June 2005 
Volume 04, Issue 2 
Tech Talk

Good vibrations in test technology

Good vibrations in test technology


Innovative applications of advanced computing and software analysis tools, along with process improvements that enhance teamwork among engineering, manufacturing and testing personnel, have helped Boeing achieve a breakthrough in the time required for airplane ground vibration testing.

The new technology and processes were validated during the ground vibration test (GVT) on the first KC-767A tanker/transport, conducted in March in Wichita, Kan. The airplane is scheduled for delivery by Integrated Defense Systems to the Italian air force next year.

Ground vibration tests are critical tests that first-of-a-kind airplanes and derivatives must pass before further production proceeds. The tanker's GVT was completed in just three days—two days faster than similar GVTs. The achievement not only saved time during an important point in the manufacturing process, it also provided significant cost savings.

The savings in time and money came primarily from a dynamic test and process improvement initiative led by the Enterprise Laboratories and Test Technologies organization in Phantom Works. A key part of the initiative involved the development of sophisticated pretest simulations, said Brent Hayward, strategic technology leader for Structural Test in Enterprise Labs. These simulations help engineers and technicians avoid using more shakers, sensors and other test equipment than necessary to get the job done. They call it the "sparse sensors" concept.

"Through use of the improved simulation tools, we are able to better plan and organize the test up front. This leads to time savings during installation and takedown, as well as being more efficient in getting needed information during the actual test," Hayward said.

Equally important to the improved tools was "the organizational effort and enterprisewide coordination of people involved in the effort," said Charlie Pickrel, a technical fellow and expert on ground vibration testing technology with Boeing Commercial Airplanes' Structural Dynamics Laboratory in Seattle.

An "extraordinary level" of coordination among the team of engineers, technicians and manufacturing experts from Seattle, Wichita, St. Louis and Long Beach, Calif., helped to eliminate a least a day from the test, Pickrel said.

This team included test director Mark Miller of BCA's Structural Dynamics Lab; Laurette Lahey of the 767 Tanker program; David Seidel of BCA's Engineering Loads and Dynamics organization; and Andy Harber, senior airframe structures engineering manager on IDS's C-17 program in Long Beach.

"A lot of work went into streamlining this test program, including building the team and preplanning the details," said Harber, who served as the KC-767A ground vibration test coordinator. "And it really paid off."

Pickrel and Hayward said the evolution of GVT test tools will continue, allowing for even more adaptive test plans in the future. This will eliminate unnecessary testing, while highlighting any anomalies that require investigation.


How it works

Aircraft are flexible. They naturally bend and vibrate in flight when confronted by the aerodynamic forces of the airstream. At high speed, under certain conditions, this can lead to an unstable and destructive oscillation called "flutter." When designing new aircraft, aerospace engineers can predict these forces and an aircraft's "aeroelastic" behavior using sophisticated tools and analyses gleaned from years of training and experience. Engineers use ground vibration tests (GVTs) to verify or update the analytical models used in flutter analyses. A variety of equipment is required to conduct a GVT: electrodynamic shakers are the primary method of "exciting" an aircraft structure; high-sensitivity accelerometers are sensors used to measure the vibrations; and computers acquire the data and generate reports.

At Boeing, GVTs are conducted on first-of-a-kind models or derivatives. The tests must be performed on a structurally complete airframe, so the testing occurs at a critical time in the production process—usually just before rollout or first flight. Boeing testing experts are continuously exploring ways to reduce the flow time for these tests while still meeting the requirements of identifying structural dynamic behavior.



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