The Lockheed XV-4 Hummingbird (originally designated VZ-10) was a U.S. Army project to demonstrate the feasibility of using VTOL for a surveillance aircraft carrying target-acquisition and sensory equipment.

[1] But the performance was far below estimates with only a 1.04 thrust-to-weight ratio, and the prototype crashed on 10 June 1966, killing the pilot.

Rockwell's XFV-12 would be even less successful at producing lift by using engine exhaust to entrain cold air, in this case through flaps on the wings.

The two Pratt & Whitney JT12 engines were replaced with six General Electric J85 turbojets, four of these units acting as lift jets.

This aircraft crashed in Georgia on 14 March 1969; pilot Harlan J. Quamme escaped uninjured, using the ejection seat.