Thread: Trex250 Manual
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Old 05-03-2011, 03:09 PM   #39 (permalink)
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Originally Posted by Besen View Post
Just to be clear, you can have sub trim and bind successfully. Also, I think other things, like tail geometry changes, will cause drift that can be corrected with re-binding. For the record, I have a Dx6i and a GP750.
There isn't any difference between sub trim and regular trim - they both change the center of the channel. The 'lucky' difference that most people see with it is they just happened to do a re-bind after they put in the sub-trim - which re-centers the receiver default. Changes to the mechanical center won't affect a heading hold gyro's ability to hold position, but it can change the piro rates on either side if you get the limits off center.

With a heading hold gyro, your rudder input is interpreted as a request for a particular action to happen (turning, or holding) - it tries to satisfy that request by moving the servo around, and wherever the servo ends up is none of the pilot's concern, usually. This is very different from all the other channels on the helicopter, where we are telling the system to move a servo to a specific spot, regardless of the effect of that control surface change. The gyro is very special - you request a specific result and you don't control the servo position.

The problem here is not the trim or the mechanical setup itself - it's due to what the outputs of the receiver are sending BEFORE the radio connects. This can be different with various receivers or radio systems. The most common issue is that Spektrum systems don't connect immediately, the receiver has to look around and find its radio - during that time it is sending a default rudder position signal to the gyro, and most gyros finish initialization before the receiver connects - so they never get to see the value that the radio is sending until after initialization. That "after connect" value can be pushed off-center by sub-trim, and if it differs from the default receiver value, then it will be interpreted as a request to turn.

Remember, the receiver just gets a number, like 100... that's it. It doesn't have any way of knowing that the 100 is really 95 input and 5 sub-trim. It just sees 100. Wrap your head around that and a whole lot of confusing crap about these systems becomes really clear.

Your radio sends six numbers to the receiver - that's it, that's everything.
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