The U.S. Navy has quietly added new sea trials to its X-47B demonstration, but an actual hook-up and passing of gas through an automated aerial refueling (AAR) system remains an elusive goal.

Navy officials are focusing AAR flight testing on a Learjet surrogate aircraft, sparking some in industry to push for an actual contact and fuel transfer with one of the service's two Northrop Grumman X-47B demo aircraft.

“The Navy has determined that demonstrating AAR technologies and standard refueling procedures can be accomplished utilizing a Learjet surrogate aircraft,” says Kelly Burdick, a spokeswoman for the Unmanned Combat Air System (UCAS) project. “Data from the demonstration will be used to assess system performance for multiple AAR refueling technologies, validate the AAR procedures and concepts, and support further development of future unmanned systems.”

The Learjet is being used to test end-to-end X-47B AAR operations, including autonomous rendezvous, approach, plugging and safe separation. Earlier in the UCAS program, one of the air vehicles was outfitted with an AAR receptacle to allow for full testing with the actual X-47B aircraft. The Navy, however, has yet to back full testing using the aircraft. One industry official says that while the Learjet testing will validate the X-47B software, the use of the actual aircraft would better validate, among other things, how it behaves in the wake of the refueler.

Service officials are still examining the results of a third—previously unplanned—round of sea trials for the stealthy, unmanned X-47B after the team returned to port Nov. 19.

The demonstrator aircraft conducted 26 deck touchdowns, 21 of which were touch-and-go tests. It also executed five catapult launches and arrested landings, five wave-offs by the landing signal officer (two planned and three for wind conditions) and two autonomous wave-offs, Burdick says. They were executed due to excessive wind gusts. “The aircraft responded with a preplanned flight maneuver and executed a wave-off,” Burdick says.

The third round of sea trials was hastily added to the program. Tests were conducted on the USS Harry S. Truman late last year, and the first-ever arrested landing of a tailless, stealthy unmanned aircraft occurred in July on the USS George H. W. Bush.

During the most recent flights on the USS Theodore Roosevelt, operators were seeking to conduct X-47B operations on and around the carrier in a broader on-deck envelope than before. Nominal conditions include calm seas and wind blowing straight down the deck at up to 25 kt. Operations this month included a 35-kt. relative headwind and 7 kt. of crosswind, Burdick says. “We learned how the X-47B responds to ship air-wake during approach and landing phases with higher off-axis winds,” she says. “We also gathered data that will help improve ship air-wake models for use with other carrier-based aircraft.”

The test team also determined that electromagnetic interference was the cause of a control issue that prevented a planned takeoff Nov. 10, when media were invited to observe operations on the ship, Burdick says. Tail No. 502 failed to execute a launch when operators were unable to command it to exceed flight idle power, which is required for takeoff. The crew tried both the primary and back-up arm-mounted controllers operated for taxiing and commanding the aircraft on deck and also switched batteries out on one set, with no results. The controller troubleshooting led Capt. Beau Duarate, UCAS program manager, to surmise that day that the issue was with the aircraft.

After a 90-min. “reboot” of the X-47B, a launch was performed. Operators had not encountered this issue before.

Rear Adm. Mat Winter, program executive officer for unmanned aircraft and weapons for the Navy, says the service is seeking funding to conduct additional flight trials.

The UCAS project is intended to provide sailors hands-on experience with an unmanned aircraft on carrier decks, with an eye toward eventually fielding a yet-to-be-designed unmanned system in carrier air wings. A long-awaited request for proposals for that system, the Unmanned Carrier-Launched Airborne Surveillance and Strike (Uclass) program, is set for release this month. Designs from General Atomics, Lockheed Martin, Boeing and Northrop Grumman are expected to be pitched.

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