Lockheed XV-4 Hummingbird

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The Lockheed XV-4 Hummingbird was a United States Army experimental jet designed in the 1960s to test if a fixed-wing aircraft could take off and land vertically while carrying surveillance sensors. Only two were built, and both were destroyed in testing.

How it worked
Lift came from engine exhaust directed downward through multiple nozzles, helped by a secondary flow of cold air. The design produced a thrust-to-weight ratio of about 1.04, which researchers found disappointing compared to expectations.

Development and flights
- The first prototype, XV-4A (tail number 62-4503), made its first conventional takeoff on July 7, 1962. It did tethered tests on November 30, 1962, hovered freely for the first time on May 24, 1963, and transitioned from hover to forward flight on November 8, 1963.
- This aircraft crashed in Cobb County, Georgia, on June 10, 1966, killing the pilot.

A second prototype followed
- Between 1966 and 1968, Lockheed rebuilt the second airplane to the XV-4B standard, replacing the two Pratt & Whitney JT12 engines with six General Electric J85s, four of which acted as lift jets.
- The XV-4B crashed in Georgia on March 14, 1969; the pilot, Harlan J. Quamme, escaped by using the ejection seat.

Overall outcome
None of the early American V/STOL designs led to production aircraft. Other programs around the same era explored different lift concepts, such as the British Hawker Siddeley Harrier and the Soviet Yakovlev Yak-38. The later F-35 Lightning II uses a shaft-driven lift fan instead of exhaust-based lift.