Together with UCAS prime contractor Northrop Grumman, the Navy team proved the aircraft's ability to land safely on the deck of the USS George H.W. Bush twice on July 10. Though a significant first in aviation history, the team also demonstrated the aircraft's ability to land in all conditions up to sea state 5, including waves up to 13 ft. high, and various crosswind conditions in flight and simulated testing quietly conducted ashore at NAS Patuxent River, Md.
The two sea-based landings were seminal for the program and carrier aviation, but perhaps more revealing about the system's capabilities was a series of seven successive touch-and-goes done during a single day's worth of trials on the USS Bush May 21, says Carl Johnson, Northrop Grumman UCAS vice president. These trials were conducted in advance of the carrier landings to prove the bolter logic—an autonomous function that kicks in to conduct a quick—safe takeoff in the event one the X-47Bs were to miss all three wires on the Bush. In a manned aircraft, the pilot engages the throttle to gain enough speed and manually directs the aircraft down the centerline of the runway.
During the bolter trials for the X-47B, engineers collected video of the landings to analyze not only how precise the touchdowns were based on predictions, but also to study how precisely the air vehicle accelerated down the centerline on its takeoff.
For the touch-and-goes, the nose gear landed all seven times with no more than 1 ft. in lateral deviation and 8 ft. in longitudinal deviation, a space slightly larger than a yoga mat. These seven landings are representative of thousands of simulations and other trials done ashore to vary the wind conditions and other factors. “Our performance at sea was equivalent to our performance on shore-based landings,” Johnson says. Also notable is the consistency with which the nose gear rolls precisely along the centerline as the aircraft speeds up for its takeoff to rejoin the carrier's traffic pattern, Johnson says.
In a video of clips showing the seven touch-and-goes compiled by Northrop Grumman, the air vehicle shows a slight deviation in its sixth landing, where the nose gear touches down slightly closer to the centerline. This was a result of a shift in crosswinds, Johnson says. The aircraft, however, compensated and conducted its takeoff precisely along the centerline as shown in the video.
The precision on landing is the culmination of calculations performed by the air vehicle's software far in advance of touchdown. The X-47B uses a relative GPS system that constantly judges its position in space relative to equipment onboard the carrier, which is often moving at more than 20 kt. and rolling with the sea. By comparison, a UAV landing ashore would have a fixed GPS target point on which to home and would not require a “relative” reading.
In the case of the Bush trials, the X-47B engaged its relative GPS system a half-mile from the ship. At that point, the aircraft tracked its position relative to the precise spot on the deck it is programmed to impact. “If your eyes were good enough, you would see the airplane move like it is on water,” Johnson says, noting the vehicle mirrors the carrier's motion as it rolls on the waves owing to the relative GPS readings. The aircraft then travels on a straight line, gliding to that point until touchdown. “It is never much more than a foot off of that line” during the approach, he says.
At this point in the operation, latency is a major issue, so the system is designed to constantly update the relative positions of the carrier and aircraft. Once aloft, if an operational unmanned aircraft were on a mission far from the carrier, that latency in communications could be dramatically different.
Through the $1.4 billion experimental UCAS program, the Navy proved that a tailless, unmanned aircraft can operate on the carrier and interoperate with the carrier wing. “From December 2012 until now, we were out there putting the aircraft into the real environment and validating that the real environment acted exactly like the simulations,” Engdahl says. “One thing we have done is to show we can characterize naval aviation digitally and take electronic commands and send them to autonomous vehicles [that] operate like an aircraft in an air wing.”