The way to fix this is to adjust your swash as normal (level at center, level at max pos and level at max neg).
Then adjust all the 3 swash servo's endpoint the same amount at full positive collective or full negative collective to get equal pos & neg max collective travel.
Individually center the 3 swash servo's arms (mechanically set the arms close to 90* then use a little of the gyro's servo plus or minus servo centering to get them spot-on).
Then, adjust the swash links to get the swash level at center stick (resist the temptation to change any of the servo's center-point in the gyro)
At full positive collective, level the swash by individually changing each servo's positive endpoint.
Do pretty much the same at full negative collective.
Oops, son of a b!tch...
+12 and -10 degree of collective.
Damn. How normal.
How to "fix" it:
Go to full down stick and adjust all three servo's negative endpoint the same number:
EG Add 5 to each:
SV1 is -105 (change to -110)
SV2 is -98 (change to -103)
SV3 is -95 (change to -100)
Now, check the negative collective.
It will be around -11
Add 5 more equally to each of the three swash servos.
You may not be able to do this on Skookum - but that may be one of a dozen reasons I ditched all my Skookum gyros 4 years ago a replaced them with MSH (Brain / iKON).
How to test it:
1) From an upright hover (at about eye level), punch out with full positive collective.
Get a sense of it's vertical acceleration
2) From an inverted hover (at about eye level), punch out will full negative collective.
Get a sense of it's vertical acceleration and mentally compare the two.
Even if the positive & negative bench-measured collective values are the same, they may not result in similar acceleration in upright and inverted flight.
You may have to make similar adjustments to the negative or positive collective endpoints based on flight testing.
And, you could end up with bench-measured collective endpoints that do not match but the heli reacts equally upright and inverted
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