Biomedical Engineering Reference
In-Depth Information
outline with the same general shape as the
hawkmoth wing but without any notches or
discontinuities. They discovered that at these
Reynolds numbers, under constant rotation
speed and angle of attack, a leading-edge vortex
periodically forms and breaks down, whereas
for rotors at a lower Reynolds number and for
the flapping motion of hawkmoth wings, a
spanwise flow is developed that stabilizes the
leading-edge vortex and enhances the lift.
Several hovering MAVs based on scaled-
down single main rotor and coaxial helicopter
configurations have been successfully built and
flight tested
[24, 25, 48, 49]
. These MAV-scale
rotors typically operate in the Reynolds number
range from 10,000 to 100,000. Consequently, they
experience much higher viscous drag than con-
ventional helicopter rotors. As a result, MAV-
scale rotors suffer from an inherent limitation in
aerodynamic efficiency, which translates into
poor endurance. By careful design of rotor blade
geometric parameters such as solidity, twist,
taper, camber, and tip shape, the maximum
figure of merit achieved to date for a rotor of
diameter 9 in. (22.86 cm) is around 0.64, and for
a rotor of diameter 6 in. (15.24 cm) is around 0.55
[26]
, at a tip Reynolds number of 40,000. In com-
parison, a conventional helicopter rotor with a
figure of merit of 0.64 is considered poor in
terms of aerodynamic efficiency. In fact, most
modern helicopter rotors have a maximum
figure of merit of about 0.8
[27, 28]
.
These hobby flyers are available in several sizes;
the smaller vehicles are suited for indoor flight,
while the larger ones can fly outdoors. The
largest ornithopter is a human-powered aircraft
with a 32 m wingspan that was recently flown
by a group at the University of Toronto Institute
of Aerospace Studies.
A few research groups have focused on
improving the performance of flapping-wing
microflyers with the goal of achieving high-
endurance, fully autonomous flight in the small-
est possible dimensions. One of the earliest
flapping-wing microflyers was the Caltech
Microbat, developed as part of the DARPA MAV
program
[50]
, which had a total mass of around
11 g. Keennon
et al
.
[51]
developed a mechanical
hummingbird as part of the DARPA NAV pro-
gram. This microflyer has a wingspan of 16.5 cm
and a total mass of 19 g. It can hover for around
4 min and can fly at a speed of 6.7 m/s. The
flapping-wing mechanism is powered by DC
motors and the wing-flapping frequency is 30
Hz. The remarkable feature of this microflyer is
that it has a fuselage shaped and painted to
make it look like a real hummingbird, which
makes it ideal for covert operations. The elec-
tronics and control system were developed in-
house and are enclosed within the body
(
Figure 5.11
). All the control inputs are gener-
ated by varying the lift on the wings, and the
vehicle does not rely on a tail for stability. The
microflyer can fly stably outdoors under the
control of a human pilot and transmits live
video to a ground station.
deCroon
et al
.
[52]
developed a family of flap-
ping-wing microflyers powered by DC electric
motors. These consisted of the DelFly I with a 50
cm wingspan and a mass of 21 g, the DelFly II
with a 28 cm wingspan and a mass of 16 g, and
the DelFly Micro with a 10 cm wingspan and a
mass of 3 g (
Figure 5.12
). The wings consisted of
thin membranes attached to carbon fiber spars.
Different types of tails were explored, and the
main goal of these prototypes was to provide a
stable platform for carrying a camera.
5.5.2 Flapping-Wing Microflyers
Nowadays, there are a large number of remotely
controlled, flapping-wing microflyers being
sold as toys or hobby aircraft. Most of these
microflyers are powered by an electric DC motor
and feature simple wings with a rigid spar and
thin membrane. Controls are incorporated in
terms of a movable tail or by modifying the lift
produced by each wing using a mechanism that
changes the tension on the wing trailing edge.
