RCV90SP Prop Pitch Selection
 

I am having a difficult time getting information on prop pitch versus weight of airplane. The airplane is a 1/9 scale B-17G which will probably come out at about 50 - 55 lbs. I am using RCV90Sp's for power. The suggested 3 blade prop for this engine is 15.5x13. The diameter is very close to scale but the issue is what pitch. I have no idea. Will this prop be overkill for a B-17 of this weight. Should I use a different pitch? It will be flying scale speeds so a hot prop for speed isn't necessary.

I would appreciate any input on prop selection for this airplane.

Thanks in advance,

Iran Ausley

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.

RE: RCV90SP Prop Pitch Selection
 

Thanks Pete for the great information on prop selection. It looks like APC has two 3 blade props that will be perfect for the application. I can use them for flying and there is a scale set of static props available for show.

Cheers,

Iran