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Thursday, October 1, 2015

Leading Edge

Shaking Things Up

By Frank Lombardi

Vibration analysis on rotorcraft blades isn’t easy. Helicopter engineers go through great lengths to ensure each blade is nearly identical in shape, weight distribution and stiffness.

Although they seem to whiz around rigidly, blades actually are quite flexible. It’s important that none of their frequencies are in resonance with airframe components. This is a difficult task since each component, including the occupant, has its own natural frequency. If the natural frequencies are similar to those produced by the rotor, the component will respond by rattling at high amplitude and become fatigued at a much higher rate.

Once blades are in service, mechanics work feverishly to eliminate the vertical and lateral vibrations that plague helicopters. Vertical vibrations are usually due to improper blade track; lateral ones are usually caused by uneven mass distribution in the rotor disk’s plane. Even with current technology, track and balance is still somewhat of a “black art.” Maintenance personnel adept at minimizing vibes should be revered.

As a helicopter pilot, I am in fine tune with my machine. This seems more true every day. One time on final approach, I suddenly picked up a lateral shimmy. After a safe landing and immediate shutdown, I found a small, plastic, snack-cake wrapper clinging to the outer portion of a blade. I was both surprised and intrigued that such a small amount of mass could throw off the balance of my rotor to a noticeable degree.

In another instance, during a routine post-inspection track and balance the analysis tool asked me to add an unacceptably high amount of counterweight. Eventually, I discovered that one blade tip was simply dressed with a bit more new paint than the others.

Rotor blades are quite flexible and it is important that none of their frequencies are in resonance with airframe components. Illustration courtesy of Frank Lombardi
After these events, I thought more about my gained sensitivity and susceptibility to unwanted rotating-system vibration resulting from subtle differences in lift and mass distribution. I began seeing the phenomenon in everyday life. I saw it in my wobbling ceiling fan one evening and spent the next few hours doing the math to find the static balance point of each blade and create a polar plot of the dynamic motion. I solved this by attaching small washers to cancel the vibe. Although it now spins with no discernible wobble, I am considering adding more blades to decrease the n per rev, just for good measure.

But that’s not all. The unbalanced load of towels in my washing machine plagues me. Clearly the counterbalance inside is poorly designed. My plan is to stalk eBay for a discarded S-76 bifilar vibration damper to improve the machine. Until then, I am washing my clothes without any water to eliminate its centrifugal-force contribution.

It now seems that my young niece and nephew are avoiding me. There was no way I was going to let them ride the merry-go-round at the park without sitting opposite each other at the exact prescribed distances from the ride’s hub, which I calculated. The real trouble came when they started crying on the swings. They wanted me to push them higher.

“No can do,” I told them. “For that, we’d have to be a system in resonance, and resonance is bad.” It did not take too much of me pushing them out of phase before I heard, “Can we go home now?”

Perhaps I should leave track and balance to the appropriate professionals.


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