94 MODEL AVIATION
The classic 2-4 Stunt setup
Dean Pappas | DeanF3AF2B@If It Flies ... pappasfamily.net
Hi, gang. Every once in awhile we look
back on our own work with great
satisfaction—maybe it’s a particularly
good landing or a nice bit of woodwork in
the workshop. The satisfaction that comes
from taking a well-considered, critical look
at our pleasing work is real.
I’ll tell you what else is real. It’s that
little twinge of dissatisfaction when you
look back at something you did and it
doesn’t quite “measure up.” Fortunately
this provides motivation to get it right next
time.
I recently reread October’s “If It Flies … ”
and realized that my thoughts must have
been scattered. At least that’s the way it
looked to me upon reviewing it more than
a month after submission. How to make it
right?
I thought about that for a while. It
occurred to me that merely explaining
things a different way would work, but that
repeating myself would be a waste of your
time.
Then it hit me; explaining the basics of
a classic CL Precision Aerobatics (Stunt)
“run” would allow me to tie things
together, and it would also give me an
opportunity to write about something other
than RC while still helping fix the leaks in
the last column.
The classic “2-4 break” CL Stunt engine
setup is a marvel of subtle sophistication.
It’s a quintessential aeromodeler’s solution
to a difficult problem.
Long before computer radios,
autopilots, and all manner of electronic
wizardry, this simple-looking but
fiendishly clever arrangement with the
barest minimum moving parts, and nothing
that looks like a control system, evolved to
keep the speed of a CL Stunter “nearly
constant,” despite the vertical climbs and
dives of the Stunt pattern.
What’s a Stunt pattern? That’s a fair
question, especially if you’ve never seen it
flown. You can find the online AMA rule
book at the address that I have included in
the “Sources” listing.
The “old” rule book illustrations were
perspective drawings that showed how the
maneuvers were drawn on the surface of a
hemisphere; they show the schedule better
than the new “flat” drawings in the rule
book. The updated descriptions do a better
job of describing some of the geometric
nuances.
YouTube has numerous videos showing
CL Stunt patterns, although the quality of
the videography sometimes leaves much to
be desired. Search under “CL Stunt” and
“F2B,” and a mess of results pops up.
Several show world-class pilots flying. The
site addresses are under “Sources.”
One video depicts an electric-powered
F2B flight. It might seem strange to open
with this, but the film is unusual in that the
fixed camera installation was done properly
and in the right spot to see the entire
pattern flown well. The pilot is doing a
decent job of it too!
Because CL fliers often perform with
several circles next to each other, it is
difficult to film clips without the sound of
other models. I found a video of a pilot
who does not fly a “book” schedule, or
even rule book maneuvers, but the engine
setup is a classic Stunt run, with a deep
break in maneuvers.
If you listen carefully, you can even
hear the engine briefly break lean after the
corners of the square loops, precisely when
the airplane scrubs off speed in the corners.
“Nice Flying Footage of Kevin’s
Tsunami” shows a performance that does
not demonstrate a full pattern. The old
“G”-series SuperTigre light-case .60 has a
nice deep break between rich and lean.
Notice how the needle setting results in the
engine oscillating between a fast four-cycle
and a rich two-cycle while the pilot walks
out to the handle.
“Bill Werwage Classic Flight VSC14”
shows a “killer” engine run from the threetime
World Champion during the Vintage
Radio Control Society nostalgia meet.
Unfortunately other flights are taking place
at the same time, making this difficult
listening music, but the setup does
everything right in what looks to have been
windy conditions.
Al Rabe was world vice-champion in
1978. “C/L Stunt Al’s Snaggletooth” is
recent footage of him with his semiscale P-
51 “Snaggletooth.”
This video shows the low-break setup
described in the text. The benefit is that the
system is less laggy when responding to
changes in propeller load.
“Fitzgerald” illustrates the more
modern tuned-pipe Stunt setup, flown by
several-time World Champ Dave
Fitzgerald. It sounds different from the
classic 2-4 break setup and many betweenmaneuver
laps have been deleted, so the
clip is short.
In this video, the changes in scavenging
The choke area (the two crescent-shaped openings on either side of the spraybar) is
much smaller than that of the RC carburetor that is normally fitted to the O.S. FP40
shown. This is necessary to generate a strong rpm-dependent fuel draw needed to
control the engine’s running characteristics. The baked-on castor oil is a hint. Slurpythick
castor oil helps maintain the crankshaft/crankcase seal, especially in plain-bushed
power plants such as this. If you look carefully you can see the half-closed crankshaft
induction valve.
12sig3x.QXD_00MSTRPG.QXD 10/21/10 11:57 AM Page 94
and torque are dictated by exhaust tuning.
A crazy person must have invented this
setup!
If you watched and listened to all of the
preceding film clips (and maybe a few
others), did you notice how the engines
appear to lean out going uphill and then
richen and “go flat” coming downhill?
That’s what keeps the model from slowing
too much in a climb and then from coming
downhill in a screaming-fast dive.
You might even notice that the changes
sound like they happen a bit late compared
to when the airplane actually changes
direction. This is less than ideal, but, still,
this basic arrangement has worked well for
more than a half century.
What arrangement? For one thing,
notice that all of these engines sound
somewhat rich. The fuel-to-air mixtures are
not set for maximum rpm; instead they are
set where the torque that the power plant
delivers changes abruptly for very small
needle valve movements. This is “the
break.”
Just on the rich side of the break, the
rich mixture delays the onset of combustion
and excess fuel in the combustion chamber
keeps the maximum temperature and
cylinder pressure low. This means less
torque and, as a result, horsepower.
If you are poised just on the rich side of
the break, one lousy little click leaner on
the needle valve will change the whole
picture. The compression heating lights the
fire a bit sooner, and the combustion
process progresses more quickly.
The more advanced ignition timing
produces more torque. And if the rpm rises,
the compression heating effect is enhanced,
making even more torque and horsepower.
Do you remember how one film clip
showed the engine hopping back and forth
across the break? What’s with that?
The explanation for that is to describe
the other key to the classic 2-4 Stunt setup:
the small carburetor or venturi used with a
CL Stunt engine. All engines are air
pumps, and our single-cylinder two-strokes
are particularly interesting.
Think of them as two-stage air pumps.
Air is sucked into the crankcase through
the crankshaft valve every time the piston
is on the upstroke. Stunters have no need
for throttles, so they have simple fullthrottle-
only carburetors or venturis. They
are not actually so simple, but that is
another matter.
If you look into the venturi, or open
carburetor of an RC engine, you’ll see that
the crankshaft valve is completely open for
roughly one-eighth of a revolution before
the piston reaches TDC (Top Dead Center).
This is, in part, because it takes time to
accelerate the air through the venturi, down
the central passage in the crankshaft, and
up into the crankcase.
The crankshaft valve takes
approximately one-half of a revolution
from when it starts to open until it finally
closes. So by the time it does close, the
piston is already on its way downward and
squeezing the air/fuel mixture from the
crankcase upward through the bypasses
into the combustion chamber.
The momentum of the air/fuel mix
matters during this transition from
crankcase suction to compression.
At nearly the same time that this
transition occurs, the exhaust port opens
and exhaust gases are expelled at high
pressure and velocity. As a result of this
sudden acceleration, they have plenty of
momentum. And as the last of the exhaust
rushes out into the muffler, its momentum
keeps it moving even after the pressure in
the cylinder has dropped.
The result is actually a vacuum, and 10°
or 20° of rotation later the bypasses open,
allowing the intake mixture to be sucked
into the combustion chamber by the
vacuum that is left there.
That is called “scavenging.” And
depending on the time relationship of all
port and valve openings and closings, the
engine will have its best scavenging in
some rpm range that the manufacturer has
designed.
The venturi (remember the venturi?) is
typically designed to maximize the amount
of vacuum generated in it as the air is
sucked through it into the crankcase. This
is what draws the fuel into the engine in the
first place.
Assuming that the engine is running in
that rpm range in which the designer has
provided good scavenging, the vacuum that
draws in the fuel will be strongly
influenced by changes in rpm. More rpm
will draw a better vacuum in the venturi,
which will improve fuel draw and provide
more fuel.
If the cross-sectional area or opening of
the venturi is small enough, it will drive the
engine rich with increased rpm. If it is
slightly larger, it will simply be adequate
for the increased needs of the engine at
high rpm. If it is too large, the engine will
suffer from poor fuel draw during climbs
and maneuvers.
That is why you can’t always put a bigger
carburetor on an engine to get more
horsepower. It also explains why the tiniest air
leak in the crankcase will ruin the fuel draw.
Where are those potential leak sources?
Loose backplate screws, for one. Loose
screws or worn O-rings under the
carburetor/venturi will ruin things as well,
but the thing that causes the most trouble is
the hardest to see.
The crankshaft and the way it fits into
the crankcase create a long rotating seal.
Fuel and condensed oil between the two
parts form the seal. This works great if the
gaps are only two or three thousandths of
an inch!
Worn bushings or even worn/loose ball
bearings allow the shaft to wiggle and
break this oil seal. Sometimes a loose
bearing will allow a bit of the aluminum
crankcase to wear away, and even a bearing
replacement will not fix the problem. This
leads to the engine being unable to hold a
consistent mixture as it warms up.
So with a small venturi providing a
strong rpm-dependent fuel mixture, and the
break providing a strong change in torque
with small changes in mixture, we have the
makings of a stable system for keeping the
airplane’s flying speed within bounds
during maneuvers.
The rpm must increase some during
dives and drop during climbs, but the
change in torque that the break provides
allows the horsepower to increase despite
the reduced rpm in a climb. Similarly, the
reduction in torque as the rich mixture
cools the fire and retards the ignition
timing will reduce the delivered
horsepower despite increased rpm in dives.
If you go back and listen to some of
those on-line videos, you might be able to
pick out the difference between the “flat”
note of a rich mixture from the slight
increase in diving rpm that caused it. And
with a little more careful attention you can
pick the “clean” note during climbs apart
from the slight rpm drop.
The first time you heard it, the
dominance of high harmonics in the
exhaust note might have convinced you
that the rpm is higher during the climb, but
that isn’t the case. In-flight measurements
with data loggers have proven this to be the
case.
If you take an engine that has been set
up this way and increase the compression
ratio slightly, it will “break” at slightly
lower rpm and with a slightly richer needle
valve setting. The change in torque that the
break provides will be greater too, but the
engine will also lag behind the changes in rpm
more. Once the engine breaks lean, the added
power creates more heat, which tends to keep
it from richening as easily.
In the dive, after the engine breaks rich,
the cooling of just a few degrees caused by
the cooling effect of the added fuel will delay
the break back to lean. Add a little too much
compression, and the first time the engine
breaks lean, the added heat is enough to keep
it there until the entire tank is emptied.
That is what the Stunt pilots call a
“runaway,” and the strong-breaking setups
will run away if the needle valve is set only a
few clicks too lean. Yes, this is an important
skill for Stunt fliers!
If instead you drop the compression a bit,
you get slightly less break and an engine that
performs this bit of magic at higher rpm,
which can be used to run a lower-pitch
propeller. You can use those to provide dive
brakes instead of a deep break rich. This
describes the run of the Snaggletooth featured
in one of the videos.
Another time I should write about fuel tank
design. The Stunt pilots have a treasure trove
of knowledge here, and much of it is
genuinely useful in RC as well.
Right now I am going to organize some of
the work I’ve been doing recently on my true
constant-rpm electric-powered Stunt setup.
I’ll be back in the February 2011 issue.
Until then, have fun, and do take care of
yourself.
Want to learn more about two-stroke
scavenging? It’s quite a subject. A great
book that has been out of print for years is
Gordon Jennings’ Two-Stroke Tuner’s
Handbook. You can probably find online
copies in a variety of places; I have
included the Web site address for one
resource in the “Sources” listing. MA
Sources:
AMA CL Stunt rules:
http://bit.ly/db6WQt
Electric F2B video:
http://bit.ly/aqpz6x
“Nice Flying Footage of Kevin’s Tsunami”
http://bit.ly/awAeRr
“Bill Werwage Classic Flight VSC14”
http://bit.ly/9wLOJj
“C/L Stunt Al’s Snaggletooth”
http://bit.ly/9ZS8XX
“Fitzgerald”
http://bit.ly/c5payH
Two-Stroke Tuner’s Handbook
http://edj.net/2stroke/jennings
Edition: Model Aviation - 2010/12
Page Numbers: 94,95,96
Edition: Model Aviation - 2010/12
Page Numbers: 94,95,96
94 MODEL AVIATION
The classic 2-4 Stunt setup
Dean Pappas | DeanF3AF2B@If It Flies ... pappasfamily.net
Hi, gang. Every once in awhile we look
back on our own work with great
satisfaction—maybe it’s a particularly
good landing or a nice bit of woodwork in
the workshop. The satisfaction that comes
from taking a well-considered, critical look
at our pleasing work is real.
I’ll tell you what else is real. It’s that
little twinge of dissatisfaction when you
look back at something you did and it
doesn’t quite “measure up.” Fortunately
this provides motivation to get it right next
time.
I recently reread October’s “If It Flies … ”
and realized that my thoughts must have
been scattered. At least that’s the way it
looked to me upon reviewing it more than
a month after submission. How to make it
right?
I thought about that for a while. It
occurred to me that merely explaining
things a different way would work, but that
repeating myself would be a waste of your
time.
Then it hit me; explaining the basics of
a classic CL Precision Aerobatics (Stunt)
“run” would allow me to tie things
together, and it would also give me an
opportunity to write about something other
than RC while still helping fix the leaks in
the last column.
The classic “2-4 break” CL Stunt engine
setup is a marvel of subtle sophistication.
It’s a quintessential aeromodeler’s solution
to a difficult problem.
Long before computer radios,
autopilots, and all manner of electronic
wizardry, this simple-looking but
fiendishly clever arrangement with the
barest minimum moving parts, and nothing
that looks like a control system, evolved to
keep the speed of a CL Stunter “nearly
constant,” despite the vertical climbs and
dives of the Stunt pattern.
What’s a Stunt pattern? That’s a fair
question, especially if you’ve never seen it
flown. You can find the online AMA rule
book at the address that I have included in
the “Sources” listing.
The “old” rule book illustrations were
perspective drawings that showed how the
maneuvers were drawn on the surface of a
hemisphere; they show the schedule better
than the new “flat” drawings in the rule
book. The updated descriptions do a better
job of describing some of the geometric
nuances.
YouTube has numerous videos showing
CL Stunt patterns, although the quality of
the videography sometimes leaves much to
be desired. Search under “CL Stunt” and
“F2B,” and a mess of results pops up.
Several show world-class pilots flying. The
site addresses are under “Sources.”
One video depicts an electric-powered
F2B flight. It might seem strange to open
with this, but the film is unusual in that the
fixed camera installation was done properly
and in the right spot to see the entire
pattern flown well. The pilot is doing a
decent job of it too!
Because CL fliers often perform with
several circles next to each other, it is
difficult to film clips without the sound of
other models. I found a video of a pilot
who does not fly a “book” schedule, or
even rule book maneuvers, but the engine
setup is a classic Stunt run, with a deep
break in maneuvers.
If you listen carefully, you can even
hear the engine briefly break lean after the
corners of the square loops, precisely when
the airplane scrubs off speed in the corners.
“Nice Flying Footage of Kevin’s
Tsunami” shows a performance that does
not demonstrate a full pattern. The old
“G”-series SuperTigre light-case .60 has a
nice deep break between rich and lean.
Notice how the needle setting results in the
engine oscillating between a fast four-cycle
and a rich two-cycle while the pilot walks
out to the handle.
“Bill Werwage Classic Flight VSC14”
shows a “killer” engine run from the threetime
World Champion during the Vintage
Radio Control Society nostalgia meet.
Unfortunately other flights are taking place
at the same time, making this difficult
listening music, but the setup does
everything right in what looks to have been
windy conditions.
Al Rabe was world vice-champion in
1978. “C/L Stunt Al’s Snaggletooth” is
recent footage of him with his semiscale P-
51 “Snaggletooth.”
This video shows the low-break setup
described in the text. The benefit is that the
system is less laggy when responding to
changes in propeller load.
“Fitzgerald” illustrates the more
modern tuned-pipe Stunt setup, flown by
several-time World Champ Dave
Fitzgerald. It sounds different from the
classic 2-4 break setup and many betweenmaneuver
laps have been deleted, so the
clip is short.
In this video, the changes in scavenging
The choke area (the two crescent-shaped openings on either side of the spraybar) is
much smaller than that of the RC carburetor that is normally fitted to the O.S. FP40
shown. This is necessary to generate a strong rpm-dependent fuel draw needed to
control the engine’s running characteristics. The baked-on castor oil is a hint. Slurpythick
castor oil helps maintain the crankshaft/crankcase seal, especially in plain-bushed
power plants such as this. If you look carefully you can see the half-closed crankshaft
induction valve.
12sig3x.QXD_00MSTRPG.QXD 10/21/10 11:57 AM Page 94
and torque are dictated by exhaust tuning.
A crazy person must have invented this
setup!
If you watched and listened to all of the
preceding film clips (and maybe a few
others), did you notice how the engines
appear to lean out going uphill and then
richen and “go flat” coming downhill?
That’s what keeps the model from slowing
too much in a climb and then from coming
downhill in a screaming-fast dive.
You might even notice that the changes
sound like they happen a bit late compared
to when the airplane actually changes
direction. This is less than ideal, but, still,
this basic arrangement has worked well for
more than a half century.
What arrangement? For one thing,
notice that all of these engines sound
somewhat rich. The fuel-to-air mixtures are
not set for maximum rpm; instead they are
set where the torque that the power plant
delivers changes abruptly for very small
needle valve movements. This is “the
break.”
Just on the rich side of the break, the
rich mixture delays the onset of combustion
and excess fuel in the combustion chamber
keeps the maximum temperature and
cylinder pressure low. This means less
torque and, as a result, horsepower.
If you are poised just on the rich side of
the break, one lousy little click leaner on
the needle valve will change the whole
picture. The compression heating lights the
fire a bit sooner, and the combustion
process progresses more quickly.
The more advanced ignition timing
produces more torque. And if the rpm rises,
the compression heating effect is enhanced,
making even more torque and horsepower.
Do you remember how one film clip
showed the engine hopping back and forth
across the break? What’s with that?
The explanation for that is to describe
the other key to the classic 2-4 Stunt setup:
the small carburetor or venturi used with a
CL Stunt engine. All engines are air
pumps, and our single-cylinder two-strokes
are particularly interesting.
Think of them as two-stage air pumps.
Air is sucked into the crankcase through
the crankshaft valve every time the piston
is on the upstroke. Stunters have no need
for throttles, so they have simple fullthrottle-
only carburetors or venturis. They
are not actually so simple, but that is
another matter.
If you look into the venturi, or open
carburetor of an RC engine, you’ll see that
the crankshaft valve is completely open for
roughly one-eighth of a revolution before
the piston reaches TDC (Top Dead Center).
This is, in part, because it takes time to
accelerate the air through the venturi, down
the central passage in the crankshaft, and
up into the crankcase.
The crankshaft valve takes
approximately one-half of a revolution
from when it starts to open until it finally
closes. So by the time it does close, the
piston is already on its way downward and
squeezing the air/fuel mixture from the
crankcase upward through the bypasses
into the combustion chamber.
The momentum of the air/fuel mix
matters during this transition from
crankcase suction to compression.
At nearly the same time that this
transition occurs, the exhaust port opens
and exhaust gases are expelled at high
pressure and velocity. As a result of this
sudden acceleration, they have plenty of
momentum. And as the last of the exhaust
rushes out into the muffler, its momentum
keeps it moving even after the pressure in
the cylinder has dropped.
The result is actually a vacuum, and 10°
or 20° of rotation later the bypasses open,
allowing the intake mixture to be sucked
into the combustion chamber by the
vacuum that is left there.
That is called “scavenging.” And
depending on the time relationship of all
port and valve openings and closings, the
engine will have its best scavenging in
some rpm range that the manufacturer has
designed.
The venturi (remember the venturi?) is
typically designed to maximize the amount
of vacuum generated in it as the air is
sucked through it into the crankcase. This
is what draws the fuel into the engine in the
first place.
Assuming that the engine is running in
that rpm range in which the designer has
provided good scavenging, the vacuum that
draws in the fuel will be strongly
influenced by changes in rpm. More rpm
will draw a better vacuum in the venturi,
which will improve fuel draw and provide
more fuel.
If the cross-sectional area or opening of
the venturi is small enough, it will drive the
engine rich with increased rpm. If it is
slightly larger, it will simply be adequate
for the increased needs of the engine at
high rpm. If it is too large, the engine will
suffer from poor fuel draw during climbs
and maneuvers.
That is why you can’t always put a bigger
carburetor on an engine to get more
horsepower. It also explains why the tiniest air
leak in the crankcase will ruin the fuel draw.
Where are those potential leak sources?
Loose backplate screws, for one. Loose
screws or worn O-rings under the
carburetor/venturi will ruin things as well,
but the thing that causes the most trouble is
the hardest to see.
The crankshaft and the way it fits into
the crankcase create a long rotating seal.
Fuel and condensed oil between the two
parts form the seal. This works great if the
gaps are only two or three thousandths of
an inch!
Worn bushings or even worn/loose ball
bearings allow the shaft to wiggle and
break this oil seal. Sometimes a loose
bearing will allow a bit of the aluminum
crankcase to wear away, and even a bearing
replacement will not fix the problem. This
leads to the engine being unable to hold a
consistent mixture as it warms up.
So with a small venturi providing a
strong rpm-dependent fuel mixture, and the
break providing a strong change in torque
with small changes in mixture, we have the
makings of a stable system for keeping the
airplane’s flying speed within bounds
during maneuvers.
The rpm must increase some during
dives and drop during climbs, but the
change in torque that the break provides
allows the horsepower to increase despite
the reduced rpm in a climb. Similarly, the
reduction in torque as the rich mixture
cools the fire and retards the ignition
timing will reduce the delivered
horsepower despite increased rpm in dives.
If you go back and listen to some of
those on-line videos, you might be able to
pick out the difference between the “flat”
note of a rich mixture from the slight
increase in diving rpm that caused it. And
with a little more careful attention you can
pick the “clean” note during climbs apart
from the slight rpm drop.
The first time you heard it, the
dominance of high harmonics in the
exhaust note might have convinced you
that the rpm is higher during the climb, but
that isn’t the case. In-flight measurements
with data loggers have proven this to be the
case.
If you take an engine that has been set
up this way and increase the compression
ratio slightly, it will “break” at slightly
lower rpm and with a slightly richer needle
valve setting. The change in torque that the
break provides will be greater too, but the
engine will also lag behind the changes in rpm
more. Once the engine breaks lean, the added
power creates more heat, which tends to keep
it from richening as easily.
In the dive, after the engine breaks rich,
the cooling of just a few degrees caused by
the cooling effect of the added fuel will delay
the break back to lean. Add a little too much
compression, and the first time the engine
breaks lean, the added heat is enough to keep
it there until the entire tank is emptied.
That is what the Stunt pilots call a
“runaway,” and the strong-breaking setups
will run away if the needle valve is set only a
few clicks too lean. Yes, this is an important
skill for Stunt fliers!
If instead you drop the compression a bit,
you get slightly less break and an engine that
performs this bit of magic at higher rpm,
which can be used to run a lower-pitch
propeller. You can use those to provide dive
brakes instead of a deep break rich. This
describes the run of the Snaggletooth featured
in one of the videos.
Another time I should write about fuel tank
design. The Stunt pilots have a treasure trove
of knowledge here, and much of it is
genuinely useful in RC as well.
Right now I am going to organize some of
the work I’ve been doing recently on my true
constant-rpm electric-powered Stunt setup.
I’ll be back in the February 2011 issue.
Until then, have fun, and do take care of
yourself.
Want to learn more about two-stroke
scavenging? It’s quite a subject. A great
book that has been out of print for years is
Gordon Jennings’ Two-Stroke Tuner’s
Handbook. You can probably find online
copies in a variety of places; I have
included the Web site address for one
resource in the “Sources” listing. MA
Sources:
AMA CL Stunt rules:
http://bit.ly/db6WQt
Electric F2B video:
http://bit.ly/aqpz6x
“Nice Flying Footage of Kevin’s Tsunami”
http://bit.ly/awAeRr
“Bill Werwage Classic Flight VSC14”
http://bit.ly/9wLOJj
“C/L Stunt Al’s Snaggletooth”
http://bit.ly/9ZS8XX
“Fitzgerald”
http://bit.ly/c5payH
Two-Stroke Tuner’s Handbook
http://edj.net/2stroke/jennings
Edition: Model Aviation - 2010/12
Page Numbers: 94,95,96
94 MODEL AVIATION
The classic 2-4 Stunt setup
Dean Pappas | DeanF3AF2B@If It Flies ... pappasfamily.net
Hi, gang. Every once in awhile we look
back on our own work with great
satisfaction—maybe it’s a particularly
good landing or a nice bit of woodwork in
the workshop. The satisfaction that comes
from taking a well-considered, critical look
at our pleasing work is real.
I’ll tell you what else is real. It’s that
little twinge of dissatisfaction when you
look back at something you did and it
doesn’t quite “measure up.” Fortunately
this provides motivation to get it right next
time.
I recently reread October’s “If It Flies … ”
and realized that my thoughts must have
been scattered. At least that’s the way it
looked to me upon reviewing it more than
a month after submission. How to make it
right?
I thought about that for a while. It
occurred to me that merely explaining
things a different way would work, but that
repeating myself would be a waste of your
time.
Then it hit me; explaining the basics of
a classic CL Precision Aerobatics (Stunt)
“run” would allow me to tie things
together, and it would also give me an
opportunity to write about something other
than RC while still helping fix the leaks in
the last column.
The classic “2-4 break” CL Stunt engine
setup is a marvel of subtle sophistication.
It’s a quintessential aeromodeler’s solution
to a difficult problem.
Long before computer radios,
autopilots, and all manner of electronic
wizardry, this simple-looking but
fiendishly clever arrangement with the
barest minimum moving parts, and nothing
that looks like a control system, evolved to
keep the speed of a CL Stunter “nearly
constant,” despite the vertical climbs and
dives of the Stunt pattern.
What’s a Stunt pattern? That’s a fair
question, especially if you’ve never seen it
flown. You can find the online AMA rule
book at the address that I have included in
the “Sources” listing.
The “old” rule book illustrations were
perspective drawings that showed how the
maneuvers were drawn on the surface of a
hemisphere; they show the schedule better
than the new “flat” drawings in the rule
book. The updated descriptions do a better
job of describing some of the geometric
nuances.
YouTube has numerous videos showing
CL Stunt patterns, although the quality of
the videography sometimes leaves much to
be desired. Search under “CL Stunt” and
“F2B,” and a mess of results pops up.
Several show world-class pilots flying. The
site addresses are under “Sources.”
One video depicts an electric-powered
F2B flight. It might seem strange to open
with this, but the film is unusual in that the
fixed camera installation was done properly
and in the right spot to see the entire
pattern flown well. The pilot is doing a
decent job of it too!
Because CL fliers often perform with
several circles next to each other, it is
difficult to film clips without the sound of
other models. I found a video of a pilot
who does not fly a “book” schedule, or
even rule book maneuvers, but the engine
setup is a classic Stunt run, with a deep
break in maneuvers.
If you listen carefully, you can even
hear the engine briefly break lean after the
corners of the square loops, precisely when
the airplane scrubs off speed in the corners.
“Nice Flying Footage of Kevin’s
Tsunami” shows a performance that does
not demonstrate a full pattern. The old
“G”-series SuperTigre light-case .60 has a
nice deep break between rich and lean.
Notice how the needle setting results in the
engine oscillating between a fast four-cycle
and a rich two-cycle while the pilot walks
out to the handle.
“Bill Werwage Classic Flight VSC14”
shows a “killer” engine run from the threetime
World Champion during the Vintage
Radio Control Society nostalgia meet.
Unfortunately other flights are taking place
at the same time, making this difficult
listening music, but the setup does
everything right in what looks to have been
windy conditions.
Al Rabe was world vice-champion in
1978. “C/L Stunt Al’s Snaggletooth” is
recent footage of him with his semiscale P-
51 “Snaggletooth.”
This video shows the low-break setup
described in the text. The benefit is that the
system is less laggy when responding to
changes in propeller load.
“Fitzgerald” illustrates the more
modern tuned-pipe Stunt setup, flown by
several-time World Champ Dave
Fitzgerald. It sounds different from the
classic 2-4 break setup and many betweenmaneuver
laps have been deleted, so the
clip is short.
In this video, the changes in scavenging
The choke area (the two crescent-shaped openings on either side of the spraybar) is
much smaller than that of the RC carburetor that is normally fitted to the O.S. FP40
shown. This is necessary to generate a strong rpm-dependent fuel draw needed to
control the engine’s running characteristics. The baked-on castor oil is a hint. Slurpythick
castor oil helps maintain the crankshaft/crankcase seal, especially in plain-bushed
power plants such as this. If you look carefully you can see the half-closed crankshaft
induction valve.
12sig3x.QXD_00MSTRPG.QXD 10/21/10 11:57 AM Page 94
and torque are dictated by exhaust tuning.
A crazy person must have invented this
setup!
If you watched and listened to all of the
preceding film clips (and maybe a few
others), did you notice how the engines
appear to lean out going uphill and then
richen and “go flat” coming downhill?
That’s what keeps the model from slowing
too much in a climb and then from coming
downhill in a screaming-fast dive.
You might even notice that the changes
sound like they happen a bit late compared
to when the airplane actually changes
direction. This is less than ideal, but, still,
this basic arrangement has worked well for
more than a half century.
What arrangement? For one thing,
notice that all of these engines sound
somewhat rich. The fuel-to-air mixtures are
not set for maximum rpm; instead they are
set where the torque that the power plant
delivers changes abruptly for very small
needle valve movements. This is “the
break.”
Just on the rich side of the break, the
rich mixture delays the onset of combustion
and excess fuel in the combustion chamber
keeps the maximum temperature and
cylinder pressure low. This means less
torque and, as a result, horsepower.
If you are poised just on the rich side of
the break, one lousy little click leaner on
the needle valve will change the whole
picture. The compression heating lights the
fire a bit sooner, and the combustion
process progresses more quickly.
The more advanced ignition timing
produces more torque. And if the rpm rises,
the compression heating effect is enhanced,
making even more torque and horsepower.
Do you remember how one film clip
showed the engine hopping back and forth
across the break? What’s with that?
The explanation for that is to describe
the other key to the classic 2-4 Stunt setup:
the small carburetor or venturi used with a
CL Stunt engine. All engines are air
pumps, and our single-cylinder two-strokes
are particularly interesting.
Think of them as two-stage air pumps.
Air is sucked into the crankcase through
the crankshaft valve every time the piston
is on the upstroke. Stunters have no need
for throttles, so they have simple fullthrottle-
only carburetors or venturis. They
are not actually so simple, but that is
another matter.
If you look into the venturi, or open
carburetor of an RC engine, you’ll see that
the crankshaft valve is completely open for
roughly one-eighth of a revolution before
the piston reaches TDC (Top Dead Center).
This is, in part, because it takes time to
accelerate the air through the venturi, down
the central passage in the crankshaft, and
up into the crankcase.
The crankshaft valve takes
approximately one-half of a revolution
from when it starts to open until it finally
closes. So by the time it does close, the
piston is already on its way downward and
squeezing the air/fuel mixture from the
crankcase upward through the bypasses
into the combustion chamber.
The momentum of the air/fuel mix
matters during this transition from
crankcase suction to compression.
At nearly the same time that this
transition occurs, the exhaust port opens
and exhaust gases are expelled at high
pressure and velocity. As a result of this
sudden acceleration, they have plenty of
momentum. And as the last of the exhaust
rushes out into the muffler, its momentum
keeps it moving even after the pressure in
the cylinder has dropped.
The result is actually a vacuum, and 10°
or 20° of rotation later the bypasses open,
allowing the intake mixture to be sucked
into the combustion chamber by the
vacuum that is left there.
That is called “scavenging.” And
depending on the time relationship of all
port and valve openings and closings, the
engine will have its best scavenging in
some rpm range that the manufacturer has
designed.
The venturi (remember the venturi?) is
typically designed to maximize the amount
of vacuum generated in it as the air is
sucked through it into the crankcase. This
is what draws the fuel into the engine in the
first place.
Assuming that the engine is running in
that rpm range in which the designer has
provided good scavenging, the vacuum that
draws in the fuel will be strongly
influenced by changes in rpm. More rpm
will draw a better vacuum in the venturi,
which will improve fuel draw and provide
more fuel.
If the cross-sectional area or opening of
the venturi is small enough, it will drive the
engine rich with increased rpm. If it is
slightly larger, it will simply be adequate
for the increased needs of the engine at
high rpm. If it is too large, the engine will
suffer from poor fuel draw during climbs
and maneuvers.
That is why you can’t always put a bigger
carburetor on an engine to get more
horsepower. It also explains why the tiniest air
leak in the crankcase will ruin the fuel draw.
Where are those potential leak sources?
Loose backplate screws, for one. Loose
screws or worn O-rings under the
carburetor/venturi will ruin things as well,
but the thing that causes the most trouble is
the hardest to see.
The crankshaft and the way it fits into
the crankcase create a long rotating seal.
Fuel and condensed oil between the two
parts form the seal. This works great if the
gaps are only two or three thousandths of
an inch!
Worn bushings or even worn/loose ball
bearings allow the shaft to wiggle and
break this oil seal. Sometimes a loose
bearing will allow a bit of the aluminum
crankcase to wear away, and even a bearing
replacement will not fix the problem. This
leads to the engine being unable to hold a
consistent mixture as it warms up.
So with a small venturi providing a
strong rpm-dependent fuel mixture, and the
break providing a strong change in torque
with small changes in mixture, we have the
makings of a stable system for keeping the
airplane’s flying speed within bounds
during maneuvers.
The rpm must increase some during
dives and drop during climbs, but the
change in torque that the break provides
allows the horsepower to increase despite
the reduced rpm in a climb. Similarly, the
reduction in torque as the rich mixture
cools the fire and retards the ignition
timing will reduce the delivered
horsepower despite increased rpm in dives.
If you go back and listen to some of
those on-line videos, you might be able to
pick out the difference between the “flat”
note of a rich mixture from the slight
increase in diving rpm that caused it. And
with a little more careful attention you can
pick the “clean” note during climbs apart
from the slight rpm drop.
The first time you heard it, the
dominance of high harmonics in the
exhaust note might have convinced you
that the rpm is higher during the climb, but
that isn’t the case. In-flight measurements
with data loggers have proven this to be the
case.
If you take an engine that has been set
up this way and increase the compression
ratio slightly, it will “break” at slightly
lower rpm and with a slightly richer needle
valve setting. The change in torque that the
break provides will be greater too, but the
engine will also lag behind the changes in rpm
more. Once the engine breaks lean, the added
power creates more heat, which tends to keep
it from richening as easily.
In the dive, after the engine breaks rich,
the cooling of just a few degrees caused by
the cooling effect of the added fuel will delay
the break back to lean. Add a little too much
compression, and the first time the engine
breaks lean, the added heat is enough to keep
it there until the entire tank is emptied.
That is what the Stunt pilots call a
“runaway,” and the strong-breaking setups
will run away if the needle valve is set only a
few clicks too lean. Yes, this is an important
skill for Stunt fliers!
If instead you drop the compression a bit,
you get slightly less break and an engine that
performs this bit of magic at higher rpm,
which can be used to run a lower-pitch
propeller. You can use those to provide dive
brakes instead of a deep break rich. This
describes the run of the Snaggletooth featured
in one of the videos.
Another time I should write about fuel tank
design. The Stunt pilots have a treasure trove
of knowledge here, and much of it is
genuinely useful in RC as well.
Right now I am going to organize some of
the work I’ve been doing recently on my true
constant-rpm electric-powered Stunt setup.
I’ll be back in the February 2011 issue.
Until then, have fun, and do take care of
yourself.
Want to learn more about two-stroke
scavenging? It’s quite a subject. A great
book that has been out of print for years is
Gordon Jennings’ Two-Stroke Tuner’s
Handbook. You can probably find online
copies in a variety of places; I have
included the Web site address for one
resource in the “Sources” listing. MA
Sources:
AMA CL Stunt rules:
http://bit.ly/db6WQt
Electric F2B video:
http://bit.ly/aqpz6x
“Nice Flying Footage of Kevin’s Tsunami”
http://bit.ly/awAeRr
“Bill Werwage Classic Flight VSC14”
http://bit.ly/9wLOJj
“C/L Stunt Al’s Snaggletooth”
http://bit.ly/9ZS8XX
“Fitzgerald”
http://bit.ly/c5payH
Two-Stroke Tuner’s Handbook
http://edj.net/2stroke/jennings
