18. TIED DOWN AIRCRAFT

We frequently had a need to do testing under load and since we did not have a test stand, we put an aircraft in a cradle and cabled it down. This also does not require a pilot to operate the machine. We tied down an HRP-1 and also an HUP-1.  The later one was used for several tests but primarily we had to run 500 hrs at max power to qualify the major components.

A failure of any major component meant starting over.  Some day’s one of us would run six of our 8 hrs. We had a fuel line connected to the aircraft so it was not necessary to have a fuel truck to refuel.       

We had several incidents happen.  I can only remember a few of the important ones. On one occasion, I was running the aircraft and Bill Shisler was sitting in. It was at night and we were churning away at high power when suddenly without warning the engine quit. To go from extremely high noise level to nothing is as startling as a sudden  noise.

To make matters worse, the aircraft filled with smoke.  We were making a rapid departure,  Bill was ahead of me, I was trying to shut things off. We had a battery secured to the floor just inside the cabin door, Bill fell over the battery and I fell over Bill, and we both fell out of the aircraft. No injuries; the engine failure was caused by a sheared accessory drive gear. That shuts off everything instantly.

The engine problem did not affect our test, since we were not testing the engine, it was government furnished, and was a continuing problem throughout the life of the HUP aircraft. It was originally 550 HP but was derated to 475 HP.

We had periodic tear down inspections.  Transmissions were removed, and inspected along with rotor hubs, and other components. During one of the inspections a hairline crack was found in a web, which was part of the aft transmission case.

Engineering labored over this problem for some time, it could mean a start over if replaced. A decision was made to bond four very small wires across the crack and connect  four lights in the cockpit so that if the lights started  to go out it could  mean the crack was progressing.

If  this happened we should shut down immediately. We were running again and watching the lights. It wasn’t long and one light blinked a few times and out.  We shut down immediately. After careful consideration Engineering decided that one light out wasn’t all that important so go ahead and run.  This continued every time another light went out. 

It became something of a joke with some of us. We finished the 500 hours, tore down the transmission, the crack had not progressed even the wires were in tact.

Another incident on the HRP tie down.  I don’t remember the test we were

running but I came in on day shift to take over running from Bill Shisler, he had just shut down. I got in the seat and fired it up. I engaged the rotor, brought it up to power, and suddenly I saw a long piece of rope sucked up into the forward  rotor followed by a large tarp. It wrapped itself around one of the rotor blades and created an unbelievable shake.

I was thrown out of the pilot seat onto the floor of the cockpit. The only thing I could get to was the mixture control, and I was able to pull it to cutoff.  All the tie down cables were stretched, I was lucky it stayed in the cradle.

The one rotor blade was destroyed, due to a bent spar. I wasn’t found at fault since I didn’t stow the tarp under the aircraft. 

Charles Kessler is a retired flight test engineer for Boeing’s Vertol helicopter division (formerly Piasecki Helicopter Co.). He joined Piesecki in 1947, in the company’s fourth year, and retired from Boeing in 1983. During his 37-year career he took part in the testing of prototypes and alterations of such models as the CH-47 Chinook and Sea Knight, the H-16, HRP-2, and the V-107. He taught the stability augmentation system to the German Luftwaffe. He has written about his experience in a blog called “Early Helicopter Years,” which can be found at http://helicopterstory.blogspot.com/.