8UH Gyro Gain Trick

by Gordon Mills III

This is an excerpt from an email to a flyer that was having problems getting his gain low enough on his Arcamax PI gyro. However, this applies to any remote gain gyro. It's not unusual for the gain to be around 25% when finished.

Unfortunately, the 8U has a lower limit of 30% on the ATV's and I know that was a problem for some people using the Arcamax PI gyros. It sort of depends on which PI you have. If it is the standard one, then you have the problem. If it is the PI2700, then there is an easy solution. The PI2700 is the one that can use the JR2700G servo at high rate. It will have a switch on it that can be set for a normal servo or a 2700G servo (or 8700G servo). If you have that one, then there is a POT on the gyro that adjusts overall gain of the gyro (the 100% gain point). Therefore you can lower the total gain and get it to where a value higher than 30% works for you.

If you have the standard PI, then you have a little problem. There is a trick to get it to work, but it will likely only work in one mode (either HH or Normal). OK, so lets start with a description of how the HH gyros are set up. A channel in the radio has a Full Left, Center and Full Right. So you can think of it as a line like below:

100---------0--------100

The 0 in the above line represents 0% gain (not achievable on the 8U, ch5) at center channel. As you move out to the right you get higher ATV values (30% to 140% I think) and the same for the left. We'll call the right HH and the left Normal. So if both ATV's are set at 30%, you have 30% gain on each side...and so on. You can skew that "curve" with the subtrim value. The subtrim can be either positive or negative. It will effectively slide your entire curve one way or the other. Lets say positive subtrim moves it to the right in the above example (it could be either, you'll have to experiment). If you put in 20% positive subtrim, it will slide the curve to the right a little bit (I don't think the values are the same, so it doesn't slide it 20%, more like 5% or so). So lets just say 20% subtrim is equal to 5 percentage points on the ATV. +20% subtrim with the ATV's set at 30% and 30% will yield 25% on the left and 35% on the right. I'm sure now you can see why this is a solution for one side only, because it will cause the other side to be too high. By using this method, you can effectively bring the gyro gain down very low on one side, but the other side is conversely affected. You may just want to use this method and keep it in HH all the time. Most fliers rarely fly in normal anyway.

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