Sources tell me DARPA is about to award Bell Boeing a small contract to study a stop-fold tiltrotor. Or restart studies, I should say, as both companies have previously looked at tiltrotors that stop and fold their rotors to go faster.
In 1972, Bell tested a 25ft-diameter stop-fold rotor in a NASA Ames windtunnel. This demonstrated the feasibility of stopping the rotor and folding the blades back along the nacelle to reduce drag, and then redeploying the blades and spinning up the rotor.
The concept was shelved because the technology was not available to overcome the added complexity and weight of the stop-fold mechanism, and the lack of an engine able to convert from driving the rotor to producing thrust.
Now it looks like tiltrotor experts Bell Boeing will get a chance to dust off the concept, apply technology now available, and perhaps run another windtunnel test. With the demise of Boeing’s stopped-rotor Canard Rotor Wing, it’s another way to get to a high-speed rotorcraft.
Tilt to take off, fold to go faster (Boeing artwork)