RogerBW's Blog

Lockheed XV-4 Hummingbird 03 May 2014

The XV-4 was a prototype V/STOL aircraft built for the US Army.

It's not entirely clear what the Army planned to do with it, other than have something like the British P.1127 which was clearly going to define the future of combat aircraft; it seems to have been a testbed rather than a prototype, since it had no provision for weapons, sensors, or any sort of payload. It seems likely that the Army ultimately wanted a close-support aircraft that could operate from small forward bases.

The aircraft design was an unusual one, with two Pratt & Whitney JT12 engines mounted over the mid-body wing and close to the fuselage. A diversion of the jet exhaust fed into a complex arrangement of vanes in the centre of the fuselage. Doors at top and bottom would open to allow cold air to be drawn down through the vanes by the exhaust flow (presumably a venturi effect), thus augmenting thrust compared with what one would get purely by pointing the engine exhausts downwards.

The first prototype flew in 1962, with free flight in 1963. The thrust augmentation was much less than had been hoped for, and it only achieved a thrust to weight ratio of about 1.04, making its handling in the hover decidedly marginal; in 1964, it crashed, killing the pilot. Data are scant, but it appears that this was caused by a failure of one of the exhaust diverters.

The second prototype was modified: the diverters were removed, and four GE85s were mounted vertically in the fuselage cavity to act as lift jets. (It's possible that the two main engines were also replaced with GE85s.) The plane was then transferred to the USAF. This did a little better, but was used as a research aircraft rather than a testbed for future development, and crashed in 1969. It still had effectively no useful payload.

Thrust augmentation would be tried again by Rockwell in the 1970s, but never quite managed to deliver on its promises. Even if it had worked, the only payload space on the Hummingbird would have been under the wings; the fuselage was crammed full of a limited fuel supply, the vanes, and the cockpit.

Survivability would also be a concern; while it could be flown to a runway landing irrespective of engine damage, a vertical landing on a single engine would be impossible.


  1. Posted by John Dallman at 10:52am on 03 May 2014

    Thrust augmentation never seems to work properly at full scale, but lab tests look great. I wonder if there's some scaling law being missed? I don't know enough aerodynamics to tell.

  2. Posted by RogerBW at 11:27am on 03 May 2014

    Ditto. I notice that it hasn't been tried as much since the 1970s, and I wonder whether the shift to digital rather than small-scale physical modelling has helped. (Though of course VTOL has relatively speaking gone out of fashion anyway.)

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