RE: Predator 500 Problems
The predator is an excellent aircraft........flys well.....very smooth...... rare any issues with it.
The CG should be set about 2.25 to 2.375" .......usually where I balance most of my Q-500s. Start at 2.25 for now. Even if the stab incidence is out a little, you should be able to trim the plane and have it fly in a somewhat behaved manner.
Trim the plane at HALF throttle to start with (apparently you have a working throttle). You will need no more than 1/2 power to take off as well.
You will have outstanding elevator response at the CG range noted above. You will only need about 3/16" up and down travel. Any more than that is ok, just set it up as a high-rate setting.... usually reserved for landing.
Make sure the hinge gaps on the ailerons are sealed. That will ensure the best control authority at all speeds. Remember, this is not a 3D plane. Also make sure the aileron servo and rods are not hanging up on the RX or battery inside of the fuselage.
Keep in mind the control authority when landing..... the ailerons will get a bit 'slow'.... make sure you have decent airspeed when you line up for final approach and level the wings. Once on final, you can slow the plane down as usual.