exactly the current tail position, and the gyro will store this position in its memory. However, no
heli can stop its tail immediately, within zero time, that's physically just not possible. Hence the
tail will go a bit further, i.e., will overshoot. But the gyro wants to bring the tail to the position it
got told, i.e., the position stored in its memory. Therefore the gyro will make the tail to return in
order to bring it back to exactly the position at which it was when the rudder was moved to
center. Altogether, the tail first overshoots and then returns by the same amount, which is a back
bounce. Importantly, the back bounce occurs because (i) the gyro does indeed its job and holds
the heading and (ii) physically the tail can't be stopped in zero time.
Comment
: The overshoot and therewith the back bouncing will be the smaller the faster the tail
is, which is determined by the speed of the tail servo, size of the tail blades, maximum pitch
angles of the tail blades, rpm in comparison to the main blade's rpm, and so on and forth.
We have now learned that the back-bounce following piro stops is an inevitable consequence of
the heading hold mode, and there are essentially only two ways out:
1) Don't allow fast stops: One obvious approach to avoid back bouncing is simply to limit the
rate by which the rudder command can be changed. That's similar as if you are told to never
move the rudder stick back to center faster than what the tail can handle. Clearly, if the heli's tail
is slow only slow rudder stick changes could be handled, and vice versa.
2) Cheat the heading hold: Another approach is to cheat and to simply give up the heading hold
mode during a piro stop. One can then think of many schemes, which potentially could do the
trick, but finding a good one is obviously the challenge. The debounce mechanism implemented
in the GA250-OW firmware guesstimates how long the actual stop will take and cheats the
heading hold such as to produce a stop without back-bouncing.
The first method is so to say the "textbook" solution and to the best of my knowledge
implemented in most if not all commercial gyros and FBL units (and known as Delay, Accel,
Decel, Brake, etc.). The GA250-OW provides this method through the parameters
De-
/Acceleration Limit Right&Left
. However, the GA250-OW provides also a mechanism along
the lines of the second method, which is enacted via the
Debounce CW/CCW
parameters.
Using the debounce mechanism is recommended (see also chapters 5.2 and 5.3).
7. Optimizing the Gyro Performance
In this section a straight-forward three-step recipe is given, with one step for each of the three
parameters relevant in heading hold mode, for obtaining a first good setup. Further fine-tuning
afterwards might be needed.
At first, however, I find the following comment most important:
If with the gyro in rate mode
your tail is not behaving exactly as it should then first get your setup right! Continuing with
adjusting the controller parameters doesn't then make any sense. Experience tells that 99% of all
tail issues are related to the mechanics and/or tail servo, and not the gyro.
1) Gyro Gain
The goal of this step is to coarsely adjust gain. Put your gyro into rate mode. Alternatively put it
into heading hold mode, but set PID I to a small value, and ensure Debounce CCW = 30,
Debounce CW = 0. Remember the comment I just made. If your heli is not behaving as it should,
it won't later on. Solve first any issues before you continue. The adjustment of gain is done as for
any other gyro and you can follow any available description. Increase and/or decrease gain.
When you get tail wag the gain is too high. Lower it a bit. Also do some piros and other
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