“MOMENTS OF SHEER TERROR”
by B. J. Schramm
There’s an old saying which defines the life of a professional test pilot. “Hours & hours of boredom and moments of sheer terror.”
When you have limited training and are forced to learn things the hard way, you somehow remember every detail with vivid clarity. I take no credit for my survival in the many close calls I’ve had over the years. I have survived entirely because of my Heavenly Father’s Grace and protection. The flight test experience related here will never be repeated by any of our future customers, so please continue reading with no trepidation what-so-ever.
To understand what happened on this particular flight, we’ll need to fill you in with some background information.
Up until the late seventies our helicopter rotor head design consisted of a patented system known as a “cable controlled tractable rotor”. It was a semi-rigid rotor, but with an important twist. The blade collective pitch was actuated independently from the cyclic control. In a Bell type 2-bladed rotor, as in the HELICYCLE rotor, the blades are feathered in relation to the hub. The rotor hub only teeters on one axis. Collective pitch is inter-mixed through the cyclic system.
In the tractable rotor the blades were scissored together and actuated by a push-pull cable that ran up through the main shaft and hooked over to attach to a position on the hub plate. The opposite end of the cable attached to the collective stick. The hub plate was mounted on a universal joint connected to the top of the main shaft. The plate was linked to the swash plate and could therefore be tilted in any direction you chose with the cyclic control. The collective cable was very flexible and would follow any angle the hub plate was positioned. We called this method of control “Flying the Hub”, because the hub angle was all the cyclic control directed.
It took me several years to get this type of rotor hub working correctly and it still was an engineering compromise as all things of a mechanical nature are. I was forever tinkering with this system to improve handling qualities, reduce vibration and increase forward speed. One of the systems deficiencies had to do with increased vibration at higher airspeeds. This occurred due to the greater angle between the hub plate and the shaft at the higher speed. Automotive engineers call this “Hooks Joint Effect” and have designed around it with the modern day “constant velocity joint”. We couldn’t use a C.V.J. because of the size and weight.
Now let’s digress for a moment. There are many well defined rotor system instabilities which an unsuspecting novice can get into while trying to learn how to build a better rotor system. I was not unaware of these, I just didn’t realize how devastating they could be. The condition involved here is called “whirl mode instability”. Normally the rotors whirl around in a “tracked” condition. Each blade follows the other through nearly the same slot of air even when pitched cyclically. On any helicopter the blade pitch is first ground adjusted to achieve an even “track”. During forward flight, if somehow the track began to separate widely apart, the blade system will whirl in an angular path unresponsive to cyclic input. If the track continues to separate and in “whirl mode instability” it usually does; it will catastrophically self destruct.
If you’re not a helicopter pilot you need to understand that even a 1" or 2" “out of track” condition produces a pronounced vertical bounce that gets your attention right away and forces an adjustment.
Back to the story. In my quest for smooth high speed flight, I got the inspiration to create a softer linkage between the swash plate and the hub. I reasoned that by doing so, the feed back into the cyclic stick which increased with airspeed would be dampened and therefore less objectionable. I knew the vibration would still be there, it just wouldn’t feel as pronounced and I could increase airspeed without discomfort. I designed a hard rubber connection which could be adjusted for freedom and installed it on the connecting linkage between the hub and swash plate. I planned a series of flights in which I would incrementally increase airspeed and rachet in more freedom in the linkage. My first flight was conservative. I loosened the control freedom one notch and flew to a 70 mph target speed. I maneuvered in pitch and roll and found the rotor stability to be excellent. I continued this process in increments of 5 mph up to 95 mph and found no changes in stability. On the first flight, I wasn’t completely sure but it seemed the vibration at the higher airspeeds was lessened somewhat. Back on the ground I collected my thoughts and decided to make another minute adjustment. This experience is so clearly etched into my memory I can remember the exact flight path I took.
I took off from our training facility helipad next to I-10 and followed the freeway south of Phoenix, Arizona. (At that time this was open country farmland. I could autorotate to an easy touch down anywhere and catch a ride back up the freeway ) Each subsequent adjustment seemed to improve things and I was getting pretty excited about it. Something began to bother me though as I continued the tests but I decided to make one last run. Everything was going smoothly as I began to increase airspeed. I passed through 85, 90 and finally up to 95 mph.
Then it happened! Instantly and without warning the blade track split at least 2 feet. The cyclic stick whirled violently with the rotor in a large circle and was very nearly ripped out of my hand. I know I didn’t say it out loud but in that brief moment in my mind I cried out “God help me!” He did and instantly! Instead of the cyclic being wrenched from my grasp and the rotor blowing off the machine I was able to hang on. I carefully reduced power and collective and was able to slow up to 65 mph. The stick was still slamming me around in a 2-3" diameter circle and the blades were about a foot out of track, but the machine was still in one piece and I was controlling it. I was near our facility, just across the freeway from it when all this happened so I was able to set back down on one of our helipads.
A close inspection revealed no damage to the rotor hub or blades, however they were removed from the ship and discarded. The cyclic control system had some bent components and lots of end play in it. (The stick could be moved in a 2" diameter circle without moving the rotor hub.) What had happened to cause this event was now obvious. As I progressively softened the linkage from the swash plate to the hub plate I had been courting disaster. In effect I was trying to disconnect the cyclic input. When this process was carried far enough, the rotor simply went off on its own and was influenced only by the increasing Hooks joint effect and aero dynamic considerations. I had been searching for smoothness and higher speeds at the expense of stability. It was at this point that I gave up on the “tractable control” design forever. I realized finally that my airspeed would always be limited with it. While it was relatively inexpensive to manufacture it would never amount to anything but a toy. This experience was humbling for me in more ways than one and yet clearly God had spared my life in that “moment of terror.” He also showed me that He wanted me to continue developing my rotorcraft designs.
The HELICYCLE is what it is today because its rotor system has been forged in the furnace of a life time of “moments like these”.