94 MODEL AVIATION
turbine jet fliers to kit designers, and they are
all just “plane” crazy.
With your help, the questions and
problems we work on will be as varied as the
wonderful resources we have at hand. Until
then I will fill in by discussing subjects I
think deserve to be general knowledge. I
hope that won’t last too long!
The First Link: In the last installment I
mentioned how the tuned-pipe exhaust for
two-stroke engines has evolved from the
groundbreaking but
finicky and
unreliable CL Speed
setups pioneered by
the late Bill
Wisniewski into
several different
uses.
In one important
application the
tuned pipe is
useful for creating
quiet. Yes, quiet:
that tacit quality
that helps protect
our flying sites
and maybe our
sense of hearing as
well. Let’s cover
noise abatement
for a bit in this
installment.
Let’s look at the
big picture. Why
would you want to
make your model
airplane quieter?
Some things in life
were not meant to
be quiet. I can’t
imagine a quiet Top
Model aircraft and noise abatement
If It Flies... Dean Pappas | [email protected]
Low inverted pass shows full length of extended exhaust
system. Judicious experimentation with extension length
and diameter can optimize performance and sound.
The relationship between propeller tip speed and sound can be reduced to a simple rule of thumb. The rpmand-
diameter combinations above and to the right of the double line are noisy; those below and left are quiet
because they avoid transonic airflow.
IF IT FLIES, I’m interested in it! It’s always
been that simple. This column is dedicated to
the notion that there is a great deal of
commonality throughout aeromodeling, so
that fliers in one group, say CL Combat, will
have something to offer, in technique or
practice, to another group, such as those who
are looking for their first building project after
learning to fly with ARFs.
The approach we’d like to take is to try to
answer questions and observations that come
from you—yes, you. The assumption is that
A simple expansion-chamber muffler can
be made extra quiet with an extension.
This was made from 1/ 2-inch-insidediameter,
1/16-inch-wall PTFE (Teflon)
tubing and a Du-Bro silicone
exhaust elbow.
Propeller Diameter Vs. RPM
0
5
10
15
20
25
0 5000 10000 15000 20000 25000 30000
RP
Propeller Diameter (Inches)
RPM
your questions will almost inevitably come
from a variety of experience and skill levels in
all facets of the hobby/sport, and that in
answering them some useful cross-pollination
between the far-flung corners of the
hobby/sport might take place. Accidentally
we might actually come up with a helpful
answer.
I use the second-person “we” because I
intend to pull information together from the
great many experts I have met through the
years. These experts range from FFers to
10sig3.QXD 8/23/07 3:02 PM Page 94
Fuel dragster, nor do the words “speedboat”
and “quiet” go together. Monster trucks
wouldn’t be monsters without the roar, and I
can’t imagine an RC Pylon race or a CL
Combat meet without the sound and fury.
If you’ve never been to either of those two
events, you must make the effort to get to one
of each. Pylon, as was almost any form of
racing, was created to be a spectator event,
and CL Combat will make you feel like an
ancient Roman watching gladiators.
As I mentioned, some events just need the
soundtrack that can only be provided by
screaming engines! All those events have
their fans, but, as with almost everything else,
for everyone who thinks a Formula 1 engine
revving 19,000 rpm is music, someone else
thinks it is simply noise.
The problem is not limited to model
aviation. At nearby Englishtown Raceway
Park, home of one of the big national-class
drag races, there are those who endlessly try
to put a stop to the racing.
Fortunately good management at the track
and some common sense on the part of the
local courts maintains the status quo. (Who
builds a retirement home right next to an
airport and a drag strip and then complains
about the noise? Only a fool!) Still, most
model airfields have neighbors, and some of
them prefer the quiet.
Everything we do when it comes to
making our airplanes quiet as part of being
good neighbors is aimed at satisfying a
reasonable person. Maybe someday I’ll tell
you about the time I got thrown out of a local
park for a noise complaint while flying a
glider. For now let’s limit ourselves to useful
talk.
The Pareto Principle—Fix the Biggest
Problem First: The sound quality that annoys
the neighbors most is the “snarl” of the
propeller tips. Sometimes you’ll hear it called
propeller howl or a “ripping” sound, but if it
is there you’ll know it. When it is absent, the
airplane’s sound is decidedly “softer.”
This is a matter of opinion, but, noise
meters and property-line noise ordinances not
withstanding, the goal of simply being a good
neighbor is best served by getting rid of the
propeller-tip snarl and fitting a half-decent
muffler. Taking those steps ahead of time can
help avoid the irritation that comes with
angry neighbors. On to the technical stuff.
Propeller howl is caused by transonic
airflow around the propeller tips. What’s
transonic? Depending on the blade shape and
airfoil, the airflow around the propeller tip is
faster than the propeller tip itself.
Somewhere near six-tenths to threequarters
Mach, or the speed of sound, the
local airflow will actually break the sound
barrier because of the slightly greater distance
it must travel to get around the propeller
blade. These small sonic booms emanate
from the propeller blade, and as the propeller
whirls around an observer will hear the
repeated beating of the sonic booms.
Drop that propeller-tip velocity slightly
below the threshold of transonic behavior,
and the noise changes dramatically. It drops
in volume and changes sound quality. That’s
the “soft” sound we want.
Propeller-tip howl usually carries much
farther than the noise coming from the
muffler. This is true for most sport engines
above a 30 or 35 size and for almost all highrpm
racing engines.
Where is the dividing line? It depends on
the propeller, so the critical Mach number
isn’t a hard-and-fast number. It is likely
somewhere between 0.60 and 0.75 times the
speed of sound.
You could calculate the tip velocity and
divide by the speed of sound and all that, but
I know a simple method. I call it the rule of
130, and Ed Izzo first suggested it in the early
1980s.
Ed was the first AMA sound-reduction
director and was credited with inventing the
foam wing for model-airplane use more than
40 years ago. (There’s a link we’ll have to
pursue in another installment.)
You multiply the propeller diameter in
inches by the propeller rpm expressed in
thousands, and compare the number to
roughly 130. When the result is higher than
140, the result is usually loud; if it’s less than
130, the result is typically extremely quiet.
A 12-inch-diameter propeller at 10,000
rpm gives a product of 120. That’s good. A
20-inch propeller at 7,000 rpm is bound to be
loud unless throttled back a great deal. It’s a
simple rule of thumb.
It’s time to pick a propeller that will fly at
approximately the same airspeed as before
the noise-reduction project while reducing the
rpm and tip speed. You can try a propeller
with the same diameter and add pitch. This
will load down the rpm, while the added pitch
will allow the airplane to fly as fast or faster,
provided the available horsepower is
adequate.
Instead of a 10 x 6 on a 40, try a 10 x 7 or
even a 10 x 8 if the engine is particularly
strong. Although the traditional propeller size
for a 60 is an 11 x 7, you might try an 11 x 8
or an 11 x 9. This approach works well for
light, low-drag airplanes, but it is not
generally the best for heavy or draggy models.
The approach that tends to work better
with many airplanes is to add a little propeller
diameter and either maintain the same pitch or
add just enough to get the rpm down to where
the rule of 130 is satisfied. An 11 x 6 or 11 x 7
would do nicely on a 40- to 45-size engine,
and a 12 x 8, 12 x 9, or 13 x 8 would harness
the power of a 60 while making plenty of lowspeed
thrust and staying relatively quiet.
Why would we add diameter if we were
trying to slow the tip speeds? It seems
counterproductive because the bigger
diameter would lead to a higher tip speed for
any given rpm.
It turns out that propeller blades are wings.
As with all other wings, you can get higher
efficiencies with a larger aspect ratio (AR).
The AR is the relation of the wingspan to the
average wing chord (the length from LE to
TE).
The whole AR discussion is yet another
link that will have to be pursued someday.
Until that much longer discussion happens,
the CliffsNotes version is that the efficiency is
better with a large-diameter propeller because
its tips are farther apart. This improved
propeller efficiency can more than make up
for the reduced horsepower the engine
produces at the lower rpm.
Three- and four-blade propellers force you
to pay an efficiency penalty, but they are
useful when the engine rpm needs to be
higher to suit the engine’s running
characteristics. (That’s two more discussions:
one about four-strokes’ power bands and
another devoted to two-strokes. If we keep
chasing these links we’ll never stop!)
If you are faced with an engine that needs
to run at too high an rpm for the propeller
diameter needed to harness its horsepower,
changing from a two-blade propeller to a
three- or four-blader might be the ticket.
Three-blade propellers typically end up with
93% of the equivalent two-blade diameter,
and four-blade propellers typically run at
85% of the original two-blade propeller.
They look cool too!
Hushed Tones out of the Box: If your
engine-and-propeller combination already
met the rule of 130, you’d be here already!
It is time to discuss mufflers.
The stock mufflers that are coming with
engines these days are better than the
fancy, sometimes expensive aftermarket
items of a decade ago. The Evolution and
O.S. AX-series mufflers are standouts.
I helped a clubmate set up an AX .55
with a 12 x 8 propeller. It turned that 12 x
8 in the mid-10,000 rpm range and pulled
like a freight train, while it whispered like
a freight train doesn’t.
Four-stroke engines have always had a
reputation for being quiet, mostly because
of the reduced exhaust-note frequency.
They fire only half as often as a two-stroke
for the same running rpm.
Even so, if you make horsepower you
will make noise, and the newer fourstrokes
are finally coming from the factory
with larger-volume, and even baffled,
mufflers. Even here, simple improvements
can be had.
Reduce, Reuse, Recycle: It is possible to
spend all kinds of money on aftermarket
mufflers, and some of them are quite good.
However, if your engine came from the
factory with a simple expansion-chamber
muffler that has no baffles in it, there is
still a simple, inexpensive option.
An exhaust extension makes the exhaust
system longer, effectively “tuning” or
filtering the harshest, high-frequency, parts
of the exhaust note. All you have to do to
prove this is play trumpet with a long
cardboard tube, such as the kind heat-shrink
covering material is wrapped around.
Cut the tube shorter, and if you have the
lung power the note will be higher and contain
more high-frequency harmonics. In general,
high notes are more annoying—except for the
staccato howl of a racing engine!
This technique was used effectively on the
Tiger 60 ARF that was reviewed on MA’s
Sport Aviator. A silicone exhaust elbow and
61/2 inches of 1/2-inch-inside-diameter tubing
turned a noisy 20-year-old muffler into an
extremely quiet exhaust package.
The extension more than doubled the
length from the exhaust port to the
atmosphere. Although the exhaust note was
dramatically mellowed, the horsepower was
barely affected.
Larger engines respond beautifully to the
large canister-type mufflers, sometimes even
enjoying a horsepower boost. The
mechanism behind this would take more ink
to describe than we have left here, but we
will go there if and when someone writes in
looking for an explanation of tuned exhausts.
Similarly, the four-strokes benefit from
having a long header pipe and then a
muffler. Go to the local drag strip on
“muffled-car night” and poke around. Some
of those race setups make beaucoup
horsepower! The header pipes on almost all
the racecars are tuned to improve the torque
in some desired rpm range.
In our case we are looking to boost the
torque at rpm that will allow us to meet the
rule of 130. That often means a header pipe
that is roughly a foot long, made from
aluminum, steel, or sometime Teflon tubing.
Teflon works better than you might
guess. Clamp a piece with the right inside
diameter around the end of the header stub
that came with the engine, find a way to
attach it to a small muffler, and you are in
business.
Teflon is slippery, so it takes a fair bit of
work to figure out how to keep things
clamped together, but it holds up well once
you get it. Industrial supply houses such as
McMaster-Carr and Grainger stock the
tubing.
Time is up, and we already have a
handful of dangling links in addition to
never having addressed the tuned-pipe
discussion that got us started on noise
abatement. There are other aspects to noise
abatement, such as soft engine mounting,
intake muffling, and shrouding the entire
power train, but propeller noise and muffling
take care of the lion’s share.
Until next time, enjoy your time at the field
or in the workshop, and please feel free to
send in any questions or observations you
might have. This column is all about making
the remote corners of this hobby/sport work
for you
Edition: Model Aviation - 2007/10
Page Numbers: 94,95,96
Edition: Model Aviation - 2007/10
Page Numbers: 94,95,96
94 MODEL AVIATION
turbine jet fliers to kit designers, and they are
all just “plane” crazy.
With your help, the questions and
problems we work on will be as varied as the
wonderful resources we have at hand. Until
then I will fill in by discussing subjects I
think deserve to be general knowledge. I
hope that won’t last too long!
The First Link: In the last installment I
mentioned how the tuned-pipe exhaust for
two-stroke engines has evolved from the
groundbreaking but
finicky and
unreliable CL Speed
setups pioneered by
the late Bill
Wisniewski into
several different
uses.
In one important
application the
tuned pipe is
useful for creating
quiet. Yes, quiet:
that tacit quality
that helps protect
our flying sites
and maybe our
sense of hearing as
well. Let’s cover
noise abatement
for a bit in this
installment.
Let’s look at the
big picture. Why
would you want to
make your model
airplane quieter?
Some things in life
were not meant to
be quiet. I can’t
imagine a quiet Top
Model aircraft and noise abatement
If It Flies... Dean Pappas | [email protected]
Low inverted pass shows full length of extended exhaust
system. Judicious experimentation with extension length
and diameter can optimize performance and sound.
The relationship between propeller tip speed and sound can be reduced to a simple rule of thumb. The rpmand-
diameter combinations above and to the right of the double line are noisy; those below and left are quiet
because they avoid transonic airflow.
IF IT FLIES, I’m interested in it! It’s always
been that simple. This column is dedicated to
the notion that there is a great deal of
commonality throughout aeromodeling, so
that fliers in one group, say CL Combat, will
have something to offer, in technique or
practice, to another group, such as those who
are looking for their first building project after
learning to fly with ARFs.
The approach we’d like to take is to try to
answer questions and observations that come
from you—yes, you. The assumption is that
A simple expansion-chamber muffler can
be made extra quiet with an extension.
This was made from 1/ 2-inch-insidediameter,
1/16-inch-wall PTFE (Teflon)
tubing and a Du-Bro silicone
exhaust elbow.
Propeller Diameter Vs. RPM
0
5
10
15
20
25
0 5000 10000 15000 20000 25000 30000
RP
Propeller Diameter (Inches)
RPM
your questions will almost inevitably come
from a variety of experience and skill levels in
all facets of the hobby/sport, and that in
answering them some useful cross-pollination
between the far-flung corners of the
hobby/sport might take place. Accidentally
we might actually come up with a helpful
answer.
I use the second-person “we” because I
intend to pull information together from the
great many experts I have met through the
years. These experts range from FFers to
10sig3.QXD 8/23/07 3:02 PM Page 94
Fuel dragster, nor do the words “speedboat”
and “quiet” go together. Monster trucks
wouldn’t be monsters without the roar, and I
can’t imagine an RC Pylon race or a CL
Combat meet without the sound and fury.
If you’ve never been to either of those two
events, you must make the effort to get to one
of each. Pylon, as was almost any form of
racing, was created to be a spectator event,
and CL Combat will make you feel like an
ancient Roman watching gladiators.
As I mentioned, some events just need the
soundtrack that can only be provided by
screaming engines! All those events have
their fans, but, as with almost everything else,
for everyone who thinks a Formula 1 engine
revving 19,000 rpm is music, someone else
thinks it is simply noise.
The problem is not limited to model
aviation. At nearby Englishtown Raceway
Park, home of one of the big national-class
drag races, there are those who endlessly try
to put a stop to the racing.
Fortunately good management at the track
and some common sense on the part of the
local courts maintains the status quo. (Who
builds a retirement home right next to an
airport and a drag strip and then complains
about the noise? Only a fool!) Still, most
model airfields have neighbors, and some of
them prefer the quiet.
Everything we do when it comes to
making our airplanes quiet as part of being
good neighbors is aimed at satisfying a
reasonable person. Maybe someday I’ll tell
you about the time I got thrown out of a local
park for a noise complaint while flying a
glider. For now let’s limit ourselves to useful
talk.
The Pareto Principle—Fix the Biggest
Problem First: The sound quality that annoys
the neighbors most is the “snarl” of the
propeller tips. Sometimes you’ll hear it called
propeller howl or a “ripping” sound, but if it
is there you’ll know it. When it is absent, the
airplane’s sound is decidedly “softer.”
This is a matter of opinion, but, noise
meters and property-line noise ordinances not
withstanding, the goal of simply being a good
neighbor is best served by getting rid of the
propeller-tip snarl and fitting a half-decent
muffler. Taking those steps ahead of time can
help avoid the irritation that comes with
angry neighbors. On to the technical stuff.
Propeller howl is caused by transonic
airflow around the propeller tips. What’s
transonic? Depending on the blade shape and
airfoil, the airflow around the propeller tip is
faster than the propeller tip itself.
Somewhere near six-tenths to threequarters
Mach, or the speed of sound, the
local airflow will actually break the sound
barrier because of the slightly greater distance
it must travel to get around the propeller
blade. These small sonic booms emanate
from the propeller blade, and as the propeller
whirls around an observer will hear the
repeated beating of the sonic booms.
Drop that propeller-tip velocity slightly
below the threshold of transonic behavior,
and the noise changes dramatically. It drops
in volume and changes sound quality. That’s
the “soft” sound we want.
Propeller-tip howl usually carries much
farther than the noise coming from the
muffler. This is true for most sport engines
above a 30 or 35 size and for almost all highrpm
racing engines.
Where is the dividing line? It depends on
the propeller, so the critical Mach number
isn’t a hard-and-fast number. It is likely
somewhere between 0.60 and 0.75 times the
speed of sound.
You could calculate the tip velocity and
divide by the speed of sound and all that, but
I know a simple method. I call it the rule of
130, and Ed Izzo first suggested it in the early
1980s.
Ed was the first AMA sound-reduction
director and was credited with inventing the
foam wing for model-airplane use more than
40 years ago. (There’s a link we’ll have to
pursue in another installment.)
You multiply the propeller diameter in
inches by the propeller rpm expressed in
thousands, and compare the number to
roughly 130. When the result is higher than
140, the result is usually loud; if it’s less than
130, the result is typically extremely quiet.
A 12-inch-diameter propeller at 10,000
rpm gives a product of 120. That’s good. A
20-inch propeller at 7,000 rpm is bound to be
loud unless throttled back a great deal. It’s a
simple rule of thumb.
It’s time to pick a propeller that will fly at
approximately the same airspeed as before
the noise-reduction project while reducing the
rpm and tip speed. You can try a propeller
with the same diameter and add pitch. This
will load down the rpm, while the added pitch
will allow the airplane to fly as fast or faster,
provided the available horsepower is
adequate.
Instead of a 10 x 6 on a 40, try a 10 x 7 or
even a 10 x 8 if the engine is particularly
strong. Although the traditional propeller size
for a 60 is an 11 x 7, you might try an 11 x 8
or an 11 x 9. This approach works well for
light, low-drag airplanes, but it is not
generally the best for heavy or draggy models.
The approach that tends to work better
with many airplanes is to add a little propeller
diameter and either maintain the same pitch or
add just enough to get the rpm down to where
the rule of 130 is satisfied. An 11 x 6 or 11 x 7
would do nicely on a 40- to 45-size engine,
and a 12 x 8, 12 x 9, or 13 x 8 would harness
the power of a 60 while making plenty of lowspeed
thrust and staying relatively quiet.
Why would we add diameter if we were
trying to slow the tip speeds? It seems
counterproductive because the bigger
diameter would lead to a higher tip speed for
any given rpm.
It turns out that propeller blades are wings.
As with all other wings, you can get higher
efficiencies with a larger aspect ratio (AR).
The AR is the relation of the wingspan to the
average wing chord (the length from LE to
TE).
The whole AR discussion is yet another
link that will have to be pursued someday.
Until that much longer discussion happens,
the CliffsNotes version is that the efficiency is
better with a large-diameter propeller because
its tips are farther apart. This improved
propeller efficiency can more than make up
for the reduced horsepower the engine
produces at the lower rpm.
Three- and four-blade propellers force you
to pay an efficiency penalty, but they are
useful when the engine rpm needs to be
higher to suit the engine’s running
characteristics. (That’s two more discussions:
one about four-strokes’ power bands and
another devoted to two-strokes. If we keep
chasing these links we’ll never stop!)
If you are faced with an engine that needs
to run at too high an rpm for the propeller
diameter needed to harness its horsepower,
changing from a two-blade propeller to a
three- or four-blader might be the ticket.
Three-blade propellers typically end up with
93% of the equivalent two-blade diameter,
and four-blade propellers typically run at
85% of the original two-blade propeller.
They look cool too!
Hushed Tones out of the Box: If your
engine-and-propeller combination already
met the rule of 130, you’d be here already!
It is time to discuss mufflers.
The stock mufflers that are coming with
engines these days are better than the
fancy, sometimes expensive aftermarket
items of a decade ago. The Evolution and
O.S. AX-series mufflers are standouts.
I helped a clubmate set up an AX .55
with a 12 x 8 propeller. It turned that 12 x
8 in the mid-10,000 rpm range and pulled
like a freight train, while it whispered like
a freight train doesn’t.
Four-stroke engines have always had a
reputation for being quiet, mostly because
of the reduced exhaust-note frequency.
They fire only half as often as a two-stroke
for the same running rpm.
Even so, if you make horsepower you
will make noise, and the newer fourstrokes
are finally coming from the factory
with larger-volume, and even baffled,
mufflers. Even here, simple improvements
can be had.
Reduce, Reuse, Recycle: It is possible to
spend all kinds of money on aftermarket
mufflers, and some of them are quite good.
However, if your engine came from the
factory with a simple expansion-chamber
muffler that has no baffles in it, there is
still a simple, inexpensive option.
An exhaust extension makes the exhaust
system longer, effectively “tuning” or
filtering the harshest, high-frequency, parts
of the exhaust note. All you have to do to
prove this is play trumpet with a long
cardboard tube, such as the kind heat-shrink
covering material is wrapped around.
Cut the tube shorter, and if you have the
lung power the note will be higher and contain
more high-frequency harmonics. In general,
high notes are more annoying—except for the
staccato howl of a racing engine!
This technique was used effectively on the
Tiger 60 ARF that was reviewed on MA’s
Sport Aviator. A silicone exhaust elbow and
61/2 inches of 1/2-inch-inside-diameter tubing
turned a noisy 20-year-old muffler into an
extremely quiet exhaust package.
The extension more than doubled the
length from the exhaust port to the
atmosphere. Although the exhaust note was
dramatically mellowed, the horsepower was
barely affected.
Larger engines respond beautifully to the
large canister-type mufflers, sometimes even
enjoying a horsepower boost. The
mechanism behind this would take more ink
to describe than we have left here, but we
will go there if and when someone writes in
looking for an explanation of tuned exhausts.
Similarly, the four-strokes benefit from
having a long header pipe and then a
muffler. Go to the local drag strip on
“muffled-car night” and poke around. Some
of those race setups make beaucoup
horsepower! The header pipes on almost all
the racecars are tuned to improve the torque
in some desired rpm range.
In our case we are looking to boost the
torque at rpm that will allow us to meet the
rule of 130. That often means a header pipe
that is roughly a foot long, made from
aluminum, steel, or sometime Teflon tubing.
Teflon works better than you might
guess. Clamp a piece with the right inside
diameter around the end of the header stub
that came with the engine, find a way to
attach it to a small muffler, and you are in
business.
Teflon is slippery, so it takes a fair bit of
work to figure out how to keep things
clamped together, but it holds up well once
you get it. Industrial supply houses such as
McMaster-Carr and Grainger stock the
tubing.
Time is up, and we already have a
handful of dangling links in addition to
never having addressed the tuned-pipe
discussion that got us started on noise
abatement. There are other aspects to noise
abatement, such as soft engine mounting,
intake muffling, and shrouding the entire
power train, but propeller noise and muffling
take care of the lion’s share.
Until next time, enjoy your time at the field
or in the workshop, and please feel free to
send in any questions or observations you
might have. This column is all about making
the remote corners of this hobby/sport work
for you
Edition: Model Aviation - 2007/10
Page Numbers: 94,95,96
94 MODEL AVIATION
turbine jet fliers to kit designers, and they are
all just “plane” crazy.
With your help, the questions and
problems we work on will be as varied as the
wonderful resources we have at hand. Until
then I will fill in by discussing subjects I
think deserve to be general knowledge. I
hope that won’t last too long!
The First Link: In the last installment I
mentioned how the tuned-pipe exhaust for
two-stroke engines has evolved from the
groundbreaking but
finicky and
unreliable CL Speed
setups pioneered by
the late Bill
Wisniewski into
several different
uses.
In one important
application the
tuned pipe is
useful for creating
quiet. Yes, quiet:
that tacit quality
that helps protect
our flying sites
and maybe our
sense of hearing as
well. Let’s cover
noise abatement
for a bit in this
installment.
Let’s look at the
big picture. Why
would you want to
make your model
airplane quieter?
Some things in life
were not meant to
be quiet. I can’t
imagine a quiet Top
Model aircraft and noise abatement
If It Flies... Dean Pappas | [email protected]
Low inverted pass shows full length of extended exhaust
system. Judicious experimentation with extension length
and diameter can optimize performance and sound.
The relationship between propeller tip speed and sound can be reduced to a simple rule of thumb. The rpmand-
diameter combinations above and to the right of the double line are noisy; those below and left are quiet
because they avoid transonic airflow.
IF IT FLIES, I’m interested in it! It’s always
been that simple. This column is dedicated to
the notion that there is a great deal of
commonality throughout aeromodeling, so
that fliers in one group, say CL Combat, will
have something to offer, in technique or
practice, to another group, such as those who
are looking for their first building project after
learning to fly with ARFs.
The approach we’d like to take is to try to
answer questions and observations that come
from you—yes, you. The assumption is that
A simple expansion-chamber muffler can
be made extra quiet with an extension.
This was made from 1/ 2-inch-insidediameter,
1/16-inch-wall PTFE (Teflon)
tubing and a Du-Bro silicone
exhaust elbow.
Propeller Diameter Vs. RPM
0
5
10
15
20
25
0 5000 10000 15000 20000 25000 30000
RP
Propeller Diameter (Inches)
RPM
your questions will almost inevitably come
from a variety of experience and skill levels in
all facets of the hobby/sport, and that in
answering them some useful cross-pollination
between the far-flung corners of the
hobby/sport might take place. Accidentally
we might actually come up with a helpful
answer.
I use the second-person “we” because I
intend to pull information together from the
great many experts I have met through the
years. These experts range from FFers to
10sig3.QXD 8/23/07 3:02 PM Page 94
Fuel dragster, nor do the words “speedboat”
and “quiet” go together. Monster trucks
wouldn’t be monsters without the roar, and I
can’t imagine an RC Pylon race or a CL
Combat meet without the sound and fury.
If you’ve never been to either of those two
events, you must make the effort to get to one
of each. Pylon, as was almost any form of
racing, was created to be a spectator event,
and CL Combat will make you feel like an
ancient Roman watching gladiators.
As I mentioned, some events just need the
soundtrack that can only be provided by
screaming engines! All those events have
their fans, but, as with almost everything else,
for everyone who thinks a Formula 1 engine
revving 19,000 rpm is music, someone else
thinks it is simply noise.
The problem is not limited to model
aviation. At nearby Englishtown Raceway
Park, home of one of the big national-class
drag races, there are those who endlessly try
to put a stop to the racing.
Fortunately good management at the track
and some common sense on the part of the
local courts maintains the status quo. (Who
builds a retirement home right next to an
airport and a drag strip and then complains
about the noise? Only a fool!) Still, most
model airfields have neighbors, and some of
them prefer the quiet.
Everything we do when it comes to
making our airplanes quiet as part of being
good neighbors is aimed at satisfying a
reasonable person. Maybe someday I’ll tell
you about the time I got thrown out of a local
park for a noise complaint while flying a
glider. For now let’s limit ourselves to useful
talk.
The Pareto Principle—Fix the Biggest
Problem First: The sound quality that annoys
the neighbors most is the “snarl” of the
propeller tips. Sometimes you’ll hear it called
propeller howl or a “ripping” sound, but if it
is there you’ll know it. When it is absent, the
airplane’s sound is decidedly “softer.”
This is a matter of opinion, but, noise
meters and property-line noise ordinances not
withstanding, the goal of simply being a good
neighbor is best served by getting rid of the
propeller-tip snarl and fitting a half-decent
muffler. Taking those steps ahead of time can
help avoid the irritation that comes with
angry neighbors. On to the technical stuff.
Propeller howl is caused by transonic
airflow around the propeller tips. What’s
transonic? Depending on the blade shape and
airfoil, the airflow around the propeller tip is
faster than the propeller tip itself.
Somewhere near six-tenths to threequarters
Mach, or the speed of sound, the
local airflow will actually break the sound
barrier because of the slightly greater distance
it must travel to get around the propeller
blade. These small sonic booms emanate
from the propeller blade, and as the propeller
whirls around an observer will hear the
repeated beating of the sonic booms.
Drop that propeller-tip velocity slightly
below the threshold of transonic behavior,
and the noise changes dramatically. It drops
in volume and changes sound quality. That’s
the “soft” sound we want.
Propeller-tip howl usually carries much
farther than the noise coming from the
muffler. This is true for most sport engines
above a 30 or 35 size and for almost all highrpm
racing engines.
Where is the dividing line? It depends on
the propeller, so the critical Mach number
isn’t a hard-and-fast number. It is likely
somewhere between 0.60 and 0.75 times the
speed of sound.
You could calculate the tip velocity and
divide by the speed of sound and all that, but
I know a simple method. I call it the rule of
130, and Ed Izzo first suggested it in the early
1980s.
Ed was the first AMA sound-reduction
director and was credited with inventing the
foam wing for model-airplane use more than
40 years ago. (There’s a link we’ll have to
pursue in another installment.)
You multiply the propeller diameter in
inches by the propeller rpm expressed in
thousands, and compare the number to
roughly 130. When the result is higher than
140, the result is usually loud; if it’s less than
130, the result is typically extremely quiet.
A 12-inch-diameter propeller at 10,000
rpm gives a product of 120. That’s good. A
20-inch propeller at 7,000 rpm is bound to be
loud unless throttled back a great deal. It’s a
simple rule of thumb.
It’s time to pick a propeller that will fly at
approximately the same airspeed as before
the noise-reduction project while reducing the
rpm and tip speed. You can try a propeller
with the same diameter and add pitch. This
will load down the rpm, while the added pitch
will allow the airplane to fly as fast or faster,
provided the available horsepower is
adequate.
Instead of a 10 x 6 on a 40, try a 10 x 7 or
even a 10 x 8 if the engine is particularly
strong. Although the traditional propeller size
for a 60 is an 11 x 7, you might try an 11 x 8
or an 11 x 9. This approach works well for
light, low-drag airplanes, but it is not
generally the best for heavy or draggy models.
The approach that tends to work better
with many airplanes is to add a little propeller
diameter and either maintain the same pitch or
add just enough to get the rpm down to where
the rule of 130 is satisfied. An 11 x 6 or 11 x 7
would do nicely on a 40- to 45-size engine,
and a 12 x 8, 12 x 9, or 13 x 8 would harness
the power of a 60 while making plenty of lowspeed
thrust and staying relatively quiet.
Why would we add diameter if we were
trying to slow the tip speeds? It seems
counterproductive because the bigger
diameter would lead to a higher tip speed for
any given rpm.
It turns out that propeller blades are wings.
As with all other wings, you can get higher
efficiencies with a larger aspect ratio (AR).
The AR is the relation of the wingspan to the
average wing chord (the length from LE to
TE).
The whole AR discussion is yet another
link that will have to be pursued someday.
Until that much longer discussion happens,
the CliffsNotes version is that the efficiency is
better with a large-diameter propeller because
its tips are farther apart. This improved
propeller efficiency can more than make up
for the reduced horsepower the engine
produces at the lower rpm.
Three- and four-blade propellers force you
to pay an efficiency penalty, but they are
useful when the engine rpm needs to be
higher to suit the engine’s running
characteristics. (That’s two more discussions:
one about four-strokes’ power bands and
another devoted to two-strokes. If we keep
chasing these links we’ll never stop!)
If you are faced with an engine that needs
to run at too high an rpm for the propeller
diameter needed to harness its horsepower,
changing from a two-blade propeller to a
three- or four-blader might be the ticket.
Three-blade propellers typically end up with
93% of the equivalent two-blade diameter,
and four-blade propellers typically run at
85% of the original two-blade propeller.
They look cool too!
Hushed Tones out of the Box: If your
engine-and-propeller combination already
met the rule of 130, you’d be here already!
It is time to discuss mufflers.
The stock mufflers that are coming with
engines these days are better than the
fancy, sometimes expensive aftermarket
items of a decade ago. The Evolution and
O.S. AX-series mufflers are standouts.
I helped a clubmate set up an AX .55
with a 12 x 8 propeller. It turned that 12 x
8 in the mid-10,000 rpm range and pulled
like a freight train, while it whispered like
a freight train doesn’t.
Four-stroke engines have always had a
reputation for being quiet, mostly because
of the reduced exhaust-note frequency.
They fire only half as often as a two-stroke
for the same running rpm.
Even so, if you make horsepower you
will make noise, and the newer fourstrokes
are finally coming from the factory
with larger-volume, and even baffled,
mufflers. Even here, simple improvements
can be had.
Reduce, Reuse, Recycle: It is possible to
spend all kinds of money on aftermarket
mufflers, and some of them are quite good.
However, if your engine came from the
factory with a simple expansion-chamber
muffler that has no baffles in it, there is
still a simple, inexpensive option.
An exhaust extension makes the exhaust
system longer, effectively “tuning” or
filtering the harshest, high-frequency, parts
of the exhaust note. All you have to do to
prove this is play trumpet with a long
cardboard tube, such as the kind heat-shrink
covering material is wrapped around.
Cut the tube shorter, and if you have the
lung power the note will be higher and contain
more high-frequency harmonics. In general,
high notes are more annoying—except for the
staccato howl of a racing engine!
This technique was used effectively on the
Tiger 60 ARF that was reviewed on MA’s
Sport Aviator. A silicone exhaust elbow and
61/2 inches of 1/2-inch-inside-diameter tubing
turned a noisy 20-year-old muffler into an
extremely quiet exhaust package.
The extension more than doubled the
length from the exhaust port to the
atmosphere. Although the exhaust note was
dramatically mellowed, the horsepower was
barely affected.
Larger engines respond beautifully to the
large canister-type mufflers, sometimes even
enjoying a horsepower boost. The
mechanism behind this would take more ink
to describe than we have left here, but we
will go there if and when someone writes in
looking for an explanation of tuned exhausts.
Similarly, the four-strokes benefit from
having a long header pipe and then a
muffler. Go to the local drag strip on
“muffled-car night” and poke around. Some
of those race setups make beaucoup
horsepower! The header pipes on almost all
the racecars are tuned to improve the torque
in some desired rpm range.
In our case we are looking to boost the
torque at rpm that will allow us to meet the
rule of 130. That often means a header pipe
that is roughly a foot long, made from
aluminum, steel, or sometime Teflon tubing.
Teflon works better than you might
guess. Clamp a piece with the right inside
diameter around the end of the header stub
that came with the engine, find a way to
attach it to a small muffler, and you are in
business.
Teflon is slippery, so it takes a fair bit of
work to figure out how to keep things
clamped together, but it holds up well once
you get it. Industrial supply houses such as
McMaster-Carr and Grainger stock the
tubing.
Time is up, and we already have a
handful of dangling links in addition to
never having addressed the tuned-pipe
discussion that got us started on noise
abatement. There are other aspects to noise
abatement, such as soft engine mounting,
intake muffling, and shrouding the entire
power train, but propeller noise and muffling
take care of the lion’s share.
Until next time, enjoy your time at the field
or in the workshop, and please feel free to
send in any questions or observations you
might have. This column is all about making
the remote corners of this hobby/sport work
for you