The Coast Guard had an urgent World War II need for at-sea rescue of crews from torpedoed ships along the U.S. east coast.
There was much skepticism in the Navy about helicopters. Their experience with autogyros showed that weight empty as a percentage of gross weight rapidly increased as the rotor size increased. Therefore, it was felt that no helicopter could be designed to meet the Navy’s 1800 pounds useful load requirement.
Piasecki proposed a tandem configuration (rotors front and rear) to best meet the design conditions of this increased size helicopter, three times larger than any other helicopter flying. Two rotors permitted a low disk loading, yet allowed the blade spars to be within available material length. The tandem design provided a significant increase in center of gravity travel, thus negating the need for shifting ballast, as was necessary in single rotor helicopters.
The Coast Guard, convinced, took the lead and a contract was signed on New Year’s Day 1944, for the XHRP-1 (experimental, helicopter, transport, Piasecki, model one) to meet the Coast Guard requirements.
Fourteen months later, Piasecki piloted the world’s first successful tandem helicopter. The first tandem was called the "dogship" because it was a flying mock-up, and was flown as a control demonstrator, without its fabric cover. Since a tail rotor was not needed to counter the main rotor torque, more weight could be lifted with a given engine. It carried 10 men and reached 95 miles per hour without its fuselage covering.
Critics predicted that downwash from the front rotor in forward flight would cause severe rear rotor turbulence, spoiling its control capability. This never happened when the proper differential collective pitch was added to the longitudinal control. 1,800 lb. External loads (world’s first log lift) were lifted with two, then one load line. An autorotative test was made with the interconnecting shaft between the rotors disengaged, as well as disengaging the engine.
|Rotor diameter:||41 ft|
|Fuselage length:||48 ft|
|Weight empty:||5,041 lb|
|Useful load:||2,000 lb|
|Max. speed:||110 mph|
|Cruising speed:||100 mph|