Cats and traps lie ahead for unmanned X-47B as work-up to 2013 carrier demonstration begins
Spring 2012 at NAS Patuxent River, Md., and an unusual shape joins the Joint Strike Fighters flying the pattern at the U.S. Navy's test center. The tailless flying wing is 's X-47B unmanned combat air system demonstrator (UCAS-D), being prepared for autonomous landings on an aircraft carrier in 2013.
The Navy may be late to the unmanned-aircraft game, but it is pushing the technology in terms of both capability and autonomy. In addition to UCAS-D, the service is launching the Autonomous Aerial Cargo/Utility System (Aacus) program to prototype advanced capabilities for vertical-takeoff-and-landing (VTOL) unmanned aircraft systems (UAS).
Just as Aacus is expected to feed technology into the Navy's program to deploy the shipborne VTOL Medium-Range Multi-Role UAS by 2019, UCAS will inform its plans to field the Unmanned Carrier-Launched Airborne Surveillance and Strike System (Uclass) by 2018, and develop a sixth-generation F/A-XX to replace the/F after 2030.
Operational studies under the UCAS program have shown that a long-endurance, aerial-refueled unmanned combat aircraft could significantly extend the surveillance and strike reach of a carrier battle group. But first the Navy must get comfortable with bringing an unmanned aircraft on to the flightdeck.
Though UCAS-D is a demonstration, and not a prototype for Uclass, the system architecture and operating concepts developed to enable the 44,000-lb. X-47B to land safely on a carrier—and particularly changes to the ship's command-and-control system—could carry over.
The first of two X-47Bs completed Block 1 envelope-expansion flight tests at Edwards AFB, Calif., on Nov. 17, and air vehicle 1 (AV-1) is to be shipped to Pax River by year-end to begin Block 2 carrier-suitability testing, including land-based catapult launches and arrested landings. The second X-47B, AV-2, made its first flight at Edwards on Nov. 22.
In 16 sorties since its initial flight on Feb. 4, AV-1 has expanded the envelope to 220 kt. airspeed and 15,000 ft. altitude—a task that was originally expected to take a year and require 49 flights. “AV-2 will continue to expand the envelope, and when it ships [to Pax River] all the necessary corners to go to the carrier will have been cleared,” says Carl Johnson, Northrop Grumman vice president and UCAS-D program manager.
While gathering flying-qualities data, AV-1 has flown simulated carrier approaches at altitude. “All X-47B flight-test data look very good and will support our carrier demonstration objectives,” says Capt. Jaime Engdahl, Navy UCAS program manager. “We found no technical issues during any of the flights and it took considerably less flight time than predicted to execute all of our planned test points.” As a result, AV-2 could be moved to NAS Pax early, in spring 2012.
The speed of envelope expansion is due in part to the accuracy and predictability with which the 62.1-ft.-wingspan X-47B executes the preprogrammed test points. But it is also due to Northrop's familiarly with its signature cranked-kite planform, and to extensive modeling and simulation. Engdahl says the aircraft simulation model accounts for about a third of the 3.4 million lines of software code for the UCAS-D program.
“The modeling and simulation is correlating so well with flight-test data that we can use it to add confidence and reduce on-aircraft testing. It significantly reduced the number of flights required to expand the envelope,” says Johnson. “The future for UAS with robust modeling and simulation is we will not have to fly the platform as much as manned systems, which are less predictable.”
“The aircraft is flying exactly the way the model said,” says Engdahl, adding no flight-control changes have been required. “Control-law development has been very robust,” agrees Johnson. “We've had no issues, but then our developers have quite a bit of experience with this planform design.”
Confidence in the aircraft's behavior will be crucial at Pax, where theF-35B and C are being flight-tested and where disruption to normal operations when the X-47B is flying must be minimized. “When we begin flying there, operating an unmanned aircraft from an active naval air station, it will be a significant step forward,” says Johnson.
The Navy has experience operating the unmannedMaritime Demonstrator from Pax. “They started conservatively, keeping all other traffic away. As they became comfortable with the system, they gradually integrated it into the airspace,” he says, predicting “It won't be too long before it will be hard to tell the X-47B from other flights.”
While the X-47B is conducting cats and traps at Pax, additional tests of two manned surrogates—a King Air and F/A-18D equipped with UCAS-D avionics—will certify the software and systems for the 2013 demonstration. “We will install the system on the ship and take the King Air and F/A-18 out to certify the carrier, so when we plug in the X-47B it will be relatively seamless,” says Engdahl.
The F/A-18D surrogate conducted the first autonomous arrested landings on a carrier in July. During the at-sea tests, the aircraft made 36 approaches to the USS Eisenhower, 16 touch-and-go landings and six coupled approaches to arrested landings. All were conducted “hands off,” and in the same way the X-47B will land, but with a pilot onboard for safety and redundancy.
“We've exercised all the functionality with the surrogates,” says Engdahl. The Eisenhower tests included straight-in, or Case 1, instrument approaches where the unmanned system took over control 8 nm behind the ship; and visual, or Case 3, approaches where the system took over as the F/A-18 passed the carrier on the downwind leg and then turned the aircraft on to its final approach.
The autonomous landings demonstrated the precision-GPS ship-relative navigation technology at the heart of UCAS-D. The carrier sends its GPS position to the aircraft via a low-latency, high-integrity Tactical Targeting Network Technology data link. The aircraft, which has triple-redundant GPS/inertial navigation systems, calculates its position relative to the moving ship and guides itself to a touchdown on the flightdeck.
“Surrogate testing with the F/A-18 using prototype software validated the algorithms. Now we are turning it into production software to do all the rigorous qualification and certification testing required by Navair [Naval Air Systems Command], to ensure we have thought of every contingency,” says Johnson. “Over the next year we will go through the work-up to validate the software in the lab, on the vehicle and in flight.”
Surrogate trials also validated the distributed control concept, in which a UCAS mission operator on the ship always has positive control of the aircraft, but the carrier air traffic control (ATC) center, primary flight control (Pri-Fly) or “air boss” in the tower, and landing signals officer (LSO) on the flightdeck can send commands to the unmanned vehicle as they would to a manned aircraft.
“Over the last 10 years the Navy has been digitizing its carriers. ISIS—the integrated ship information system—has automated and digitized the information flow around the ship, so ATC and Pri-Fly can share a picture of who's flying, how much gas they have, etc.,” says Engdahl. For UCAS-D, a ship interface processor is installed to act as gateway between the X-47B mission control element and the carrier network. This allows ATC to pull in data such as fuel state and send commands to the vehicle, while the UCAS mission operator has access to all ATC and deck information.
When the aircraft is inside the 50-nm-radius carrier control area, but outside 10 nm from the ship, ATC sends digital commands to the mission operator. The aircraft checks in with its position, airspeed and altitude, and ATC sends a message back with marshal position and push time. Inside visual range, control passes to the Pri-Fly, and the mission operator monitors as the tower sends messages to the aircraft and it automatically responds. On final approach, control passes to the LSO, who can hit the pickle switch and wave off the aircraft at any time without having to go through the tower, ATC or mission operator.
Key to the control philosophy is a level of air-vehicle autonomy beyond that in today's unmanned aircraft. “UCAS-D represents a new generation of UAS due to the level of autonomy developed to do a carrier landing or automated aerial refueling,” says Johnson. “Other systems are remotely piloted, and only do what they are told from the ground. All the decisions are on the ground, and the system architecture makes it hard to move to a higher level of autonomy.
“With UCAS-D we start with a fundamentally different architecture that puts capability on the aircraft. We won't release it all for flight test, but the architecture is designed for higher levels of autonomy we can use as we expand capability,” he says.
“The vehicle knows it needs to refuel and in a machine-to-machine process talks to the tanker and gets permission to move to the tanking position. It develops a relative-navigation coordinate system so it knows where the tanker is and uses that frame of reference to move itself to where it needs to be.”
Increasing levels of autonomy will be demonstrated as the X-47B moves from land-based testing to the carrier and eventually automated aerial refueling. “In land-based testing, it flies a validated mission plan in a fixed frame of reference. When we move to the ship it introduces additional variables the system must react to,” he says. “Ultimately we will look at inflight replanning permission. We will tell the vehicle where the tanker should be and it will adapt its mission to find the true position.”
As with envelope expansion, clearing the X-47B to land on a carrier is expected to benefit from the vehicle's predictability. “A pilot can get off-nominal but, because of the digital interface with the ship and real-time updates to the vehicle, an unmanned aircraft is not going to get very far off track,” says Johnson. But as it will be the first tailless aircraft to land on a carrier, tests are focused on ensuring the X-47B has good flying qualities, says Engdahl.
At-sea testing will evaluate handling qualities in crosswinds and headwinds, control power as the vehicle passes though the airflow “burble” behind the carrier, touchdown dispersion on the deck and lateral dispersion on “bolter” touch-and-goes. More than one carrier is to be outfitted to work with the X-47B for the 2013 demo. “We will work with the carrier schedule to get as much test time as we can. That's when it will get interesting,” Engdahl says.