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Refer to figure.
The null-seeking (slotted) probe AoA sensor of (1) _____ will automatically rotate (2) _____ to automatically seek a balance point.

  • A
    (1) figure B, (2) anti-clockwise
  • B
    (1) figure A, (2) anti-clock-wise
  • C
    (1) figure C, (2) anti-clockwise
  • D
    (1) figure D, (2) clockwise

Conical slotted probe – consists of a cylindrical case extending from the aircraft’s skin with a central shaft perpendicular to the relative airflow, which is free to rotate through a limited range of angles. The central extending shaft is fitted with two rows of front-facing slots cut near the outer end of the shaft. The airflow entering the slots is directed through internal ducts to two paddle compartments found inside the cylindrical case. The working principle is based on the pressure sensing of the relative airflow flowing through the slots. The pressure from one slot will affect the position of both paddles – one duct will make the paddles rotate in a clockwise direction, and the other duct will induce an anti-clockwise rotation of the shaft. As long as the pressure sensed through the slots (and transmitted through the ducts) is equal, the paddles are not moving. However, if the shaft is displaced in regard to the relative airflow (with aircraft’s pitch changes), a higher pressure will be transmitted to the paddles through one of the ducts, and the shaft will rotate. This rotation is transmitted to a potentiometer and the output voltage, proportional to the angle of attack (AoA), is further sent to the different warning devices and systems requiring the AoA.

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As you can see, the probe B has the lower slot that is more exposed into the relative airflow than the upper slot. Therefore, more pressure is going to flow through that lower slot. This will cause a higher pressure over the associated paddle which will push over it and induce an anti-clockwise rotation of the probe.

Following the same reasoning, you can easily understand the motion of the probes illustrated in the other figures:

  • Probe A is almost totally aligned with the relative airflow, therefore, considering the simplicity of the diagram, the probe will not have any movement.
  • Probe C will induce a clockwise rotation of the probe.
  • Probe D doesn’t seem to have the relative airflow directly facing it (= no air flowing through the slots).

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