Biomedical Engineering Reference
In-Depth Information
The other two mechanisms, rotational lift and
wake capture, occur during stroke reversal. The
flip and rotational lift mechanisms rely heavily
on the aeroelastic deformation of the wing. Typi-
cally, the ribs in insect wings make them rela-
tively stiff in bending but very flexible in torsion.
Sane
[36]
reviewed the different mechanisms of
lift production in insect flight, and Ellington
[37]
summarized these effects, including estimates of
lift, power, and flight speed for potential appli-
cation to microflyers.
The degree of unsteadiness in the flow is typi-
cally expressed in terms of the Strouhal number
(St), given by
computational fluid dynamics are used to calcu-
late the forces and power. Descriptions, analytical
models, and reviews of the flight of different types
of animals, in addition to their morphology, mus-
cle energy consumption, and other physiological
aspects, can be found in several references—for
examples, Azuma
[38]
, Pennycuick
[42]
, Norberg
[30, 43]
, Rayner
[44]
.
5.5 AIRFRAMES
A wide variety of airframe configurations have
been proposed for MAVs/NAVs. Two fixed-
wing MAVs were developed under the DARPA-
funded MAV project. The primary advantages
of fixed-wing configurations are their relatively
high lift-to-drag ratio (
L
/
D
) in cruise, mechani-
cal simplicity, and high cruise speed. Their
main disadvantage is their inability to hover.
In addition, their high cruise speed makes it
difficult to maneuver and avoid obstacles in
indoor, cluttered environments. This can be
seen in
Figure 5.10
which plots the mass of
several MAVs as a function of their endur-
ance. The fixed-wing MAVs typically have a
lower mass due to their mechanical simplicity
and higher endurance, and their superior
L
/
D,
compared to rotary-wing MAVs. The DARPA
specification is also indicated on this figure,
which shows that the endurance requirement
was very stringent, while the total mass speci-
fication was achievable. Subsequent versions of
the fixed-wing MAVs were able to achieve sig-
nificantly improved endurance by optimizing
several of their subsystems. Also shown in the
figure are vehicles that were designed to hover
and transition to forward flight in fixed-wing
mode (Hoverfly, Microcraft OAV). The penalty
for this additional ability is an increased mass
and marginal improvement in endurance. The
parameters of these MAVs are summarized by
Bohorquez and Pines
[45]
, who also reviewed
the state-of-the-art in MAVs and discussed the
challenges for future development.
fA
V
∞
(5.4)
ST =
,
where
f
is the frequency of motion (such as flap-
ping) in Hz (beats per second),
A
is the ampli-
tude of motion, and
V
∞
is the flight velocity or
freestream velocity. In classical discussions of
unsteady aerodynamics, the reduced frequency
k
is used as a measure of the unsteadiness of the
low
[39, 40]
and is closely related to the Strou-
hal number; here,
ω
c
V
∞
k
=
,
(5.5)
where
ω
is the frequency of the motion (in
radians/s) and
c
is a chord length (typically par-
allel to the freestream). Many unsteady effects
are directly related to the Strouhal number and
reduced frequency. For example, Taylor
et al
.
[41]
found that flying and swimming animals, over a
range of sizes, cruise at a Strouhal number between
0.2 and 0.4, which gives them the best propul-
sive efficiency. Classical aerodynamic theories
involving the Theodorsen function or the Wagner
function, for example, can be used to model the
unsteady aerodynamics; however, they are only
valid for attached flow
[39, 40]
. Consequently,
these theories are often used to analyze the flap-
ping flight of wings undergoing small motions.
For higher-amplitude motion involving sepa-
rated flow, vortex theories, indicial methods, and
