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and velocity for the prop. The best measured efficiency
was 83%, while the code predicted a peak efficiency of
82%. The propeller actually operates at 78% efficiency,
even though the peak efficiency at 25 MPH is 81%.
Since the motor efficiency is higher at higher speeds, a
slight sacrifice in propeller efficiency increased the
efficiency of the total propulsion system. Also note that
the peak efficiency increases with increasing freestream
velocity due to higher blade Reynolds numbers.
0
5
10
15
20
25
30
2,500 3,000 3,500 4,000 4,500 5,000 5,500 6,000 6,500 7,000 7,500
Prop RPM
15 MPH
20 MPH
25 MPH
30 MPH
35 MPH
15 Data
20 Data
25 Data
30 Data
35 Data
Figure 10: Thrust vs. RPM and freestream velocity
for 3.81-inch diameter propeller
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
2,500 3,000 3,500 4,000 4,500 5,000 5,500 6,000 6,500 7,000 7,500
Prop RPM
15 MPH
20 MPH
25 MPH
30 MPH
35 MPH
15 Data
20 Data
25 Data
30 Data
35 Data
Figure 11: Efficiency vs. RPM and freestream
velocity for 3.81-inch diameter propeller
Airframe Structural Design
The structural design of an MAV presents several
unique challenges. Because of the square-cube law, the
inertial loads induced on an MAV during accelerations
and decelerations (such as takeoff and landing) are
quite small relative to larger aircraft. During a typical
landing, the MAV flies into the ground at a shallow
angle and survives a few bounces with no damage. In
fact, the worst case design loads for many parts of the
structure are the handling loads imposed by people.
The first generation Black Widow design used a
solid foam wing structure with some internal
reinforcements in high-stress areas. The wing structure
consists of the basic wing, the internal rigid structure,
and the vertical fin assembly, as shown in Figure 12.
Figure 12: Solid foam wing structure
The wings are fabricated from expanded
polystyrene (EPS) foam. The foam has many desirable
qualities, including ease of shaping, light weight,
strength, and ease of bonding.
The main pieces in Figure 12 are shaped by cutting
using a hot wire tool. The ends of the wire are moved
by CNC-controlled stepper motors, such that precision
cuts can be made from CAD drawings.
A cavity is cut from the leading edge of the center
wing section. This is the area where the internal rigid
structure is embedded. The internal rigid structure is
designed to hold the most massive parts of the MAV,
(batteries and motor) together, and tie into the high load
points of the MAV, such as the launch lug. The rigid
structure is mainly fabricated from fiberglass sheet.
The vertical fin assembly is hand fabricated from
balsa wood. The rudder is also made of balsa wood and
is hinged with Kevlar cloth.
American Institute of Aeronautics and Astronautics
7
Avionics
One of the objectives for the Black Widow MAV is to
achieve autonomous flight so that the vehicle can be
easily operated by an unskilled operator. The first step
toward autonomous flight is to sense the state of the
vehicle and pass the data to the flight computer. For this
reason, the Black Widow has a two-axis magnetometer
to sense compass heading. A pitot-static tube is
connected to an absolute pressure sensor to sense
altitude and a differential pressure sensor to sense
dynamic pressure. The vehicle also uses a piezoelectric
gyro to sense the turn rate.
The MAV must also receive commands from the
ground station, and translate the commands into control
surface movements and throttle changes. This requires a
command uplink receiver, a flight computer, and
control actuators. The uplink receiver has a mass of 2
grams, and is about the size of two postage stamps. It
operates at 433 MHz. The aircraft uses two
microprocessors to perform onboard computations. The
rudder and elevator control surfaces are moved with
custom-developed 0.5-gram actuators.
Video Camera Payload
The Black Widow MAV was developed as a platform
to deliver live color video images in real time to an
observer on the ground. The video payload evolved
from a current off-the-shelf (COTS) video transmitter,
and a modified COTS black and white CMOS camera,
to a custom video transmitter and a custom color
CMOS camera, as shown in Figure 13.
Figure 13: Black & white camera, COTS
transmitter (top); custom color camera,
　
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