Operator Painted Blades
Recently we received a number of main rotor blades sent to Aviation Blade Services with balance problems. These blades were for the S-58 and S-61 helicopters. Apparently these operators thought they were properly maintaining their helicopter by painting the blades, however other problems were created by doing this.
By repainting the tops of the blade with a polyurethane paint approximately 1 to 2 mils of paint was added to the top or approximately 1 to 2 lbs of weight to the blade (polyurethane is a high solids paint and weighs more than the original lacquer finish). Most operators 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. Considering 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. You don’t really know what condition the balance of the blade is in before you start. As repairs are completed on blades which have been in operation for years such as field repairs and paint touch ups, the weight of the blade is increased and the cord wise balance changes. One or more blades could be heavy to start with, thus when blades are painted any out of balance problem may be exacerbated.
While such 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 will arise 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.
Some of the items that will suffer from blades being out of dynamic balance:
- Rod ends and bearings in the main rotor head
- Rotating star bearing
- 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.
A 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.
This information letter has been provided to call operators’ attention to a potential problem that needs to be considered before starting a painting process. In spite of the fact that blades seem to be robust and require very little maintenance, they are in fact operationally delicate. By paying attention to the static balance of the blades operators will see significantly lower maintenance and vibration in the area of the rotor head, flight controls and airframe.