XV-15 litho - NASA's History Office

modes of the tilt rotor aircraft involved natural (or wake) turbulence excitation.
The results of these initial structural dynamic evaluations are presented in reports
by Bell and Government researchers. 39,40
An extensive series of airplane mode aeroelastic stability tests were conducted in
March and April of 1987 by Wally Acree, the Ames TRRA principal investigator.
The analysis of these test results revealed several problems. Many of the important
mode-shape natural frequencies were closely spaced and some modes were not easily
excited, especially with the natural turbulence excitation. Most significantly, the
resulting damping-estimate scatter, although always indicating positive stability, was
too extensive for meaningful correlation with, and validation of, the analytical predictions.
The addition of left-hand flaperon and collective-pitch actuators similar to
those on the right side of the aircraft enabled the excitation of specific symmetric
and anti-symmetric mode shapes but the damping level scatter remained too large.
Another modification to the excitation system provided the capability to input
“frequency sweeps,” the continuous variation of the excitation frequency from a
pre-selected low setting to a pre-selected high setting (over a period of 23 seconds),
at a chosen amplitude. Each test point required the test pilot to maintain
the flight condition for about 30 seconds. Again, using the prior analytical methods,
the damping level for many modes was poorly defined.
The search for improved aeroelastic stability test and data analysis technology
led to the application of frequency-domain methodology by Dr. Mark B. Tischler
of the Army Aeroflightdynamics Directorate at Ames. 41 This work improved the
quality of the flight test results, improved the identification of the modes and,
coupled with the frequency sweep excitation, was demonstrated to reduce the
total flight time required for flight envelope expansion stability evaluation.
The aeroelastic stability flight program at Bell, led by Jim Bilger, evaluated various
experimental methods and conducted extensive investigations of two configurations
of titanium proprotor hub yokes and one steel hub. No significant effects
on stability were detected for the three hub configurations.
An important result of the aeroelastic stability flight test evaluations 42 done at
Ames and Bell was that positive damping (i.e. positive stability) was verified for
39 J. M. Bilger, R. L. Marr, Ahmed Zahedi, “Results of Structural Dynamic Testing of the XV-15 Tilt
Rotor Research Aircraft,” Presented at the 37th Annual AHS Forum, New Orleans, Louisiana, May 1981.
40 L. Schroers, “Dynamic Structural Aeroelastic Stability Testing of the XV-15 Tilt Rotor
Research Aircraft,” AGARD Paper No. 339; also NASA TM-84293, December 1982.
41 C. W. Acree, Jr., M. B. Tischler, “Using Frequency-Domain Methods to Identify XV-15
Aeroelastic Modes,” NASA TM-100033, Nov. 1987, and; C. W. Acree, Jr., Mark B. Tischler,
“Determining XV-15 Aeroelastic Modes from Flight Data with Frequency-Domain Methods,”
NASA TP-3330 and ATCOM Technical Report 93-A-004, 1993.
42 W. L. Arrington, M. Kumpel, R. L. Marr, K. G. McEntire, “XV-15 Tilt Rotor Research Aircraft
Flight Test Report,” Vol. I-V, NASA CR 177406 and USAAVSCOM TR-86-A-1, June 1985.
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