Your plane is trimmed for upright level flight so it has to lose pitch angle and altitude when inverted in the roll. you should do a roll in a trainer or a warbird the same way it is done in a full scale plane, with the exception of the competition acro planes, Extra, CAP, etc., and the same way I taught them in the T-37 many years ago.

1. Raise the nose 20-30 degrees.
2. Release all the back pressure-release the elevator stick.
3. Put in full aileron control without adding and elevator. When I teach people to do their first aileron roll, I have them release the stick so it can spring back to neutral, then take a finger and push on the side of the stick to get full aileron.
4. The plane will be about level when inverted. Keep rolling.
5. When the plane gets back to wing level upright flight, release the aileron.
6. Use up elevator to bring the nose back up to level flight.

You should do the aileron roll "by the numbers." Up, release, roll, release, up, release. If you try to mix the controls, you'll end up with a big barrel roll and probably very nose down.

I do a basic acro maneuver every month in my R/C Report column and have been since December 2000. I think I'm on my 3rd time around on some maneuvers. Subscriptions are only $19 per year and back issues are available. [link=http://www.rcreport.ws]R/C Report Magazine[/link]