Purdue University engineers are helping the U.S. Coast Guard deal with a possible 10-fold increase in vibration that could result from installing a larger observation window in a search-and-rescue aircraft to improve visibility during missions.
Presently, the Lockheed Martin HC-130J Hercules search-and-rescue plane has a circular observation window that is about a foot in diameter, and the Coast Guard would like to install a window that is about 4 feet long by 3.5 feet wide.
"The potential disadvantage of making a change like this to an aircraft's fuselage is that it could increase the noise and vibration environment inside the aircraft to dangerous levels, making it difficult for the rescuers to complete their missions," said Douglas E. Adams, an associate professor of mechanical engineering at Purdue.
The engineers have concluded, however, that the impact of increased vibration on personnel could be managed by installing special vibration-isolating seats.
"The bottom line is that the benefits of increased visibility probably would outweigh the potential dangers posed by increased vibration," Adams said.
Mechanical engineers worked with Ronald Sterkenburg, a professor in the Purdue Department of Aviation Technology, to recreate two mock panels of the aircraft's fuselage, one with the large window and one with no window. The engineers analyzed the two panels, comparing how much vibration and sound would be created in each.
Findings will be detailed in a research paper to appear in the November/December issue of Experimental Techniques magazine. The paper was written by Adams and graduate students Spencer Ackers, Jennifer Daley, Kelly Hanson and Brandon Zwink.
"The students completed this project in a single semester while taking a course on experimental vibration," Adams said. "That's an impressive accomplishment for such a challenging project."
The purpose of the course, sponsored by the National Science Foundation, is to give students valuable lab experience in real-world problems to prepare them for jobs in engineering.
"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 that has the same number of engines but only four blades on each engine.
"You are pushing more air at a higher rate with the six blades, which creates more vibration and pressure from sound waves," Adams said. "Because of the vibration issue, the Coast Guard wants to determine if adding windows is going to create a problem for search-and-rescue personnel."
The engineering students attached a network of four types of devices called accelerometers to each fuselage panel. They then struck the panels in 77 locations with a special "modal hammer" equipped with a sensor, measuring both the force exerted by the hammer and the resulting vibration, which was recorded by all of the accelerometers.
Aircraft vibration is caused three ways: The motion of the spinning engines transmits vibration through the aircraft's frame; sound from the rotating propellers transmits noise directly through the fuselage; and the propeller blades also push air across the plane's exterior, causing vibration.
The engineers used the hammer strikes to recreate the type of vibration caused by the spinning engines, and the 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.
"The tests showed that adding the window is likely to produce about 10 times more vibration and 10 times less noise," Adams said. "Because we know that occupants are sensitive to certain vibration frequencies and certain sound levels, we were able to determine whether increases in vibration and sound would pose problems.
"We've recommended that if the window is added, the Coast Guard should think very carefully about adding vibration-isolation seats for the people sitting inside the aircraft."
Although the effects of the added vibration on people could be managed with the special seating, the vibration could cause damage over time to the aircraft itself.
"We have also recommended that they probably should monitor the vibration and make sure that it's not getting worse over time, because if it is that means the structure is weakening," he said.
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