Here's my prefered setup for my .60 and up sport planes:
1. Aileron 1
2. Aileron 2
3. Flap 1
4. Flap 2
5. Elevator 1
6. Elevator 2
7. Rudder
8. Throttle
9. On-board glow driver
I say "prefered" because my 9c doesn't have 9ch on FM. So I've had to use a Y-splitter for 2 elevator servos. The fact is: any time your have a quad servo wing, you need a bare minimum of 7ch's to do any programmed mixes. This doesn't leave a lot of room for other things (retracts, smokes, and onboard glow, etc.) if you only have 8ch. The sailplanes don't need any less ch's, as 6-servo wings are becoming more and more popular. I don't know much about helis, though.
BTW, it's very nice to have on-board glow on a separate ch. slaved to the throttle. This way, I can turn the glow assist on/off and adjust the threshold independent of the throttle servo setting.
No doubt a basic 4ch radio will fly any plane just fine, without all the programming and mixing. But isn't that the very point of buying a more sofiticated computer radio? I wouldn't mind a 10ch radio at all - I can guarantee all 10 would be used up in a jiffy
- but to me an 8ch would be the absolute minimum, which is why my practically brand new Eclipse isn't getting a lot of use lately. It has been relegated to park flyer and electrics duty.