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motor/gearbox efficiency, and the combined
motor/gearbox/propeller efficiency vs. thrust at a
freestream velocity of 25 MPH for the geared propeller
configuration. The vehicle drag at 25 MPH (the
optimum loiter velocity) is 9.9 g for the geared prop
configuration. Therefore 9.9 g of thrust is required for
level flight. Notice that the slopes of the propeller and
motor/gearbox efficiency curves are in opposite
directions near 10 g of thrust. Therefore the best total
propulsion system efficiency is obtained by making the
optimum tradeoff between the two efficiencies. Note
that the optimizer did this automatically, achieving a
total propulsion system efficiency of 40%. Figure 2
shows the experimental data, which agrees well with
the predicted performance shown in Table 4.
Figure 3 shows the propeller, motor, and combined
propeller/motor efficiencies vs. thrust at 25 MPH for
the direct drive prop. Since the direct drive
configuration is slightly lighter than the geared prop
configuration, it has less induced drag, and the required
thrust is 9.4 g. Notice that the propeller and motor
efficiencies both decrease with increasing thrust at the
design point of 9.4 g. Therefore the combined
propulsion system efficiency does not have an
unconstrained maximum at the design point like the
geared prop test results. The maximum attainable
efficiency is constrained by the required thrust to be
43%. The propeller wind tunnel tests have increased
our confidence in the validity of the optimization code,
and they have shown that the direct drive propeller
configuration outperforms the geared prop
configuration. Therefore we chose to use a direct drive
prop for the first generation Black Widow
configuration.
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
0 5 10 15 20 25
Thrust (g)
Propeller
Motor/Gearbox
Motor/Gearbox/Propeller
Figure 2: Propeller, motor/gearbox, and combined
efficiencies vs. thrust at 25 MPH for geared prop
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
0 5 10 15 20 25
Thrust (g)
Propeller
Motor
Motor/Propeller
Figure 3: Prop, motor, and combined efficiencies vs.
thrust at 25 MPH for direct drive prop
Table 5 presents a performance summary for the
first generation Black Widow configuration. Figure 4
shows the mass breakdown.
Table 5: Performance summary for the first
generation Black Widow MAV
Total mass 56.5 g
Loiter drag 9.4 g
Lift/drag ratio 6.0
Loiter velocity 25 mph
Loiter lift coefficient 0.42
Loiter throttle setting 70%
Endurance 33.4 min
American Institute of Aeronautics and Astronautics
4
Propulsion
62%
Structure
17%
Controls
9%
Payload
12%
Figure 4: Mass breakdown for the first generation
Black Widow configuration
After arriving at the optimum configuration, a brief
sensitivity analysis was performed. This analysis
showed that an additional 1 gram of drag would
decrease the endurance by 3 minutes, and an additional
1 gram of mass would decrease the endurance by 30
seconds.
The first generation MAV configuration performed
a 22-minute flight with a black and white video camera
on March 3, 1999. This vehicle weighed 56 grams, and
had a cruise speed of 25 mph. The next step was to add
color video and increase the endurance to our goal of 30
minutes. In the summer of 1999, we performed another
design iteration, and further refined the Black Widow
design. The final vehicle is shown in Figure 5. The
vehicle is controlled by a rudder on the central fin, and
a small elevator in the middle of the trailing edge. The
pitot-static tube can be seen extending forward from the
right wing tip.
Figure 5: Final Black Widow MAV configuration
Energy Storage
In the beginning of the MAV program, we evaluated a
wide range of power sources, including internal
combustion engines, fuel cells, micro turbines, and
solar power, but the best source of energy among
currently available technologies turned out to be
modern lithium batteries. Fossil fuels have a much
higher energy density than batteries, but the currently
available small internal combustion engines are
extremely inefficient, difficult to throttle, and generally
quite unreliable. Small fuel cell technology looks
promising, but it is not here yet. Microturbines also
look promising, but they may take even longer to
mature. Solar cells cannot supply enough energy to
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Development of the Black Widow Micro Air Vehicle(3)
