The Next X-plane
Tilt rotor, lift fan, tail sitter: What’s the best technology for both vertical lift and high speed?
development, production, and operating these V/STOL concepts,” writes Mike Hirschberg, who developed a modern V/STOL Wheel of Misfortune while working on the F-35 program and is now the executive director of AHS. The only four to go into production are the Harrier, the Yak-38 and F-35B jet fighters, and the V-22 Osprey tiltrotor troop transport, which swivels two large wingtip “proprotors” upward to take off and land like a helicopter and forward to fly like an airplane. That enables the Osprey, now in service with the United States and, soon, Japan, to cruise at 275 knots, roughly double the cruising speed of most military heli- copters. Though the Osprey can hover, it doesn’t do so very efficiently, in part because speedy forward flight requires the proprotors’ blades to be short and heavily twisted, and in part because the craft has high disk loading (the total weight of the aircraft divided by the area of the circle its rotor blades describe). “Nearly all of the aircraft shown on the V/STOL Wheel—all of which were built and flown—could have been fielded, given enough time, money, and willingness to accept compromise,” says Hirschberg. “But they needed to have a compelling capability over less expensive, less complicated conventional aircraft of their day.” What the VTOL X-plane program, also known as the VXP, is trying to do is to come at the problem by combining the lessons learned over the past 75 years with new technology and new ideas.
DARPA’S Bagai says a key VXP requirement is “the ability to hover and to perform sustained hover. That’s an attribute of vertical flight that’s important. That’s why we have helicopters today, because nothing else can do those missions that helicopters can do.” Speed, he says, can be gotten pretty much just by adding power: “We know how to fly barn doors fast.” He should know. When at Sikorsky, he was principal engineer on the X2 high-speed technology demonstrator, a compound helicopter—coaxial rotors with a pusher propeller—that won the Collier Trophy in 2011 for setting an unofficial helicopter speed record: 250 knots in level flight.
In March 2012 Bagai came to DARPA and started the VTOL X-plane project, the sort of experimental project DARPA was made for. The agency doesn’t produce military equipment; instead, it funds farsighted, sometimes far-fetched research to help keep the military on the cutting edge of technology. If DARPA’S projects bear fruit, the results are handed over to the military for possible development. For example, the agency did some early work on the lift fan technology used in the F-35B, but has no direct role in F-35 production.
The VTOL project kicked off in 2013, when the agency awarded preliminary design contracts of $11 million to $17 million each to four competitors. Two of the awards went to some of America’s top military aviation manufacturers: Boeing and a Sikorsky–lockheed Martin team. The other contracts went to smaller companies headed by two of the country’s most inventive aeronautical engineers: Aurora Flight Sciences, whose chief designer, John Langford, got his start heading a team that set a world record for human-powered flight, and Karem Aircraft, founded by Abraham Karem, who among other achievements invented the Predator drone.
The four contestants are offering dramatically different solutions—all unmanned, an option DARPA offered to speed the work.
says, adding that Phantom Swift uses a “four-post lift approach” for vertical flight and hover. After vertical takeoff, louvers at the bottom of the lift fans in the fuselage can be used for maneuvering in a hover or closed entirely for forward flight. The body lift fans then shut down while the wingtip thrusters tilt forward to make the craft fly like a conventional airplane.
“We have the ability to control thrust [independently] on all four fans,” Ritter says. “We have louvers in the bottoms of the body lift fans which can provide directional control. We have the ability to rotate our wingtip thrusters independently and help provide directional control. And then, by controlling the thrust out of each of those fans, all of those different things combined provide the means for low-speed directional control.” At high speed “you’ve got full flying tails and flaperons on the aircraft as well,” Ritter adds. “So we’ve got maneuverability and agility, in addition to just the wingtip thruster control in forward flight.”
Power for Phantom Swift comes from two GE CT7-8 engines, a variant of GE’S 700 series, which are used in Sikorsky’s S-92 multi-role helicopter. Boeing built no subscale demonstrator to test its concept in flight, but Ritter says some “young engineers” at the Phantom Works built a six-foot model as “a marketing tool during the proposal stage”; a video of it flying in a lab can be found on Youtube.
It would be “very easy” to build manned or unmanned versions of the Phantom Swift, Ritter ventures, scaled down from the VXP’S required 10,000 to 12,000 pounds to as little as 4,000 pounds. Scaling Phantom Swift up to 24,000 pounds also could be done, he says, though “it’s going to take some engineering design.” Nevertheless, Ritter adds, “when we look at this, we see a great X-plane and fun opportunity to demonstrate new technology—but also the makings of a new product line.”