That Means "Shake 'Em Until They (Almost) Break"

As part of the C-5 Galaxy's Reliability Enhancement and Re-engining Program, the aircraft is undergoing dynamic taxi testing to check the structural strength and flexibility of the four-engine modification -- specifically, the C-5 structure's movement when traveling over rougher surfaces with its new engines and pylons.

When the C-5 was developed, it featured the first-generation, high-bypass turbofan engine, the TF-39. The new CF6 engine is a third-generation, high-bypass turbofan engine that produces 20 percent more thrust and consumes less fuel than the TF-39s.

Unlike the TF-39's mechanical control, the CF6 responds to throttle commands via a digital computer. Its increased fuel efficiency allows more cargo to be carried over a given route since less fuel is required to make the trip.

"We are looking at the entire aircraft with these new engines," said Jessica Wojtanowski, a flight test engineer with the 418th Flight Test Squadron. "We are trying to prove that the new engines won't negatively affect the structural modes of the airplane."

The pylons and the engine are a bit heavier, so the structure's movement has to be checked, said Mariusz Wisniewski, a 418th FLTS Det. 4 flight test engineer.

"The dynamic testing is interesting because it involves hard-core engineering analysis, and it allows me to operate the airplane on the ground while controlling to very tight parameters," said Capt. Aaron Tucker, a 418th FLTS C-5 experimental pilot.

"It is a challenge to steer the plane within a couple of feet at different speeds and control the ground speed within one knot," the captain said. "It is also interesting how flight test and structural engineers interact with each other to ensure we have a safe test."

The 412th Test Wing Instrumentation Branch designed a ramp while the 412th Equipment Maintenance Squadron team built four large ramps out of plywood and metal, and installed them on Roger's Dry Lakebed here. The ramps are designed to shake the aircraft structure in a certain way when the C-5 rolls over it at a specific speed. Instrumentation, such as accelerometers and strain gauges, are installed to detect the movement of the C-5's parts.

"We make sure the C-5 is in a tight weight band, and the instrumentation is really precise," he said. "We designed our test so the aircraft can hit the bumps at different speeds. We started with 30 knots and will conclude with a 110-knot data point. We drive across the lakebed and have to hold a precise ground speed while the aircraft has to hit the exact center of the ramp."

The testing will measure how well the airplane operates on an unimproved runway in the combat area, Captain Tucker said. Runways there tend to be bumpier sometimes due to repaired battle damage.

"As far as the results go, the team is still working on the numbers," he said. "As we increase the ground speed, we are getting closer to the limit of the structure, which is what we want. However, the test team is being more and more careful as we proceed."

The dynamic testing is also a prerequisite of the "flutter testing" and only a fraction of the RERP testing, Ms. Wojtanowski said.

With flutter testing, pilots fly the C-5 at certain altitudes and speeds out to the very limits of the aircraft envelope, she said. The control surface will be moved rapidly, and the test team will observe the aircraft's structural response.

"The testing on the ground helps predict the result in flight," Mr. Wisniewski said. "It also helps us to know if the instrumentation works as they should in flight."

(Aero-News salutes Airman 1st Class Julius Delos Reyes, 95th Air Base Wing Public Affairs)

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