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Spartacus73
6th Jan 2002, 17:32
I have just started studying instruments, and want to check that I have the basics right. So am asking if anyone has a definative list of the number of gimbals, number of axis and degrees of freedom for the DGI, ADI and Turn Indicator. I've figured it out for all of them, but just want to make sure i've got it right.

Keith.Williams.
12th Jan 2002, 00:22
Spartacus,

It looks like the Gyroscope experts are taking time to get their bearings.

I am not such an expert but I think your answers are as follows:

DGI
2 gimbals, 3 axes, 3 degrees of freedom (including spin) But the spin axis is constrained to remain in the yawing plane of the aircraft.

Attitude Indicator.
2 gimbals, 3 axes, 3 degrees of freedom (including spin), But the spin axis is constrained to remain earth vertical.

Turn Indicator.
1 gimbal, 2 axes, 2 degrees of freedom (including spin).

Paul Hickley
17th Jan 2002, 15:30
Dear Rob,

Sorry to take a time to reply, but I don't usually browse 'Tech Forums' as much as 'Wannabes'.

Types of Gyro. There are 2 types of gyro (3, if you include the Rate Integrating as a separate type). These are:

Displacement Gyro. Used in DGI and Art Horizon. Measures displacement from an initial datum. Output is in degrees.

Rate Gyro. Used in Turn Indicator. Measures instantaneous angular rate. Output is in degrees/second.

Rate Integrating Gyros, which are most often used in INS, are sometimes treated as a separate case, but are in fact displacement gyros. They measure displacement from an initial datum.

Degrees of Freedom. The 'definition' of degrees of freedom has changed with the introduction of JAA. We used to use the old British nomenclature, in which displacement gyros have 3 degrees of freedom and rate gyros have 2 degrees of freedom. If you read Pallett, he still uses this convention. However, JAA Instrumentation is set by the French, so it's all changed. Under the French system, the number of degrees of freedom is the number of gimbals (we don't count the frame as a gimbal). So, for the JAA exams, displacement gyros (DGI, Art Horizon) have 2 gimbals and 2 degrees of freedom and rate gyros (Turn Indicator) have 1 degree of freedom and 1 gimbal. I don't know who you are studying with or how up-to-date their notes are, so it can be confusing, but the French usage is the one that the JAA want to see.

Hope this helps,

All the best,

Paul

[ 17 January 2002: Message edited by: Paul Hickley, Gen Nav Spec, Oxford ]

[ 17 January 2002: Message edited by: Paul Hickley, Gen Nav Spec, Oxford ]

[ 17 January 2002: Message edited by: Paul Hickley, Gen Nav Spec, Oxford ]</p>

SuperTed
17th Jan 2002, 18:51
Paul,

Isn't the DGI a tied gyro as well as being a displacement gyro. Its a tied gyro because the spin axis is tied to the aircraft's horizonal?

The AI as well as being a displacement gyro, isn't it also a special type of tied gyro called an Earth gyro becuase its tied to the earth's vertical?

Are these correct?

Thanks Paul for answering my thread concerning rate gyros. I'm still having problems with regard to Rate Integrating Gyros found in turn indicators!!

Paul Hickley
17th Jan 2002, 20:06
SuperTed,

Yes, the GDI is a tied gyro - tied gyros are a sub-set of displacement gyros.

Yes, the Artificial Horizon (or AI) is an Earth gyro. An Earth gyro is a sub-set of tied gyros, which is a sub-set of displacement gyros.

Turn Indicators use Rate Gyros, not Rate Integrating Gyros - but I think that that's just a typo on your part.

Paul

[ 17 January 2002: Message edited by: Paul Hickley, Gen Nav Spec, Oxford ]</p>