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The CH-37 Mojave (aka S-56) was a heavy-lift helicopter developed by the Sikorsky helicopter company. The first flight of the S-56 took place in 1953 and eventually the S-56 would be developed into the S-60. The S-56 was developed to be an assault transport. In other words, it was designed to rapidly deploy or redeploy combat forces and to get troops past obstacles that are problematic for ground vehicles. One notable fact about the S-56 is that it is one of the very last piston-powered heavy helicopters. The rise of turbine engines and their far superior power production essentially made piston technology obsolete as far as heavy helicopters were concerned. The specific unit used in the S-56 was the Pratt & Whitney Double Wasp 2100 horsepower R-2800-54. Two of these were installed in the S-56. Because of the angle of the drive train the engines were installed in a non-standard position, which led to oil foaming and related issues. This was eventually corrected with the addition of baffles. The S-56 was composed of an aluminum structure with a magnesium skin. It also made use of reinforced plastic or fiberglass panels. Rather advanced for the time period. The main rotor used a five-bladed design that made use of a two-stage hydraulic servo control system and later automatic stabilization on some variants. A six-bladed main rotor design was tested and found to be generally superior, but was never implemented on any variant of the S-56. Surplus S-56 helicopters were sold on the open market and at least ten main variants were produced over the span of its production life. The S-56 was however built in relatively low volumes, with only 154 units every produces. Understandable, given that the US army and marine corps were the only customers. The S-56 was retired in the late 1960s.
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