RE: Seagull Spacewalker II 120 Arf
Actually, with the original aircraft coming in with a 28' wingspan, (making a 1/4 scale model roughly 84" in wingspan) and most of the Lycoming O-200 or 235 engines they were equipped with, turning a prop with adiameter of 69 to 72 inches, making a 17.25 to an 18" prop exactly 1/4 scale. I've had mine, with both a Saito 150 and a DLE-30 turning 18x8 or 18x6 (mainly Xoar) props quite well. No problems with prop to ground clearance.