C-5 Galaxy undergoes dynamic runway testing

EDWARDS AIR FORCE BASE, Calif. -- As part of the C-5's Reliability Enhancement and Re-Engining Program, the C-5 Galaxy is undergoing dynamic taxi testing to check the structural strength and flexibility of the four-engine modification.

The main focus of the testing was 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 turbo fan engine, the TF-39. The new engine is a third-generation high bypass turbo fan engine, the CF6, which 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. The CF6's 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, 418th Flight Test Squadron Detachment 4 flight test engineer. "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, 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, 418th FLTS C-5 experimental pilot. "It is a challenge to steer the plane within a couple of feet at different speeds and to control the ground speed within one knot. 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 members built four large ramps out of plywood and metal, and installed them on Rogers Lakebed here, Captain Tucker said. The ramps are designed to shake the aircraft structure in a specific 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 various 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 significance of the testing is it will allow the airplane to operate on an unimproved runway in the combat area, said Captain Tucker. The 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.

"We are doing the testing on the ground that would help predict the result in flight," Mr. Wisniewski said. "It also helps us to know if the instrumentation works as they should in flight."

Prior to C-5's RERP phase, the aircraft underwent an Avionics Modernization Program that installed digital controls in the airplane, including computer instrument display, Captain Tucker said.

"We updated our technology to be in line with what the commercial sector had," Captain Tucker said. "Instead of doing voice communication, we can do datalink communication with the air traffic control and the new instrument displays are easier to maintain."

The RERP, on the other hand, was designed to improve the reliability of the C-5 weapon system.

"If we are going to extend the C-5 for another 20 years and support strategic airlift operations, we need to exploit new technologies in order to make the C-5 better," Captain Tucker said. "The reason we are doing this is to prove the reliability of the aircraft so we can better support the warfighter. With RERP, the C-5 can move more cargo, faster, using less fuel."