Aerospace & Defense Technology - October 2021 - 35

Tech Briefs
tions - but all focused mostly on performance
measurements and fall far
short from being comprehensive. Similarly,
discrepancies in analyses identified
both in lifting-line and CFD have still not
been systematically addressed due to the
scarcity of reliable and comprehensive
test data. Thus, both lack of experimental
data and validated analyses can become
significant technical barriers towards effective
and efficient use of slowed rotors
in the development of next-generation
high-speed compound rotorcraft.
Coaxial compound has emerged as one
of several potential solutions for highspeed
rotorcraft - along with tilt-rotors
and slowed-rotor compound - since the
successful resolution of critical technology
shortcomings associated with the earlier
XH-59A demonstrator. These shortcomings
- low efficiency/high fuel
consumption, high empty weight fraction,
high vibration, and challenges associated
with reducing rotor speed - have
now been mitigated in the X2 Technology
Demonstrator by innovative use of modern
technologies such as advanced airfoils
(double-ended at root and super-critical at
tip), advanced materials (titanium to
graphite-epoxy blades), active vibration
control in the fixed-frame, and advanced
propulsion (high efficiency pusher propeller
instead of turbojet thrust).
The potential of modern refined analytical
tools that have matured over the
last fifteen years - if brought to bear on
this advanced coaxial rotor system - can
bring about dramatic improvements in
its capabilities. Some of the current technical
challenges are: (1) reduced efficiency
due to large reverse flow area in
high-speed flight (80% of retreating side,
at µ=0.8, 20% rpm reduction, 250 knots),
(2) high weight penalty due to active
force generators in the fixed-frame to
cancel the high vibration levels due to
stiff blades, and (3) weight and drag
penalty due to high root stresses as well
as a large hub. Recently, the X2 Technology
Demonstrator achieved a level flight
speed over 250 knots, proving the basic
concept of the coaxial compound configuration.
However, the rotor of this aircraft
was designed assuming an equivalent
single rotor in conjunction with
blade element momentum theory. Further
development of this concept and opAerospace
& Defense Technology, October 2021
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timization of its performance requires refined
analytical tools and detailed test
data. It becomes necessary to generate a
benchmark set of experimental hub
loads; blade surface pressures; and rotor
wake velocity measurements with which
the analytical tools can be validated.
This work was done by Inderjit Chopra of
the University of Maryland - College Park for
the Army Research Office. For more information,
download the Technical Support
Package (free white paper) at www.
aerodefensetech.com/tsp under the DAQ,
Testing & Sensors category. ARL-0242
È
http://www.aerodefensetech.com/tsp http://info.hotims.com/79418-799 http://info.hotims.com/79418-967
Aerospace & Defense Technology - October 2021

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