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National Advisory Committee for Aeronautics, Technical Notes - Saoring Flight in Guinea

New methods have been devised for studying the irregu-
larities of the wind and the horizontal and Vertical components
01' its velocity in those regions where the birds are flying.

For this purpose we used the Service doe Inventions anemomster
kites which permit measurement of the speed and irregularities

of the wind by means of a force indicator whose readings may be
controlled by various means. Also. for the study of the vertical
component. we need kites which. carried. a pivoted arm, the angle
between which and the vertical was afunction of the angle of the
wind with the horizontal. This angle was determined from the
33011116. by the use of a Rochon telescope and trigonometric calcu—
lations. This enabled us, knowing the relocity of the wind, to
determine its vertical or horizontal components.

We also used. with some modifications, the sounding bal-
loon method described in the above mentioned Notes. The balloon
Was adjusted to have a low ascensional velocity and a Roohon tel-
escope was used instead of a camera in order to investigate con-
ditions at greater distances and altitude. A complete study of
the sources of error introduced by this method Justified its use.

These different experimental methods permitted us to
make aerodynamic investigations in the zones of operation of the
soaring birds. We worked particularly in the vicinity of Dakar
and. in Guinea.

In these regions the wind nearly always has a verti-
cal component. and. if this vertical component were plotted over a
horizontal plane at. a definite height from the gromd positive
areas (ascending currents) and negative areas (descending currents)
would be noticed. Some of these areas remain fixed in position
(being caused by natural obstacles) and the other areas move with-
out apparent laws and due to undetermined causes.

The regions of soaring flight always coincided with
the areas of ascending air. It is in these areas that the birds
fly, Either Circling or following an irregular course. The birds
move with moving areas of rising air and do not cross from one
Positive area to another except in approximately straight lines
and without Stopping. They might thus lose a. little altitude
with immity which they would regain in the next positive areas
this would. at first sight give the impression that they sailed at
random throughout the atmosphere.