XV-15 litho - NASA's History Office

understand and resolve the cause of the
dynamic instability. Flight testing
resumed on March 29, 1956, but on
July 25 the instability occurred again,
causing Bell to conduct another series
of ground tiedown tests which lasted
until late September of that year.
It is important to note that the ability of
the rotorcraft dynamicists of that period
to analyze complex systems (such as
the rotor/pylon/wing of the tilt rotor)
was quite primitive compared to the
computational capabilities of the 1990s.
The attempts to correct the instability
that occurred on the XV-3 had to be done by combining the available analytical
methods with experimental data. Therefore, ground tiedown tests were needed to
expand the database documenting the fundamental characteristics of the tilt rotor as
well as to evaluate configuration changes.
Following the second ground test effort, flight testing continued with the goal of
expanding the speed and conversion envelope of the XV-3. On October 25, 1956, as
Bell test pilot Dick Stansbury moved the rotor shaft 17 degrees forward from the vertical,
a severe rotor instability occurred that resulted in extremely high cockpit vibrations
and caused the pilot to black out. The subsequent loss of control caused the number 1
XV-3 ( aircraft tail number 4147) to crash, seriously injuring the pilot (figure 19).
The XV-3 program faced a crisis. The inability to solve the instability using
traditional analyses, experimentation, and trial-and-error empirical methods
made even some of the tilt rotor’s most avid supporters question the readiness
of this technology. But the believers held on. A satisfactory solution to the
rotor/pylon/wing dynamic instability problem had to be found. Advocates of
the tilt rotor at Bell and the Government decided to continue the work and
authorized the initiation of a major design change as well as plans for testing
the XV-3 in the NACA Ames Aeronautical Laboratory 6 40- by 80-foot wind
tunnel. The original three-bladed, 25-ft diameter articulated rotor was
replaced with a two-bladed stiff-inplane rotor. By July 18, 1957, with isolated
two-bladed rotor static tests and rotors-installed XV-3 tiedown tests completed
(figure 20), investigations of the performance and dynamic behavior of the
modified XV-3 began.
6 The NACA (National Advisory Committee for Aeronautics) was the predecessor to the NASA
(National Aeronautics and Space Administration). The NACA became the NASA in October
1958 and the Ames Aeronautical Laboratory was renamed the Ames Research Center (ARC).
ARC is located at Moffett Federal Airfield, formerly Moffett Naval Air Station, about 40 miles
south of San Francisco, California.
Figure 19.
Crash of the XV-3 on
October 25, 1956.
(Bell Photograph 217259)
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