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appears to have originated in England during the Second World War. Of
course, none of their reports were published. Their techniques were
communicated to the National Research Council in Canada and similar
tests were begun there. The wavy lines drawn on the film by steady
lights as used initially permitted only evaluation of angular vibration
in roll, i.e., about the fore and aft axis of the aircraft. By replac-
ing the steady lights with lights flashed rapidly at an accurately
known rate, the possibility of measuring pitch and yaw components was
added. This was first done with lights flashed by means of rotating
sectors, as reported in 1951. When similar studies were resumed much
more recently electrically flashed discharge lamps were used instead.
The General Radio Stroboslaves operating at 300 flashes per second had
only enough intensity to permit good imagery for a 153 mm focal length,
f/5.6 lens, if the aircraft was flown at about 400 feet above ground.
This low altitude complicated flight operations because of air
traffic control regulations.’ To avoid this difficulty, the optical
systems shown in figure 2 were designed, and built, to concentrate
practically all the light from the flash tubes in an upward cone of 31°
semi-angle. These permitted operation at 1000 feet above ground with
ample image density on the negatives.
Subsequently, to permit testing of inertial systems for recording
camera position and orientation, arrangements were made to tie the
flashing lamps to the CHU Canadian time signals, and a third flashing
lamp was added to provide enough data to permit calculation of the six
elements of exterior orientation of the camera at each flash.
3.1 MEASUREMENT DIFFICULTIES
This method of measuring angular motion of cameras in flight has
several disadvantages which should be mentioned. 1) The ground instal-
lation is fixed, requiring the aircraft to come to it. 2) The ground
operation is costly in man-hours. 3) Flying conditions are not typical
of aerial survey operations. 4) Data analysis is time-consuming and
hence the length of time sampled is small. 5) Accuracy is limited by
pointing accuracy on the images of the point light source on the film,
with granularity a significant factor. 6) Accuracy of angular velocity
values can be improved by increasing the time between flashes, but data
on high frequency vibrations may be lost. If an alternative measuring
method could be developed, based on instrumentation which could be used
in an aircraft without a ground reference, it would be most helpful in
improving knowledge of vibration problems.
4. RESULTS OF FLIGHT TESTS
Figures 3 to 10 show in graphical form some examples of vibration
data obtained in flight tests. The plots are of angular velocities of
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