Lab uses vibration testing to solve aircraft problems

  • Published
  • By Jeanne Grimes
  • 72nd Air Base Wing Public Affairs
For every test Bill Fleenor conducts in the vibration test facility here, there is one overriding intention.

“If an item is going to fail, I want it to fail on my table and not with the warfighter,” said Mr. Fleenor, a mechanical engineer.

Mr. Fleenor, whose degree is in physics for industrial applications, works alone in a lab where aircraft components, accessories and systems are subjected to vibration, shock and temperature extremes.

His customers include weapons system engineers, project managers and equipment specialists at the Oklahoma City Air Logistics Center.

Some of the work is pretty straightforward.

For example, when a replacement item is procured from a new source, it may be necessary for Mr. Fleenor to run vibration tests as part of the quality requirements, making sure the new item stands up to the test “just as the original part did.”

Other tests address specific problems encountered on an aircraft. When an air cycle machine experienced a problem with a bearing, engineers turned to Mr. Fleenor to determine if a prototype fix would withstand the aircraft’s vibrations.

“(The bearing) was known to overheat, and that caused the oil lubricating it to explode,” Mr. Fleenor said. “This was on a KC-135 (Stratotanker), and the machine is located right below the fuel cell.”

In one instance, the exploding part sent out hot shrapnel that caused a fire on an aircraft on the ground.

The fix included mounting a heavy plate to serve as a scatter shield between the machine and the fuel cell, as well as replacing the metal bearing with a ceramic one.

“I try as close as possible to simulate the environment it’s going to see on a plane,” he said.

Another problem involved the bomb release on a B-52 Stratofortress bomber. The release has two levers, one to arm the bomb and the second to release it. The second lever was malfunctioning, releasing the bomb prematurely.

Understandably, crews said they were nervous about a bomb “rolling around on the bay door.”

Engineers reworked the mechanism on the release and sent it to Mr. Fleenor for testing. The part tripped prematurely. It was reworked and retested and again failed. The third time, though, the lever held.

“The planes were the first in the fight in Afghanistan after 9/11,” Mr. Fleenor said, recalling the e-mail reports he received from aircrews in the field. “They were thrilled.”

Sometimes the tests do not turn up answers, but they do eliminate suspected causes to problems. Such was the case when excessive vibration was behind engine rejection in the B-2 Spirit. Engineers suspected a mount was reacting to a vibration which set off its resonant frequency -- a state of excitement in the material.

Mr. Fleenor ran the test with the lab’s vibration control system, a Polytac laser to measure displacement, and a small ball peen hammer.

“I managed to aim the laser off the top of the accelerator so when I tapped the engine, the laser could pick up any displacement,” he said.

The test found resonant frequency for the mount was 515 hertz (cycles per second); however, the problem was at 275 hertz.

“I didn’t solve their problem, but I eliminated one suspected cause,” he said.

Elements of typical vibration testing are sine, which is a single tone at one frequency, and sweep sine, which starts with a slow frequency and speeds up, Mr. Fleenor said.

There is also random vibration testing, a simulation of the vibrations occurring on an aircraft.

“It sounds just like a jet engine. There’s no particular frequency because all frequencies are occurring simultaneously between 10 and 2,000 hertz,” Mr. Fleenor said. “Random vibration is like sitting in the bed of a pickup going 60 mph down a dirt road.”

Work at the lab includes performance, endurance and shock testing.

In a typical year, Mr. Fleenor works on “probably eight to 10 projects.” Preparation takes up to 99 percent of the time for a given test. The lab’s high-tech equipment includes two Polytac lasers, a dynamic shaker, a 48-inch skip table and a data acquisition system operating on 16 channels.

“That’s a lot of data coming in,” Mr. Fleenor said. “She will rock ‘n’ roll.”

It is a challenge to stay current with the software to keep all the equipment operational.

“I’m constantly learning about the system and how things react,” he said.

“I don’t perform production work here,” he said. “Every test is something new and different.”