YH-16A / H-27
The H-27 was initially the designation for the second YH-16 w/T-38 turbine engines, later redesignated as the YH-16A prior to the aircraft's first flight in 1955. 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.
The size of the YH-16A fit the power output of the existing 1800 HP Allison T-38 gas shaft turbine. However, the T-38 was not a free-wheeling turbine and thus not ideally suited for multi-turbine interconnection. A concept was developed to tune the two fixed shaft turbines to act in consonance. (This concept was later utilized in Air Geep II). The YH-16A established an unofficial speed record of 166 mph in 1956.
During the development of the Chinook, originally Piasecki, and then Vertol, were the companies that manufactured the Chinook and its modifications and/or improvements. When they were testing that helicopter, they had an H-16 completely instrumented so that all of the critical parts, such as the blades, rotor heads,and everything else, were instrumented. The vertical drive shafts were quite big and they were hollow. The wiring to the rotor blades for the strain gages went through this hollow shaft and they were in conduits. The conduit was secured by a bearing in there so when the shaft went around, the conduit didn't move. It was factory lubricated, supposed never to fail.
But on 05 January 1956 it failed, and the shaft on the inside began to wobble. As a result it hit the edge, side, or the conduit insame place on each rotation until it finally wore it in two. When it snapped in two, it desynchronized the rotor systems, and the rotor blades hit each other. The rotors contacted each other cutting each rotor apart. There was such a terrific force wrenching the helicopter that it twisted it right in two. The crash killed both crewmembers.
Investigators determined that a frozen bearing in the test instrumentation had precipitated the failure by allowing a steel-tube standpipe, placed within the aluminum rotor shaft to guide wires from the instrumented blades, to undetectably inscribe a deepening groove within the shaft.
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