"Jump Jet" Engine Fix Designed at UDRI

Bigger isn't always better. A lightweight "simple fix" designed by researchers at the University of Dayton Research Institute to resolve a problematic jet engine ring has proven successful in real-use testing. This fix, designed to prevent cracking in the engine inlet ring on AV-8B (Harrier) aircraft, will now be applied to the entire Harrier fleet --saving the Navy significant maintenance dollars and man hours and improving fleet readiness.

The Harrier engine inlet ring is an aluminum sheet-metal "duct" attached to the face of the engine through which outside air is funneled. But engine noise creates excessive vibration in the ring, causing the ring and its fasteners and stiffeners to crack.

Not only does the cracking compromise the integrity of the structure, but parts that crack and come off during operation can damage the engine, said Bill Braisted, senior research engineer in the structures group of UDRI's aerospace mechanics division and one of several researchers who worked on the AV-B8 project. Because of damage risk, jets are pulled from service for repair as soon as cracks are discovered --a process which has become a maintenance and cost burden and impedes readiness of the Harrier fleet.

The manufacturer's first approach to resolve the problem was to increase the ring's wall thickness and add more stiffeners during production. That added more weight to the structure, but didn't solve the problem. When cracking returned after additional flight time, Naval Air Systems Command (NAVAIR) and the manufacturer explored a structural fix. The team proposed a modification to beef up existing rings by replacing their fasteners and most stiffeners with stronger parts --but the replacement procedure posed its own risk to the rings.

To explore the feasibility of alternates to structural fixes, NAVAIR came to UDRI, where researchers have had more than two decades of success controlling vibration with a passive damping system. Instead of bulking up the structure, researchers damped its vibration by adhering a set of simple tiles they designed and created from a readily available acrylic polymer and aluminum.

"When a structure tends to crack, the response is often to throw more material on it," Braisted said. "That adds significant weight --which isn't desirable in aircraft design --and often doesn't solve the problem. The damping system works well because it's light, inexpensive and can be easily applied to retrofit existing engine rings.

Using an inlet ring and fuselage provided by NAVAIR, Braisted and his colleagues performed tests to see where, when and just how much the ring would vibrate. Information in hand, they selected a kind of acrylic polymer with ideal properties for the job. At room temperature, this polymer has a self-adhesive property and can be stuck right to the engine ring. And when the material is stretched, it tends to heat up --so the polymer dissipates much of the engine vibration by converting its kinetic energy to heat.

After initial tests to see if the material would hold up in performance, researchers created two sets of tiles --squares of polymer with a layer of aluminum on top --which were placed on the engine rings of two Harriers for flight testing. One of those jets recently completed more than 120 military service hours in Iraq, and the system was declared effective.

Because of its success, NAVAIR has approved incorporation of the damping system into the entire Harrier fleet, and UDRI will investigate the possibility of addressing a similar problem on S-3 Viking inlet rings.

March 3, 2004