Now that Joint Heavy Lift has been redefined as the VTOL candidate for the US Air Force/Army Joint Future Theater Lift requirement, the Army's Aviation Applied Technology Directorate is awarding contracts to update the JHL concepts to the latest performance specs.
Contracts are going to Bell Boeing for the Quad Tilt Rotor, Karem Aircraft/Lockheed Martin for the Optimum Speed Tilt Rotor and Sikorsky for the X2 High Speed Lifter. The coaxial-rotor X2 didn't make the original cut for JHL, but is being looked at again to see what it can do in the heavy intra-theater lift mission.
There is another candidate concept being looked at, one that was not in the original JHL mix - the Variable Diameter Tilt Rotor (VDTR). With JHL funding, NASA has awarded Sikorsky a contract to answer questions on the viability, performance and risk areas associated with a VDTR configuration. Sikorsky studied the VDTR for NASA, but shelved the concept around 2000. Images are hard to find...
Sikorsky VDTR. Photo: NASA
VDTR tries to balance the power requirements for vertical and forward flight in a tiltrotor by varying the diameter of the rotor in helicopter and airplane mode. Sikorsky's design uses a jackscrew-actuated telescopic blade that extends in the hover to reduce disk loading and retracts in forward flight to reduce tip speed.
Karem proposes to balance vertical and forward flight requirements by using a multi-speed transmission to optimise rotor speed over a range of 50-100%, and Bell Boeing will probably do what it does on the V-22 - slow the rotor a bit in forward flight. But Sikorsky's VDTR is a more radical approach.