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 th
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