The X2 has an entirely new airframe, with new fly-by-wire controls, so several challenges have presented themselves so far. In the early stages, flight simulation software could speed development, but it could not predict every detail of the turbulence that might result as the two main rotors worked together. Following the first flight, in August 2008, test pilot Kevin Bredenbeck suggested some fine-tuning. Says advanced programs manager Jim Kagdis: “It was hot”—too sensitive to the pilot’s control inputs. “We adjusted the controls.”
Even if the X2 proves out, the drag its rotors create as they turn will probably hold speeds down to 300 mph. The limitation posed by rotors is the reason the Defense Advanced Research Projects Agency is paying Boeing and partner Virginia Tech University to develop the DiscRotor, a helicopter in which the main rotor can be packed away during flight into a circular housing, reminiscent of the radome atop an AWACS aircraft. In this phase of flight, fixed wings would provide the lift, perhaps assisted modestly by the smartly shaped rotor housing.
Such a craft has never taken to the air, so DARPA, says program manager Phil Hunt, is taking one careful step at a time. The project is presently in the conceptual stage, having completed a Phase 0 contract and entered Phase I.
In another edgy rotorcraft program—the X-50A Dragonfly, which would stop its rotors in flight and employ them as fixed wings—both unmanned prototypes crashed during test flights. So for this phase of the DiscRotor’s development, DARPA has pegged modest goals, asking only that wind tunnel tests through 2011 establish whether the design deserves more development money. In
theory, the helicopter could hit 400 mph, making it highly attractive for search-and-rescue in combat settings.
Exciting stuff, but the last word belongs to those who have to fly the things. Greg Lengyel, based at Florida’s Hurlburt Field, says faster machines need to come in large versions, capable of missions such as transport, external lift, and long-distance rescue: “If you have to trade lift for speed—if you can only go fast with helicopters the size of the Cobra or Apache [gunships]—you don’t gain very much.”
I asked Wade Hasle about tradeoffs that pilots would watch for, should faster helicopters hit the airways. “I don’t think it would be well received if a faster helicopter had limited vertical performance,” he says. In combat, good hover performance is a must, since a helicopter in the field often lacks the opportunity to gather speed with a rolling takeoff. No matter how fast it goes, in other words, the next-generation helicopters will still have to have the talents of the old-school ones.
James R. Chiles is the author of The God Machine: From Boomerangs to Black Hawks, the Story of the Helicopter (Bantam Dell, 2007).