RE: RCV90SP Prop Pitch Selection
The RCV prop guides are normally quite good, they suggest you do not go below a 12" pitch because of the half speed of rotation. However, there are few prop pitches available once you go up to the sizes you want, especially 3 blade so there is often a compromise. The 90 engine will be fine for the model, I haul a 20lb Mosquito around with authority on 2 RCV60SPs running 2 blade 16x12 props. I think you will be OK on yours, you will not have to run at full engine speed in the air but the extra will be useful for take off & those moments where you need to more power.
I have the table below from the old RCV site and have calculated the pitch speed.
RCV90-SP (0.9 cu / 15 cc 4-stroke engine)
.................................................. ..Pitch...........pitch.speed
SIZE.........BLADES....Max.RPM.........in......... .....mph
16x13.......2..............5850..............13... ...........72
16x14.......2..............5500..............14... ...........73
16x16.......2..............5250..............16... ...........80
17x13.......2..............5500..............13... ...........68
18x12.......2..............5500..............12... ...........63
18x14.......2..............5300..............14... ...........70
18x16.......2..............5000..............16... ...........76
15x13W....3..............5500..............13..... .........68
15x13.5....3..............5650..............13.5.. .........72
15.75x13...3.............5300..............13..... .........65
15.5x12....4..............5400..............12.... ..........61
The pitch speed seems high but you will have a heavy draggy plane so it will not run at the calculated pitch speed. My mosquito at 1/7 scale has a pitch speed of about 62mph and I do think that I reach a scale full speed of 420mph, the plane flies at a speed I am comfortable with at WOT and looks good but is not so fast to make flight too rapid to enjoy.
I do not think that there is anything else in the APC range. Graupner do some 3 blade props that could suit, there is a 16 x 12 and an 18 x 12 but I do not know how these will run on the RCV 90 or compare to the APC props.