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Coupled wing/rotor whirl-mode aeroelastic
instability is the major barrier to increasing tilt-rotor
speeds. This research investigated the unusually
simple approach of adjusting the chordwise positions
of the rotor blade aerodynamic center (a.c.) and
center of gravity (c.g.) to improve the stability boundary
of the full aircraft. The XV-15 tilt-rotor research
aircraft was modeled with the CAMRAD II rotorcraft
analysis program; the model was then modified to
simulate a thinner wing, which had lower drag but
also a lower stability boundary than the baseline
wing. Numerous rotor modifications were studied to
determine their effects on whirl flutter for the XV-15
with the new wing.
Small stepwise, rearward offsets of the a.c. over
20% of the blade radius created large increases in the
stability boundary, in some cases by over 100 knots.
The effect grew progressively stronger as the offsets
were shifted outboard. Forward offsets of the c.g. had
similar but less dramatic effects. An example of a
stepped a.c. offset is shown in figure 1, with the
unmodified blade for reference. The a.c. offsets were
modeled by shifting the entire airfoil section with
respect to the elastic axis (EA), as shown in figure 1.
The research was extended to include swept
blades, which give benefits similar to those of the
stepped offsets, but for a much more practical blade
configuration. Figure 1 shows an example swept
blade with 10-degree a.c. sweep and 5-degree c.g.
and a.c. sweep. Although unorthodox, the design is
feasible. Control-system (pitch) stiffness was also
increased for a further improvement in stability.
Figure 2 summarizes the combined benefits of
the example swept blade with doubled control
stiffness. Damping of the three unstable whirl modes
is plotted against airspeed; the wing/rotor structure is
aeroelastically unstable below zero damping. The
rotor modifications completely eliminated the
instabilities.
Point of Contact: C. W. Acree, Jr.
(650) 604-5423
wacree@mail.arc.nasa.gov
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Fig. 1. XV-15 rotor blade planforms: unmodified,
stepped offset, and swept (45 degree twist and
1 degree baseline sweep not shown).
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Fig. 2. Whirl-mode damping versus airspeed for the
original rotor blade and control system and for the
example swept blade with double control-system
stiffness. (Modes that were always stable are not
shown.)
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