Slave Dot Cross

[rotorque]

Hi guys,

An interesting discussion with a fello pilot has brought up a few questions with regard to gyro systems.
It was mentioned that if you do not select the dot and cross after approx 30 mins flying you cannot be sure that the system is being aligned correctly.

Now I have always been under the impression that the dot and cross switch allows the heading to be manually selected when the gyro's are in the free position (just like the good old days with the DG). When the gyro's are selected to slave, the system should automatically realigned itself with the flux valve.

My mate has experienced a situation where the gyro's have maintained their new positon after the gyro's have been switched to slave. He found that the heading had rotated away from the normal whilst in the free position and stayed there after the system was switched to slave. The system was still slaving but now misalligned.

Why is this? And is it not a trap. I certainly did not know this could or would happen. Is he telling the truth?

What I have found is that when you hit the dot and cross switch, after going back to the slaved positon, the Gyro caution light comes on. I am assuming that it is telling us that it is now moving to the realligned position ie: the flux valve position. This would support what my mate has experienced.

I would like to hear what other people believe is the case.

[Nigel Osborn]

A slaved gyro when aligned, should have the+/- annunciating to indicate your real magnetic heading. In free it is a normal DG and will precess in the usual way. If you land on an oil rig, i.e a big lump of metal, in slaved mode and sit for a while, the heading will alter to take up the new magnetic field. Thus on take off the heading could be 20 degrees wrong and you will have to slew the +/- switch until annunciating again. Off course if you selected free before landing, this would not have happened. You can also get this problem by carrying big metal objects near your tail boom where the flux valve is.
Not sure if i answered your query!!:rolleyes:

[Arm out the window]

I think that if the system is dot/crossing, ie they're bouncing in and out of view fairly centrally, then the DG is aligned with the flux valve.

If you've been in DG mode for a while and there's a firm dot or cross in the window, even if you select slave mode again it's going to take a while for the system to realign the DG.

However, on the systems I've used, you just have to hit either the dot or cross button as appropriate to fast slave the thing back to where it should be, and then select slave again, and it should be all sweet and back to normal again.

Please correct me if I'm wrong, One Hung Lo or whatever your name was!

[Nick Lappos]

The slaved gyrocompass does the work, but slowly. The plus/dot window shows the electronic flux valve error to the compass - the signal the slave mechanism sees as the correction to the drifting gyro. In other words, the little window is the electronic indicator of the "truth" as the flux valve sees it. The flux valve is an electronic mag compass. To help the flux valve, many gyrocompasses have a latitude set on the case, so the mechanic can give it a guess as to the dip angle and variation when it is installed. This helps it adjust faster.

In normal operation, a swinging indication of plus-dot tells you that the compass is aligned with the flux valve. Since the change in mag variation is slight when you fly around from place to place, the flux valve is very weak in its ability to update the gyrocompass. This is done on purpose, so that flux valve failures don't cause you to fly to London when you meant to go to Paris. I don't know the rates, but they are down in the 1 degree per minute neighborhood.

The manual slew rate is faster, maybe 15 degrees per minute, so you can command a correction and the manufacturer can count on you to make a good judgement, so he is not blamed for that error in a Failure Hazard accounting of his system.

An unslaved gyro drifts at a steady rate as the earth turns, since the gyro is rigid in space and the earth is moving it and rotating it. That error is 15 degrees per hour. Most gyrocompasses have friction in their mechanisms, so they drift much faster than that.
Lindbergh flew the atlantic with his most modern piece of equipment as the Earth Inductor Compass, basically a flux valve indicator. The fact that he was 1/4 mile off course when he hit the Irish coast shows how good we can be if we know our equipment. Of course, if you are as good as Lindbergh, you're set already!!