Biomedical Engineering Reference
In-Depth Information
a simple, single-bladed helicopter. Azuma
[38]
provided a comprehensive review of several
types of autorotating seeds along with their
lift and drag characteristics. One of the earliest
autorotating devices that was used in a submu-
nition deployed from an airplane was described
by Kline
[55]
. However, this was a completely
passive device. As part of the DARPA NAV pro-
gram, Lockheed Martin developed a vehicle
based on the samara, called the Samarai
[56, 57]
.
Early concepts of this vehicle featured a wing
with a flap for control, driven by a fuel-powered
pulsejet engine at the wing tip. More recently,
this concept developed into a family of vehicles
with a range of sizes from 17 cm to 72 cm, pow-
ered by electric motors driving propellers.
Ulrich
et al
.
[58, 59]
developed mechanical
samaras incorporating a rapid-prototyped pol-
ymer body and a propeller driven by a DC
brush- less motor. Three sizes of mechanical
samara microflyers were developed, with a
total mass of 75 g, 38 g, and 9.5 g, having a
maximum dimension of 270 mm, 180 mm, and
75 mm (
Figure 5.13
). The largest of these had a
flight time of around 20 min. Control is achieved
by varying the angle of incidence of the wing
with respect to the fuselage. It was envisaged
that these microflyers would be deployed from
a fixed-wing unmanned aerial vehicle and
would then fly autonomously to execute their
mission. Extensive experiments were performed
characterizing the dynamics of this microflyer
configuration and evaluating the effect of plan-
form geometry
[60]
.
5.5.4 Flap Rotors
There have been several attempts to harness
unsteady mechanisms similar to those found in
nature, such as flapping and pitching, to enhance
the performance of conventional lift production
mechanisms. For example, in a conventional heli-
copter rotor in hover, an airfoil at any spanwise
location on the rotor blade experiences a steady
aerodynamic environment. At higher levels of
rotor thrust, as the airfoils operate close to their
static stall angle, they experience a loss of lift and
an increase in drag, resulting in a decrease in the
hover efficiency of the rotor. It may be possible to
improve this efficiency by creating an unsteady
aerodynamic environment at the airfoil. The
unsteady motion can be created mechanically in
different ways. Due to the large forces involved
in creating such motion, this approach is only
feasible at the microscale.
Bohorquez and Pines
[61]
developed an active
flapping and pitching mechanism for a 20 cm
diameter, two-bladed helicopter rotor. The
mechanism enabled the rotor blades to be
actively pitched and flapped in an oscillatory
fashion at a frequency independent of the rotor
speed. The goal of the oscillatory pitching was
to induce dynamic stall on the rotor blade,
resulting in a large increase in lift coefficient.
The goal of the flapping motion was to generate
a radial flow along the rotor blade, which was
expected to stabilize the leading-edge vortex
created during the dynamic stall event. An
appropriate combination of flapping and pitch-
ing amplitudes as well as frequencies was deter-
mined by experiments.
FIGURE 5.13
Mechanical samara microflyer, total mass
9.5 g, maximum dimension 75 mm
[59]
. Credits: E.R. Ulrich,
D.J. Pines, J.S. Humbert, From Falling To Flying: The Path To
Powered Flight Of A Robotic Samara Nano Air Vehicle,
Bioinspiration & Biomimetics, 5, 045009, 2010.
