80 M ODEL AVIATION
Gary A. Shaw
S a f e t y C ome s F i r s t
Box 4520, Milton FL 32572; E-mail: [email protected]
fingers will remind me of that April first
when I was the fool.”
From the Mailbag: In previous columns
(August-December 2000), I’ve written
about the dangers of propellers and
included a few readers’ questions
regarding safety concerns about using
current propeller technology.
The original question posed to me was
why AMA allows the use of the current
variety of propellers (carbon fiber,
fiberglass-filled, plastic), but not the use
of metal propellers.
A strong opinion about my response
arrived from modeler and engineer John
T. Deden.
“Just read your [December 2000]
column in Model Aviation, in specific the
‘From the Mailbag’ on why metal props
were—and are—unlawful, and plastics are
relatively (?) safe.
“I’m an old USAF [United States Air
Force] pilot, a collector of prop-driven
aircraft photos and data, and an
aerospace engineer of some 40 years
experience, dating from propeller jobs,
to jets, to space.
“The comment that metal props are
more dangerous because ‘they don’t
stress well,’ and the other
statement about the
main factor for their
being banned was
because they have
more mass, are heavier,
of blood, they would not stick. With my
hand held high to try to slow the bleeding,
she finally managed to get both fingers
wrapped with paper toweling and
electrician’s tape.
“We both then set about disassembling the
airplane and the field-box stand. It took about
10 minutes to get everything into the car and,
with directions from other fliers on the field,
another 10 to drive to the nearest hospital.
“The emergency room doctor found
when he unwrapped the bandaging that
the bleeding had stopped, but because the
skin was shredded, it could not be
stitched. He ordered X-rays and found the
bones had hairline cracks. He then
rebandaged the fingers while the nurse
gave me a tetanus shot.
“I was placed on penicillin and told to
see my family doctor in two days for
rebandaging. The ER doctor said it could
be up to a year before the skin healed
completely and I regained full use of the
fingers.
“Because I had read so many articles
about safety with props, I was very aware
of what not to do, but somehow I was
traumatized by the failed attempt of the
flight and forgot the number one rule:
When the airplane engine is running, never
do anything else with the airplane, but pay
strict attention to that engine. If there is
anything else that needs to be done, shut
down the engine first.
“The airplane was easy
to repair, but
these damaged
A FOOL’s DAy: It’s been some time
since I provided a hand-in-the-prop story.
However, since the spring flying season is
around the corner, the following letter
from Albert Isaacson is a good reminder
for those who might become careless
around running engines.
“April 1, 2000 was the day I had been
waiting for. At the beginning of the year, I
had finished my 1⁄7 semiscale model of a
Savoia-Marchetti S.56 amphibian. Tom
Hunt had agreed to be my test pilot, but we
needed a relatively mild morning with
winds no stronger than five miles per hour.
April first was just that sort of day.
“In August of 1996, RC Modeler had
printed my construction article of a scratchbuilt
Curtiss SBC-4 Helldiver of my design.
The magazine had shown an interest in the
Savoia-Marchetti story, but since it too was
my design, it had to fly successfully. Thus
my rush for that first flight.
“Unfortunately, the field Tom uses,
almost an hour and a half from my home, is
a rough, grassy one, and the underside of
the hull of the Savoia has only about 3⁄4-
inch ground clearance. The wingtip floats
are about 1⁄4-inch higher than that.
“Because of inadequate ground
clearance, and about 2° of down-thrust, the
two attempts that were made to take off
failed, and on the second one, the left wing
float caught some grass on the ground and
the airplane nosed over. The damage was
slight, and in a couple of weeks it would be
repaired. The damage to my confidence
would take longer.
“I placed the airplane on the field-box
stand and started the engine to run the tank
dry, but that is not the end of the
misadventure, only the beginning. My
wife waited patiently in the car as I
separated the interplane struts from the top
wing and went about looking for more
structural damage.
“Sighting down the fuselage from in
front of the airplane, in my emotional state
I forgot completely that the airplane
engine was running.
“I reached forward with my left hand to
touch a slight wrinkle in the covering at the
gear when it happened.
“The ‘thwack’ I heard and the blood I
saw came from the knuckles of my pinky
and ring fingers, and were caused by the
tip of the blade making a series of thin
slices in my skin. I ran to my wife and
yelled to get out the first-aid kit.
“She applied some antibacterial gel and
Band-Aid™s, but because of the amount
and have more kinetic energy, which
causes them to have pieces fly/go
farther upon breaking is horse manure!
“Examples: Some years back I was alone,
flying my sport/Pattern biplane, which used a
Saito 80 and a major manufacturer’s plastic
14-inch-diameter, six-inch pitch prop at
approximately 9,500 rpm.
“I had completed one fine flight, then
gassed up, checked batteries and controls
and, using an electric starter, fired up the
Saito with throttle in the full-retarded
position as usual.
“Engine started, then dead silence, and
the engine stopped. Fortunately, my habit is
that as quickly as the engine starts, I move
to behind the prop arc, which I had done.
“Examining the now off and silent
engine, I found the entire prop missing,
including the prop washer and retaining
nut. Hmmmm. Where did the prop go, if
anywhere?
“Searching the immediate area, I found
a six-inch slice of prop and tip buried in
the dirt beneath the edge of my flight box,
which was some six to eight feet away.
(Later I found a neat slit in the lower edge
of my left pants leg where that piece of
blade passed me by.)
“Now to the rest of the story: Where
was the balance of the prop? Looking
within 10 or so feet of the model, no luck.
Walking straight out from the thrustline of
the airplane, I found the balance of the
prop partly buried in the earth, halted by a
small clump of tough grass.
“How far? That remainder of the prop
had traveled some 40 feet in a fraction of a
second! So, plastics have low angular
momentum, eh? Baloney! I know a little
about it, much more when I was younger
and sharper, but the bottom line is: plastic
or unobtainium, that major piece of prop
had a lot of rotating forward momentum to
travel some 40 feet straight ahead of the
model in less than, say, two seconds.
Sounds pretty dangerous to me!
“Some others of our club have had
broken plastic prop blade experiences
I’ve been told.
“Aluminum metal too easily fatigable?
Another false/incorrect statement.
“I have in my possession dozens of
photos of bellied-in P-47s, P-51s, and P-
38s, not to mention various bombers. All
these airplanes had metal props which
typically were rotating at least at 1,000
rpm on impact or were feathered (on the
P-38 and bombers).
“Every photo shows the prop blades
neatly bent back and often twisted
severely, but not broken! And not fatigued
and flying off in various directions!
“Yes, it is true that under certain
conditions, like a power-on or full-bore
full-scale airplane crash, prop blades, as
well as many other aircraft parts, fly off in
various manners and to various distances,
but usually not far at all—and those are
very heavy full-scale props.
“And yes, it is true that there are
known instances of full-scale airplane
April 2001 81
props throwing a blade while the engine is
running, typically airborne at medium-tohigh
rpm settings, and indeed when this
happens the prop blade can travel many
hundreds of feet and can slice through
fuselages and wings like a hot knife
through butter.
“(I saw the results of this happening on
a B-24 in Colorado. Really bad! The blade
cut off the back of the pilot’s head, as well
as going completely through the fuselage
cockpit area and out. But it was more of a
hub failure rather than a blade failure.)
“The old original Hurricane and Spitfire
props were of high-class laminated wood,
and when they broke for whatever reason
while rotating, they sent splinters, both
large and small—many yards.
“Wood and plastic model props can do
the same and that’s one very good reason
for the safety admonishment to start
engines then quickly get behind the prop
arc, but also for the safety rule of starting
engines, regardless of prop type, with the
engine and prop pointed to the runway.
(That way only the modeler on the
flightline flying is in hazard!)
“Now I readily admit that the metal
model props I saw long ago were of cast
aluminum, which indeed does not have any
significant bendability without breaking,
but I would hope that anyone producing
metal model props today would have sense
enough to either forge the blades, or use
highly ductile and highly stressable metal
formulations and manufacturing
techniques, as do full-scale
manufacturers.”
John’s point that wood, plastic, and
carbon props develop enough energy when
parting from the hub to cause bodily harm
is noteworthy, although I’ll need to get
better information before responding to
comments regarding metal flexibility.
My plan is to forward the question to
AMA for a qualified answer. (Editor’s
note: The question has been turned over
to the AMA Safety Committee for
consideration.)
that’s it for this month. Hope all your
landings are on the spot intended! MA
