XV-15 litho - NASA's History Office

Flight Research
In 1981, after a number of maintenance
test flights, the Project Office began a
series of ground and flight investigations
to acquire a comprehensive data
base to meet the fundamental and
advanced technical goals of the TRRA
project. These test activities would
eventually address structural loads, handling
qualities, flight dynamics, structural
dynamics and stability, acoustics,
performance, and proprotor downwash.
Hover Performance
One of the first experiments at Ames explored several characteristics of the
TRRA in the hover mode. The scope of this hover test included an evaluation of
performance, acoustics, and the documentation of the “outwash” (the flow parallel
to the ground generated by the proprotor downwash) at various hovering
heights. These data were required by the Navy for the planned operational evaluation
of the XV-15 onboard an aircraft carrier. To measure the proprotor wake
flow in the vicinity of the hovering aircraft, the Naval Air Test Center of
Patuxent River, Maryland, provided data acquisition equipment and a supporting
research team. The outwash test apparatus consisted of a remote-controlled
motorized cart that carried an array of sensitive electronic (ion-beam) anemometers
(to measure the low-speed airflow) mounted on a 10-foot high pole. While
the aircraft hovered (figure 50) over a point on the hover pad at a selected height,
the instrumented cart was moved to various predetermined positions along a
track radiating from the point below the XV-15. To survey the region around the
hovering aircraft, the heading orientation of the TRRA was varied 180 degrees in
30-degree increments, thereby documenting the outflow from the region directly
forward of, to the region directly aft of the aircraft.
The outwash test required that the aircraft hover at a precise height, heading, and
position for a 15- to 20-second data acquisition period. The method devised to
accomplish this involved the use of sets of visual targets mounted on tall poles
around the hover pad. By lining up two sets of selected targets, the aircraft was
positioned at the desired point in space (figure 51). Hover conditions for these
tests ranged from an in-ground-effect (IGE) 2-foot wheel height to an out-ofground
effect (OGE) 50-foot wheel height. In addition to the outwash data, these
steady hovering operations conducted in near-zero wind conditions enabled the
simultaneous acquisition of excellent performance data. 34,35
34 M. Maisel, D. Harris, “Hover Tests of the XV-15 Tilt Rotor Research Aircraft.” Presented at the
1st Flight Testing Conference, Las Vegas, Nevada, AIAA Paper 81-2501, November 11-13, 1981.
35 D.J. Harris, R.D. Simpson, “Technical Evaluation of the Rotor Downwash Flow Field of the
XV-15 Tilt Rotor Research Aircraft.” NATC Report No. SY-14R-83, July 28, 1983.
Figure 50.
XV-15 hovering in-groundeffect
during 1984 performance
and downwash test.
(Ames Photograph
AC81-0165-152)
65