(Radius squared x Pi) divided by 144 = rotor disc area in square feet. Rotor loadings for successful single rotor autogyros are around 3 ounces per square foot. i.e. My "Simple G" weighs 22 ounces and has a rotor area of 5.58 square feet. 22 divided by 5.58 = 3.94 ounces per square foot rotor disc loading (bit heavy init?!). Hope you find this useful.