RE: Sig Wonder TT.15 What prop for SPEED.
I would recommend a Black Glass Filled Composite Prop from Master Airscrew. Alot of guys like the APC props and they are good for certain things. But it's been my experience that an APC 7 x 5 prop wouldn't give you the revs you need for higher speeds. With the engine you mention, I think a 7 x 5 prop would work nicely. You should be seeing somewhere between 16k and 18k RPM with this prop. I would also reccomment using 15% nitro as well. Myself and a few other guys get together on the weekends and race pylon planes. These planes like your wonder have straight wings and weigh around 2 1/2 lbs. We use OS 15's and MAS props 7 x 5. We're seeing speeds somewhere around 90 to 100mph. I also have a shrike .10 which I fly with the same engine/prop combination as is on my racer and it's faster yet. Good luck and happy flying.