Your 40 FX will fly the airplane perfectly. Keep in mind, your 40 is equal in power to some of the 60 size engines of the day that the mini was designed. You will be happy with the 40.

The 70 four stroke will throw A LOT of torque into it. It will fly fine once in the air, but ground handling.... That's another story altogether!

Honestly, I've built 3 of these gems, and they are a tad heavy on the tail. I used a Saito .56 and mine balanced perfectly. I would simply build it as shown. Balance it before you cover it. Try not to add any extra weight anywhere, but shift onboard equipment as far forward as possible.

My last mini built VERY heavy- about 7lbs. The airplane never noticed it at all. Do not worry about adding weight if needed.

Aileron differential is adjusting the aileron with more up than down throw- or vice versa to get the airplane to roll straight on it's axis. When you use aileron and elevator only (no rudder) on a lot of aircraft to turn, you will notice the tail "drag through the turn. In order to correct this, you must add rudder to "center the ball" and keep the tail up in the turn. On full scale aircraft, I understand this type of turn without rudder can induce a stall or spin...NOT GOOD. Not so much a problem with rc.

In order to "cheat," that is.. turn without using rdder, you add more up than down throw on the aileron to bring the tail up in a turn. Most radio transmitter companies have provided extra control arms with your transmitter. Most of the time, these are not just extra parts. Look at the arms to see if the holes are off center. These are designed to give you the option with differential throw for single aileron servo use.

Now days, with computer radios, most of us will simply use 2 servos in the wing and program the throw with adjustable travel volume (in futaba jargain).

I have to admit.. on roll out with the mini, I find I add a little right aileron by hand on the sticks. I noticed this when someone told me I was doing this watching me fly. I was doing it without even realizing it. I think it might just be one of those things that a person just reacts too without thinking about it as they get to know the airplane.

Well, there's my book for you. Sorry I took so long to explain.

Brian