Lockheed began private research into ejector augmention systems for VTOL aircraft in 1959. They received a US Army contract in July 1961 to build two of their Hummingbird aircraft as the XV-4A. The Hummingbird had a wingspan of 26 ft, and a length of 32 ft, primarily consisting of a boxy fuselage that housed the ejectors and augmentors. Along each side of the aircraft, a Pratt & Whitney JT12A turbojet engine produced 3,300 lb thrust either for horizontal flight, or diverted into the augmentor ejectors for vertical take-off and landing. The two engines fed interleaved ejectors in case of engine failure. In transition, one engine was diverted from the ejectors to providing forward thrust, until wing-borne lift allowed the second engine to do the same; the augmentors doors were then closed. The augmentors were constructed of stainless steel and titanium, accounting for a significant portion of the 5,000 lb empty weight and the 7,200 lb gross weight. Actual vertical thrust after installation losses was about 7,500 lb for a 1.04 thrust-to-weight. This was only a 14% net augmentation.

First conventional flight was on 7 July 1962. First tethered hover on 30 November of that year was followed by first free hover on 24 May 1963. First transition was not completed until 8 November 1963. The first aircraft crashed on 10 June 1964, killing the pilot.

Source: AHS V/STOL Wheel