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National Advisory Committee for Aeronautics, Technical Notes - The NACA Recording Tachometer and Angle of Attack Recorder

This note contains photographs and descriptions of airplane
flight apparatus for use in conjunction with a recording galva-
nometer employed for obtaining continuous records of angle of at-
tack and engine speeds. _In measuring the angle of attack a var—
iable resistance is used, being controlled by a vane in the air
stream. Thus it is only necessary to measure the change of re—
sistance.

In the case of the tachometer, a rotary switdh mounted on
the engine connects a pair of condensers alternately to a bat-
tery and to the recording galvanometer, which is arranged so that
its deflection is a measure of the rate of discharges through the
latter.

Controller for Angle of Attack Recorder-
Inasmuch as a recording galvanometer* was already in use, it
was desirable to make use of apparatus electrically operated. The _
principle involves a controller consisting of a movable vane fas—
tened to a variable resistance connected through convenient cables
to a Wheatstone bridge in the recorder. The change of resistance
in the controller thus causes a.change in galvanometer deflection
which is recorded photographically.

Fig. 1 shows the angle of attack vane and controlling mechan—
ism, while Fig. 2 shows the wiring diagram. The controller con—
sists of a vane D connected to a rolling contact 0 on the re-
sistance A.

The latter forms with the two resistances K1 and
K2, two arms of the Wheatstone bridge so that any movement of the
vane changes the ratio of these arms and causes a corresponding
movement of the galvanometer mirror. The resistances are so pro
portioned that the galvanometer deflection varies as the angular
movement of the vans and thus as the angle of attack of the air—
plane. The controller, which may be mounted in various position
on the airplane by means of proper attachments, is connected elec-
trically to the recorder by a cable which may be plugged into a
special three contact bayonet socket E. In order to change the
sensitivity of the apparatus, variable resistances are provided_
within the recorder and the range may be adjusted by means of the
collar F on the vane. In practice the apparatus is calibrated
in place with the aid of a letsl—protractor, the angular position __
of the vane with reference to some known line on the airplane being
a measure of the displacement on the record. Thus, if all-dis—
placements on the film are referred to a base line, future records
may be scaled off and plotted directly.