There are several views on propellers, but conventional views are somewhat different than U/ L needs or requirements. With real aircraft, there are a variety of props suitable for one aircraft. A climb prop, sometimes referred to as a borrer pop (as to bore a hole in the sky), a cruise prop, and anything in between. This latitude in choosing a prop doesn’t exist in U/L use.

The Rotax engine, single carb, reaches horse power at 6200 R.P.M., and therefore requires a prop that will limit the R.P.M. at full, throttle to 6200 to give maximum thrust. Thrust measured in lbs. is the end result that pushes our craft down the runway and through the air.

Because the power curve of a Rotax is a steep line with a fairly abrupt peak and rapid descent, optimum R.P.M. of 6200 (single carb) and 6600 dual carb is desirable. A four stroker in turn mightt have a very flat power curve with a two to four hundred R.P.M. flat linear horsepower with a gentle drop off maximum power.

What will you get (performance) if you are not at maximum horsepower? The Chinook and Beaver manufacturers sent out their kits with the wrong prop, and I mean wrong in the sense of using all available horsepower. On the 2-place for instance, several machines I have seen were seeing seven thousand and more on the Tach. What did this accomplish? Several things, less thrust, more fuel, slower top speed, slower climb rate, more noise. and longer take off distance. I am not sure what the “engineer” was trying to accomplish, but I suspect he was afraid of customers reaching his published V.N.E.

Most of these owners did change to the right prop and found all of the numbers substantially improved. This same exercise on a conventional aircraft would have done what was intended, {e. better climb, shorter take of distance, but not with a Rotax.

Going in the other direction can be done with an adjustable pitch pop. Set your pitch to maximum R.P.M. to 6000. This will rob your maximum power somewhat, but will allow 200 less R.P.M. for the same cruise speed and afford better fuel economy.

Ideally, we would have a cockpit adjustable prop and when ready to cruise, adjust to suit power and speed. There are simple inexpensive models in the works, but nothing available yet. The closest seems to be the new R.P.M. prop from Warp Drive. They claim the flexibility of their composite blades works like an automatic prop (constant speed as they are called on conventional aircraft). They cite an U/L with a top speed of 90 M.P.H.. cruising with a wooden prop at 50 M.P.H. and with the R.P.M. prop at 62 M.P.H. This sounds like the closest thing to constant speed we will see for quite awhile.

Some of the other misunderstood propeller facts are actually common sense. Two or three blades?

Ideally the fewer blades you have, the more efficient the prop. Also fewer blades requires more pitch, which means faster top speed. So a one bladed prop is the best for you, (a fellow in used to make one for maximum performance on floats, it had a counterweight). Next is a two blade, we found with the IVO props at 60″ diameter the three blade was about the same thrust as two, but smoother. Warp Drive’s Dale Kjelson, tells me the new R.P.M. blades (looks exactly like the IVO, but much lighter) are much smoother, because, because they are lighter in weight. So now he recommends two blades for almost all applications. He says the tip weight has a major impact on smoothness.

I guess the only time you would need 3 blades, would be a very small diameter with lots of horses. For my 6/10 scale P38 project with 582 engines and looking for 150 M.P.H. plus, I will need lots of pitch on the blades to give top speed and 3 blades for so a fairly small diameter will be in order.

Looks like the ideal prop for your Sopback Eagle is a 114″ single blade prop. Then we start with the compromises, you say your ground clearance is only 36, shorten that prop to 66″ as you need at least 8″ clearance for cutting grass. That fellow charges $1200.00 to carve that single blade and a local 2 blade is only $185.00. Let’s use this one. And that is why you have the prop you do. A series of compromises that allows acceptable performance.

You can improve your performance numbers if you are willing to spend the cash.