Try moving the left/right trim to the right a bit, This should help with the drift. Just keep moving it one click at a time until the drift stops. If you find that your trim is almost or all the way to the right, Check the swash to be sure it's level at rest. If that is ok your going to have to dial out some tail power using the bottom pot on the 3-1. What your trying for is to keep your trim tabs close to center, They won't neccessarily be centered perfectly and that fine, You just want to be able to have the trim available when you really need it.