Advanced rotorcraft configurations are being
investigated with the objectives of identifying
vehicles that are larger, quieter, and faster than
current-generation rotorcraft. A large rotorcraft,
carrying perhaps 150 passengers, could do much to
alleviate airport capacity limitations, and a quiet
rotorcraft is essential for community acceptance of
the benefits of VTOL operations. A fast, long-range,
long-endurance rotorcraft, notably the tilt-rotor
configuration, will improve rotorcraft economics
through productivity increases.
A major part of the investigation of advanced
rotorcraft configurations consists of conducting
comprehensive analyses of vehicle behavior for the
purpose of assessing vehicle potential and feasibility,
as well as to establish the analytical models required
to support the vehicle development. The analytical
work of FY99 included applications to tilt-rotor
aircraft.
Tilt Rotor Aeroacoustic Model (TRAM) wind
tunnel measurements are being compared with
calculations performed by using the comprehensive
analysis tool (Comprehensive Analytical Model of
Rotorcraft Aerodynamics and Dynamics (CAMRAD
II)). The objective is to establish the wing and wake
aerodynamic models that are required for tilt-rotor
analysis and design. The TRAM test in the German-Dutch
Wind Tunnel (DNW) produced extensive
measurements. This is the first test to encompass air
loads, performance, and structural load measurements
on tilt rotors, as well as acoustic and flow-visualization
data. The correlation of measurements
and calculations includes helicopter-mode operation
(performance, air loads, and blade structural loads),
hover (performance and air loads), and airplane-mode
operation (performance). Figure 1 shows an
example of the comparison of TRAM-measured
performance with calculations. The figure shows the
difference in calculated rotor power obtained by
using an aerodynamic model (wing and wake)
appropriate for helicopter rotors, instead of a tilt-rotor
aerodynamic model. The span loading and wake
formation are very different on tilt rotors and helicopters,
so it is essential to use model features specific to
tilt rotors in order to adequately predict the behavior.
Future analyses will be concerned with TRAM tests in
the Ames 40- by 80-Foot Wind Tunnel, which will
produce data over a much larger operating envelope
for two rotors and the airframe, including advanced
flow-visualization results, and with XV-15 rotor tests
in the Ames 80- by 120-Foot Tunnel in order to
obtain tilt-rotor data at a larger scale (although no air
loads data) with different blade planform and twist.
The quad tilt-rotor configuration has been
proposed in order to meet the objective of a large
rotorcraft; it is hoped there will be fewer difficulties
associated with scaling the rotors to large size (since
there are four rather than two rotors to lift the gross
weight). The technical issues in the development of
quad tilt rotors include aerodynamic interference
(performance, control, and handling qualities; wing-to-
wing, rotor-to-wing, and wing-to-rotor); vibration
and blade loads; and whirl flutter. Figure 2 shows
results from CAMRAD II calculations for a quad tilt
rotor, illustrating the aerodynamic interference issues.
The calculations were performed with a rigid airframe
and a free-wake model (for 2 wings, and
12 blades on 4 rotors).
The views in figure 2 are from forward/port, for
two rotor azimuth angles. Shown are the wing
section lift, the rotor blade section thrust, and the
wing and rotor-tip vortices. The wing-to-wing
aerodynamic interference is evident in the influence
of the front-wing tip vortices on the rear-wing span
loading, producing an increased loading at the rear-wing
tip; this interference will affect efficiency and
handling qualities. The rotor-to-wing interference is
evident in the loading at midspan of the rear wing.
Three-per-revolution loading variation is produced by
the rotor-tip vortices, hence the different loading in
the two pictures. This interference will produce
increased vibration and structural loads. The wing-to-rotor
interference is evident in the loading on the
rotor blades when in front of the wing, compared to
outside the wing, as in the two pictures; this interference
will affect proprotor efficiency, vibration, and
blade loads. Also of concern is whirl flutter (coupled
aeroelastic stability of the rotor on the flexible wings),
since the rear wing has a larger span than the front
wing, and thus lower frequencies and less stability if
the two wings have the same sectional stiffnesses.
Investigations being conducted at Ames Research
Center promise design solutions other than stiffer and
heavier wings as a means to produce an acceptable
level of whirl flutter stability.
Point of Contact: W. Johnson
(650) 604-2242
wjohnson@mail.arc.nasa.gov
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