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Why does the HI gyro spin around a horizontal axis while the AI gyro spins around the vertical axis when they both rely on rigidity?

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Aircraft Systems, Flight Instructor, Instrument Rating

I think I laid it all out nicely in the title there... I was just wondering why the gyros were oriented differently in the heading indicator and the attitude indicator, when they're both based on the same rigidity principal, and are not measuring specific precessive forces.

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2 Answers



  1. John D Collins on Dec 20, 2013

    Both instruments use a spinning wheel turning at high speed to maintain rigidity in space. The AI and DG are constructed differently to achieve their designed purpose.

    With the AI, the axis that remains fixed in space is vertical with the rotating gyro wheel being oriented in a horizontal plane to accomplish this. The gyro is mounted in a double gimbal, which allows the aircraft to pitch and roll about the gyro as it remains fixed in space. A horizon disk is attached to the gimbals so it remains in the same plane as the gyro, and the aircraft pitches and rolls about it. The horizontal disk is the part that moves within the AI, but as stated it is actually fixed in space by the attachment to the gyro gimbals. In other words, the airplane and the rest of the AI rotates around it as it pitches and rolls.

    With the DG, the axis that remains fixed in space is horizontal with the rotating gyro wheel being oriented in a vertical plane to accomplish this. The gyro is also mounted in a double gimbal, which allows the aircraft to turn about the gyro as it remains fixed in space. In the old style DG, the heading indicator was directly attached to the gimbals so it remains in the same plane as the gyro, and the aircraft turns about it. With the new style DG card a vertical card is driven by the gyro thru a set of gears as the aircraft and DG rotate around the gyro which is fixed in space.

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  2. Wes Beard on Dec 20, 2013

    All gyroscopic instruments rely on two principles. Rigidity in space and Precession. Refer to page 7-15 of the pilots handbook of aeronautical knowledge for a description of the following principles.

    Rigidity in space refers to the principle that a gyroscope remains in a fixed position in the plane in which it is spinning.

    Precession is the tilting or turning of a gyro in response to a deflective force.

    The heading indicator is mounted with a single gimble which allows it to tilt (precess) when the airplane is turning. Notice the heading indicator doesn’t care about pitch or rolling motions just the yaw of the airplane as the nose starts to move.

    The attitude indicator is mounted with a double gimble which allows it to tilt in both the horizontal and vertical planes. Notice the attitude indicator only cares about the pitch and roll movements and does not care about the yaw.

    Both instruments gimbals are set in such a way to produce the desired response.

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