If It Flies... - 2009/02

Also included in this column:
• Quiet the loudest part of
the airplane
• Propeller efficiency
Goldberg Tiger 60
ARF with the muffled
YS .61 and APC 13 x
7 Sport propeller.
The muffler and Teflon tube
extender combination robs
power but allows the power
plant to pass a noise test of 92
decibels at 10 feet. The .61 will turn a
13 x 10 at the same 10,000 rpm using
a tuned pipe!
IF IT FLIES, it just might crash. I’m not
referring to our model airplanes, although,
sadly enough, it is sometimes true;
sometimes columns crash. Okay, maybe
this time I’m referring to a bad landing
rather than an outright crash. There was an
error in the October issue that needs
correction.
I don’t know exactly where it happened,
but I do know how: pilot error. It’s always
pilot error, because the preflight inspection
is supposed to catch things such as this.
The editor had requested that I shorten
what had originally been submitted,
because it would have filled roughly twice
the allotted space. Yes, that was pilot error
number one. So I shortened it in a hurry and
skipped the final proofreading because
these things always seem to happen when
you’re up against a deadline.
Putting things together in a hurry and
not double-checking is a great way to cause
a crash. I once beat up a good airplane just
two weeks before leaving for the AMA
Nats that way. Anyhow, more than a few of
you wrote to call my attention to the dumb
thumbs I had committed. Thanks.
The October column (near dead center
Rpm Vs. Diameter for Propeller-Tip Noise Reduction
This diagram encapsulates the 130 rule of thumb. Its purpose is to prevent the
noise generated by transonic and supersonic propeller-tip airflow. Diameter and
rpm combinations in the upper right make the in-air snarl or howl that frequently
annoys neighbors.
Propeller Airspeed Values
This nomograph is not too useful for predicting
airspeed because of rpm increase in the air and
because similar pitch numbers stamped on
different brand propellers don’t always mean
the same thing. It is useful for predicting the
higher same-brand pitch that will be needed
after a noise-friendly rpm reduction.
of page 86) reads, “Your typical foot-long
propeller turning in the neighborhood of
12,000 rpm has a blade tip speed that just
exceeds the speed of sound. The propeller
tip is moving at roughly 425 mph,
compared to the nose of the aircraft.”
As it turns out, the blade tip is moving at
approximately 425 mph (the actual
calculation is slightly higher, and I
rounded), but the speed of sound at
“normal” temperatures is close to 767 mph.
The propeller tip is moving along at
nearly Mach 0.56, or 56% of the speed of
sound, so it isn’t exceeding the speed of
sound. Oops.
With all the fervor of a National
Transportation Safety Board aviationaccident
investigator, I went digging for an
earlier version of the computer document.
There it was: “Your typical foot-long
02sig3.QXD 12/22/08 12:33 PM Page 85
propeller turning in the neighborhood of
12,000 rpm has a blade tip speed of just
over half the speed of sound. The propeller
tip is moving at roughly 425 mph,
compared to the nose of the airplane.”
At least it was right at some point,
although it would have been good if I’d
proofread just one more time. As I
mentioned, it’s almost always pilot error.
In the spirit of making lemonade with
unexpected lemons, this started me
thinking. I originally chose the example
because propeller tip speed of Mach 0.56 is
noise-friendly. I like to stick with noisefriendly
examples when I can.
The sound caused by the propeller tips
as they approach and exceed the speed of
sound is what causes that loud snarl or howl
that typically characterizes a noisy airplane.
I’ve heard many call it the “ripping sound.”
Whatever you call it, it sounds like raw
performance to many of us, and it sounds
like a nuisance to our flying-site neighbors.
Nothing you ever do will satisfy an
unreasonable neighbor. That’s true both as
an aeromodeler and as a homeowner, but
only the unreasonable will complain when
we take reasonable steps to reduce our
annoyance factor.
This propeller-tip noise is often much
louder than the engine’s exhaust, and it can
cause high-performance electric-powered
models to be quite loud as well. This is
because of the rotating sonic booms that
emanate from a rotating propeller when
parts of the blade (mostly out near the tip)
go transonic or supersonic.
The shockwave and loud sound caused
by an object that moves faster than the
speed of sound (or supersonic) is called the
“sonic boom,” but what does transonic
mean and why is it important? I’ll get there
in a moment.
For most airplanes with engines sized
.40 and over, the loudest noise source is the
propeller, unless you’ve taken steps to
change that. High-rpm racing engines,
ducted fans, CL Combat engines, and the
like tend to have very high propeller-tip
speeds no matter how small the
displacement. The next most prominent
noise source is the exhaust, and then finally
the carburetor and airframe.
Actually, the airframe noise can vary
wildly, depending on the airplane’s
construction. The important thing is that
attacking a lesser noise source such as the
muffler is not going to help much if the
propeller tips are howling, so in general we
start by dealing with the propeller noise.
When an airfoiled wing section moves
through the air, the airflow around it speeds
up locally. I wrote about that in the last
column.
That locally accelerated airflow can
break the sound barrier even though the
wing itself is moving through the air at
substantially less than Mach 1.0. That local
breaking of the sound barrier tends to start
near the airfoil’s high point, especially on
the top surface.
Propellers that are carefully optimized
for noise reduction can be operated at tip
speeds that are close to Mach 0.6 (maybe
slightly more) without transonic effects and
noise. But for most purposes, we need to
stay at less than Mach 0.55, as measured on
the ground.
With a small allowance for round-off
error, that corresponds to a 12-inchdiameter
propeller turning at 12,000 rpm.
This is actually close to the edge, as far as
noise is concerned, because the in-air rpm is
usually a bit higher than what we measure
under static conditions.
Rather than calculating propeller tip
speeds and comparing them to Mach 0.55
(767 mph, 1,125 feet per second, or 343
meters per second), we can use a rule of
thumb. Multiplying the diameter in inches
by the rpm (in thousands) for that 12-inch
propeller at 12,000 rpm, we get 12
multiplied by 12, or 144. Taking 10% off
for that in-air unload I mentioned and
rounding off a bit, we get 130.
If either the diameter or the rpm
increases, the tip speed will be higher and
vice versa. If the diameter doubles and the
rpm halves, the tip speed and the multiplied
product will stay the same, so we can use
simple multiplication and a rule of thumb to
substitute for all that calculation. A figure
of 140 is really the borderline. If the
multiplied product is less than 130, the
propeller will not howl unless the in-air
rpm rises a lot, as in a full-throttle dive.
This rule works well in practice, and its
effect is to set an rpm limit for any given
diameter. Using the more conservative
figure of 130, you see that a 10-inchdiameter
propeller is limited to
approximately 13,000 rpm. A 20-inch
propeller must turn slower than 6,500 rpm
to be quiet.
Now it becomes easier to understand
why canister mufflers with their low-end
torque-boosting characteristics have
become popular on the big gas burners. Not
only do they muffle the exhaust note well,
but they also help the engines to breathe
effectively at low rpm. Two-stroke exhaust
scavenging is a subject for another day—
maybe sometime soon.
A little 6-inch propeller can turn at close
to 22,000 rpm without howling, but the
exhaust will have a piercing note at that
number. For a 10-inch-diameter propeller,
the arithmetic is easy, at 13,000 rpm, and
for 11- and 12-inch diameters, the
revolutions are limited to roughly 12,000
and 11,000 respectively.
Now that we know that we need to limit
the rpm based on the diameter, we have a
tradeoff to juggle. Let’s say you have an
airplane with a .40-size engine that was
originally bought for Quickie 500 racing.
Such an engine was probably intended by
its manufacturer to deliver its best power
and handling characteristics at fairly high
revolutions.
It would do to keep such an engine
propped for close to 13,000 rpm with a 10-
inch diameter and keep adding pitch until
the engine was loaded down to that figure.
How much would it take? I can’t say for
sure, but although a 10 x 6 propeller is
often used on a .40, it might take a 10 x 8 or
a 10 x 9 on a really strong engine. Yes,
these propeller sizes are available, even
though many hobby shops don’t stock
them.
On the other hand, let’s say that you
have a .40 or .45 engine with a reputation
as a torquer. Such a power plant might be
happy in the mid-10,000 rpm range, where
a 12-inch diameter is usable. You would
then change the pitch to load the engine
down to less than 11,000 rpm. The question
remains: how much pitch does it take to
load the engine to just less than 11,000 rpm
with that big of a propeller?
You’ll have to experiment. But if you
can run enough pitch to get the desired
airspeed, and the load produces the desired
noise-friendly rpm, the greater efficiency of
a large-diameter propeller will probably
outweigh the slight power reduction caused
by running the engine at reduced rpm.
The important thing is not the
horsepower at the crankshaft, but the
horsepower that comes out of the propeller.
The difference is propeller efficiency.
Propeller efficiency is better when the tip
speeds are reduced and when the tips are
farther away from each other, as in more
diameter. In that last respect, they are much
like wings.
Glider wings have high aspect ratios,
because that makes them more efficient.
How that works is the subject for yet
another day. The aspect ratio of a wing is
the ratio of the wingspan to the average
wing chord. That’s the distance from LE to
TE.
The reduction in tip speed is also an
efficiency booster, because the drag losses
on a part of the propeller vary with the
square of the speed of that part of the blade.
It all depends on how low of an rpm at
which your engine will run without lugging.
Although the “standard” propeller for a
.60-size engine has been an 11 x 7 for as
long as I can remember, 12 x 8s are great
for most sport models. Most .60s will turn a
12 x 8 at roughly 11,000 rpm. If yours is
stronger, try a 12 x 9.
Only the airplanes that need to fly fast,
such as some warbirds, or aircraft you want
to fly as fast as possible for fun need to
stick with the 11-inch diameter and 12,000
rpm. There you might need a pitch as high
as 10 inches to keep the revolutions limited.
Expect the takeoff roll to be slightly
longer than with an 11 x 7. But even though
the model sounds quieter, it will go faster
once it is “on the step.”
I have been flying a Carl Goldberg Tiger
60 ARF with a muffled YS .61 turning an
APC 13 x 7 Sport propeller, with great
results. The engine turns that propeller right
at 10,000 rpm, while my O.S. .70 Surpass
turns it at just more than 9,000 rpm. The
YS-and-13 x 7 combination is only as quiet
as the muffler, because propeller noise is no
longer an issue. Now I need to find a decent
muffler to fit this engine that doesn’t weigh
a ton.
A diagram shows the rpm-and-propellerdiameter
relationship that satisfies the rule
of thumb of 130. If your rpm-and-diameter
combination lies to the upper right of the
curves, the propeller is likely to be noisy. If
it is below and or to the left, you are in the
quiet zone.
For those who are mathematically
inclined and the Algebra 1 students out
there, the curve is a hyperbola. The formula
for the “130” curve is Y (or rpm) = 130,000
divided by X (or diameter). Kids, no matter
what some adults will tell you, algebra can
be useful.
Also shown is a nomograph for airspeed
vs. rpm for different values of propeller
pitch. The speeds it predicts are subject to
substantial errors, because there is no
explaining how some manufacturers come
up with the pitch numbers they stamp on
the propellers.
However, the graph is useful for
figuring out how much you would have to
change the pitch to fly at the same speed if,
for example, you wanted to change the rpm
from 14,000 to 12,000.
All you have to do is draw a horizontal
line through the point where the diagonal
pitch line (for the propeller you are using
now) crosses the rpm you measure. As you
move across that horizontal line, you can
see what rpm would correspond to an
increase or decrease in pitch. Then move to
the diagram showing rpm vs. diameter to
see if the new combination is in the quiet
zone.
When you do this exercise, you may
find several propeller sizes that “work” as
far as noise is concerned. Start with the
higher-diameter alternatives; they offer
better takeoff performance.
It’s time for me to sign off. Until next time
we get together, have fun and take care of
yourself. MA