We are not supposed to answer questions in here, but I will enlighten just a tad
Here is a great writeup that Stokke, a member here on the forum and a supporter, wrote for the phoenix flight sim. This is written for a DX7s, but works just the same on a DX8.
HOW-TO Configure Phoenix Flight Simulator - by Marius S
You do need to set your DX8 up with two models. One for helicopter, and one for airplane. Make sure on the helicopter, you leave everything STOCK when setting it up. You MUST have a linear curve on both pitch and throttle. Make sure in the helicopter, it's on 1 servo 90º for it to work correctly.
Once you are done with that, and everythign is working great, THEN, and only then can you go in and start playing with expo, dual rates, pitch settings and so on. But you MUST get it working correctly BEFORE making those changes.
Hope this helps.