The US Army has begun conceptual work on the Air Maneuver Transport (AMT), an aircraft capable of vertical take-off and landing with a twenty ton payload (the max weight of the Future Combat System armored vehicles) over a range of 900 kilometers. Only one operational helicopter comes close to this requirement, the Russian Mi-26 Halo, hauling 20 tons only 550 kilometers or 15 tons for 900 kilometers. Bell Helicopter has informally proposed a four-rotor helicopter. Boeing and Sikorsky will likely contribute variants of their CH-47 and CH-53 designs, respectively.
In its request for proposals, the Army deliberately avoided the term "helicopter" to encourage creativity amongst the entrants. Perhaps the most innovative concept thus far comes from a small Utah firm called Groen Brothers Aviation. This outfit specializes in gyrodynes, aircraft with wings, propellers, and a helicopter-type main rotor for lift. Their AMT concept uses the proven Lockheed Martin C-130 transport with a graphite composite four-blade rotor mounted where the wing meets the fuselage. This rotor, larger than the Hercules' wingspan, is powered by small rocket motors mounted at the tip of each rotor blade. For take-off and landing, the rotor is powered. Once airborne, however, the C-130's four conventional turboprops propel it forward and the wings generate lift, allowing the rotor jets to be shut off and the rotor to autorotate or freewheel, saving fuel and increasing the vehicle's range compared to a traditional helicopter.
The major technical challenge for Groen is designing blades of such a large size that can be controlled to provide maximum lift during ascent and descent, while creating minimum drag during forward flight. Overall the gyrodyne concept is much simpler than the mechanically complex tilt-rotor, such as the V-22 Osprey. The army project, however, is another vote of no-confidence in the V-22 (which the army was originally going to buy, but backed off when the V-22 ran into development problems.)
So far the AMT has no funding allocated. Its planned to be operational in the 2015 timeframe. --AJ Wagner