Evening star., 1938-10-23
SNENC|NG PLANE
. 80
ARMY CORPS AIM
Search for Way to Reduce
Propei|er's Noise Is
Spurred by Games.
BY JOSEPH S. EDGERTON.

Muff1ing the reports of pistols is an
a easy task compared with the silencing
of the modern military airplane.
Although the Army Air Corps long
has sought some means of reducing the
sound output of its aircraft and the
search has been intensified as re-
suit of the recent North Carolina anti
aircraft exercises, which show ground
observers can hear airplanes at high
attitudes. the solution still is remote
The Army's quest for the silent air
plane is no half-hearted affair 1t
is realized that only through some form
sof muff1ing can bombardment and at
tack airplanes hope to take their OBJ
lectives by surprise or hope to escape
the combined fury of airplane inter-
ception and anti-aircraft defensive
fire
The muffling of an airplane engine
is not difficult, though it can be ac-
complished only through a serious loss
in power The chief airplane noise
maker, however, is not the engine but
the propeller. The silencing Of aircraft
ing of engines is easy, the Air Corps
has found.
2,10O Revolutions a Minute.
Take the case of the Douglas Bib
bombardment airplane, now the most
numerous type in use by the Air Corps
This airplane swings two three -bladed
propellers IL feet 6 inches in dame
ter. The maximum permissible speed
of the engines is 2,100 revolutions per
minute for normal high-speed opera
tions. This speed may be exceeded in
dives. The propeller is operated
Through reduction gears at ratio of
is to H. Nevertheless, at high speed
the propeller tips are moving at a
speed of approximately 865 feet per
second The speed of a .38-caliber
Army Colt revolver bullet is $50 feet
per second.
IF you ever had stood close to a
target and listened to the crack Of a
Colt revolver bullet going by" a
veteran Air Corps officer observed,
you can realize that to silence an
eirplane propeller tip. which is a
tremendously larger object, moving at
even greater speed. is something Of a
problem. And when you figure that
with three blades, you are hearing the
tips 60 by at rate of something like
5.400 times a minute, you begin to
appreciate what We are up against"
Manufacturers of airplane propel-
lers and users of propellers find it
necessary to test them thoroughly
under over-loads, which sometimes
are carried to the point of destruction.
For these tests the propellers are turned
.by silent electric motors. Yet the pro-
peller sounds are so terrific that the
tests cannot be made within five
miles of any residential area. The
shrek of large propeller being
whirled on the great test stands at
Wright Field, Dayton, Ohio, is so
tremendous that the effect h acutely
nauseating to most persons, despite
every effort to muffle the sound by
insulating the enclosure. Tests cannot
be carried on while people are on
duty in the laboratories on the field;
1t is impossible for them to converse
or even to work through the din.
Air Raiders Heard.
During the North Carolina exercises
one of the greatest surprises was the
ability of civilian observers to hear
and track invading air raiders. even
when they climbed to their maximum
ceilings of $4,000 feet or more.
The invading black" commanders
endeavored to escape the keen ears OF
the observers on one mission by fitting
engine siiencers, somewhat similar to
automobile engine mufflers, to the en-
gine exhausts of a four-engined Bit
flying fortress" bomber. The silenc-
ers made little difference to the OBJ
servers, the airplane was tracked sue
cessfully from the Atlantic Coast,
where it entered the warning net, to
Fort Bragg. the target. more than 200
miles inland. The ability of the OBJ
servers to hear the bomber was not
due to engine noise, but to propeller
noise.
Of course, it is theoretically possible
to silence an airplane propeller. This
may be done by slowing down its speed.
To do this, however it would be neces-
sary to increase the size of the pro-
peller or it would become useless for
propulsive purposes.
Practical Limitations.
There are. however. practical limi-
tations to propeller sizes. Weight in
Creases tremendously with size. More
serious than this, however, is the fact
that the airplane must be equipped
. with ''longer-le2ged' landing gear to
keep the larger propeller off the
ground. This, coupled with the in
creased propeller weight, tends to nose
the airplane over on the ground. The
larger landing gear also offers in
creased resistance to airflow, in itself
creates greater weight and increases
the difficulty of retraction in fight.
Another solution often advanced is
the uee of propellers with more blades.
TIE three -bl"ded propeller now is
strn6ard in this country. Coupled
A'''i'h c01trollable pitch equipment, it
T. 90 "do-i as far more efficient than
'"-N'7lcd prOpellers used in Europe
"" E; nrcpel,erS with four or more
''~'"~ u0u'f ro-ut only in chewing
80 q" u-"ieSelv. it is held. unless
- '"' >o"~tely were made less effec-
t''~1 our present adiustabl pitch
= 'oi'"r' at p"escnt airplane speed.
G>~,, 0,ryro,e yIroc,ooJ