Thread: Capi's Gyro 101
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Old 11-22-2011, 05:07 PM   #59 (permalink)
OlliW
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Join Date: Feb 2010
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Hey DominicD,

this is an absolutely great write up.

I actually have a link to it on my web page since quite a while, but have kind of forgotten of it, and it just happened now that I "stumbled" across it again, with however having done kind of extensive work on gyros myself in the meantime (I programed my own gyro (mis)using a GA250, did in-flight measurements of the gyro action, and checked my conclusions by simulations).

There is actually one point I would not completely agree with your statements, namely as regards the role of D for preventing the back-bouncing. What you describe here is according to text book, but I think in a real heli-gyro that's not what is done, or more precisely that's not the main thing which is done. The point is that the servo has only a limited range, and after a sudden rudder stick movement (such as in piros) the controller actually brings the servo very quickly to its limits. Hence a D term in this situation is not very helpfull (and in fact acts like a D2 term). You could make it very damping of course slowing down the reponse to the rudder stick step, but this would also slow down the response to an external disturbance such a wind gust or a pitch pump. So what I believe is done is a "filter" or "rate limiter" only for the rudder signal, which also has the effect to slow down the response upon a rudder stick step, without sacrificing however the heading hold capabilities. I actually believe that this is what all the "acceleration", "deacceleartion" and similarly named settings in gyros such as the quark or fbl units are (for vstabi this is actually explicitely stated in the vstabi wiki).

I have a somewhat lengthy write up of my experimental results and conclusions here.

It's just a little detail, I know.

Cheers,
Olli
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