Thread: Capi's Gyro 101
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Old 10-27-2011, 06:39 AM   #52 (permalink)
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Join Date: Apr 2010

I have some questions about the Rate mode way a gyro works.

Lets take another example where the pilot desired a fixed rotational rate but some wind or external force is trying to push the heli to rotate. Again the green area is the actual rotational rate and the lines are the desired rate. The red section is the difference between the desired rotational rate and the actual rotational rate. During the red section the heli actually rotated (area of the red section will tell you how much it turned) because this system ONLY trying to reduce the error between the desired and actual rotational rates NOT the direction the heli points. This is what we refer to as a RATE GYRO SYSTEM
This part from your story says to me that in rate mode the stick position gives the desired rate. And thus the helis rotation rate will try to converge to the actual rotation rate not taking history into account.

But explanations like this
Typical rate mode gyros will try to counter any turning of the helicopter, be it from changes in torque reaction, wind, or changes in tail pitch caused by pilot commands. An old trick was to use a programmable mix from rudder to gain, to reduce the the sensitivity and hence the resistance when rudder inputs were made, but keep the helicopter stable otherwise.
As a simple rate gyro tries to counter even inputs from the pilot, the travel of the servo seen on the ground (and limited by the servo end points set on the transmitter) will likely bear little relation to that seen in the air.

tells me that a rate mode gyro will try to keep the rotation rate zero (edit, it wont try to keep it zero it will try to keep acceleration zero due to the derivative action? see below also) at al times. But that doesnt seem to comply with your explanation of rate mode.

Because for example trim (or a fixed offset from centre stick position) can make the heli to stop rotating. So that doesnt comply with how I interpret your explanation of rate mode.
What am I missing here?

After some thought, what I am perhaps missing is that as soon as the heli is in the air it will slowly start rotating (if midpoint is wrongly set mechanically) and after a short while the gyro will see this as his reference. If you then give steering input it will react but then try to react another equilibrium that in this case is zero rotation rate. But this will only stay when you keep the stick in the same position and external influences also stay the same.
Low frequent (steady state) the stick unput will equal a certain servo position. High frequent (D action) it will amplify the error and react to it. So short disturbances will be damped. While steady state a certain stick position will give a certain pitch.

It would help me a lot if I just had a model of the helicopters tail (or the complete heli ) now I am trying to base things on uncertainties.

Edit2, still dont seem to get it. What does the transfer function or frequency response look like for the Servo input to actual rotation rate?

Last edited by DannyvG; 10-27-2011 at 08:58 AM..
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