The experimental Lockheed XH-51A Compound in flight. It reached 423 km/h (263 mph) at 3660 m (12,000 ft) in 1965.
U.S. Department of Defense
<a rel="nofollow" class="external text" href="http://www.navy.mil/ah_online/department_arch.html">U.S. Navy All Hands magazine</a> <a rel="nofollow" class="external text" href="http://www.navy.mil/ah_online/archpdf/ah196512.pdf">December 1965</a>, p. 35.