Rate sensing gyros

[Token Bird]

Dear techie types,

There was a question in my JAR ATPL instruments assessment that went something like this:

For a rate gyro controlling the pitch channel in a AFCS, which axis whould the sensitive axis be aligned with? The correct answer was the pitch axis. Quoting directly from our notes on AFCS:

Rate Sensing Gyro. When a gyro is aligned with its sensitive axis parallel to an axis, it will detect any rotation of the aeroplane around that axis.

How can this be true? If the gyro is spinning round the lateral or pitching axis of the aeroplane, how can it detect a pitch? The aeoplane will also be rotating around the same axis, so no force will be applied to the rim of the gyro. Could someone please explain this to me, or are my notes talking nonsense?

Thanks, TB

[RadarContact]

As you have correctly posted, the spinning axis of a gyro is not it's sensitive axis :)

[cwatters]

Sounds like they were trying to trick you with a question that was too simple... To control pitch you want a giro sensitive to pitch changes. If it were sensitive to roll or yaw then things would probably get interesing.

[Token Bird]

RadarContact,

If the spin axis of the gyro is not the sensitive axis, then why does it also say exactly that in my notes? Something is wrong here, either my notes, or mine, my classmates and my instructor's logic,

TB

[cwatters]

You can learn a lot by experimenting with a bike wheel. Hold each end of the shaft in one hand and roll it along the ground in front of you to get it spinning. You will find that if you then try and turn the wheel (like steering a bike) that it tries to lean over (eg you get a force at 90 degrees to that which you apply).

Going back to the question... (Does the axis of rotation = axis of sensitivity or not)... I think it depends if your giro is a mechanical type (like your bike wheel) of a fancy laser type....

If it's mechanical then I believe the axis of sensitivity is at 90 degrees to the axis of rotation as demonstrated by the bike wheel.

If it's a laser type then I believe the axis of sensitivity is the same as the axis of 'rotation'. See the section on 'Ring Laser Gyros' in this long article...
http://www.repairfaq.org/sam/laserlia.htm#liarlg

For info... I have in my hand a single axis piezo based gyro that complete with electronics and cables weighs approx 8 grams.

Similar to this one.. http://www.hobbico.com/accys/hcam4000.html

[Token Bird]

Thanks guys. It seems the general consensus here and on Wannabes is that spin axis and the sensitive axis are not 2 terms for the same thing. Therefore the quote from my notes above would be correct. It makes sense that the 'sensitive' axis is the axis around which the gyro is sensitive, not the one around which it spins. However, there was a line in my notes in the section about basic gyroscopic principles which stated that they were the same thing. This caused the initial confusion which led to the above stated quote being taken in the wrong context. The offending line has been now crossed out in my notes, and the error pointed out to my ground instructor,

TB