Purdue University mechanicalengineering-graduate student Harold Kess (standing) uses a special "modal hammer" to strike a mock section of a U.S. Coast Guard aircraft. Mechanical engineering Professor Douglas E. Adams monitors vibration data on a computer screen. Photo: Purdue News Service /David Umberger


Currently, the Lockheed Martin HC-130J Hercules search-and-rescue plane has a circular observation window that is about a foot in diameter. The Coast Guard would like to install a window that is about 4 X 3.5 ft.

The problem: "It could increase the noise and vibration inside the aircraft to dangerous levels, making it difficult for rescuers to complete their missions," said Purdue Associate Professor of Mechanical Engineering Douglas E. Adams.

Purdue engineers have concluded, however, that the impact of increased vibration on personnel could be managed by installing special vibrationisolating seats.

"The bottom line is the benefits of increased visibility probably would outweigh the potential dangers posed by increased vibration," Adams said.

Mechanical-engineering students worked with Purdue Aviation Technology Professor Ronald Sterkenburg to recreate two mock panels of the aircraft's fuselage, one with the large window and one without. Researchers analyzed the two panels, comparing how much vibration and sound would be created in each.

"Vibration is a serious problem in search-and-rescue aircraft," Adams said. "In helicopters, for example, vibration-related fatigue can become so high that it has actually forced pilots to ditch their craft in the water."

The HC-130J has four engines, and each engine spins a propeller containing six blades. Having six blades increases vibration inside the airplane, compared to the HC130-H aircraft, a similar plane with the same number of engines but only four blades per prop. "You are pushing more air at a higher rate with the six blades, which creates more vibration and pressure from sound waves," Adams said.

Engineering students used hammer strikes to recreate the type of vibration caused by spinning engines. Researchers also bombarded the fuselage sections with sound from a loudspeaker to recreate the vibration caused by propellers pushing air across the plane's exterior.

"Tests showed that adding the window is likely to produce about 10 times more vibration and 10 times less noise," Adams said. "We know occupants are sensitive to certain vibration frequencies and certain sound levels, so we were able to determine whether increases in vibration and sound would pose problems."