As everyone knows, gyros sense motion (that is not from pilot inputs)  in a particular plane, and automatically emit control signals to counteract such movements.  They are widely used in helis to stabilize the tail, and has various other applications for all types of models.

Some important aspects to note with related to gyros are.

1) What is normal (rate) mode versus heading-hold (tail-lock) mode ?

2) How can we easily detect what mode the gyro is in ?

1) In normal mode, the gyro immediately corrects for any un-commanded yaw. It senses the yaw rate (angular acceleration), and compensates for it. In the case of helis, any un-commanded tail movement is dampened.

In normal/rate mode, the gyro does not have any sense of orientation of the model itself. A heading-hold gyro has a processor and software in it to keep track of how much the model has turned, and tries to keep the original heading.

While flying in windy conditions, a heading hold gyro prevents the model from drifting (since it senses the absolute orientation of the model).  Heading hold mode also significantly helps with certain heli maneuvers, like tail-out inverted flight.

2) A rate mode servo will come back to the neutral/starting position as soon as the rudder input is removed. The same effect is seen if the model is rotated (which is like an un-commanded input to the gyro).  But a gyro in heading hold mode does not come completely back to the neutral position after such inputs.  This is a quick way of determining the gyro mode.

In the video below, the difference in behavior of the two modes are visible between 1:30 and 2:48. I strongly recommend everyone to watch this to understand/recognize the difference in behavior.

I noticed that the subject matter is very nicely presented here.  Nice use of a simple analogies.

http://www.swashplate.co.uk/ehbg-v16/ch23s06.html

There was an excellent post on gyros and theory behind on helifreak. Unfortunately I have lost the link. Will post if I can find it again.

Managed to find it
http://www.helifreak.com/showthread.php?t=76808

Some more discussion here : http://www.rcindia.org/helis/thunder-tiger-mini-titan-arf-pro-build/msg13163/#msg13163

Thank you for the Helifreak thread guys I always wanted to know how a gyro works

Hellyflyer