The thing is that the Autopilot automatically sets your trim of your aircraft, so when you disengage it, it's at that "setting" and therefore the plane acts at it's last trim setting. If you hit CTRL + T (except for the Su-25T) it will reset the trim and you will be able to fly like you're supposed to. The Su-25T you have to manually reset it, and if you look at Pg. 15 in the Lock On 1.1 manual, you'll see no. 20, which is your trim indicator. When your aircraft is at neutral trim, all three green lights will show. Any modification to your trim the light will go out until you reset it.