Ameri-King AK-451-ELT - Manual (Page 125)

125
APPENDIX E
FAA ADVISORY CIRCULAR AC43.13-2B, SECTION 303
a. The antenna’s structural load, plus any required allowances, may not exceed the
design capacity of the structure intended to support it. It is important to understand
the operational characteristics of the aircraft and consider forces that occur during
flight (dynamic loading) as well as those that occur when the aircraft is not in
motion (static loading). For example, an aircraft designed without flaps may
employ a side slip procedure to lose altitude, during which the direction of airflow
across the fuselage is not in line with the aircraft longitudinal axis. Antenna
mountings on these aircraft need to be designed and evaluated for the direction of
airflow that occurs during such an operation.
b. Whenever possible, an antenna should be mounted to a flat surface. Minor aircraft
skin curvature can be accommodated with the use of an appropriate gasket but if
gaps over 0.020" appear between the base plate and mounting surface, use of a
mounting saddle is recommended.
c. Since antenna systems typically require a ground plane (this may be a conductive
surface that the antenna mounts to) any separation of an antenna from its ground
plane may impact performance. Contact the manufacturer for recommendations if a
gasket or mounting saddle is needed (see figure below).
d. Mounting screws must never be over torqued in an attempt to distort aircraft
structure to reduce gaps between the antenna base plate and aircraft-mounting
surface.
e. Consider the factors of flutter, vibration characteristics, and drag load. The
approximate drag load an antenna develops may be determined by the formula:
D=0.000327 AV
2
(The formula includes a 90% reduction factor for the streamline shape of the antenna.)
D is the drag load on the antenna in lbs.
A is the frontal area of the antenna in sq. ft.
V is the V
NE
of the aircraft in mph.
Example: Antenna manufacturer specification frontal area = 0.135 sq. ft. and V
NE
of
aircraft is 250 mph.
D=0.000327 x .135 x (250)
2
=0.000327 x .135 x 62,500
= 2.75 lbs
f. The above formula may be adapted to determine side load forces by substitution of
the apparent frontal area value for A, when the aircraft motion and antenna
orientation are not the same.