VTOL UCAV tailsitter design, as I’ve long pictured it
Sikorsky Sees Missions For Rotor Blown Wing VTOL UAS
Jun 2, 2016 Graham Warwick | Aviation Week & Space Technology
Believing a long-range vertical-takeoff-and-landing (VTOL) unmanned aircraft could play a key role in anti-access/aerial-denial (A2/AD) operations, Sikorsky is continuing to study the configuration proposed for DARPA’s VTOL X-Plane program.
The Rotor Blown Wing (RBW) concept was not selected for the high-speed VTOL flight demonstration, but the design was taken to a preliminary design review under the first phase of the program. Sikorsky is exploring a range of mission applications, says Glenn Tiongson, a senior operations analyst at Sikorsky Innovations, who was presenting at the AHS Forum 72 conference in West Palm Beach, Florida, in May.
The VTOL X-Plane program is seeking an aircraft with a speed of up to 400 kt. but a hover efficiency similar to that of a helicopter and a substantial useful load. The competition to build and fly a demonstrator was won by Aurora Flight Sciences with its LightningStrike distributed hybrid, electric ducted-fan design.
Air refuelable ISR/strike Rotor Blown Wing carries targeting sensors and Hellfire missile under fuselage and wingCredit: Sikorsky
The RBW, meanwhile, is a tailsitter with dual proprotors and a traditional fixed-wing configuration for speed, range and endurance without the complexity of hardware reconfiguration between vertical and forward flight. Sikorsky is studying platform designs for intelligence, surveillance and reconnaissance (ISR), strike, escort and aerial refueling missions, Tiongson says.
The aircraft sits nose up, the wing perpendicular to the ground. One or two turboshaft engines drive the helicopter-like, semiarticulated proprotors to provide lift for vertical flight. Similar to a helicopter, collective blade pitch is used to change lift and cyclic pitch to move forward.
As the aircraft accelerates into forward flight, the wing generates more lift, the aircraft pitches over, and the rotors provide more propulsive thrust. At 100-130 kt., the RBW is fully wingborne and flies like a propeller-driven fixed-wing aircraft. The rotors are used for forward flight control, simplifying the design.
Carrying external tanks to increase its fuel capability, an RBW tanker aerial refuels and ISR/strike aircraft. Credit: Sikorsky
Tiongson explains that fly-by-wire flight control and autonomy technologies overcome the disadvantages of previous tailsitters, including the difficulty the pilot experienced while taking off and vertically landing the Convair XFY-1 experimental VTOL fighter of the mid-1950s.
Boeing’s mid-1990s Heliwing unmanned aircraft was a tailsitter, but had control issues and crashed. Northrop Grumman has selected a tailsitting VTOL flying-wing design for DARPA’s Tern ship-based, medium-altitude long-endurance unmanned aircraft, but this has a coaxial rotor mounted centrally.
Sikorsky studies indicate the RBW configuration is scalable to gross weights of 2,000-10,000 lb., Tiongson says. The company sees four key mission applications in the 10,000-25,000-lb.-size class: ISR and ISR/strike, a directed-energy weapon platform and a “unique-access” tanker.
Laser-armed RBW could operate from small-deck warships with speed to engage manned and unmanned aircraft. Credit: Sikorsky
The ISR aircraft has a high-aspect-ratio wing for efficient cruise and loiter, and castering, wheeled landing gear for ground maneuvering and handling on small-deck warships. With a 13,000-lb. gross weight and two 1,750-shp. engines, the RBW can cruise at up to 250 kt. and carry a 600-lb. ISR payload over a 1,600-nm radius with an 18-hr. time on station.
The larger ISR/strike RBW weighs about 16,500 lb. with two 2,200-shp engines. Weapons are mounted on fuselage and wing stations, and the aircraft can deliver a 1,000-lb. payload over ranges up to 4,000 nm at speeds up to 300 kt., Tiongson says. Both the ISR and ISR/strike configurations include aerial refueling and auxiliary power units to extend operations.
The directed-energy RBW has a 23,000-lb.-gross-weight, 3,000-lb.-payload capability and two 3,000-shp engines to generate the excess power required to provide a strike capability with a directed-energy weapon and the speed required to engage manned and unmanned aircraft as well as surface targets.
This version has a maximum cruise speed of 350 kt., a range of 3,500 nm and a maximum time on station of 19 hr., but it is still designed to be compatible with small flight decks. Tiongson explains that the speed also enables the directed-energy RBW to target cyber networks on the ground.
The unique-access tanker variant is intended to provide fuel and cargo to supply combat units at forward arming and refueling points. Weighing in at 21,300 lb., with two 3,000-shp “advanced technology” engines, this variant would carry 500 gal. (3,350 lb.) of fuel in three external tanks plus 700 gal. internally.
The tailsitter RBW would be able to operate into locations too small for U.S. Marine Corps Bell Boeing V-22s, and Tiongson says a single external tank would allow refueling of three Bell UH-1Y Venom or AH-1Z Viper helicopters in a conflict zone.
The RBW tanker also would be able to use a hose-and-drogue system to refuel other unmanned or manned aircraft, he notes. Maximums are: range, 3,500 nm; endurance, 19 hr.; and speed, 250 kt.
In future A2/AD operating environments, Tiongson argues, a substantial portion of aerial assets must be capable of autonomous operations from small-deck ships such as destroyers and amphibious assault vessels, but with fixed-wing range and endurance to launch farther from shore and penetrate deeper.
With their increased speed, range and autonomy, advanced unmanned-VTOL concepts such as RBW have the potential to mitigate the A2/AD threat, he concludes, and permit operations in a high-threat environment with less risk of casualties.
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