DARPA does not want bidders for its new VTOL X-Plane program to "just revisit old concepts" for vertical take-off and landing that combine the high-speed performance of fixed-wing aircraft with the low-speed agility of helicopters.
A look at the American Helicopter Society's wonderful V/STOL Wheel (of fortune or of shame, take your pick) shows how hard that could be, as it lists no fewer than 45 different concepts that have been tried, of which only three can be considered successes -- the Hawker Harrier, Yakovlev Yak-38 and Bell Boeing V-22 (four if you include the Lockheed Martin F-35, but I won't go there).
VTOL X-Plane program manager Ashish Bagai says DARPA has seen "a few isolated and novel approaches" that might work, but he acknowledges the answer could lie in "more astute integration" of concepts that have been tried before.
All graphics: DARPA
A clue might be found in Bagai's background. Before he joined DARPA, he was principal engineer at Sikorsky, responsible for aerodynamic design of the rotor blades on the X2 Technology Demonstrator. The X2, and follow-on S-97 Raider, are essentially the "astute integration" of technologies first flown in the 1980s in the Sikorsky XH-59 Advanced Blade Concept demonstrator.
The XH-59 achieved 263 kt, but needed four engines and four hands to fly and vibrated like a jackhammer. The X2 demonstrator exceeded 260 kt on one engine, with one pilot and vibration levels no worse than a Black Hawk at 150 kt. That feat was achieved through the synergistic integration (sorry!) of rigid rotors, propulsor, fly-by-wire controls, active vibration control and other technologies.
Bagai sees similar potential to improve the integration of other concepts tried before, but discarded. "The V/STOL wheel of fortune offers a lot of insight into what was done in the past, and examples of concepts that were reinvigorated," he says. "There is a lot of technology available to advance previous concepts." He cites new design tools and subsystem technologies and "the potential for far more hybridization".
The 52-month, $130 million VTOL X-Plane program to build an experimental aircraft that, compared to a conventional helicopter, is able to: fly faster (300kt or more), hover more efficiently (75% or better), cruise more efficiently (lift/drag ratio of 10 or better) and carry a useful load of 40% or more of gross weight. The X-plane itself is to be a reasonable size -- 10,000-12,000lb gross weight, or the size of Sikorsky's S-97 Raider -- so that it can be reliably scaled to the objective vehicle (a tricky thing with rotorcraft).
Bagai has some other objectives. "This is an aggressive program," he says. "We cannot afford to get bogged down. We will not suffer non-A teams. We want small teams of personalities, with outstanding leadership." Perhaps his thinking is influenced by DARPA's checkered history with high-speed VTOL.
Remember the Sikorsky X-Wing circulation-control rotor concept, cancelled in 1988 before it flew? Or the Boeing X-50A Dragonfly canard rotor/wing stopped-rotor concept, cancelled in 2006 after both subscale unmanned demonstrators crashed? Or the Groen Heliplane gyroplane, with tipjet-driven rotor, cancelled in 2008 after encountering problems with tipjet noise?
Better luck this time, DARPA.