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.
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