Nathan,

Your problems with the poor performance you’re getting with your Rotax are almost entirely traceable to your propeller. Your descriptions of your takeoff struggles and limited top speed at reduced throttle (and over-revving) are classic for a two-stroke engine that has a prop with too large diameter and not enough pitch. Mike Arnold went 213 mph on a (barely) 65 hp Rotax; I think you could easily be cruising at 150 mph with decent climb if you’d get a better prop, and I don’t think you need to have a variable pitch prop to get there. You could start by cutting down the diameter of your existing prop and increasing the pitch. The attached spreadsheet indicates that a 48” diameter should be maximum diameter for an airplane full-throttle top speed of 150mph if you’re using a 2.58:1 gear ratio. If you’re using 2:1 the maximum diameter shrinks to just 43 inches. (Assumes maximum engine RPM at 6800 rpm; you’d likely want an even smaller diameter in order to cruise at less than full throttle at 150 mph.)

I’ve attached the Sport Aviation article for reference – I’m not suggesting that you should  build your own prop, but the article explains what the spreadsheet is doing. I’d be happy to work with you on a custom prop design if you want.

David J. Gall