11/10/89

DESIGN
To make the design work of the blimp easier an ellipsoidal
shape was chosen rather than a teardrop shape. An ellipsoid is a
good shape for a blimp because it is aerodynamic and has a large
volume for a given surface area. The volume of an ellipsoid is:

where a is the radius along the semi-minor axis and b is the
radius along the semi-major axis. The surface area is more
difficult to calculate so the area of a cylinder which is 2b long
and a radius of a was used for the lift calculations. A computer
program using iterative techniques generated a table of blimp
dimensions based on lift required. Several parameters were
varied to see the effect on the blimp size. The parameters
included: skin weight, length to width ratio, temperature and
pressure of the helium. Page 1 of the appendix shows the
calculation of the lift generated for a given volume of helium.
After some consideration a length to width ratio of 3 to 1 was
chosen because it has a low drag coefficient and dimensions which
are not unwieldy. For the 10 kg target lift, the blimp size must
be at least 1.8m x 5.4m and should be less than 2.0m x 6.0m. The
final size of the blimp is dependent on the other varying
parameters such as temperature and pressure of the helium, and
the weight of the skin material.

A specific skin material has not been decided on. One
possible material is Polyethylene. This material is strong and
can be heat-sealed. Samples are being sought for testing. Tests
need to be made to evaluate the strength of heat-sealed bonds.

The blimp will be propelled by two fans or propellers. The
fans will be mounted so that they can be swiveled to point upward
or downward. The electric motor drawing diagrams one possible
configuration of the propulsion system. There will be one fan on
each side of the gondola. This arrangement will allow
maneuverability at any speed. The blimp will turn when the
motors are run at two different speeds. The separation of the
motors determines the moment which turns the blimp and this
distance has not yet been evaluated. See the attached gondola
frame drawing for possible fan placement.

Calculations of the drag on the blimp at 5 miles-per-hour
are on page 2 of the appendix. These calculations are for a
blimp size of 2m x 6m. The flow was determined to be partially
turbulent based on the Reynolds number calculation.
Interpolation was used to find CD from standard values for ReL of
105 and 107. Using this value of CD the drag was found to be
0.466 newtons.

The power required to propel the blimp is calculated on page
3 of the appendix. A blimp size of 2m x 6m was used in these
calculations. The calculations show that 5.0 watts are needed to
propel the blimp at 5 miles-per-hour. The motors presently
slated for use are rated at 10 watts using 7 volts. Because two
motors are being used, up to 20 watts will be available for
propulsion. With 20 watts a maximum speed of 7 mph may be
obtained.

Some stability analysis must be done to determine the size
of the tail fins. In addition, the stability analysis should
help determine the maneuverability of the blimp.

The gondola will carry the radio-control equipment,
batteries, propulsion motors, camera, and video transmitter. A
sketch of the gondola is in the gondola frame drawing. The
camera mounting is shown in the camera holder drawing. The
camera will be mounted so that it can be rotated to view from
straight ahead to straight down. The video signal will be
transmitted using a citizen band TV transmitter, the particular
one is still being investigated.

The gondola will be attached to the blimp with two cords.
Each cord will be attached to grommets which will encircle the
blimp. The number of grommets and the exact placement of the
gondola on the blimp have not been determined. Thus the weight
of the gondola will be spread about the surface of the blimp.
Remaining considerations include the maneuverability of the blimp
based on the gondola placement and the effect of the deformation
of the blimp by the grommets. The tail fins will be attached in
a similar manner. The weight of the gondola and the tailfins
will have a tendency to distort the blimp shape. Therefore the
blimp must pressurized enough to prevent any distortion. Further
analysis must be made before the actual pressure required can be
known. Installation of a valve in the envelope will allow a hose
to be attached to the blimp during inflation. The actual
apparatus for the inflation has not been determined. An accurate
means of measuring the pressure must be incorporated into this
apparatus.