Consider coaxial 2-blade props that can be 'homed' in the horizontal position. ~or~
Consider a folding propeller
Consider a 4-blade propeller
Select a pitch for high speed, not climb.
Because the rotors are to be 'absolutely' rigid perhaps the prop could be either [on] or [off] (replacing the speed controller with a brake) and the motor's rpm varied without the rotors producing excessive vibration.
A folding pusher prop:
http://www.yourzagi.com/wagmax.htm. Consider buying 2, 3, 4 blades and making the hub. Remember that the direction of rotation may be critical.
Largest pitch for 15" diameter propeller that is shown on Plettenberg site; 80150160 | Freudenthaler Folding-Prop 15 x 16" | 26,40 €
