on the
next flight, I started looking for the real
cause.
The throttle servo was steady, the engine
mount was not loose, and the fuel flow was
fine, so I removed the plug. Sure enough, it
was frosty-white and a little bit lumpy in
appearance. With the fresh four-stroke plug,
I was back to a reliable 1,600 rpm idle.
A cooler-heat-range plug, in conjunction
with a switch to 5% or 10% more nitro in the
fuel might have been a good idea—provided
I was careful to run slightly rich, because the
Tiger’s 91/4-pound weight hinders vertical
performance in the summer heat.
That’s what I often used to do with my
two-strokes. When the weather got hot, I
would “tip the nitro can” and set the engine
rich before I started it. If it liked a hot plug in
the cool weather, I’d change it for a plug that
was one heat range cooler, in the heat.
If you look at listings for different glow
plug manufacturers, you’ll see that they are
rated hot to cold. Between one brand and
another, you are on your own: it’s not in their
interest for you to be able to easily compare
them. Hmm.
I’ve mentioned the idle, but the other
thing that changes with summer heat is the
idle mixture. It typically has to be leaned out
a tiny amount as warm weather hits, and it
must be fattened back up when the
temperature drops. Getting the idle mixture
setting right tends to frustrate many; it isn’t
as easy as listening for the peak at full
throttle.
If you are having problems with the idle
to full-throttle transition, or with prolonged
idling, a simple test will help you correctly
set the idle mixture. Remove the cowl if
necessary, but you must be able to reach the
fuel line to the carburetor while the engine is
running.
Set the high end, and then slowly reduce
the engine to idle. At first you might have to
settle for a slightly faster idle to keep the
engine running, but that is okay. Pinch the
fuel line for a full two or three seconds. This
is unlike the full-throttle pinch test that some
people do.
If the engine sags straight away, the idle
mixture is too lean. If it speeds up
immediately, the idle mixture is too rich.
Proper idle mixtures will normally continue
to run exactly the same for a good three or so
seconds after you have pinched the fuel line,
and then the rpm will rise no more than 100
or 200 rpm immediately before the engine
quickly sags and quits. If it speeds up more
than that, it is still too rich at idle.
If you have set the idle mixture this way
and the engine still quits cold and rich or
blubbers rich when the throttle is advanced to
full, you probably need a hotter glow plug.
This is where many pilots get frustrated; the
choice of glow plug is often made by price
rather than heat range.
When cool weather returns, the engine
will need to be richened, both at idle and topend,
so don’t trust the settings simply
because they worked during the last outing.
At the start of each flying day I suggest that
you open, or richen, the needle the same
three or four clicks (or one-quarter turn).
After fully warming up the engine, listen
carefully for the perfect setting while slowly
leaning the mixture.
The worst that can happen is that you
confirm the older setting. The best is that you
catch a bad setting before it ruins an engine
or airplane. The important thing is that you
set the needle starting from an obviously rich
condition. These engines respond strongly to
small changes in weather, so take the time to
develop that first-start engine-setting ritual.
If you want to spend some time reading
about compression heating, visit the NASA
Web site. In just a few pages it describes the
relationship between the temperature,
pressure, and volume of a gas.
If you want to delve more deeply into the
subject, try the HyperPhysics Web site
address. There you can read descriptions of
how adiabatic compression produces even
higher temperatures and pressures, but it
starts to make use of more complicated
math. MA
Sources:
Compression heating:
http://bit.ly/bEjWso
HyperPhysics:
http://bit.ly/9QeKVW
Edition: Model Aviation - 2010/10
Page Numbers: 70,71,73
Edition: Model Aviation - 2010/10
Page Numbers: 70,71,73
70 MODEL AVIATION
The elements of glow-engine adjustment
Dean Pappas | DeanF3AF2B@If It Flies ... pappasfamily.net
HI, GANG. The weather is a never-ending
source of inspiration. This time it was a
string of warm flying days that reminded
me of airframe and engine setup
adjustments that come with the hot days of
summer. I made few adjustments to the
airplane itself as weather changes, and most
of them were part of the fine trimming
needed for Aerobatics competition.
Since I haven’t competed in years, I
don’t have much use for that sort of detail
fussing, but changes in weather still require
adjustments to keep the engine running
powerfully and reliably. The need for
reliable horsepower never changes.
Glow and, to a lesser extent, gas/ignition
engines respond dramatically to substantial
weather changes. As I mentioned, this
subject came to mind with the onset of
warm but tolerable flying weather, but this
week has seen unrelenting 100° days that
make you wait patiently for that little bit of
relief just before sunset.
Despite the heat, it is the nature of the
magazine business that this will be
published in an issue labeled “October,”
that many of you will read in September.
That is good, because many of the changes
we make for hot weather have to be undone
when summer ends. I might get ahead of
the curve.
Language is a funny thing. The names
we give things sometimes describe them
very well, and other times they confuse you
or hide what is actually happening. Take
the term “Figure Skating,” for instance.
You might have seen it on television.
Did you know that the event took its
name from the first round of the
competition, in which skaters had to
perform precise figures such as circles,
figure eights, intersections, and
combinations of them? Then the judges
would walk out and closely inspect the
skate marks in the ice to assign scores.
The entire “figures” round was dropped
from the event maybe 40 years ago. It was
mind-numbingly boring to watch. The
“figures” have been gone since before half
of the world’s population was born, yet we
still call the event by that name.
I guess it’s what you’d call tradition. As
far as I can tell, the real question is whether
or not it should be called “artistic skating”
or “athletic skating.”
“Glow ignition” is a similarly
misleading term. Thermal or
thermocatalytic ignition is more
descriptive, and in some languages that’s
exactly what glow ignition translates as.
Yes, I’ve had enough of the vocabulary
lesson too!
A properly controlled spark-ignition
system is superior to glow ignition in all
regards but one: weight. When it comes to
flying, weight is the enemy. It always has
been. For that reason alone, the
combination of alcohol fuel and a hot
catalytic metal glow plug has been
dominant in aeromodeling starting nearly
60 years ago.
So much has changed since the advent
of the glow plug. Gasoline engines with
lightweight, reliable, electronically timed,
capacitive-discharge systems are available,
and even electric power plants threaten to
topple the glow plug from its lofty perch.
But still, more glow engines are used at
the flying field every day than anything
else. Their care and feeding is an acquired
skill, and it perplexes some of us
aeromodeling “lifers” that some good pilots
out there haven’t learned it.
The glow plug is a deceptively simple
device. Simplicity does not mean that all
glow plugs are the same; one size does not
fit all, as the old Frank Zappa song title
suggests.
Picking the right glow plug makes an
engine run better, but several variables
must be juggled in a good-running
combination. By paying proper attention to
managing those factors, we can avoid
unnecessary frustration.
This juggling act is far from difficult.
You actually have a fair bit of flexibility in
the variables, which are:
1. Compression ratio
2. Fuel and nitro content
3. Air density
4. Glow plug heat range
5. Fuel/air mixture or needle-valve setting
Of those, probably the most powerful is
the needle valve. Any setup can be ruined
by mishandling that dainty little thing. How
can such a small item be so important?
In a glow-ignition engine, the “fire” is
started when the fuel/air mixture is heated
by compression of the fuel/air mixture. The
heating of a gas that is suddenly
compressed is described as “adiabatic.”
That means that there is little or no heat
lost, at least in the short term.
The resulting sudden temperature rise is
then enough to start the combustion
process, with help from the catalytic action
of the glow plug element. A “catalyst” is a
substance (often a metal) whose surface
takes part in a chemical reaction in a way
that promotes the chemistry without being
changed in the process. (More about this in
a minute.)
The catalyst, in this case the hot surface
of a platinum-alloy coil, lowers the
activation-energy “hump,” so that the
Only 10 years ago, the idea that a meaningful percentage of model aviation enthusiasts
would not have owned a glow-ignition engine would have occurred to only a handful of
visionaries. The YS .61 shown used to power the author’s sport workhorse Tiger 60. The
old standby K&B 1-L glow plug was the ticket in the summer heat; when temps dropped
below 70°, a hotter plug such as the Enya #3 improved idle reliability.
10sig3.QXD_00MSTRPG.QXD 8/20/10 12:45 PM Page 70
sudden compression heating is able to light
the alcohol and nitro. This combustion
process does not take place all at once; it is
not an explosion in that sense of the word.
How many times have you heard the
workings of an internal-combustion engine
described as an “explosion”? Yet again, the
words we use to describe things can confuse
rather than inform. Combustion rate
depends on the peak compression-induced
temperature, type of fuel, and strength of the
fuel/air mixture. Higher compression ratios
tend to start the combustion process sooner
and more quickly because of the greater
compression heating.
That is especially useful when an engine
is running at high rpm, because if it takes
too long to light the fire, the engine won’t
run well at high rpm. Conversely, good lowrpm
operation tolerates a wider range of
compression ratios.
Nitro content has an interesting effect.
For one thing, adding more nitro to your
fuel behaves similar to adding compression;
ignition occurs earlier, but the difference is
that nitro prolongs the burn.
That is because nitro releases oxygen as
it burns, allowing the remaining alcohol and
nitro to burn hotter. This makes more
horsepower, provided—and this is a big
“if”—the fuel/air mixture is rich enough. If
it is lean, combustion is quick and
extraordinarily hot.
Combine a too-high compression ratio
with too much nitro and a lean needle-valve
setting, and you get the model airplane
engine version of a thermonuclear
meltdown. Nothing hurts my ears the way a
death wail of tortured metal does.
The air density is a contributor too. Low
barometer readings mean that there is less
air to compress in the cylinder. Leaning the
needle valve a little bit restores the proper
fuel/air mix and the leaner mixture burns
more quickly, so that the combustion still
happens at approximately the right time.
Still, this means less torque, even when the
ignition is just right.
When higher-density air returns, failure
to fatten the needle will make for more
trouble. Low-barometer days don’t have to
mean lost performance. More about that
later.
Glow plugs come in a variety of heat
ranges. That mostly has to do with the plug
construction. If there is a longer, skinnier
path of metal between the platinum element
and the relatively cool metal of the cylinder
head, the plug’s element will run hotter,
helping promote the start of combustion.
The same is true when thinner-gauge
wire is used for the element itself, especially
in conjunction with a larger number of coils.
The center of the wire runs hotter than the
ends because of the heat-sinking action of
the glow plug body.
If you hook up your glow driver to a
plug and look at it in the sunlight, you’ll see
that only the middle glows hot enough to be
seen until you create some shade with your
hand. Cooler plugs generally have thicker
elements with fewer coils.
If the catalytic element runs too cool, it
does its job poorly. If it gets too hot, the
surface gets corrupted. Tiny bits of metal
from inside the engine bond to the surface
of that expensive platinum alloy, and all of a
sudden it isn’t a good catalyst anymore.
As it turns out, that happens to all glow
plugs with running time. And even before a
plug burns out completely, you might lose
the reliability of the idle or the needle valve
will become inconsistent.
The telltale sign of a bad plug is the
surface of the platinum wire having a lumpy
grayish-white appearance rather than a
shine. (Even a soft shine is okay.) This is
tough to describe adequately, but I spent
most of the evening a few days ago trying to
take pictures of a worn-out glow plug with a
magnifying glass and bright light. I needed a
photomicroscope similar to the one I had at
an old job for taking pictures of tiny
microcircuits.
Eventually I gave up. I couldn’t produce
a picture that would help you tell the
difference between merely used and used
up.
In the case of extreme heat, and maybe
slight engine vibration, the element will
self-destruct. Sometimes it leaves the engine
without much fuss, but just as often it makes
it only halfway out of the exhaust port
before the piston comes up and tries to chop
it in half. The piston, ring, and cylinder liner
usually loses this battle, and the resulting
scratch kills the compression. Replacement
parts aren’t cheap.
If you blow glow plugs every 10 or 20
flights, it’s time to change to the nextcooler-
range plug or set the needle valve
richer.
The needle valve is the 900-pound
gorilla in all of this. If you set it a bit rich,
the combustion process starts late and
progresses extra slowly because the excess
fuel has to be heated to get it to burn. I
already described the consequences of an
overly lean setting: even the glow plug
element gets too hot to do its job properly.
What happens when the weather changes
considerably, such as when comfortable
spring days give way to the heat of summer
and then back to fall? For one thing,
summer heat means less dense air, both for
the wings and engine with which to work.
This means lost performance.
The first temptation is to regain that
performance by leaning the needle valve a
bit more. But higher temperatures already
mean that the combustion process starts
earlier and gets hotter, so even if you don’t
set the engine too lean, you will wear out
glow plugs prematurely.
This happened to me recently. Two
Saturdays ago I flew maybe a half-dozen
flights with the old Tiger and its normally
dead-reliable O.S. .91 four-stroke. On the
first flight the next day, the engine quit as I
did a spin.
Okay, I thought, I’ll speed up the idle
100 rpm, and that problem will go away.
But when it quit in the same place on the
next flight, I started looking for the real
cause.
The throttle servo was steady, the engine
mount was not loose, and the fuel flow was
fine, so I removed the plug. Sure enough, it
was frosty-white and a little bit lumpy in
appearance. With the fresh four-stroke plug,
I was back to a reliable 1,600 rpm idle.
A cooler-heat-range plug, in conjunction
with a switch to 5% or 10% more nitro in the
fuel might have been a good idea—provided
I was careful to run slightly rich, because the
Tiger’s 91/4-pound weight hinders vertical
performance in the summer heat.
That’s what I often used to do with my
two-strokes. When the weather got hot, I
would “tip the nitro can” and set the engine
rich before I started it. If it liked a hot plug in
the cool weather, I’d change it for a plug that
was one heat range cooler, in the heat.
If you look at listings for different glow
plug manufacturers, you’ll see that they are
rated hot to cold. Between one brand and
another, you are on your own: it’s not in their
interest for you to be able to easily compare
them. Hmm.
I’ve mentioned the idle, but the other
thing that changes with summer heat is the
idle mixture. It typically has to be leaned out
a tiny amount as warm weather hits, and it
must be fattened back up when the
temperature drops. Getting the idle mixture
setting right tends to frustrate many; it isn’t
as easy as listening for the peak at full
throttle.
If you are having problems with the idle
to full-throttle transition, or with prolonged
idling, a simple test will help you correctly
set the idle mixture. Remove the cowl if
necessary, but you must be able to reach the
fuel line to the carburetor while the engine is
running.
Set the high end, and then slowly reduce
the engine to idle. At first you might have to
settle for a slightly faster idle to keep the
engine running, but that is okay. Pinch the
fuel line for a full two or three seconds. This
is unlike the full-throttle pinch test that some
people do.
If the engine sags straight away, the idle
mixture is too lean. If it speeds up
immediately, the idle mixture is too rich.
Proper idle mixtures will normally continue
to run exactly the same for a good three or so
seconds after you have pinched the fuel line,
and then the rpm will rise no more than 100
or 200 rpm immediately before the engine
quickly sags and quits. If it speeds up more
than that, it is still too rich at idle.
If you have set the idle mixture this way
and the engine still quits cold and rich or
blubbers rich when the throttle is advanced to
full, you probably need a hotter glow plug.
This is where many pilots get frustrated; the
choice of glow plug is often made by price
rather than heat range.
When cool weather returns, the engine
will need to be richened, both at idle and topend,
so don’t trust the settings simply
because they worked during the last outing.
At the start of each flying day I suggest that
you open, or richen, the needle the same
three or four clicks (or one-quarter turn).
After fully warming up the engine, listen
carefully for the perfect setting while slowly
leaning the mixture.
The worst that can happen is that you
confirm the older setting. The best is that you
catch a bad setting before it ruins an engine
or airplane. The important thing is that you
set the needle starting from an obviously rich
condition. These engines respond strongly to
small changes in weather, so take the time to
develop that first-start engine-setting ritual.
If you want to spend some time reading
about compression heating, visit the NASA
Web site. In just a few pages it describes the
relationship between the temperature,
pressure, and volume of a gas.
If you want to delve more deeply into the
subject, try the HyperPhysics Web site
address. There you can read descriptions of
how adiabatic compression produces even
higher temperatures and pressures, but it
starts to make use of more complicated
math. MA
Sources:
Compression heating:
http://bit.ly/bEjWso
HyperPhysics:
http://bit.ly/9QeKVW
Edition: Model Aviation - 2010/10
Page Numbers: 70,71,73
70 MODEL AVIATION
The elements of glow-engine adjustment
Dean Pappas | DeanF3AF2B@If It Flies ... pappasfamily.net
HI, GANG. The weather is a never-ending
source of inspiration. This time it was a
string of warm flying days that reminded
me of airframe and engine setup
adjustments that come with the hot days of
summer. I made few adjustments to the
airplane itself as weather changes, and most
of them were part of the fine trimming
needed for Aerobatics competition.
Since I haven’t competed in years, I
don’t have much use for that sort of detail
fussing, but changes in weather still require
adjustments to keep the engine running
powerfully and reliably. The need for
reliable horsepower never changes.
Glow and, to a lesser extent, gas/ignition
engines respond dramatically to substantial
weather changes. As I mentioned, this
subject came to mind with the onset of
warm but tolerable flying weather, but this
week has seen unrelenting 100° days that
make you wait patiently for that little bit of
relief just before sunset.
Despite the heat, it is the nature of the
magazine business that this will be
published in an issue labeled “October,”
that many of you will read in September.
That is good, because many of the changes
we make for hot weather have to be undone
when summer ends. I might get ahead of
the curve.
Language is a funny thing. The names
we give things sometimes describe them
very well, and other times they confuse you
or hide what is actually happening. Take
the term “Figure Skating,” for instance.
You might have seen it on television.
Did you know that the event took its
name from the first round of the
competition, in which skaters had to
perform precise figures such as circles,
figure eights, intersections, and
combinations of them? Then the judges
would walk out and closely inspect the
skate marks in the ice to assign scores.
The entire “figures” round was dropped
from the event maybe 40 years ago. It was
mind-numbingly boring to watch. The
“figures” have been gone since before half
of the world’s population was born, yet we
still call the event by that name.
I guess it’s what you’d call tradition. As
far as I can tell, the real question is whether
or not it should be called “artistic skating”
or “athletic skating.”
“Glow ignition” is a similarly
misleading term. Thermal or
thermocatalytic ignition is more
descriptive, and in some languages that’s
exactly what glow ignition translates as.
Yes, I’ve had enough of the vocabulary
lesson too!
A properly controlled spark-ignition
system is superior to glow ignition in all
regards but one: weight. When it comes to
flying, weight is the enemy. It always has
been. For that reason alone, the
combination of alcohol fuel and a hot
catalytic metal glow plug has been
dominant in aeromodeling starting nearly
60 years ago.
So much has changed since the advent
of the glow plug. Gasoline engines with
lightweight, reliable, electronically timed,
capacitive-discharge systems are available,
and even electric power plants threaten to
topple the glow plug from its lofty perch.
But still, more glow engines are used at
the flying field every day than anything
else. Their care and feeding is an acquired
skill, and it perplexes some of us
aeromodeling “lifers” that some good pilots
out there haven’t learned it.
The glow plug is a deceptively simple
device. Simplicity does not mean that all
glow plugs are the same; one size does not
fit all, as the old Frank Zappa song title
suggests.
Picking the right glow plug makes an
engine run better, but several variables
must be juggled in a good-running
combination. By paying proper attention to
managing those factors, we can avoid
unnecessary frustration.
This juggling act is far from difficult.
You actually have a fair bit of flexibility in
the variables, which are:
1. Compression ratio
2. Fuel and nitro content
3. Air density
4. Glow plug heat range
5. Fuel/air mixture or needle-valve setting
Of those, probably the most powerful is
the needle valve. Any setup can be ruined
by mishandling that dainty little thing. How
can such a small item be so important?
In a glow-ignition engine, the “fire” is
started when the fuel/air mixture is heated
by compression of the fuel/air mixture. The
heating of a gas that is suddenly
compressed is described as “adiabatic.”
That means that there is little or no heat
lost, at least in the short term.
The resulting sudden temperature rise is
then enough to start the combustion
process, with help from the catalytic action
of the glow plug element. A “catalyst” is a
substance (often a metal) whose surface
takes part in a chemical reaction in a way
that promotes the chemistry without being
changed in the process. (More about this in
a minute.)
The catalyst, in this case the hot surface
of a platinum-alloy coil, lowers the
activation-energy “hump,” so that the
Only 10 years ago, the idea that a meaningful percentage of model aviation enthusiasts
would not have owned a glow-ignition engine would have occurred to only a handful of
visionaries. The YS .61 shown used to power the author’s sport workhorse Tiger 60. The
old standby K&B 1-L glow plug was the ticket in the summer heat; when temps dropped
below 70°, a hotter plug such as the Enya #3 improved idle reliability.
10sig3.QXD_00MSTRPG.QXD 8/20/10 12:45 PM Page 70
sudden compression heating is able to light
the alcohol and nitro. This combustion
process does not take place all at once; it is
not an explosion in that sense of the word.
How many times have you heard the
workings of an internal-combustion engine
described as an “explosion”? Yet again, the
words we use to describe things can confuse
rather than inform. Combustion rate
depends on the peak compression-induced
temperature, type of fuel, and strength of the
fuel/air mixture. Higher compression ratios
tend to start the combustion process sooner
and more quickly because of the greater
compression heating.
That is especially useful when an engine
is running at high rpm, because if it takes
too long to light the fire, the engine won’t
run well at high rpm. Conversely, good lowrpm
operation tolerates a wider range of
compression ratios.
Nitro content has an interesting effect.
For one thing, adding more nitro to your
fuel behaves similar to adding compression;
ignition occurs earlier, but the difference is
that nitro prolongs the burn.
That is because nitro releases oxygen as
it burns, allowing the remaining alcohol and
nitro to burn hotter. This makes more
horsepower, provided—and this is a big
“if”—the fuel/air mixture is rich enough. If
it is lean, combustion is quick and
extraordinarily hot.
Combine a too-high compression ratio
with too much nitro and a lean needle-valve
setting, and you get the model airplane
engine version of a thermonuclear
meltdown. Nothing hurts my ears the way a
death wail of tortured metal does.
The air density is a contributor too. Low
barometer readings mean that there is less
air to compress in the cylinder. Leaning the
needle valve a little bit restores the proper
fuel/air mix and the leaner mixture burns
more quickly, so that the combustion still
happens at approximately the right time.
Still, this means less torque, even when the
ignition is just right.
When higher-density air returns, failure
to fatten the needle will make for more
trouble. Low-barometer days don’t have to
mean lost performance. More about that
later.
Glow plugs come in a variety of heat
ranges. That mostly has to do with the plug
construction. If there is a longer, skinnier
path of metal between the platinum element
and the relatively cool metal of the cylinder
head, the plug’s element will run hotter,
helping promote the start of combustion.
The same is true when thinner-gauge
wire is used for the element itself, especially
in conjunction with a larger number of coils.
The center of the wire runs hotter than the
ends because of the heat-sinking action of
the glow plug body.
If you hook up your glow driver to a
plug and look at it in the sunlight, you’ll see
that only the middle glows hot enough to be
seen until you create some shade with your
hand. Cooler plugs generally have thicker
elements with fewer coils.
If the catalytic element runs too cool, it
does its job poorly. If it gets too hot, the
surface gets corrupted. Tiny bits of metal
from inside the engine bond to the surface
of that expensive platinum alloy, and all of a
sudden it isn’t a good catalyst anymore.
As it turns out, that happens to all glow
plugs with running time. And even before a
plug burns out completely, you might lose
the reliability of the idle or the needle valve
will become inconsistent.
The telltale sign of a bad plug is the
surface of the platinum wire having a lumpy
grayish-white appearance rather than a
shine. (Even a soft shine is okay.) This is
tough to describe adequately, but I spent
most of the evening a few days ago trying to
take pictures of a worn-out glow plug with a
magnifying glass and bright light. I needed a
photomicroscope similar to the one I had at
an old job for taking pictures of tiny
microcircuits.
Eventually I gave up. I couldn’t produce
a picture that would help you tell the
difference between merely used and used
up.
In the case of extreme heat, and maybe
slight engine vibration, the element will
self-destruct. Sometimes it leaves the engine
without much fuss, but just as often it makes
it only halfway out of the exhaust port
before the piston comes up and tries to chop
it in half. The piston, ring, and cylinder liner
usually loses this battle, and the resulting
scratch kills the compression. Replacement
parts aren’t cheap.
If you blow glow plugs every 10 or 20
flights, it’s time to change to the nextcooler-
range plug or set the needle valve
richer.
The needle valve is the 900-pound
gorilla in all of this. If you set it a bit rich,
the combustion process starts late and
progresses extra slowly because the excess
fuel has to be heated to get it to burn. I
already described the consequences of an
overly lean setting: even the glow plug
element gets too hot to do its job properly.
What happens when the weather changes
considerably, such as when comfortable
spring days give way to the heat of summer
and then back to fall? For one thing,
summer heat means less dense air, both for
the wings and engine with which to work.
This means lost performance.
The first temptation is to regain that
performance by leaning the needle valve a
bit more. But higher temperatures already
mean that the combustion process starts
earlier and gets hotter, so even if you don’t
set the engine too lean, you will wear out
glow plugs prematurely.
This happened to me recently. Two
Saturdays ago I flew maybe a half-dozen
flights with the old Tiger and its normally
dead-reliable O.S. .91 four-stroke. On the
first flight the next day, the engine quit as I
did a spin.
Okay, I thought, I’ll speed up the idle
100 rpm, and that problem will go away.
But when it quit in the same place on the
next flight, I started looking for the real
cause.
The throttle servo was steady, the engine
mount was not loose, and the fuel flow was
fine, so I removed the plug. Sure enough, it
was frosty-white and a little bit lumpy in
appearance. With the fresh four-stroke plug,
I was back to a reliable 1,600 rpm idle.
A cooler-heat-range plug, in conjunction
with a switch to 5% or 10% more nitro in the
fuel might have been a good idea—provided
I was careful to run slightly rich, because the
Tiger’s 91/4-pound weight hinders vertical
performance in the summer heat.
That’s what I often used to do with my
two-strokes. When the weather got hot, I
would “tip the nitro can” and set the engine
rich before I started it. If it liked a hot plug in
the cool weather, I’d change it for a plug that
was one heat range cooler, in the heat.
If you look at listings for different glow
plug manufacturers, you’ll see that they are
rated hot to cold. Between one brand and
another, you are on your own: it’s not in their
interest for you to be able to easily compare
them. Hmm.
I’ve mentioned the idle, but the other
thing that changes with summer heat is the
idle mixture. It typically has to be leaned out
a tiny amount as warm weather hits, and it
must be fattened back up when the
temperature drops. Getting the idle mixture
setting right tends to frustrate many; it isn’t
as easy as listening for the peak at full
throttle.
If you are having problems with the idle
to full-throttle transition, or with prolonged
idling, a simple test will help you correctly
set the idle mixture. Remove the cowl if
necessary, but you must be able to reach the
fuel line to the carburetor while the engine is
running.
Set the high end, and then slowly reduce
the engine to idle. At first you might have to
settle for a slightly faster idle to keep the
engine running, but that is okay. Pinch the
fuel line for a full two or three seconds. This
is unlike the full-throttle pinch test that some
people do.
If the engine sags straight away, the idle
mixture is too lean. If it speeds up
immediately, the idle mixture is too rich.
Proper idle mixtures will normally continue
to run exactly the same for a good three or so
seconds after you have pinched the fuel line,
and then the rpm will rise no more than 100
or 200 rpm immediately before the engine
quickly sags and quits. If it speeds up more
than that, it is still too rich at idle.
If you have set the idle mixture this way
and the engine still quits cold and rich or
blubbers rich when the throttle is advanced to
full, you probably need a hotter glow plug.
This is where many pilots get frustrated; the
choice of glow plug is often made by price
rather than heat range.
When cool weather returns, the engine
will need to be richened, both at idle and topend,
so don’t trust the settings simply
because they worked during the last outing.
At the start of each flying day I suggest that
you open, or richen, the needle the same
three or four clicks (or one-quarter turn).
After fully warming up the engine, listen
carefully for the perfect setting while slowly
leaning the mixture.
The worst that can happen is that you
confirm the older setting. The best is that you
catch a bad setting before it ruins an engine
or airplane. The important thing is that you
set the needle starting from an obviously rich
condition. These engines respond strongly to
small changes in weather, so take the time to
develop that first-start engine-setting ritual.
If you want to spend some time reading
about compression heating, visit the NASA
Web site. In just a few pages it describes the
relationship between the temperature,
pressure, and volume of a gas.
If you want to delve more deeply into the
subject, try the HyperPhysics Web site
address. There you can read descriptions of
how adiabatic compression produces even
higher temperatures and pressures, but it
starts to make use of more complicated
math. MA
Sources:
Compression heating:
http://bit.ly/bEjWso
HyperPhysics:
http://bit.ly/9QeKVW
