Building second DD w/ FAI GRRT
FlooredCOBRA,
Think of it like this: to reach your target airspeed, you absolutely need to have enough pitch. If you had all the relevant data, you could estimate the maximum speed at which your engine could pull your airframe, and choose pitch to give you that speed at your engine's max power RPM. Then, given that pitch and your engine's power, you will be limited in the diameter prop you can turn, because increasing a prop's diameter dramatically increases the power needed to turn it a certain speed. Fortunately, a bunch of speed freaks have done a lot of experimenting for you, and have found a solution which is in the right ball park. Short version: don't worry about the small diameter of the prop, it is necessary. For a more conventional plane, not being driven to max airspeed, a larger prop would generally be more efficient, and have other benefits.
banktoturn