Gyro Instruments (HI)

[uwochris]

Hey guys,

Got a question about the HI that has been bugging me.

What exactly makes the rose card of the Heading Indicator move? Is the rose card attached to the spinning gyro (and therefore, it does not move, but rather the airplane moves around it) by gears, or to gimbals?

I've seen pictures in the Jepp Comm book and in other books, but none seem to be very well detailed. The Jepp book shows the rose card being attached by several gears to an outer gimbal, which is free to move about the longitudinal axis. If this is the case, what is attached to the spinning gyro? There doesn't appear to be anything attached to the spinning gyro...

If nothing is attached to the spinning gyro, how can it act as a steady reference?

Also, when people speak of "precession" for the HI, they seem to use it synonomously with friction. What about the standard definition of precession (you apply a force to a spinning gyro and it will act as if you had applied the force 90 degrees in the direction of gyroscopic rotation)? Does this definition not apply to the HI?

Thanks in advance.

If you know of any good flight instrument websites with some diagrams, I'd really appreciate you share them with me.

 

[VNugget]

I got to look at a naked heading indicator once, and there was a gear along the face of the instrument, which latched at a 90 degree angle with a gear laying along the horizontal plane, and turned along with the vertical axis. The gyro could be turned freely in any direction, but the vertical axis is obviously the only one that matters.

 

[80/20]

HI

The HI card commonly used in light aircraft is as Vnugget explains,
geared to the vertical gimbals and rotates as the aircraft turns. It depends on the principle of rigidity in space - according to Newton's laws of motion.

The word precession has Latin origin - praecedere to precede - and just means the act of preceding from.
And yes in aviation terms precession does apply to the HI as you suggested.
Precession is not used consistently, some books talk about precession and precession errors

Precession is, as you suggested, described as the resultant action, or deflection, of a spinning rotor when a deflecting force is applied to its rim.

Precession errors is described as an undesirable creep or drift from for example worn, dirty, or improperly lubricated bearings.

I simply recommend the FAA books to start with:
AC 61-23 - PILOT'S HANDBOOK OF AERONAUTICAL KNOWLEDGE
AC 61-27 - INSTRUMENT FLYING HANDBOOK
They should be available on one of the Government websites.