To: T'aint natural
When I was first exposed to Sikorsky helicopters we worked on HOS-1s and R-4s but at the level of instruction we never got into the mass balancing of the fabric blades and wooden tail rotors. The S-51s that I maintained had just been converted to having hydraulic boost and the fabric blades had been replaced by metal blades. Our helicopters still had a three blade tail rotor made from wood. These too would eventually be replaced by a two blade tail rotor made of metal.
When I went to work for Sikorsky I was involved in a long term training program that included class room instruction and working in the shop to include building blades, mass balancing blades and whirl testing of blades to establish aerodynamic properties and to establish neutral climb and or dive tendencies.
The blades were balanced on a knife edge balance beam that had a fixed weight on one end. The weight was held in a neutral position with a mechanical lock. When the blade was attached the lock was released and the weight would move down and the blade would move up. A machined metal weight was attached in a neutral spot on the blade tip. The tip cap and the attaching screws were weighed and then set aside. Smaller weights and attaching hardware were placed on the blade outboard end until the blade was in perfect balance. Then the weight of the tip cap and screws were removed and the tip cap attached which kept the blade in balance. The tolerance was ¼ inch ounce.
The blade CG was calculated and this information was stenciled on the underside of the blade.
The blade was then sent to the whirl stand. Two brand new blades would then be installed on the whirl stand rotor head and a third master blade would also be installed. The master blade had built into it all of the desirable aerodynamic and aeromechanical characteristics. The pitch link for each blade had a proof ring (strain gage) to measure the feedback forces from each of the three blades. First the blades were spun up and measurements taken along with a blade track using a canvas strip. The trailing edges of the two blades under test were bent up or down to bring the blades into track. On the second spin up they would check the feedback forces on the two test blades cross checking them against the master blade. Other adjustments were made until the forces on all three blades were the same. Then the blades were checked for a tendency to climb or dive when cyclic and collective pitch were input into the system. If the blade wanted to climb the tip cap on that blade was removed and the balance weights were moved forward. If the blade wanted to dive the opposite movement of the weight would be made. Then the tip cap was replaced and the whirl test would then verify that the two test blades had the same characteristics of the master blade.
Hopefully I didn’t leave anything out or get the sequence mixed up. This all happened in 1955-56.