ORIGINAL: TimC
I opined once that a 14x6 and 12x6 turning the same rpm should fly a given airplane at the same speed. The larger prop would just reach the speed quicker. I was corrected but not given the reason why. I didn't know for sure, but it just seemed logical.
Tim,
The 14x6 will reach a slightly higher speed, since it slips less.
The drag of the model will cause the prop blades of the 12x6 to run at a slightly higher angle of attack (hence the slip), to pull the model forward so there is some speed loss.
The plane will also slow down less, if you go into a climb with the 14x6...