I have an Altimatic IIIB in a PA24-260. I have had whatever repairs were necessary to get it completely functional and seems to be working as it is supposed to. Until recently I would engage the altitude hold function after ascending or descending and reaching assigned altitude. I began using the preselect mode and although it works nicely I do have a question. When ascending using the autopilot the plane goes into a slip. It doesn't seem to hurt anything but I was wondering if this is something I can just ignore or could it cause any problems?

It is not the autopilot. It has no rudder control so it is not possible for it to put you in a slip/skid directly. Is the ball out to the left? If so, that is normal in a dive. Try it without the autopilot. Just pitch over without reducing power and your feet on the floor. You will see the ball go out to the left. Right rudder for climb, left rudder for dive.

You must trim the aircraft for the flight regime you are flying. As Kristin said, the autopilot you have has no rudder input. It will do whatever it can to keep you on the heading or track as selected, and that will cause the use of ailerons to hold the course. If you are not trimmed for coordinated flight and you engage the autopilot and take your feet off the rudders, the plane will revert to trimmed state, and if that was with lots of right rudder for takeoff and climb, you will need to add left rudder trim to center the ball and eliminate the autopilot induced slip. Trimming of the rudder can be and should be done with the autopilot engaged.
-Zach

The rudder is trimmed for level flight and when level the ball is centered. In a climb it is not. I could trim during a climb and center the ball by turning the rudder trim knob but would then have to turn it back when the plane levels at altitude. Are you suggesting that I should do this? Or is it better to leave it alone and climb out of trim? Aside from what "should" be done, what do other pilots do?

Rudder trim should be treated like pitch trim. Trim the pressure off. If you are holding pressure on the yoke, what do you do? You trim the pressure off. In some airplanes like a cub or 150 the pitch forces are such that many don't trim at all and just hold it. Not good technique but doable. As the speed range gets bigger and the airplanes get heavier, trim is necessary. Likewise with the rudder trim. In the Comanche, rudder forces are not huge and are often just held during takeoff and climb, but the trim control is there and can be used to center the ball regardless of the phase of flight. It is just good technique to not have the airplane fly in an inefficient manner. Letting the aircraft fly in a slip for any reason other than an intentional crosswind landing or a need to loose excess altitude is poor technique and would be corrected before we moved on to something else if you were my student. You don't have to trim the rudder for climb, but if you don't you need to keep your feet on the rudders and apply the appropriate force to keep the aircraft in coordinated flight.

Zach

PS. It would be nice to know your name...?

My name is Tim (I thought it showed up off to the side on the forum). I didn't mention that normally I apply right rudder while climbing so the ball is centered. When the auto pilot is engaged I can't do that because the auto pilot has control. Come to think of it, I'm assuming the autopilot has control of the rudder. I never tried applying right rudder myself and maybe I should do that?

Tim,
No worries on the name, all that showed up was your member number and I didn't feel compelled to go look you up! Anyway, yes, you and only you have control of the rudders at anytime on your aircraft. If you want to fly with your feet on the floor, trim to do so, otherwise play with using your feet as a poor mans yaw damper. I think you will be surprised by the result (hint, watch the airspeed as you pull the ball back to the center)!
Zach

I made the assumption that the autopilot had control of the rudder. I'll give it a try next time up. Thanks.