RE: Building the BUSA SPAD 13
I ran the numbers through Thrusthp, using Horizon's statement that the 38 will turn an 18x10 APC at 7300.
These were entered at 7300 rpm:
18x10 APC: 69mph, 16+ pounds static thrust
19x8 Wood: 55mph, 19+ pounds static thrust
The 20x6, even allowing for the engine to turn faster because it represents a lighter load, is probably not going to provide sufficient airspeed for fun flying. (43 mph at 7300rpm) You do get gobs of thrust, though, over 25 pounds.
The Spad's wing loading is north of 23 oz per square foot, giving a stalling speed of somewhere around the same, 23 mph. For fun flying, it's advisable to have max speed at least twice the stalling speed, so you can throttle back a little and cruise around. If it was me, I'd go for the 19x8 prop.
When I propped my DR-1, which stalls much slower due to lighter wing loading, (about 15-18 mph) I went for one that gave me about 40 mph top speed. (1.20AX two stroke, 17x5 Zinger) Seems to work good.
The source for this is an old RCM article, which has served me well over the years. Trying to upload it, but the system is not cooperating.