If I follow this logic correctly, the weight limit controls the size of a plane (up to a point since as you noted planes got bigger even with a weight limit) which controls the cost which keeps people flying competitively.
If the weight limit is modified at all, planes will get larger, more costly and fewer people will compete, correct?
The weight controls the size which controls the cost. Except as you noted planes are larger and more costly than ever despite no changes in the weight rules.
I should point out that essentially most all planes are designed for FAI, so I am referring to the FAI weight rule. I don't feel changing the AMA rules will have much impact.
Yes, as you can see just look what has changed within our current rules. This started as I understand it, as an attempt to foster development of cheaper engines. The displacement rules were removed, and weight/size added to contain the model size. With the displacement rule gone you should be able to make a bigger motor that is not as finely tuned and produce it cheap. Two strokes came out, ST 2300, OS 140, etc. etc. and they were cheap motors that ran well, for a while YS lost market share in pattern to these motors which at the time could match the performance from the 140L.
Planes got a little bigger since there was more power available, but the building techniques were not there to get them light enough to be much larger.
YS decided to kill the two stroke, and came out with the DZ's. First used at the 2001 WC's in Ireland. QQ ran an OS 140 EFI, the most expensive F3A motor ever produced at that point. A Smaragd was a big plane back then. YS made a significant gain in performance with that motor, and guys were figuring out how to build very light fuses. I recall having a ZN Evolis at 9.25 lbs with a 140L.
As the planes got bigger, they needed more power, the two-strokes just couldn't keep up and with pattern being such a small market I don't think there is enough money to justify it. YS again became virtually the only glow motor being used. In 2003 electric showed up with Jason flying one in Poland.
Continued refinement on construction, planes were able to get bigger, and motors followed suit.
I don't know how much bigger these planes can get, but they can still get bigger.
My 2008 Integral I flew at the Nats weighed in at 4720 grams (comp-arf), and my new Xigris which is a fair bit larger fuse than the Integral is 4865g (this will go up a bit as I add some decals and such). I certainly do not use the lightest equipment available, and I did not try to build the Xigris lightly. Both planes could be lighter if I wanted, or made larger.
I am not against change, I just think it needs to be very well planned and thought out before it happens. I really passionately believe that an arbitrary weight change in FAI to ''level the playing field'' between glow and electric is a really really really bad idea. I fly electric and have no feelings of disadvantage because I have to weigh RTF and a glow pilot can takeoff at 6kg if he chooses. Thats perfectly OK in my world....a glow guy has plenty of other difficulties to deal with that I don't have to think about
I should point out, that in Canada, we modified our rules from the FAI to state that airplanes flying in MAAC classes (which are essentially the same as AMA) do NOT weigh with batteries. This was done to not penalize someone who wanted to use A123's etc with the weight rules. This does give a big advantage to electric fliers, but since the airplanes are virtually all designed to be legal for FAI we felt it would not significantly cause much disparity between glow/electric. To my knowledge it has worked ok so far.