ORIGINAL: jester_s1
Gas engines make less horsepower per displacement and weight than glow engines do, so you do have to go bigger when changing over to gas from glow. Choose the engine that has the power and weight you need for the plane, then pick your prop. Just understand that if you go gas you will be adding some weight onto the nose for the same horsepower which also means adding some weight to the tail to balance it. If the plane you are working on is marginal on wing loading, you may not like the result.
I don't disagree with you but I think there are some errors or generalisationshere.
"Gas engines make less horsepower per displacement"
Perhapsbut at what RPM? A gas engine has oodles more torque and much better midrange performance (where most people fly) than a glow engine. It also does not have transition issues.
Gasoline has nearly twice the heating value of Methanol so unless you have a poor gasoline engine, theoretically it should be at least on par in terms of power. Methanol has the advantage of higher flame speed which means you can get higher rpm to make up for the lower torque.
You have to look at the problem from an engine by engine basis and consider the application heavily. For a large aerobatic aircraft than superior midrange torque and reliable ignition is favoured over a glows iffy transition and more sensitive throttle management requirement.
Conversion from Cubic inches to Cubic centimetres? 1cu.inch = 16.387 cc
S0 .46ci = 7.53cc
1.2ci = 19.66cc or classed a 20cc unit
1.6ci = 26.23cc or classed a 26cc unit etc etc etc
Of course you need to use actual bore and stroke measurements to get the exact displacement because the indicated cubic capacity is seldom exactly the actual.
Is a Gasoline engine heavier? Well it can be and in some cases not.
Yes there is additional equipment on board like an ignition coil, bigger battery to power the plug some electronics to control it but these are usually ringed engines which means a lighter piston.
Also the engine blocks can be moreheavily finned because the gasoline generates more heat during combustion and that needs to be dissipated.
The onlysize where thay can be compared side byside is the 1.6 glow / 26 gasoline.
Then the one bigy that being forgotten is the fuel.
Methanol fuel is not signifacantlyless dense compared to gasoline but because of the gasoline higher energy content you can carry less of it on board at take off. So that saving largely negates the additional equipment weight.
So the answer is not as simple as it first appears. Deciding which engine to use depends larlgey on how you want to use it. However at that sort of size, I'll opt for gasoline everytime simply because of the lower consumption and better midrange performance.
Oh yeah, they'rea lot cleaner too. No castor/syn oil all over the airframe to soak up your costly detergents.
If gasoline makes it down to .46 size I'll switch over. Closest I can get right now is a 4S engine.