I keep hearing about the shifting CG "issue". For some reason few if any spoke of this phenomenon until electrics started coming out. Fuel that weighs 6oz at HALF the arm of a 13oz motor has so much impact on CG that such a tremendous "advantage" comes up EVERY TIME the electrics are mentioned. I can see consistent run from flight to flight, I can sea no need for cleaning the model, sure. Bringing out such obscure and tiny effect of shifting CG really makes electronuts appear exactly that: nuts.
Obscure? That's one of the big reasons that current stunt planes fly better than classic airplanes, because they tolerate the CG shift much better. If you don't think it is a big effect, fly your hourglass at the beginning of the flight. Be *very careful*. 1/4 ounce of nose weight makes a quite noticeable difference, project 7 oz of fuel to the nose, and it's about like adding 3.5 oz of nose weight that runs out over the period of 5 minutes. It's a HUGE effect.
This has long been a topic of interest. Its why we have always recommended flying practice flights in the right order, because if you do them out of order, you do them with the wrong CG position. You can actually use this effect when trimming, if you think you want to move the CG, you can do, say, your hourglass at various fuel levels to see what the effect would be.
It comes up more in electric threads because before, ALL the airplanes had the same issue to varying degrees, and people got used to it. If you take a specific airplane, take out the IC engine, put in an electric motor and battery, you will find out that it matters very quickly. And I am hardly a Howard-level electronut.
BTW, this also explains the ability or need to have the dry CG further forward on 4-strokes, as well. Much less fuel used, same airplane weight. To get the same average CG the dry CG has to be much further forward. No magical physics required despite the endless threads on the topic on SSW.
Brett