XV-15 litho - NASA's History Office

Figure 31.
Bell 25-ft. diameter proprotor
performance test in the Ames
Research Center 40- by 80-ft.
wind tunnel.
(Ames Photograph
AC70-5390)
26
For large-scale performance characteristics, the Bell
25-foot diameter proprotor was tested in the Ames
40- by 80-foot wind tunnel in November 1970 (figure
31) as part of an earlier contracted effort. Ames also
contracted with Bell and made arrangements with the
Air Force Aero Propulsion Laboratory (AFAPL) for
the March 1973 proprotor hover performance test at
Wright-Patterson Air Force Base.
While the fundamentals of tilt rotor aeromechanics
were being explored, another group of researchers
and engineers were investigating the flying qualities,
crew station, and control law aspects of this class of
VTOL aircraft. Model-scale wind tunnel tests, analytical
modeling, and piloted simulations were used to
address these issues.
A series of tests was conducted with a 1/5- scale powered
aeroelastic model of the Bell Model 300 tilt rotor
aircraft design under an Ames contract. Hover tests
conducted in September, October, and December of
1972 with this model examined the performance and
dynamic characteristics for operations near the ground.
It was discovered that, in the helicopter mode, the downward flow from the rotors
impinging on the ground produced a strong upward-moving flow below the aircraft’s
longitudinal axis. This upwash, known as the “fountain,” impacts the lower
surface of the fuselage with increasing strength as the aircraft descends to the
ground. Because this fountain is somewhat unsteady, the major portion of this air
mass is seen to skip from one side of the fuselage to the other (particularly on round
cross-section fuselages), causing this fountain-flow to impinge, alternately, on the
lower surface of the right or left wing. This condition can contribute to the lateral
darting observed during the XV-3 flight tests and lead to a considerably high pilot
workload during the landing operation. Also, the occurrence of the unsymmetrical
aerodynamic loading on the wing surfaces produces a rolling moment that increases
in magnitude, i.e. is statically destabilizing, as the aircraft descends toward the
ground. 20 Recognition of these phenomena contributed to the development of
improved stability augmentation control algorithms for future tilt rotor aircraft.
Subsequent wind tunnel tests, conducted in the Vought Aeronautics low speed
wind tunnel, Texas, from January through March 1973, documented the performance,
static stability in yaw and pitch, and determined trimmed control positions
in all flight configurations. These data were critical for the flight dynamics ana-
20 R. L. Marr, K. W. Sambell, G. T. Neal, “Hover, Low Speed and Conversion Tests of a Tilt
Rotor Aeroelastic Model.” V/STOL Tilt Rotor Study, vol. VI, Bell Helicopter Co., NASA
CR-114615, May 1973.