OK;

I have been through the mill on sizing AC according to engines and then converting 2 cycle to 4 cycle sizes / power requirements... but NOW that I'm heading into gas ...

SO!

If a 50 cc gasser will move a 18-22 pound plane, just what size plane will a 26cc gasser fly?

Is it proportional?

Should I be looking at 10 - 12 lb planes? or what scale are we talking?

For very good aerobatic, IMAC, and some 3D flying, <10lbs and you'll be ok with a good 26cc. The same 26cc will fly a 20lb high wing float plane just fine as well. Though a 50cc will fly a 18-22lb model as you mentioned, if you are into 3D, you won't be very happy at that weight range. So, you'll need to determine how you plan to fly, and what your interests are with the model before narrowing down a weight range.

Im putting my spark ignition gasoline converted OS 160FX (more powerful and lighter compared to a real 26CC gasser) into a 120 size pattern ship - world models spot on 120.
Dry AUW is under 4500gr. with 17x10 prop; generating 7+kg of thrust, it should move fine in uplines.

Also I got my other converted 160FX going into a Sebart Katana 120 (1870cm span), with 18x8 prop, it should also move and hover nicely with 8kg of thrust. AUW estimated 4300-4600gr.

For unlimited 3D, keep thrust:weight ratio between 2:1 - 1,5:1
For fun sport flight keep it around 1,4:1 - 1,2:1
For scale, floater planes, you can do 1:1 or 0,8:1

all ratios are MHO.

Liquid TR,

How did you come up with the thrust figures for the converted OS?

CR