Biomedical Engineering Reference
In-Depth Information
have been developed with higher power den-
sities; however, to operate they require special
conditions (such as high temperature) that make
them unsuitable for microflyers.
As the size of electric motors decreases, the
efficiency of converting electrical power to
mechanical power also decreases. It is not
uncommon for motors with a power output on
the order of tens of Watts to have an efficiency
of only around 50%. This means that the batter-
ies that are installed on the microflyer must be
sized for significantly higher power outputs
than required to sustain flight.
than 1 cc have been realized [13] . Further minia-
turization of sensing, control, and communication
electronics will enable a significant improvement
in microflyer capabilities in the future.
5.3.4 Aerodynamics
A fundamental challenge at the scale of micro-
flyers arises from the aerodynamics. The
Navier-Stokes equations are fundamental phys-
ical relations that govern fluid flow. When the
incompressible Navier-Stokes equations are
nondimensionalized to make them independent
of scale, they yield a dimensionless parameter
called the Reynolds number . When the Reynolds
number is kept constant between two flows at
different scales or in different media, the fluid
behavior is identical. The Reynolds number is
given by
5.3.3 Miniaturization of Electronics
The majority of microflyers are remotely piloted
by a human pilot. These could either be in the line
of sight or could be piloted in a first-person view ,
by the human pilot watching live video broadcast
from cameras on the microflyer, thus giving the
impression of being located inside the microflyer.
The miniaturization of electronics has resulted
in extremely small, lightweight microprocessors
and associated sensor packages that are powerful
enough to allow some degree of autonomy. This
could range from on-board stability augmenta-
tion systems to autonomous take-off and landing
algorithms. The ultimate goal would be to create
an on-board sensing and processing system that
replicates the nervous system and brain of natural
flyers, enabling them to autonomously sense their
environment, identify and avoid obstacles, recover
from gusts, take off and land, and navigate to spe-
cific waypoints. Sensors based on microelectro-
mechanical systems (MEMS) technology, such as
accelerometers and gyros, are enabling significant
sensing capability in a small package. However,
the drawback of such sensors is a long-term drift,
and techniques such as sensor fusion and Kalman
filtering are required to correct errors. The size
of computers decreases by a factor of 100 every
ten years, and recently, computers with ultra-
wideband transceivers having a volume of less
Vl ρ
µ ,
RE =
(5.3)
where V is the freestream velocity, l is a charac-
teristic length, ρ is the density of the fluid, and μ
is the dynamic viscosity of the fluid.
The Reynolds number represents the ratio
between inertial forces and viscous forces. If the
number is very high, then inertial effects dominate
the flow and viscosity can be neglected. For con-
ventional aircraft in cruise, the typical Reynolds
number that they operate at is on the order of 1
million-10 million. However, at the scale of micro-
flyers, the Reynolds number is on the order of
1-10,000. For small insects, the Reynolds number
can be as low as several hundred. At these small
Reynolds numbers, the flow is dominated by vis-
cous effects, and the behavior of the flow can be
quite different than at high Reynolds numbers.
As the Reynolds number is decreased, the
maximum lift coefficient of an airfoil decreases,
the profile drag coefficient increases, and the
lift-to-drag ratio decreases. These effects are
shown in Figures 5.3-5.5 . Also shown for refer-
ence are the values for a flat plate; note that
 
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