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noise emissions and the power con-
sumption of the rotor can be positi-
vely influenced by this method. Con-
ventional (collective and cyclic) heli-
copter controls cannot effectively in-
fluence the higher harmonics of the
undesired aerodynamic effects, and
are only used to control the flight
attitude. This is why as early as the
mid-20th century, researchers tried
to counter such phenomena and their
effects by means of active rotor con-
trol. In addition to the pitch angle
changes at all blades that are imple-
mented by the primary controls, ac-
tive rotor control introduces high-
frequency pitch angle changes at a
certain multiple of the rotor frequency
(rotor-harmonic frequencies). This sig-
nificantly reduces the vibrations in the
helicopter and the noise radiation,
and can also improve thrust and lift.
Active Rotor Control
Existing approaches to active rotor
control often have significant draw-
backs. Systems that create the dyna-
mic pitch angle changes by moving
the swashplate (through actuators)
are individual-blade-control (IBC)-
capable only for rotors with up to 3
blades. For rotors with four or more
blades, as are common today, this
approach is limited for reasons of
swashplate kinematics. Other systems
are fully IBC-capable but use actua-
tors in the rotating system that are
subject to high loads and have to be
supplied with energy and control sig-
nals via slip rings, which can pose a
major challenge in and of itself.
DLR test setup for developing active controls for helicopter rotors.
DLR BRAUNSCHWEIG
or a helicopter in forward flight,
the flows resulting from the for-
ward movement of the helicop-
ter and the rotation of the rotor bla-
des overlap. This creates highly un-
symmetrical flow conditions within
the rotor disk. This causes various aero-
dynamic, aeroelastic and aeroacous-
tic effects, such as dynamic stall, noise,
and vibrations. These effects usually
occur periodically with the rotor’s ro-
tational frequency and integer multip-
les thereof (rotor-harmonic frequen-
cies). One approach to counter or at
least mitigate these effects focuses di-
rectly on the helicopter’s rotor controls.
Controlling the Helicopter
The main mechanical control unit of a
helicopter is called a swashplate. It
transfers the pilot’s commands to the
rotating blades. This is done by a com-
bination of collective pitch control,
i.e., changing the pitch angle of all
main rotor blades to change the lift,
and cyclic pitch control to influence
forward and sideward thrust. The lat-
ter causes a variation of the blades’
pitch angles to occur once every revo-
lution of the rotor, or 1/rev.
Taking Action
To mitigate undesired effects, the pitch
angles of the rotor blades can be modi-
fied by stimulating the rotor blades
with an integer multiple of the rotor
frequency and a low amplitude. To re-
duce vibrations, the frequency, ampli-
tude, and phase of the control signal
is chosen such that vibrations are can-
celed out by interference. But also
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Source: © DLR
dSPACE Magazine 1/2016 · © dSPACE GmbH, Paderborn, Germany ·
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