From the Central Arizona Control Line Club
Failure Modes
by Jim Hoffman
Consider designs and procedures
that may improve the reliability and longevity of
your models ...
I have seen countless Control Line ships destroyed
or damaged needlessly due to mechanical failures
that could have been easily avoided. The cause of
the failure could have been avoided by an
alternative design or procedure. In aerospace world,
as a product is conceptualized and designed, the
team looks very hard at all possible potential
failure modes. The possible failure mode effects are
used as a guide as the design progresses and the
zillion design evaluations are made. This approach
certainly can be used with our models; it has been a
part of my modeling program for many years.
I look at every component and consider every
possible failure mode I can imagine. From there, I
next look at three parameters:
• Severity of the failure. Severity can be
ranked from high to low. If this failure occurs will
it cause personal injury or perhaps an immediate
crash? Lesser-severity failures certainly exist,
which may result in performance reductions, or
cosmetic issues.
• Likelihood of the occurrence. This is
difficult to evaluate, but after enough years of
experience and observation, one does develop a sense
of the likelihood. In most industries there are
actual records that give one a basis to compare the
likelihood of various failures.
• Detectability. Many mechanical failures can
be prevented by regular inspections of the airframe
and equipment. A loose engine mounting screw can be
detected by visual examination, torque check, or
even a change in the sound of the airplane in the
air. Usually the fastener can be tightened before
anything really bad occurs.
In order to have reliable and long-lasting models
one must address all failure modes. The magnitude of
the time, cost, and weight penalties needed to
resolve any failure mode ought to be linked to the
three parameters listed. I have seen examples where
all failure modes are given a score for severity,
likelihood, and detectability. This is overkill for
us.
The most elegant solution is to design away as many
failure modes as possible.
Examples of Possible Failure Modes.
Wimpy Bellcrank Mounting System: May be the
poster child of a most nasty failure mode. Severity
is very high—an in-flight failure will likely
destroy the airplane. Likelihood is high due to the
continuous flight loads on the bellcrank.
Detectability is moderate if you do frequent pull
tests.
An elegant solution is the hard point handle, which
simply eliminates the cable altogether. The hard
point style handle can also fail due to loose
fasters, but this failure mode is common to the
cable style handle also.
Hooking up your lines backward. Severity:
again very high. Very few airplanes survive this
error. Likelihood varies due to the individual, but
is never zero. Attention to detail and careful
preparation certainly can reduce the likelihood. We
can all cite instances where very experienced
veterans suffered the loss of an airplane due to
this error. Detectability is pretty good if you
really check up/down before you fly. This means more
than wiggling the handle and observing the elevator
wiggles.
A common solution is to have a handle and lines
dedicated to each airplane. When I roll up, the
cables and handle are never separated and are stored
together. When I disconnect the cables for the
airplane, I leave one connector on the cables. This
makes it very unlikely that the lines will be hooked
up backwards next time. I also clear the lines of
twists and check that up is up, down is down, and
neutral is neutral before each flight. Many others
color-code the up and down lines on the handle,
cables, and airplane leads out.
Cowl and landing gear mounts. Severity: this
is lower than in the previous examples. The airplane
will likely survive if the cowl or landing gear
falls off. Likelihood again varies due to the
individual but is never zero. Detectability is
pretty good if you really check the fasteners
frequently.
This is important to competition types, because the
flight is disqualified if anything falls off the
airplane.
Cowl: There are some nice design solutions to
consider. Many stunters are built without a cowl.
This makes for a little more effort to remove the
fuel tank, but is not uncommon. There are some
clever designs that restrain the cowl with a single
fastener. I choose to use several fasteners to hold
the cowl in place in the name of reliability. Should
one fastener fail, the cowl will stay put with other
fasteners.
Landing gear: Similar to the cowl. Permanently
installed landing gear are not likely to fall off,
but you give up ease of maintenance and adjustment.
Again, a very good solution is to use multiple
fasteners.
Wheel collars can loosen and allow a wheel to
depart in flight. Severity: this is lower than
in the other examples. The airplane will likely
survive if a wheel falls off. Likelihood again
varies due to the individual, but is never zero.
Detectability is pretty good if you really check the
fasteners frequently.
Again we are looking at a single set screw, which
can result in a problem. The use of Loc-Tite is
helpful. Many folks grind a flat on the axle to
allow the set screw to better register. Another
solution is to design the set screw away and retain
the wheel with a soldered washer.
I have only scratched the surface of a very complex
subject. I continuously look for possible failure
modes and ways to simply design them away. I also
pay a lot of attention when a failure occurs at the
field and try to understand the cause of the
failure. I hope that this is useful and makes your
airplanes more reliable and longer lasting.
Q
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