INFORMATION LETTER #2003-1
we have received a number of main rotor blades sent to ABS with balance
problems. These blades are for the S-58 and S-61 helicopters. It would
appear that these operators thought that they were putting the effort
into maintaining their helicopter by painting the blades, which should
be a good thing, however they are creating other problems for themselves
by doing this.
repainting the tops of the blade with a polyurethane paint (Polyurethane
is a high solids paint and weighs more than the original Lacquer finish)
this will add approximately 1 to 2 mils of paint to the top or approximately
1 to 2 lbs of weight to the blade. Most operators will apply the new
topcoat directly to the blade surface and not remove any of the old
paint. On a Sikorsky S-58 main rotor blade with a cord dimension of
16.5 inches and an S-61 blade with 18.5 inch cord, your cord wise center
of gravity is approximately 4.6 inches from the leading edge. Consider,
the amount of paint applied to the blade, most of the paint is aft
of the center of gravity thus three times more weight in paint is aft.
One of the biggest problems is that you don't really know what condition
the balance of the blade is in before you start. As blades continue
in operation for years other repairs are completed such as field repairs
and paint touch up, this increases the weight of the blade and changes
the cord wise balance. You could have one or more blades heavy to start
with. The first thing that will happen is the newly painted blades
will be out of chord wise balance. Guaranteed. When you paint these
blades, you may be exaggerating any out of balance problem.
these blades are technically out of balance they may fly together as
a set if painted at the same time, by the same painter and with the
same product. However, the problem that arises in the future is when
one of the blades needs to be replaced. The new blade is not going
to fly very well with the originals, as it is probably up to 2 lbs
lighter. Resulting in an aircraft that is rough in the air and also
will cause excess wear and tear on all of the aircraft, especially
the head, flight controls and airframe.
of the items that will suffer from blades out of dynamic balance:
1. Rod ends and bearings in the main rotor head take a beating.
2. Rotating star bearing takes a beating.
3. On a helicopter that does not provide for rotor head balance, out of dynamic
balance blades can cause lots of problems that may not be apparent at that
time but will eventually surface.
simple check of the condition of your main blades is to observe what
happens to the blade track when you turn off the primary servos during
your flight checks. Do they stay together as when both systems are
operating, or do they separate and seek their own track path. The design
of the sloppy links on the primary servos allows for the blade to run
where it needs to when the primaries are shut off, not where you force
it to run. If you have ever lost the primary system in flight, and
the helicopter was hard to fly or land, you will understand the reason
for properly balanced blades. You should notice no appreciable handling
difference with the primary system off, on the ground or in the air.
have written this information letter to call your attention to a potential
problem that you need to consider before starting the painting process.
In spite of the fact that the blades seem to be robust and require
very little maintenance, they are delicate in the way they operate.
By paying attention to the static balance of the blades, you will see
significantly lower maintenance and vibration in the area of the rotor
head, flight controls and airframe.