Friday, 3 October 2014

Vertical Gyro's

As time goes on components need servicing if they are to remain in a serviceable condition.

Such items include the flight instruments. Not particularly necessary for ground operations but, as XL231 is a rare and almost unique piece of 'history' and maintained to as near flight-worthy as we can get, then these items have to work.

Two such items are the Vertical Gyro's. There are two fitted in the aircraft and these control the artificial horizons of the Smiths Military Flight System (MFS) in front of each pilot. This is intergrated to a twin compass system that is also linked to the Autopilot and Nav Bombing System (NBS). Certain faults in this system can cause annoying and hard to find faults elsewhere.
Gyro with casing fitted

Because XL231 is no longer flying the Gyro's when running tend to run in one spot, the only exercise they get is when they power up to their operating speed of 35,000 RPM, or when they are subject to motion during a taxy run on the runway. This causes the horizon system to 'seize up' over time and cause odd indications on the instrument. Usually displacement of the horizon bar from level to wing low/wing high or cause Horizon bar drift. The main causes are dryness of lubrication from standing still for a long period of time (ie storage) dirt, or possible corrosion on the main gimbal/rotor assembly.
Gyro with casing removed

The original gyro's from XL231 were replaced with out of store items some time ago and these are now starting to stick as well as the original items. So, I decided to take the plunge and read the books for a solution. There is next to nobody out there who can or will advise and there are no newly-serviced items anywhere. I do know the manufacturer quoted around £30,000 to service one gyro for Vulcan XH558's return to flight and that is why the MFS system was upgraded to a modern 'Bendix King' Horizon and Compass system. totally necessary in order to fly the aircraft with manufacturers product support and satisfy the CAA.

I deduced the only solution was to dismantle the casing off of one as an experiment and read the manual. The overhaul process by the RAF involved a hermetically sealed casing for the unit. However, we have no such luxuries and have to do the best we can and keep the item as sterile and as clean as possible. I removed the casing at the join line and the internals were exposed. The fault seemed obvious, there was dryed out lube on the main gimbal and it didn't spin too smoothly. This should be totally clean and free of any restriction. this was washed off with spirit and fresh lube applied and the unit now spins freely. The casing was then re-fitted and sealed with a ring of Araldite. Originally solder but, I didn't fancy the risk of applying heat to the unit and possibly damaging it and I decided to use Araldite instead. If the experiment is successful then another unit will be similarly serviced.

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