Smooth Moves
Reducing fatigue and wear through better
balancing
BY STEVEN W. ELLS (From
AOPA Pilot, November 2003.)
Permission to reprint granted 12/04.
Dynamically balancing your prop won't help
you if you're flying an airplane with a worn-out engine or
beat-up propeller. But if your engine and propeller are in
airworthy condition there's a good chance that a dynamic prop
balance will transform your airplane into a smoother mover.
Rockwell Swanson spent $225 for a dynamic
propeller balance. "The minute I pushed in the throttle I knew
I'd done the right thing," he said. Swanson — like almost 90
percent of airplane owners — thought the engine in his Cessna
Turbo 210 was smooth until he rode in a very well-balanced
Beechcraft V35B Bonanza. Swanson's propeller needed balancing —
in prop-balancing terms the initial reading of imbalance was
0.714 inches per second (IPS); after balancing, the imbalance
was reduced to 0.026 IPS. We'll get to the language of propeller
balancing later in this article.
Engine Components Inc. (www.eci2fly.com),
manufacturer of Titan cylinders and an experienced engine repair
and overhaul facility located in San Antonio, suggests that the
crankshaft, connecting rods with bolts and nuts, pistons, piston
pins, counterweights, and all counterweight attaching hardware
be sent in for balancing. In addition to this general list,
Lycoming starter gear supports (also called ring gears or
flywheels), Teledyne Continental Motors (TCM) crankshaft
alternator face gears and bolts, and rear crankshaft gears for
C-series and 200-, 240-, 300-, and 360-series engines must be
balanced to achieve the smoothest- running engine. Some of these
parts are balanced statically and some are balanced dynamically.
When the weight of a nut affects engine balance, it's a strong
signal that balancing is important.
Static balancing
The engines in most of our airplanes have a
number of parts that move back and forth (reciprocate) as part
of normal operation. Weight variations between like parts (such
as pistons) create imbalances and vibrations. Engine
manufacturers and engine overhaul facilities strive to match the
weights of each piston in a set of pistons, each connecting rod
in a set of connecting rods, and each wrist pin in a set of
wrist pins. Twenty to 30 years ago, when manufacturing wasn't
well controlled, overhaul shops had to stock large quantities of
each piston part number, for instance, and weigh each piston in
an attempt to match piston weights. This practice is known as
tolerance matching or tolerance stacking. Because of advances in
manufacturing, and the power of competition, today all
manufacturers sell parts sets that are very closely matched in
weight.
Not only are connecting rods matched by
weight, they're also matched by the weight distribution between
the big end and small end of the rod. All engine manufacturers
and all engine rebuilders have written standards outlining their
weight tolerances for each component. For instance, TCM Service
Information Letter 02-1, titled "Piston Position Identification
and Piston Weights," lists piston weights by two-gram
divisions.
Static balancing becomes an issue for
owners when a cylinder needs changing. Owners and mechanics must
pay attention to the part numbers and weights of both connecting
rods and pistons when changing cylinders. Both Teledyne
Continental (in Service Information Letter 02-1) and Textron
Lycoming (in "Notes on Replacing Connecting Rods and Pistons" in
its Key Reprints and Service Instruction No. 1243) detail these
issues. Briefly, it's critical that the weights of reciprocating
components in opposing cylinders be closely matched. In the
past, when cylinders were refurbished by grinding oversize
(removing metal to return the cylinder walls to a non-worn
profile) the oversized pistons required to maintain the proper
piston-to-cylinder bore fit could present imbalance problems,
especially if the maintenance technician wasn't aware of the
importance of piston weights.
Again, competition from companies such as
Superior Air Parts and Engine Components Inc. (ECI) has lowered
new cylinder prices to the point that buying new cylinders is
often more cost effective than grinding oversized cylinders.
This is especially true if the cylinder time in service is high
or unknown. First-run, or other low-time, cylinders can be
rejuvenated using ECI's CermiNil coating process — these
advances have largely eliminated the practice of grinding
oversize cylinders and returning them to service.
Since the weights we're talking about are
very small — a single piece of typing paper weighs 4.7 grams
while TCM pistons range from a low of 706 to a high of 1,640
grams — manufacturers allow some piston weight variations. TCM's
advanced technician training course book specifies that piston
weight differences of up to one-half ounce, or approximately 14
grams, are allowable. Lycoming, in Service Instruction No. 1243,
lists certain pistons with an "S" suffix behind the part number.
These are known as service pistons and have weights that fall
midway between original piston weight limits. As such, they are
approved to replace either heavier- or lighter-weight pistons
with the same part number.
Dynamically balanced components
Crankshafts and other rotating parts should
be dynamically balanced, or checked for balance, before
installation in an engine.
TCM specifies that its crankshafts are
balanced to a maximum imbalance calculated on a two-inch radius
at a rotational speed of 600 rpm. This specification means that
the maximum imbalance would be akin to a three-quarter ounce
weight located at an arm, or distance, from the middle of the
crankshaft of one inch. Various other field overhaul facilities
advertise lower numbers such as one-half-inch ounces.
Aircraft Specialties Services, of Tulsa,
balances more than 100 crankshafts per week. The company reports
that 90 percent of new crankshafts and crankshafts from engines
that have been manufactured or rebuilt within the past five to
10 years turn out to be well balanced. Fifty percent of the
crankshafts that have been out in the field for more than 10
years need balancing.
If an owner desires balancing to closer
tolerances, or if an imbalance is detected, qualified shops have
FAA approval to grind off a small amount of metal from
noncritical surfaces of the crankshaft. It doesn't cost much to
check the balance of a crankshaft — checking ranges from $50 to
$80, and a complete dynamic balance usually runs under $200.
There is some conjecture as to whether the
manufacturer's balance tolerances are close enough. Lycoming
says that its engines are carefully balanced to the degree that
is necessary and concedes that its engines are not balanced to
the point of perfection — Lycoming company officials say it's
not necessary because of the slow crankshaft rotational speeds
of Lycoming engines. They further state, in the engine balance
section of the Key Reprints portion of the company Web site (www.lycoming.textron.com),
"Additional internal balancing contributes little to engine
smoothness."
On the other side of the issue are engine
rebuild shops that insist that close-tolerance static and
dynamic balancing is critical, and warn owners that without
combining fine-tuned balancing with their practice of balancing
all cylinder combustion chamber volumes (a practice that's
designed to match the power output of each cylinder on an
engine) an engine will never be electric-motor smooth. There are
strong advocates for both approaches, but everyone agrees that
crankshaft balancing is money well spent.
If I were overhauling my aircraft's engine,
I'd pay the small amount of extra cash to get the crankshaft
balanced to the smallest imbalance the shop could deliver. A
smooth-running engine lessens both metal and pilot fatigue,
reduces avionics and instrument maintenance, and impresses
passengers. Having said that, the smoothest-running general
aviation engine I have ever flown behind was installed in Dick
Bicknell's Cessna 182. The 182's carbureted six-cylinder TCM
O-470 engine is not renowned for its smoothness; in fact, uneven
fuel distribution because of its log-type induction manifold
causes large splits in cylinder exhaust gas temperatures (EGTs).
Split EGTs indicate that there's a variance in the strength of
the cylinder power impulses, which have an effect on an engine's
net smoothness. Bicknell's engine overhaul was done by a
competent shop, but no extraordinary balancing was specified. He
said the smoothness was the result of a dynamic propeller
balance.
Dynamic propeller balancing
Dynamic propeller balancing is accomplished
by ground running an engine-propeller installation and then
attaching small weights, most often to the backing plate of the
propeller spinner, to reduce the imbalance of the rotating
mass.
ynamic prop balancing is only effective at
decreasing vibrations that occur at propeller rpm. These
vibrations are called first-order vibrations because they occur
once for each crankshaft-propeller (assuming the propeller is
not geared) rotation. Excessive half-order (half-order
vibrations occur when the spark plugs fire and the power stroke
starts in the cylinders) and first-order vibrations are
destructive — rivets loosen, wear in avionics and instruments is
accelerated, and pilot fatigue increases — since engine-mount
vibration isolators aren't very effective at dampening these
frequencies.
Dynamic balancing is quick and easy. A
minimum of one vibration sensor is attached to the engine just
aft of the propeller. This sensor converts motion into
electrical signals that are fed into a processor box. This box
plots the magnitude of the mass imbalance, and the location
(azimuth) of the imbalance with reference to the propeller disk.
A solution is derived and simple weights (almost always in the
form of aircraft-quality screws, nuts, and washers) are attached
to the aluminum propeller spinner backing plate. The engine is
then run up on the ground; usually the tester selects an
arbitrary rpm representative of cruise power such as 2,200 or
2,300 rpm, although Jim Beech of Dynamic Solutions Systems says
that 2,000 is high enough since this rpm is above the excitement
rpm of the engine mounts. The processor is then able to plot the
effect of the initial weight solution and create a final
solution.
Kent Felkins of Felkins Aircraft Service
in Tulsa has been doing dynamic propeller balancing for 15
years. He is a believer. Using the 0.2-IPS standard as a
baseline, Felkins has kept records showing that the propellers
on 88 percent of 580 airplanes he tested were out of balance.
Manufacturers of balance equipment
recommend rechecking the balance every 400 to 600 hours. Felkins,
an experienced balancer, cautions that anytime the prop is
removed — to change an alternator belt on a Lycoming engine, for
instance — the propeller-engine combination should be
rebalanced.
The goal of balancing is to bring the
center of the rotating weight (mass) into alignment with the
rotational center of the crankshaft. Because there are
manufacturing tolerances built into propellers that allow them
to be mounted onto the crankshaft flange, removal and
reinstallation require rebalance.
If the center of the rotating weight mass
is one one-thousand of an inch out of alignment with the
centerline of the crankshaft, Felkins' rule of thumb says that
this creates a 0.3 IPS imbalance. All the manufacturers of
balancing equipment agree that 0.3 IPS is too high.
Imbalance is measured in inches per
second, which is an expression of velocity. Imbalance also can
be expressed in units of gravity (Gs) or in displacement (in
milliliters or 0.001) caused by the imbalance.
Is your airplane out of balance?
Based on Felkins' figures, most pilots have
never flown a smooth airplane. That, coupled with the fact that
personal definitions of smoothness are subjective, often makes
skeptics out of airplane owners. Yet Cirrus, Lancair, Mooney,
and other high-end airplane manufacturers balance their
propellers before delivery. There are some telltale signals that
often indicate an out-of-balance condition.
If you feel a shiver go through your
airplane as you reduce prop rpm prior to landing, it's probable
that the prop is out of balance. The vibration is caused by a
harmonic vibration between the frequency of the out-of-balance
prop and the dampening frequency of the engine mounts. Typically
this happens between 1,800 and 1,200 rpm.
If instrument needles vibrate, or the
compass never settles down, this indicates a condition that
propeller balancing may improve. Do you have to stop every two
hours during a cross-country to get some circulation back into
your body? Does your arm, or one or both feet, go to sleep
during cruise flight when rested against the cockpit or
floorboards?
All of these scenarios are out-of-balance
indicators. More than one pilot has reported that the effects of
getting his propeller balanced has made flying fun again simply
because of the reduced fatigue associated with a smooth-running
airplane.
