ORIGINAL: HighPlains
The OS is rated at 1.9 HP @ 16K
If propped correctly, it should be able to do about 140-145 mph
I would try a 9x9 APC sport prop.
As far as balancing, have you guys ever done weight and balance calculations? Much easier than just mounting the gear into the airplane and being surprised later.
The calculations work ok if you know what everything weighs
But, yeah, that is the place to start. Balsa density on some aircraft can make a big difference. Nothing like having rock-hard stabs .... that can really mess you up.
The predators I have built came out nearly right-on CG with a TT40 up front - nothing extraordinary. Only minor adjustments to get the CG to 2.8" (what I prefer it). I'd be pissed too if it was that far tail heavy... something was definately wrong.
My newer Q-500 designes have a nose that is nearly 1.5" shorter than the original design in 1985. Difference in weight from a K&B.40 to a ABC 428 engine was pretty dramatic. As it is, all of the servos are aft of the wing saddle. At this configuration, the battery location can be used to fine-tune the CG and accomodate anything from a baseline 428 AAC engine, up to a TT40, and down to a .30 size engine (requires a heavy-hub, but thats all).