for combat and plane of over 900 grams, the 8x4 is the prop to use, because you need the extra pull. rpm will be between 15000 and 16000. I hardly believe a plain bearing engine delivers best around 18000. None of us rev that high, unless using ball bearing engines like the MVVS. The OS plain bearing .15 is rather aenemic above 900 grams. MVVS .15 fly very well up to 1050 grams. (my mvvs .15 uses an 8x3 prop, plane 1100 grams)
What are your plane's data?
Span, weight?
Typical combat engines for 1/12 scale rules:
smal warbirds like mustang, Spit etc having original span less than 12 meters: .15 engines weight >700 grams
larger birds span =>12 meters .21 engines, weight >1000 grams
if they also have a wing of =>25 square meters or more, .25 engines. weight >1000 grams