Bioinspired and Biomimetic Microflyers - Engineered Biomimicry - page 124

thrust vector of the rotor can be changed almost
instantaneously.
The concept of cycloidal propulsion was first
investigated in the 1920s by Kirsten [66] and in
the 1930s by Wheatley [67, 68] . These early
cycloidal rotors were intended for use in full-
scale aircraft. Wind-tunnel tests were performed
on 8 ft diameter cycloidal rotors and significant
forces were obtained; however, due to incom-
plete theoretical knowledge of unsteady aerody-
namic effects, it was not possible to accurately
predict the performance of these devices. The
cycloidal rotor can change the direction of its
thrust vector almost instantaneously over a com-
plete circle, i.e., over an angular range of 360°.
Because of this unique ability, cycloidal rotors
eventually made their way to marine systems,
where they are used in tugboats to provide them
with low-speed maneuverability. More recently,
cycloidal rotors have made a reappearance in
aircraft applications. They have been proposed
for use on airships [69, 70] and on an UAV of
gross weight 600 lb, where the wings are replaced
by cycloidal rotors [71] . On a smaller scale,
cycloidal rotors of span around 0.8 m have been
investigated for VTOL UAVs of take-off mass
around 50 kg [72, 73] . These rotors were able to
demonstrate a power loading around 12 kg/HP
at low thrust that asymptoted to 5 kg/HP at
high thrust.
Due to the potential performance benefits of
unsteady aerodynamic effects as well as the
increased maneuverability afforded by the
instantaneous change in thrust vector, cycloidal
rotors have been explored for microflyers.
Hwang et al . [74] designed a microscale cyclo-
copter with two cycloidal rotors of radius 0.2 m.
Sirohi and Parsons [75] developed a six-bladed,
six-inch-diameter cycloidal rotor for a micro-
aerial vehicle. Experiments were performed on
a prototype to measure the flowfield in the
downwash of the rotor as well as the thrust and
torque produced at rotational speeds up to 1,200
rpm. The rotor blades had a NACA0010 profile,
and the amplitude of the oscillatory blade pitch

Ω

FIGURE 5.19 Thrust vectors at each blade cross-section.

that is perpendicular to the direction of flight
( Figure 5.18 ). The blade span is parallel to the
axis of rotation. As the blades rotate around the
azimuth, their pitch angle is varied periodically,
typically using a passive mechanism such as a
four-bar linkage. Each spanwise blade element
operates at about the same conditions—velocity,
Reynolds number, angle of attack, centrifugal
force—and thus can be designed to operate at its
optimum efficiency.
Figure 5.19 shows a cross-section of a six-
bladed cycloidal rotor rotating with an angular
velocity Ω. Each of the blades produces a lift and
a drag force. Blades at the top and bottom posi-
tions produce an almost vertical net force, while
those at the sides produce small lateral forces
because of their reduced angle of attack. The
horizontal components of the forces cancel,
resulting in a net vertical thrust. In addition, the
amplitude and phase of the maximum blade
pitch angle may be changed by modifying the
configuration of the mechanical linkage. In this
way, the magnitude and direction of the net