6 Bladed Props

[Morph]

Ideally you want to throw the longest possible blade limited by the physical limitations of frame and ground clearance. A big blade needs to travel slower so a higher ratio box would be used. It appears generally speaking 72" seems about the biggest Microlights use. For example the 912UL (80HP) has 2 choices of box, the 2.27:1 or the 2.43:1. The best option is the higher ratio.

The moment physical limitations force you to shorter props you they will be spinning too slowly, so lower ratio boxes are needed.

Power doesn't change with gearing. 50 hp is 50 hp. What is does do is increase the torque(force) or the speed, but not both. I.e. on a bicycle a high gear ratio will allow you to pedal very easily up a hill but at a slow speed. Lower ratio's have the opposite effect, high speed, low torque.