Focus on Competition - 2008/08

August 2008 179
Focus on Competition
sheer distance and is only a factor when
operating at or near airports.
We don’t have either luxury; we fly
relatively low compared to full scale and
we’re on the ground every 10 to 15
minutes at the same airport. We also
operate more than one airplane at a time
from the field, so if enough members are
flying on a Sunday afternoon there is an
engine running almost all the time for
several hours.
Needless to say, no matter how quiet
we are, it’s going to annoy someone
sometime, so we need to be aware of how
sound carries and do everything possible to
contain it and control it. Here at AMA we
don’t have specific set guidelines for noise
limits at sanctioned fields. Mostly due to
the variety of site locations and
municipalities that govern these areas, one
shoe will not fit all.
We are in the process of forming a
committee to try and address this issue and
get at least some type of workable number
system that will work a majority of the
time. The old numbers that were published
back in the early 1990s stated 90 dB at 9
feet. There never was much adherence to
that because 90 dB under perfect
circumstances was almost impossible to
attain. I know of a few Pattern fliers who
did get those numbers, but only a rare few.
In the interim, here are a few items that
may help you get your noise footprint
down to a manageable level.
1) Try a three-blade propeller. The
most important thing to remember about
airplane noise is most of what you hear
does not come from the engine; it comes
from the prop. What you hear when your
engine unloads in the air is the prop tips
going supersonic, so getting the engine
rpm down will limit much of that.
So you think a bigger two-blade prop
will slow the engine down, right? Wrong!
Yes it will slow the engine down, but now
you have longer blades and the tips are
actually going faster (physics).
The answer is to add blades so you
keep the diameter down while also
reducing the rpm. The reason you see most
THERE’S NOTHING better than hearing
a well-tuned model engine (or two)
running nicely and pulling your favorite
model through the air with ease.
Unfortunately not everyone is in
agreement that model engines make a
pretty sound or should make a sound at all.
Airplanes, whatever size and shape, are
historically noisy. Since the Wright
brothers, airplanes have been in need of
maximum power production, and large
restrictive mufflers not only cut down on
noise but also cut down on power.
Because of that, mufflers have never
been very high on the designer’s priority
list. Still today, most piston-powered
aircraft have small, inefficient mufflers if
they have any at all.
During WW II, several airplane
engines had nothing more than a 6-inch
open stack on the exhaust valve, just
enough to clear the cowling and that’s all.
Another theory being airplanes fly high
enough that the noise is controlled by
Technically Speaking ...
Greg Hahn
Technical Director
[email protected]
of the big IMAC guys now using threeblade
props is not for performance, but
noise control. If you don’t know how to
figure for a three-blade prop, you might
ask your engine manufacturer for a
recommendation. Or a rule of thumb is to
reduce diameter by 1 inch or pitch by 1
inch or both if needed.
2) Use as large as possible “Canister
type” muffler. Just about all engines, glow
and gas, sold on the US market today are
supplied with a large can-type muffler.
Most of these supplied mufflers will do an
adequate job of controlling the
combustion noise coming from the
cylinder.
Adding a piece of rubber hose to the
outlet pipe can also help in stopping some
of the ringing sound that can be produced
by the muffler. The rubber hose doesn’t
have to extend past the pipe; just cover it
like insulation.
3) Soft mount your engine. Often there
can be quite a bit of noise coming from
your airframe, especially if it contains a
lot of fiberglass or is open structure
covered in plastic film. When the engine is
hard mounted to the firewall, the vibration
will be carried back through the airframe
and essentially act as a drum unless there
is some type of absorption material in
there to soak it up.
Many of the newer 30% and 40%
aerobatic aircraft have foam turtledecks
and other foam parts that absorb much of
the vibration noise; if not, your beautiful
machine will sound more like a drum kit
than a purring kitten. There are several
soft mount systems on the market today.
By the way, to my knowledge all fullscale
piston engines are soft mounted.
Take a lesson from the big boys.
Hope these few ideas will help you
control noise at your field. It’s very
important that we stay on top of this issue
because it is one of the biggest threats we
have to saving flying sites. If we don’t
control it our neighbors will, and it won’t
be pretty.
See you at briefing. MA
National Record Updates
CONTROL LINE
A Speed (302)
Open 190.73 Chris Montagino 04/19/08
INDOOR AMA CEILING CATEGORY II
Intermediate Stick (202)
Open 33:09 Larry Coslick 04/17/08
As of June 2008 Competition Directory Updates
Racing Contest Board
District VII
David Norman
14227 Crane St. NW
Andover MN 55304
**The complete Competition Directory listings run every other month.