Safety Comes First - 2009/11

100 MODEL AVIATION
Dave Gee | Safety Comes First [email protected]
Also included in this column:
• Ashley MacArthur gets her
picture in Model Aviation
• Aerobatics from inside the
airplane are way different from
RC
• Stressing about motor mounts
A story about battery terminals: touching and short
Ashley MacArthur displays an array of battery-pack connectors.
Exposed terminals could cause shorts when stored.
Above: Judy Phelps, aerobatics flight instructor
and competitor, has the skill to fly crisp
maneuvers despite the author’s whimpering in
the rear seat.
Right: A Hammerhead stall in progress
demonstrates that full-scale stunt aircraft
must be properly designed and braced, as
must our models.
GOOD OLD MURPHY! If it weren’t for his law, I might be
writing pet obituaries somewhere instead of enjoying a position here
at the pinnacle of aeromodeling journalism. “If anything can go
wrong, it will go wrong.”
Our miniature aircraft can be complicated things and there are
plenty of ways that they can go wrong. My e-mail brings new
stories all the time.
Mark Miller wrote to me about his exciting experience with a
simple, inert box of batteries. He stores his Li-Poly cells in a plastic
box rather than in the aircraft. Check. Each pack is in its own
compartment, to prevent shorts. Check.
Mark says he closed the lid of his storage box and then, as he
wrote:
“ ... there was a loud pfffft! and the box filled with smoke. I was
baffled at first—the pack hadn’t been damaged or even handled
roughly. It hadn’t been over-discharged, and it was sitting in its own
compartment of the plastic box.
“After the smoke cleared, the pack appeared scorched but not
punctured. I use Deans Micro Connectors with only one exposed
pin, so I wasn’t concerned about shorts.
“However, the tops of many balance connectors leave part of the
balance pin exposed, although the connector itself is female. When I
closed the lid of the box, it evidently pushed the bare power-
11sig3.QXD_00MSTRPG.QXD 9/25/09 9:50 AM Page 100
November 2009 101
connector pin into the exposed pin on the
balance connector and pfffft!”
According to Mark’s report, there was no
damage beyond that one battery pack, but a
serious potential problem has been exposed!
He is now considering putting heat-shrink
tubing over the balance connector, so that
those pins aren’t exposed, or maybe
rethinking his whole connector system.
I wouldn’t have predicted trouble from
such a well-designed battery setup.
A local survey showed that some pilots put
dummy plugs in their storage cases and plug
the packs into a dead pin. Other fliers bag
each pack individually, which prevents shorts
but impedes cooling and ventilation. Even
rubber-banding the leads into a neat coil
would not be a 100% guarantee against this
problem.
One of this month’s photos shows my
granddaughter, Ashley MacArthur, breaking
into print modeling by displaying a variety of
electrical connector styles from my
workbench. All of the brands and types are
well designed for a particular use, but will not
work for every purpose.
A close examination of the connector ends
showed that some of them could be touched
by metal when disconnected. I’m going to
take a fresh look at how I manage and store
my cells, and try to out-think Murphy.
Things can “go wrong” with our airframes
also. Phil Alvirez and I corresponded about
the types of motor mounts used on electric
airplanes these days. He is cautious, with
more than a little engineering expertise, and
he recommends Allen bolts and blind nuts
behind the firewall. A layer of nylon cloth
coated with cement completes the installation
so that, as he wrote, “the motor ain’t goin’
anywhere.”
Phil and I both worry about models with
motors mounted on a plywood box glued to
the firewall. It works well most of the time,
but it’s nice to have reinforcement in the form
of corner braces or angles and some bolts.
Epoxy adheres only to the top layers of
plywood. You might have seen such a joint
“pop loose” under stress. This is why
mechanical backups such as bolts or screws
are important.
The ARF revolution is bringing in more
new pilots who fly, but do not build their
models. Some are unfamiliar with airframes
and other structures that are designed to deal
with heavy stress.
When their airplanes are damaged, they
might not know how to properly repair and
reinforce the airframes to spread and absorb
the forces of flight. Issues such as this are no
longer covered in high school shop classes!
An ARF pilot might have quite a bit of
flight time, but assembling a kit or scratchbuilding
means another learning curve. An
expert computer programmer or tax specialist
will find that doing structural work on a
model airplane is a new thing.
In the olden days, building model airplanes
was considered good training for careers in
drafting, aviation maintenance, or any
mechanical field. (When was the last time you
saw a drafting table in use?)
Now that drafting—and nearly everything
else—is done on a computer, people aren’t
trained in basic, hands-on mechanical work.
Those of us who do get this exposure
eventually learn to spot design problems even
without an engineering degree.
You might have seen structural failures on
model aircraft, sometimes because of faulty
design, but more often from poorly done
repairs or improperly stressed modifications.
By “stressed” I mean designed to withstand
flight stresses by absorbing them and passing
them throughout the rest of the airframe.
Even without formal education in airframe
design, many builders have learned what
works and what doesn’t from the good and
bad models they have seen. Experienced
model designers and builders can look at a
picture of a pioneer-era aircraft and describe
exactly why it won’t fly and where it will
break when the engine runs. Those airplanes
were designed by hit and miss, when nobody
had sufficient experience with flying
machines.
I had occasion to compare modelconstruction
styles with full-scale aircraft
structure. I was highly interested in the
construction of a certain Bellanca Citabria,
because I got to go up in it for some full-scale
aerobatics.
The original plan was to use a Pitts twoseater
biplane, but there was too much of me
and not enough cockpit, so we went with the
roomier airplane. The Citabria has a weldedsteel-
tube fuselage structure with numerous
diagonals and crossbraces to spread the Gforces
of the pilot and her large, nervous
passenger.
Models are built the same way, because
we sometimes do things with them that no
onboard pilot would try. The engine mounts
were integral with the airframe so that forces
of torque and thrust could dissipate
throughout the entire airplane.
My pilot was Judy Phelps of CP Aviation
in Santa Paula, California. She is an
experienced aerobatics instructor and
competitor. Judy took very good care of me,
because the cockpit is so hard to clean. I was
even given the stick for a while, and when she
complimented my nicely coordinated turns, I
had to confess that I was trying to fly straight.
Aerobatics are way different from inside
the aircraft! I would not have been able to
even identify some of the maneuvers we
performed if Judy had not narrated. The
airplane seemed to take forever to do moves
that my models do in an instant, but my
models don’t get airsick.
Judy asked me how I was doing after each
stunt, and there came a point when I told her
we were finished for the day—or else. Back
on the ground, she looked as fresh as a daisy
while I was draped over a chair, recovering.
Just another day at the office for her!
A full-scale aerobatics machine is
carefully and meticulously maintained, since
any failure could be fatal. Our models are
different in many ways. We can afford to be
Does Your Club
Have a Flying Site
Success Story?
If so, MA wants to hear about it!
Finding and keeping flying sites is
a paramount issue facing the Academy
and its members. Just as there are
stories about clubs losing flying sites
or being unable to find flying fields,
success stories abound as well. It is
vital that these success stories be
shared so that others may learn and
apply sound methods for finding and
keeping flying sites.
Your club can assist others in
similar situations by submitting an
article to MA that meets the following
guidelines and includes the following
information.
• Text—not to exceed 1,500 words—
should include the name and location
of the club, the chronology of events
leading up to obtaining or retaining the
site, the names of club members who
were responsible for the success story,
and the members’ opinions about what
their most important action was that
ultimately resulted in the venture’s
success. Note which government
agencies were contacted and the
process involved.
Electronic text should be submitted
in Microsoft Word on a CD or via Email.
• Photos and diagrams of the field
should be included. Pictures should
number no more than a dozen and
must complement and illustrate what is
in the text.
Glossy 35mm prints and digital
photos on CD will be accepted. MA
will not accept E-mailed photos or inkjet
printouts of digital photographs.
Please include captions on a separate
page.
• Send your article to MA Editor
Michael Ramsey at 5161 E. Memorial
Dr., Muncie IN 47302.
All submissions are subject to
review prior to acceptance. Payment is
made upon acceptance. Article
submissions will only be returned if an
SASE is included.
For more information, contact
Editor Michael Ramsey at (765) 287-
1256, extension 223 or michaelr@
modelaircraft.org.
11sig3.QXD_00MSTRPG.QXD 9/25/09 9:50 AM Page 101
slightly more casual with them, since they
are unmanned and they are usually simpler
and easier to inspect and service.
Pound for pound, they are also much
stronger than big airplanes. If you don’t
believe me, imagine picking up a full-scale
airplane by its rudder or by a propeller
blade. Oops! It would be different with a
small to moderate-size model, right? This
overengineering can lead to overconfidence
too.
Rubber Scale builders pay close attention
to structural design, since their fuselages
must bear the torque of a tightly wound
motor. Some of us are a bit tightly wound
ourselves. I proved this by carefully
examining the Citabria before my flight. The
design was close to what a midsized Rubber
model would use, swapping balsa for steel
tube.
Electric models don’t need the same
strength in the same spots, but their motor
mounts have to withstand tremendous loads
in all directions. Imagine the stress on an
outrunner motor mount when the pilot
performs a tight outside loop at full throttle!
This all comes back to the issue of what
is strong enough and good enough for motor
mounts.
Next month I will have the results of my
question about propeller-saver systems.
Meanwhile, I would appreciate your input
on motor mounts you have used. Which
ones worked, which types failed, and what is
your best technique for spacing the motor up
to the nose from the firewall?
There is just enough space here to slip in a
little e-mail quote I got from an official
AMA message:
“The safety code is not an official part of
the safety code so the rules change system
doesn’t apply ... thanks for your inquiry.”
This glitch was sent by Gregory Hahn,
who is an extremely competent fellow and
handles technical issues at AMA
Headquarters. And, yes, he told me it was
okay to print it.
If a man as sharp as Greg can slip up
now and then, we can all feel better about an
occasional lapse in attention. It’s no big deal
at the keyboard, but at the field it could be a
lot more serious, so we should try to use an
appropriate level of care with whatever
we’re doing. MA
Sources:
Dave Gee
Box 7081
Van Nuys CA 91409
CP Aviation
(805) 525-2138
www.cpaviation.com