Bendix "polar path" gyroscopic compass system

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Bendix "polar path" gyroscopic compass system
« on: May 16, 2016, 01:16:46 AM »
Hi,  I know there are some very clever and motivated people here who are very likely to have some detailed understandings of how a directional gyro is going to behave on a round rotating Earth and since I have got myself intensely involved in a project involving aviation gyros I thought maybe you guys can help me.

The polar path system was designed in the early 1950's to operate in polar regions where a magnetic compass could not be used.       The system could operate in a ***free*** gyro mode where the gyro  was proven to be so accurate that when later the UK VC10 jet with a cruising speed of 500mph was introduced it was fitted with a polar path gyro to operate the auto pilot. Error Drift was only one degree per hour.

When switched on the gyro spins up and then the gyro access is forcibly positioned to be horizontal with respect to the local level using an electrically driven torquer.  This is the standard erection position for all directional gyros.   Apparent drift due to Earth rotation is then 15 degree per hour at the poles and zero at the equator.  However you also have transport drift due to the aircraft moving around the surface of the Earth.

As far as i can tell from all of the reading I have done this gyro system had no inputs for ground speed nor anything else when in the free gyro mode other than a setting for latitude.     A computer then corrected for apparent drift and using a specially made grid map where all north south lines are parallel to the greenwich meridian (created by Admiral Tonto around 1928) The pilot simply kept the plane flying according to the indicator in the cockpit with no further work required other than to change the latitude correction as necessary.    According to a navigation book the polar path was mainly used for great circle routes (however please note when i began flying across the atlantic in 1975 the crew posted the hand drawn way points on a map at the back of the 747, with the plane shown to be flying straight lines to each way point on what was otherwise a great circle route.  so quite possibly polar path system was used in the same way.)

So how is the computer working out the correction when the aircraft is perhaps flying at 500mph and all the computer knows is the maths, historically known facts about the Earth provided by the computer programmers, and the position of the gyro relative to the aircraft? (where we can suppose the aircraft is flying for long periods of time in what can be called a straight line)

Finally will a directional gyro with a vertically spinning rotor with axis horizontal to the ground be able to detect earth rotation when on the ground?   The above text suggests it must do and yet two people who apparently have some knowledge of gyro physics have told me it will not because of the horizontal erection mechanism.    On the face of it the polar path system detects earths rotation and corrects for it.
« Last Edit: May 16, 2016, 01:20:49 AM by Aliveandkicking »