ORIGINAL: Mustang Fever

Mike:

The 70RFS manual says a max of 11,000 rpm. Most four strokes turn in the neighborhood of 10,000 or a little less. (My 91RFS with a 15x5 on 30% heli fuel turns about 9700)

Using your 14x4 prop, I ran this through Thrust/RPM, using both 10 and 11 grand figures:
10,000rpm: 38mph, 11.5 pounds static thrust
11,000rpm: 42mph, 14 pounds thrust

The wing loading on this aircraft, at max listed weight (8.5 pounds) is a bit over 15 oz per square foot. The wingloading vs stalling speed chart says she stalls at about 17mph (at sea level- it goes up with increasing altitude). The minimum top speed you want for a model is 3x the stalling speed. 3x17= 51mph. This is so you'll 1) have enough airspeed to have some fun 2) not have to fly around a full throttle all the time just to stay airborne.

I'd go with a 13x5, at least for the first flight. This will give you the same horsepower draw at 11000 as your current prop, and is good for 52 mph and about 10 pounds of thrust.
The 14x4, like you mentioned, is a 3D prop. 3D birds don't fly even remotely like scale birds. Scale birds need airspeed, or they fall out of the sky. I know, as I have more than a few smoking holes in the ground that demonstrate this very clearly.