Initially powered with two P&W R-2180-11 radial engines (each in an engine room), the YH-16 was the first twin engine helicopter. Later, the YH-16 became the world’s first twin turbine helicopter. The YH-16 became the largest helicopter in the world, having a rotor diameter of 82 feet and an overall length (rotors turning) of 134 feet. The fuselage was as large as that of a four-engine airliner and could accommodate three light trucks loaded through a rear ramp.
The experimental Piasecki-designed PV-15 (XH-16), was developed for the Army Air Forces as a long-range transport and rescue aircraft. The Piasecki YH-16 Transporter with a twin-rotor design had provisions for up to 43 troops. This helicopter was to have a direct influence on the progress of the Marine Corps' forthcoming helicopter program. It appealed to all services because it had a gross weight of 46,000 pounds and a useful load capability of 14,000 pounds or 40 passengers. The tandem design XH-16, with two engines driving the two 82-foot diameter rotors, was the largest helicopter in the world. Although development of this gigantic helicopter was started by Piasecki in 1946, almost concurrent with the HJP-1, its first flight would not occur until more than seven years later. The development of this transport and rescue helicopter was accelerated in response to the Korean War.
On 09 July 1947 the Marine Corps Commandant made an important change relating to the helicopter's characteristics by specifying only one size helicopter of a 5,000-pound minimum payload capability. It eliminated the requirement for the helicopter to be accommodated by the ship's elevator and stowed on the hangar deck and listed the overall dimensions as "small as possible." The design proposal for a 3,500-pound helicopter was cancelled. In order that the early landing may he provided with necessary continuity, it is necessary that communications vehicles, recoilless weapons, and initial resupply he provided at an early hour and, ideally, that these should be followed by artillery. This required a payload of approximately 5,000 pounds.
The Chief of the Bureau of Aeronautics cast the assault helicopter transport program into the development doldrums by linking it with the Air Force's XH-16 program. The CNO was informed by BuAer on 24 December 1947 that additional studies indicated the development of a large helicopter meeting the requirements of the Marine Corps was feasible, but it would involve a four- or five-year program and require considerably more funds than could be obtained in view of the continuing budget curtailments.
"It now appears," the Bureau Chief stated ". . . that the assault helicopter characteristics are very similar to those of the XH-16 helicopter contemplated by the Air Force . . . and insofar as the basic helicopter is concerned, are almost identical. In view of this fact, effort on the assault helicopter will be undertaken on the basis of joint Air Force/Navy development of the XH-16.
By the end of 1947, the new Marine program appeared to have a sense of direction and momentum. Organizationally, the first helicopter squadron had been formed in December for the purpose of determining the operational feasibility of the vertical envelopment concept. Plans for execution of the concept in terms of aircraft were based upon the eventual acquisition of a very large helicopter - the Piasecki XH-16. In preparation of a concept, the special group designated as the Helicopter and Transport Seaplane Board had been formed to develop a tentative doctrine for the employment of helicopters in amphibious operations.
Unfortunately, two years later, the whole process had reached a developmental plateau which jeopardized the entire helicopter program. Lack of continued progress could be attributed to the inability to realize that the helicopter manufacturers were unable to comply with their own predictions for meeting the specifications and requirements which they had so willingly accepted. Additionally, an exceptionally long developmental period was required once the decision on the type of helicopter was made and the money budgeted to coincide with its development.
By mid-1949 the Navy and Air Force were jointly developing the Piasecki XH-16. The XH-16 represented a large step forward in helicopter technology and would require extensive component and flight testing after the anticipated completion date of the first test aircraft in 1952. This would preclude the construction of production models for an appreciable length of time since only two experimental aircraft were being built.
By January 1950 the XH-16's lengthy development period was seen as seriously retarding the Marine Corps helicopter program and although it was not desired to divert funds for its support, emphasis was placed on allocation of funds toward the proposed 3,000 pound payload helicopter. Because of the time required to perfect fully such a large helicopter, and its doubtful ability to operate from small aircraft carriers, it appeared advantageous for the Marines to proceed with an additional project for the development of a small helicopter which would meet the Marines minimum requirements, which will be suitable for carrier operations, and might well be more easily and quickly obtained. The diversion of the remaining XH-16 Navy research and development funds was also viewed as meeting with CNO approval provided the Navy could be persuaded to terminate its support of the XH-16 project.
The Marine Corps lost interest in the H-16 once it became clear that it was too large for the small deck escort carriers which were envisioned as the helicopter platform of choice. After a discussion, it was determined that the the Marine Corps helicopter program of the future should be composed of two parts. First, and as the longer-range solution, the Marine Corps should continue with the program to obtain a carrier-based assault helicopter which would meet the requirements of AO-17501 (XHR2S-1), as was recommended by the March 1950 helicopter conference, and simultaneously attempt to persuade the Army and Air Force to cancel the XH-16 project and join with the Marine Corps in developing the XHR2S-1, which in Army service became the H-37 Mojave.
Piasecki continued the development of its XH-16. However, although still under construction, it would soon join the list of unsuccessful ventures. Piasecki's XH-16 had a large cabin and was the type which had initially interested the Marine Corps as an assault transport. The aircraft failed primarily because the state of power plant and transmission development had not advanced sufficiently to match the demand.
The first flight of the YH-16 was 23 October 1953. The slow turning speed of the rotors (125 rpm) almost made the blades visible in their rotation and in-flight vibration was loping in character. The three blades in each rotor were all aluminum alloy step taper milled skins keeping a ± .002 inch tolerance through their 41 foot radius by a special process developed by Piasecki. The bonded blade was made in four pieces, with two outer skins, a honeycomb filler and a leading edge balance weight which was almost a mechanical fastener of the leading edges of the skins.
The YH-16's utility was limited because it was underpowered despite two 1650hp Pratt & Whitney r-2180 radial engines. Like the Dogship before it, the YH-16 lifted as much as it did only because of very low disc loading that compromised other aspects of its performance. Greater power was clearly needed to realize the design's potential.
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