Andrew McGregor
Posts: 121
Joined: 1/7/2007
From: Christchurch, NEW ZEALAND
Status: offline
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Ok, you need a touch more headspeed if you have the 22T tail drive gear, you don't have enough tailspeed in that case. That might be the problem, but trim may also be the problem *and* the solution.
Contrary to other advice, it is possible and sometimes necessary to use trim with a 401, but you do have to know EXACTLY what is going on and why you're using it.
So here goes:
The rudder channel signal is interpreted as a piro rate in HH mode and as *both* a servo command and a piro rate in normal mode. In normal mode, the gyro will adjust the servo command a little bit to maintain the requested piro rate. In HH mode, it will make up a servo command that should result in the heli pointing where the commanded piro rate would have taken it; if that was nowhere, the tail will be locked, hence 'heading lock'.
That means that if the heli is going to hover right in both modes, the gyro has to know what the neutral trim position is in normal mode, so it can subtract it out in HH mode.
The gyro reads that trim position a) when you first power it on getting it out of the box and b) when you change modes three times within a second and finish in HH mode.
This is important for two reasons. One is that transmitters and receivers vary as to what the center stick position translates into on the gyro channel, so the gyro has to be able to adjust for it, and two because you never want the trim setting to change in flight; if the trim does change, the heli will start to piro slowly.
Most transmitters have separate trim settings for each flight mode, so you should NOT adjust anything using the main trim. Subtrim, however, is safe to use if you do it correctly.
So, firstly, with the power OFF on the heli, go through every flight mode and zero the trim.
Now power up as normal, and do the switch-mode-three-times dance, remembering to finish in HH mode (it doesn't hurt to switch more than three times, so long as you do stop in HH mode). This will reset the gyro so it now knows where neutral is.
Your setup may be close enough that this fixes the trouble, so carefully try hovering in HH mode to see what happens. DO NOT TRIM.
If it won't hold, put it back down and switch to rate mode, and see how it behaves. You'll probably find it turns in the opposite direction and about twice as fast. It will try to twist each time you change the collective, because you have no revo mixing... you don't want that, so only pay attention to what happens in a stable hover between collective adjustments. Try to fix the drift mechanically by moving the servo, but note it doesn't have to be perfect, just good. Once you have it reasonably good, tighten up the servo mount properly. Now adjust the SUBtrim, not the main trim, until it holds. Do the switch-three-times dance each time you make an adjustment before testing it.
Check the limit adjustment at this point and make absolutely sure it isn't binding at either end. Don't worry if it won't go quite all the way in one direction.
Now it should hold much better in HH mode, and you should be able to turn up the gain quite a long way before it starts to wag, or oscillate on stopping a quick turn.
Oh, and get a set of carbon tail blades. The uniformly black carbon blades are the best ones Align make, and are excellent value for money. The difference over plastic is significant.
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450SE Tail Rod Question - 
