To: Dave Jackson
Having two engines driving in parallel with each of the two engines having a lower reliability than the single engine being replaced is great on paper. However the failure rate of each of the two parallel engines would be at a higher rate so in your proposal each of the two parallel engines must be able to carry the weight of the helicopter in the event of a single power plant failure. The transmission would have to be redesigned to accept two engines and the engineers will have to do some serious analyses and testing to eliminate any vibratory problems from the two engines firing at differing intervals.
On the S-55, 56 and 58 Sikorsky had the engines driving at a 34½-degree offset to minimize vibratory effects between the engines and the transmission. Even then, there were some serious problems with the engine cooling fans at certain rotating speeds.
