XV-15 litho - NASA's History Office

The Boeing technical approach
was also evaluated for dynamic
stability in the Ames 40- by 80foot
wind tunnel. In August
1972, under Army funding,
Boeing conducted dynamics
tests of its 26-foot diameter
proprotor with the hingeless,
soft-in-plane hub on the same
semispan wing and rotor
nacelle used for the Bell fullscale
aeroelastic stability test
(figure 26). Performance tests
of that proprotor in the 40- by
80-foot wind tunnel were completed
in December 1972.
Performance and Control
In a related effort, a folding version
of the Bell 25-foot diameter
rotor (figure 27) was tested
in the Ames 40- by 80-foot wind tunnel in February 1972. The stop/fold tilt rotor
eliminated the rotor/pylon/wing aeroelastic instability by stopping the rotor while
in the airplane configuration. The aerodynamic drag of the stopped rotor blades
was then reduced by folding them back along the nacelle while a convertible
engine was used to produce the jet thrust required for airplane-mode flight up to
higher speeds than would be attainable with a rotor as the thrust-producer. This
Figure 26.
Boeing 26-ft. diameter proprotor
on semi-span wing in
the Ames Research Center
40- by 80-ft. wind tunnel.
(Ames Photograph
AC72-5255)
Figure 27.
The Bell stop/fold tilt rotor
in the Ames Research Center
40- by 80-ft. wind tunnel.
(Ames Photograph
AC85-0247-02)
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