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Operator Painted Blades

Recently 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.

By 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.

While 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.

Some 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.

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.

We 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.