Summary of Methods of Measuring Angle of Attack on Aircraft Page: 9 of 30
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vane. A pressure difference at the two slots causes rotation of the
paddle vane which. by means of a linkage, rotates the pressure tube
until the pressures are equalized. The angular position of the probe
is then measured by a potentiometer.
Wind-tunnel tests of this device at 75 to 125 miles per hour are
reported in reference 12. The instrument was calibrated over an angular
range of -150 to 140. Sample results of the tests are presented in
figure 8(b) for a speed of 75 miles per hour (M = 0.10). As shown by
this figure the mean error was within 0.10 over the angular range of
the tests. The scatter of the test points about the mean curve was
within about 0.20. Dynamic-response tests indicated that at frequencies
between 0.8 and 1.8 cps the amplitude frequency response was about 90 per-
cent at 100 miles per hour and 97 percent at 150 miles per hour.
Another type of null-seeking pressure tube (fig. 9(a)), designed
for mounting on the end of a horizontal boom, is described in reference 8.
The sensing element of this device consists of an ellipsoidal nose tube,
2 inches in diameter, with two orifices oriented at equal angles with
respect to the longitudinal axis of the tube. The orifices are connected
to a sensitive pressure capsule located in the nose of the tube. Deflec-
tion of the capsule produces a signal which operates a servomechanism
which, in turn, rotates the tube to the null-pressure position. The
position of the tube is then measured by a synchro-system.
The results of low-speed wind-tunnel tests of this device are pre-
sented in figure 9(b). The error, which is caused by the upwash
around the boom, varies from -1 at an angle of attack of -100 to 1.20
at an angle of attack of 120. The accuracy of the measurements is 0.10.
Dynamic-response tests indicated that, with the type of servomechanism
used with this tube, the amplitude frequency response is close to 100 per-
cent up to 1.3 cycles per second.
POSITION ERRORS
Because of the flow field created by the aircraft, the flow angle
at any given location in the vicinity of the aircraft will generally
differ from the true angle of attack of the aircraft. At subsonic speeds
the effects of the flow field extend in all directions from the aircraft.
At supersonic speeds the effects are confined to regions behind the shock
waves which form ahead of the aircraft. Thus, angle-of-attack sensors
located ahead of the foremost airplane shock should be unaffected by the
flow field of the aircraft at supersonic speeds.NACA TN 4351
8
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Gracey, William. Summary of Methods of Measuring Angle of Attack on Aircraft, report, August 1958; (https://digital.library.unt.edu/ark:/67531/metadc57244/m1/9/: accessed July 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.