Sponsored by the U.S. Army Air Forces/U.S. Air Force, the McDonnell XH-20 Little Henry proved that helicopters could fly using ramjets located in the tips of their rotor blades. The tip-driven rotor eliminated the need for a torque-compensating tail rotor. It did not need a transmission and was controlled with a rudder. The XH-20 was a low-cost research prototype employing open-frame, steel-tube construction. Little Henry was not a success because ramjets were too noisy and used too much fuel, so only two test models were built. One of the XH-20s is on display at the National Museum of the U.S. Air Force at Wright-Patterson Air Force Base, Ohio.