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sustain level flight, but they could be used to recharge
the batteries while the MAV is parked somewhere.
Batteries and electric motors are extremely reliable,
inexpensive, and quiet. However there are tradeoffs
among different battery chemistries. Nickel-Cadmium
(NiCd) and Nickel-Metal-Hydride (NiMH) batteries
have very high power densities, but very low energy
densities. Lithium (Li) batteries are generally designed
to have high energy densities, but relatively low power
densities. Also, NiCd and NiMH batteries are
rechargeable, while most lithium batteries are not. We
chose to use NiCd and NiMH batteries for flight testing,
and lithium batteries for demonstration flights.
As with any aircraft, the energy source is a primary
design driver. Therefore it is critical to have
quantitative data for many different batteries in order to
select the best battery for the vehicle. During the MAV
program, we characterized a variety of small batteries
using discharge tests over a range of temperatures. We
reduced this data to a series of curve fits, and we used
the curve fits in the MAV optimization code.
Motors
Throughout the MAV program, we tested and evaluated
several electric motor candidates. We built a
dynamometer specifically for small motors, and we
tested each motor over a wide range of operating
conditions. The motor test data was used to create an
analytic math model of each motor. These motor
models were then integrated into the MDO code.
The dynamometer tests showed that efficiencies as
high as 70% can be achieved with small motors. The
trends are that larger motors have higher efficiency to
power ratios, and higher voltage motors have higher
efficiency to power ratios. Unfortunately the motor
voltage is limited by the battery supply voltage unless a
power converter is used.
American Institute of Aeronautics and Astronautics
5
Micro-Propeller Design
The early MAV prototypes used plastic propellers
developed for small model airplanes. Some of these
propellers were modified by cutting and sanding
commercially available props. Since the propeller
performance is critical to the success of the MAV, we
developed a propeller design methodology which
allows us to significantly increase the efficiency of
small propellers.
The nominal mission profile for the Black Widow
is to climb to about 200 ft above ground level, and
cruise around at the optimum loiter velocity gathering
video data. Therefore at least 90% of the flight occurs
at a single flight condition. This greatly simplifies the
propeller optimization, since the off-design conditions
do not strongly affect the overall performance. It also
allowed us to use the minimum induced loss propeller
design methodology to optimize the twist and chord
distribution for the loiter flight condition. The prop
diameter was optimized by the genetic algorithm along
with the other top-level vehicle design variables.
The propeller shown in Figure 6 was optimized for
a 4:1 gearbox and a 7-gram DC motor. The pitch is 6.04
inches, and the diameter is 3.81 inches.
Figure 6: Micro-propeller designed for 4:1 gearbox
A 3-dimensional model of the propeller geometry
was created using the SolidWorks solid modeling
software. Stereolithography models of the upper and
lower mold halves were then created from the virtual
solid model. Figures 7 and 8 show the prop mold
geometry. The propeller was fabricated from
unidirectional and woven carbon-fiber composites.
To validate the propeller design code, a series of
tests were performed in the AeroVironment wind
tunnel. The wind tunnel is an open-circuit, suction
design with a test section that is 20 inches wide, 20
inches high, and 40 inches long. The tunnel is capable
of producing velocities between 5 and 80 mph in the
test section. The torque and thrust were measured using
the balance shown in Figure 9. This balance was
constructed using three load cells from commercially
available scales. The load cells have a 0.1-g accuracy,
and they are insensitive to offset loads and moments.
Figure 7: Upper half of propeller mold
Figure 8: Lower half of propeller mold
Figure 9: Balance for prop performance tests
American Institute of Aeronautics and Astronautics
6
Figure 10 shows the thrust vs. RPM and freestream
velocity for the prop. The plot shows excellent
agreement between the experimental data and the code
predictions. The propeller was designed to produce 10 g
of thrust at 25 MPH and 5,250 RPM.
Figure 11 shows the propeller efficiency vs. RPM
　
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