DARPA, the U.S. Defense Advanced Research Projects Agency, announced this week they have completed sub-scale testing of their hybrid electric VTOL design and will move ahead with developing a full-scale aircraft. “The VTOL demonstrator was designed specifically to test the aerodynamic design of the aircraft, validate flight dynamics, and develop the flight and mission-systems controls for application to the full-scale vehicle,” said Ashish Bagai, DARPA program manager. “The aircraft exhibited exceptional flight characteristics, with no loss in altitude even as it transitioned from vertical to horizontal flight. It also demonstrated aerodynamic effectiveness of the distributed propulsive system.”
The subscale aircraft flight design and mission control architectures will, for the most part, be carried over into the full-scale VTOL aircraft, DARPA said, but with a few changes. The full-scale aircraft will incorporate a triple-redundant flight control system instead of a single system. Also, a hybrid turboshaft engine will drive electric generators to power the fan units, instead of the batteries used in the demonstrator. Also, the full-scale aircraft fan units will be synchronized to the generators and turn at a constant RPM, but incorporate variable pitch, whereas the demonstrator’s fans are speed controlled.
The XV-24A will weigh 12,000 pounds, compared to 322 pounds for the demonstrator, and will aim to verify specific performance objectives, DARPA said: flight speeds in excess of 300 knots, full hover and vertical flight capabilities and—relative to helicopters—a 25 percent improvement in hovering efficiency and 50 percent reduction in system drag losses during cruise. “These are ambitious performance parameters,” Bagai said, “which we believe will push current technologies to the max and enable a new generation of vertical flight operational capabilities.”
