Boeing Darpa DiscRotor

High-Speed VTOL Goes for a Spin
Boeing has concluded that a disc-rotor vertical take-off and landing aircraft is "feasible, but not easy". The company is studying a high-speed combat search-and-rescue concept under DARPA's DiscRotor program, and will test a model of the telescoping-blade rotor in the windtunnel next year.

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Invented by Danish engineer Jacob Ellehammer early last century, the disc-rotor has a disc with blades attached that acts as a helicopter rotor in vertical flight and a wing in forward flight. Retracting the blades into the disc reduces drag in fixed-wing mode. NASA studied a Mach 0.85 disc-rotor in the late 1960s and took another look at the concept in the early 1990s.

The disc-rotor promises to combine the high cruise speed and altitude of a fixed-wing aircraft with the low-speed and hover capability of a rotary-wing aircraft, with low hover downwash and reduced radar signature when the blades are retracted. The low lift-to-drag ratio of the disc as a wing is an issue, so Boeing has added a swept wing to provide efficient lift in forward flight.

Boeing's conceptual CSAR DiscRotor has a dash speed of 360kt and radius of 400nm with a payload exceeding 2,400lb,, and at a mid-mission gross weight of around 31,000lb can maneuver at up to 4g, and hover in ground effect at 12,000ft density altitude - performance figures that are competitive with a tilt-rotor. Blades retracted and wings folded, DiscRotor takes up less deck space than a MV-22 or CH-53E

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Developing a mechanism that will reliability and repeatably retract and extend the blades under flight loads (including centripetal forces) is the central technical challenge of the DiscRotor program, Boeing technical fellow Michael McVeigh told the International Powered Lift Conference in Philadelphia earlier this month.

To address the challenge, Boeing is building a 20%-scale rotor model for windtunnel testing next summer at speeds up to 150kt. This will have four blades, each with three telescoping sections, and a rigid hub with blade retraction and pitch-change mechanisms, and an eight-segment disc fairing.

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The DiscRotor has an integrated propulsion system using two turboshaft engines fitted with fans so they can also generate forward thrust. Shaft power goes to the main gearbox to power the rotor and a pair of wing-mounted, cross-shafted ducted propellers that provide the majority of the thrust in fixed-wing mode.

Engine core and bypass exhaust goes aft to a thrust-vectoring nozzle in the tail that provides anti-torque control in helicopter mode, and during conversion between modes when the disc is being braked or accelerated, and also provides auxiliary thrust in forward flight. The disc is stopped in fixed-wing mode to avoid gyroscopic effects.

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In helicopter-mode flight, the DiscRotor has conventional rotor collective and cyclic pitch controls. Hover anti-torque and yaw maneuvering is provided 70% by the tail thruster, and 30% by differential thrust on the ducted props. Rotor controls are phased out at 120kt and yaw control provided by the thruster and winglets, and pitch and roll control by wing flaperons.

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Mid-way through Phase 1b of DARPA's program, McVeigh says "DiscRotor looks feasible, but not easy. It offers a large increase in cruise speed, but with several large technical challenges." The rotor model tests next year should help decide whether the concept is worth pursuing.

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