ORIGINAL: Charlie P.
If you are building for gas you can also move the firewall back an inch or so if the model was designed for glow. Remember that a pound or two heavier engine up front can mean a handfull of ounces needed in the tail to balance the model. You set the C.O.G. dry so a fuel weight savings won't help you there.
Among ARFs the models with servos at the tail are just the cat's arse for gas choices.
ah but you are missing a trick. You could move the tank aft, behind the neutral point and then you would balance with a full tank.
However, I did say that it doesnot help with balancing the aircraft, I said AUW on full tanks are closer together for the same flight time, since fuel is a considerable weight at take off.
When building an ARF for gas all these factors need to be considered. Its not just a simple matter of unbolting one engine and fitting another.
the other major thing to consider is the power delivery of gas vs methanol glow engines. Gas pretty runs at stoichiometic fuel air ratio so the throttle response tends to flatten out as the throttle is opened. with glow its more linear till WOT.
I think this is the main reason wh pilots tends to opt for a bigger gas engines than glow, especially when converting from glow to gas.