craft & technique
airworthiness certificate is issued.
Some just use the kit manufacturer’s
template and trust that their
airplane is identical. But weight and
balance is critical to performance.
Make a weight and balance a part
of your annual inspection…at a
minimum…as well as any time
you add or remove any component
bigger than a ham sandwich.
Balance also includes your
rotating parts: Internal engine parts
like the crankshaft, counter weights,
rods, and pistons as well as the
flywheel, alternator, and propeller
need to be in balance for good
performance. It’s surprising how
many rebuilt engines (even from
some pro shops) have mismatched
internals: incorrect crankshaft
counterweights or mismatched
connecting rods and pistons. Rods
need to be balanced at both ends,
as well as for overall weight, and
opposing pairs of pistons and rods
should match closely. The ideal
would be a set that matches, but
if you need to use mismatched
components, don’t do it in
opposing pairs.
Your propeller may balance
statically, but its aerodynamic
balance is also important. Though
both blades may weigh the same,
each could be moving a different
quantity of air, creating aerodynamic
imbalance. This effect could be
illustrated by a constant-speed
prop that has its blades arranged
at different pitches: perfect static
and radial balance, but asymmetric
aerodynamic balance. Another
example could be on a repaired
prop, where one blade had a small
amount of material removed near
the tip and another blade had a
large amount of material removed
near the hub; when rotated freely,
neither blade would appear “heavy,”
because the moment arm of each
blade would exert the same leverage
on the center (radial balance), but
if the blades could be removed
and weighed individually (static
balance), the mismatch would be
apparent.
As in all things aviation, it’s
important to make only those
modifications that actually
enhance your overall goals. Top
speed at the expense of fuel burn,
handling, and TBO may not be
useful in the everyday world, but
racers’ modifications that increase
longevity, smoothness, and
efficiency can do a lot of good.
Tim Kern is a private pilot and
certified aviation manager as well
as an aviation writer and consultant
based near Indianapolis. You can find
him online at www. TimKern.com.
