LZ wrote:
The helicopter I was referencing was a converted USN HSS-1 and was a one-off. I do not know if the modification (switching the Avco Lycoming 1820 for two T-58s) was Funded by Sikorsky or by the US Navy. It was flying in the 1955-56 time frame.
Lu is quite right. In 1956, Westland purchased the rights to build the S-58/HSS-1. Almost immediately, they began a redesign of the aircraft to accept the Napier Gazelle turbine engine. First flight was May 1957. Sikorsky certainly must have known about the mod.
(Interestingly, in France Sud-Aviation also assembled-then-license-built S-58's for their armed forces. In 1962, they had their first flight of an S-58 equipped with two Turbomeca Bastan IV engines.)
Coincidentally in 1956 (as Lu mentioned) one HSS was re-engined to accept two G.E. T-58's. First flight was 30Jan 1957 and it was designated HSS-1F. But further development of that particular model was not undertaken. Why?
Well, sometime in 1957, the U.S. Navy issued a weapons requirement to combine the "hunter" and "killer" anti-submarine roles. Contract order was placed in December of that year. Drawing on their experience with the HSS-1, Sikorsky enlargened the cabin and rotor, installed the twin T-58's, and called the new model "HSS-2."
Who funded what is probably murky. Sikorsky was probably flush with all kinds of R&D money at the time.
Development of the S-58 languished (at least in the U.S.) for awhile, until 1970 when it became available with the Pratt and Whitney PT-6 twin-pac. Sikorsky must have been as surprised as anyone at the continued popularity and use of the aircraft.
All I can say about the S-58T is...flew it, loved it. Like SASless, it's probably my favourite aircraft of all time.