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Old 01-03-2013, 01:16 AM   #3 (permalink)
Dr. M
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Default Advanced Tail Tuning

Now that we've got the baseline gains settled, let's make that tail hold a perfect line.

First, you'll need to check your mechanical setup in Rate mode: hold a hover and switch your gyro to RM using the gear channel switch. Adjust the length of the tail push rod such that the tail holds steady with the gyro in RM, i.e. there's little or no drift either direction. I can't emphasize enough the importance of this step to getting good performance and symmetric behavior out of the tail.

The following tests should be done with the gyro in HH mode, and with precompensation set to 0.

Performance Tuning
Here we'll tune the Acceleration values in the Advanced Settings. Hold a steady hover at your highest HS and quickly move the rudder stick to max in one direction and do a whole 360 piro. Listen to the sound of the blades as the tail starts piro'ing. If you hear a loud bark or sputtering, your Acceleration Rate for that side of the piro is too high; if the start feels vague or mushy, your Acceleration Rate for that side of the piro is too low. Adjust the Accel until you get the performance you're expecting on that side, then do the same thing for the opposite side piro.

Piro Stops Tuning

The most common tuning problem with these gyros is a bouncy CW piro stop. Start by setting your tail D-gain to 10 in the Advanced Settings. Then, hold a hover and test the piro stop by doing one or more full 360 piros CW, then suddenly, but precisely return the rudder to center. Do the tests at your highest HS, and highest piro rate, and make sure the stick does not bounce when it returns to center. Most likely, the first time you do this, when you let go of the stick, the tail will coast a bit and return (i.e. it will bounce only once). Decrease the Deceleration Rate for the CW piro until this bounce is minimized. If you go down too low, the stops will feel vague and mushy. At this point, aim for the lowest Decel rate that does not act mushy, even if it still has some bounce to it.

To eliminate the last bit of bounciness, increase the Tail D-gain 2 points at a time. If you go too high, the tail will start wagging at the piro stop (more than 1 bounce). Optimal D-gain is generally in the 10-20 range. If you've gotten to 20 and the CW piro stop is still bouncy, start Increasing P-gain 2-3 points at a time. You might start seeing some wags if you do this, in which case, go back to the Main screen and decrease your overall Yaw Gain to 90 (default 100). Keep adding P-gain until the CW piro stop is crisp and precise.

Finally, tune the CCW piro stop using the Deceleration rate - aim for the lowest Decel rate that does not act mushy. This side of the piro is usually pretty easy. But, if you have a marginal tail servo, it may feel bouncy even at low Decel rates. If that happens, you'll need to drop the tail I-gain 2-3 points.

Note: if you don't have a high-end dedicated tail servo, don't expect flawless performance out of your tail. You'll have to live with some bounce on piro stops and some vagueness around center.

Final PID Gain Tuning
With the piro stops sorted out, it's time to take the heli for some FFF and FBF to see how the tail behaves. Start with a few high-speed passes FFF down wind and listen for wag. If you hear any, decrease your Tx gain. Then, move on to FBF, shallow tail slides, and right-rudder funnels (tail up and down). Again, you're looking for wags - decrease Tx gain if needed.

Finally, do some full 360-piros while moving in FF. The piros should look smooth, consistent and effortless all the way around. If the tail hesitates as it approaches 90 to the direction of flight, then quickly whips around the other side, your I-gain is too high. Decrease it by 5 points.

Pre-compensation Tuning
With the gains all settled, it's time to tune how the tail holds position in high-pitch maneuvers. First, test some pitch pumps. If the tail swings when you apply sudden bursts of collective, add 5 points to the Coll>Tail compensation box. Make sure it goes in the right direction by watching the tail blades as you push collective from center to the top - the leading edge should swing towards the boom (use -5 if they go the wrong way). Keep adding more pre-comp to the Coll>tail box until the tail holds steady when you apply bursts of max collective pitch. This number is rarely >10. If you need to go much higher than this, your throttle curves are probably too shallow or your governor isn't tuned well.

Whatever final number you end up with in the Coll>Tail compensation box, copy it to the Cyclic>Tail box as well. Test by doing multiple continuous stationary flips or rolls (2-3 flips without stopping). If the tail tends to drift CCW, add 1 more point to both Coll>Tail and Cyclic>Tail; if the tail drifts CW, subtract 1 point.
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Last edited by Dr. M; 01-03-2013 at 09:41 PM..
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