XV-15 litho - NASA's History Office

flow phenomenon that caused the higher than expected download. These tests
involved the use of a new “balance” designed to provide highly accurate proprotor
thrust and torque data. The balance, mounted between the proprotor and the
drive motors, was developed by Boeing Helicopters (previously Boeing Vertol)
under the contract that provided for the development of new composite-material
proprotors for the XV-15 aircraft. The original XV-15 metal blades obtained
from Bell for performance and stability wind tunnel tests in the early 1970s were
one of the full-scale configurations tested. Data obtained from this test showed
that the XV-15 proprotor performance was, in fact, better than the earlier estimates.
The somewhat mixed blessing that came out of these investigations was
that highly twisted proprotor blades could be designed to produce high performance,
but the high download generated by the proprotor wake consumed all of
the unexpected performance gains. It was clear that the hover performance, and
therefore the effectiveness of the tilt rotor aircraft, could benefit from an understanding
and reduction of the download loss.
Aeroelastic Stability Evaluations
Of all of the technical areas to be explored in the TRRA test program, none
would be as important as the investigation of the aeroelastic stability of the XV-
15 in high-speed airplane-mode flight. The future of the tilt rotor aircraft depended
on the outcome of these tests.
The instability problem encountered by the tilt rotor aircraft is caused by elastic
deformation of the wing, pylon, and proprotor which oscillate when disturbed.
The flexing of the wing and pylon imposes a pitching and/or yawing motion on
the proprotor. This produces a proprotor in-plane force acting in the same direction
as the original motion. Under some circumstances these in-plane forces are
sufficient to make the displacements in amplitude grow with each oscillation, in
effect acting as a powerful negative spring, producing an aeroelastic instability.
Both Bell and the Army/NASA TRRA project offices produced predictions of
the structural dynamic stability of the XV-15. Bell used a company-developed
method and the Government used predicted values determined from the analysis
generated by Dr. Wayne Johnson. Both analyses indicated satisfactory stability
throughout the envelope of the XV-15 except for one operating condition. The
predicted instability occurred only at high airplane mode airspeeds and at the
high RPM that was used for the hover and helicopter mode flight. The solution
was to set an airplane mode speed limit above which the proprotor RPM had to
be reduced to a level where the “one-per-rev” excitation of the natural mode
could not occur. Fortunately, this RPM reduction was planned during the design
of the XV-15 to improve the performance of the proprotor so that it became standard
procedure to reduce RPM just after converting to the airplane mode.
To evaluate the aeroelastic stability of the TRRA in flight it was necessary to create
rotor/pylon/wing displacements at the frequencies that corresponded to the various
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