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  • Six Engines Mean Survivable VTOL
    Posted by Graham Warwick 1:38 PM on Aug 23, 2011

    Six engines seem a bit extreme, especially on an aircraft of a size that would normally only have a couple of turboprops. But for retired aeronautical engineer Richard Oliver, six engines is the solution to designing a vertical take-off and landing (VTOL) aircraft that can lose any one of its propulsion units, remain stable, and compete its mission, even at maximum weight.

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    All concept art: Oliver VTOL

    Atlanta-based Oliver VTOL is now trying to get the Hexplane idea off the drawing board, filing a patent application and working to raise $12 million to build a prototype that would be based on a Piaggio P.180 Avanti airframe to save money. Oliver believes the aircraft could fly in about two years.

    Oliver says he first conceived the idea 20 years, when he set out to design a two-seat VTOL aircraft that would allow downed aircrew to rescue themselves. The idea was to use inexpensive, lightweight two-stroke engines. "But four props are not enough - if you lose one, you lose the aircraft," he says. So he came up with the idea of adding an extra wing, and another pair of props, across the middle.

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    "It's a little odd looking," he admits. "But it has the unusual ability to survive the complete failure of any propulsion unit and remain statically stable. If you remove any one unit and wing in flight it will still fly safely, even a maximum gross weight and HOGE [hover out of ground effect]." The six engines are completely independent, with none of the cross-shafting you would see in a two- or four-prop tiltrotor.

    The Hexplane design is all about survivability, he says, and about being able to continue the mission and not having to return and land after a failure. "I don't see it as complex. There are not six throttles, just a single go lever and six-channel fly-by-wire," Oliver says.

    Oliver says he has had the Hexplane design validated by aerospace engineering consultancy DAR and Penn State University, and is now negotiating with an investment house for the funds to build a prototype. As a starting point, Oliver says he has taken on the challenge set by DARPA for the now-cancelled Groen Heliplane high-speed VTOL - to carry a 1,000lb payload 1,000nm at 400mph.

    "I want to build a 12,500lb gross-weight technology demonstrator that has the capability to transition to saleable hardware," he says. Powered by six 800shp turboprops, this aircraft would have a maximum speed of 447mph and a sea-level climb rate of more than 9,000ft/min.

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    The initial target market would be a VTOL unmanned aircraft (above), potentially as a cargo resupply vehicle or as a contender for the Navy/Army MRMUAS/MRPM medium VTOL UAV requirement. Oliver claims to have "interest from a DoD customer."

    Six engines still seem a bit over the top, but Oliver says power required for hover and for high-speed cruise are well-matched. The vehicle is designed to be capable of full gross-weight vertical operation at the HOGE limit with a failed propulsion unit, "even at 6k/95 conditions [6,000ft and 95F]," he says. "The design power is driven by that necessity."

    Conventional props are used, not the gimballed prop-rotors of a tiltrorotor, and Oliver says the Hexplane's efficiency lies somewhere between the V-22 and conventional fixed-wing aircraft. The Hexplane would fly further, faster than a V-22 on the same fuel, with the same payload "and do it safer," he claims.

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    Tags: awt, aeronautics, VTOL

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