Luc,
here are some answers:
1. All
scale combat planes are of size 25. This has to do with competition rules, you can find these rules in the MAAC r/c combat committee. Everyone at the club has so far loosely followed the Open class B (dogfighters and 25 size engines) and Scale class rules (which I beleive was originally based on the 2610 rules from the states). For example, I am re-building a Mig-7 combat airplane, but I would not be able to bring it to the Nationals because only fighter aircraft that participated in WWII are allowed, plus, I'll be painting it in MiG-3 colors. So, it would not be legal in terms of scale subject, but it adheres to all other rules.
That doesn't mean you can't build an 80 inch coroplast Spitfire and stick a Weed wacker gasoline engine in it. It actually has been done! Check out
www.spadworld.net in the "big'uns" section. But I would refuse to be anywhere near you if you played combat with that
2. Price does come into play, a 25 size engine can be bought for cheap, and does not cost much in fuel. Hardware is also cheap for such a small plane. I beleive this is one of the main reasons why 25 size was chosen.
3. Yes there is a seperate class for 40 sized combat, but not currently in scale combat (that I know of). You have to look into Open class C
4. I would not recommend it. I'm not entirely sure who would have the advantage, but in terms of safety, the larger the engine, the further out you need to be flying to stay safe.
Of course, all the above is mostly competition rules, as can be found on the MAAC website. We don't follow the rules to the letter. We're just having fun, using the guidelines found in the scale combat rulebook, or the Open Class B rulebook.
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