One night during the HRP testing, at the lot behind Philadelphia International Airport, we received some visitors from engineering. There had been an HRP-1 accident in the service and they wanted to do some testing. The test was to run the rotor at normal operating RPM, quickly close the throttle, which will cause the rotor to over- run the engine, then quickly open the throttle.
We did this a number of times
until we had a large bang. A look outside surprised us. The ply wood from the
blades was littered all over the parking lot. Very little wood was left on the
blade spars.
This is exactly what happened
in the service, during a landing the engine cut out then came back on and the
blades came apart. The aircraft was just about to touch down so no one was
hurt. This drive system incorporates a
so called jaw clutch. This device disengages the rotor from the drive system to
permit the rotor to auto rotate in the event of engine failure. The rotor will re-engage when the power is
applied. There is sometimes some free
run depending on the relation of the two halves of the jaw clutch. This causes
a very rapid acceleration to the rotor blades, causes the trailing edge to
compress and actually explode. Sounds complicated doesn’t it. The cause was simple, but the fix was tough!
We used one of the HRP-1
aircraft to test some fixes. We tried a shear pin in the engine shaft and
destroyed 6 more blades. We also tried a
friction slip joint in the shaft and destroyed
more blades. We stopped using
real blades and made some from 2x4, not
to fly. The original jaw clutch had 9 teeth, we got jaw clutches made that had
21 teeth; that cured the problem. The
new jaw clutch with smaller teeth meant that they would always be a shorter
distance from engaging, so there would be less impact.
It is hard to explain but it
worked. After several runs on the
ground, with normal blades, Frank Piasecki and pilot Bill Knapp lifted the
aircraft into the air and while hovering turned the Magneto switch off and on
again, which created hugh back fires, but that was Piasecki!
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