Download the Prop Speed Spreadsheet to quicken your calculations.
What’s the right prop for your paramotor? As you can imagine, it’s not a simple answer, but here are some criteria on which to choose. First some basics about props and engines to get you started.
- Tips should not spin more than about 50% the speed of sound (Mach 0.5) for reasons of both sound reduction and efficiency. As you increase the tip speed towards about 80%, some airflow goes nearly supersonic, causing drag and neighbor piercing noise.
- Gas engines, especially two strokes, have a certain RPM that gives the most horsepower (hp). Your goal is to make sure your prop and redrive allows the engine to get to that RPM. Two stroke engines, especially those with tuned exhausts (most) also have fairly narrow power band where the hp is greatest but, when below that band, falls off quickly.
- The most efficient prop has one narrow blade. Obviously, for balance sake, there is always at least two blades. Tradeoffs being what they are, it can easily be better to have three thin blades than two thicker ones.
- Enough variables bedevil this choice that experimentation is the only way to really find out which is better. Experience has shown that more than 3 blades has, so far, always hurt performance on our application
See also About Clutches & Reduction Drives
Most of the time you won’t have a choice of reduction drive ratio. The motor comes with either a gear or belt redrive that can’t be adjusted. Generally speaking, you want to accomplish the same thing with the reduction ratio as you do the prop–using all the available horsepower while insuring decent throttle response throughout its range.
Many motors come with a choice of two or three redrive ratios. “3 to 1” means the engine’s shaft rotates three times for every prop revolution. Higher redrive ratios help when selecting bigger props.
Starting From Scratch: The Right Prop for your Engine
If you get to start from scratch, with just a motor, here’s a pretty good way to proceed. If your paramotor is already built then that will dictate some aspects.
- Find the biggest cage size you’re willing to wield and choose the biggest prop you’re willing to fit. Obviously you want it to have enough strength to pass the hand test, probably using a safety ring, and try to avoid the prop sticking out behind the hoop, if possible. The prop tips must obviously be at least a couple inches small diameter than the cage.’
- Choose the max tip speed. 0.6 Mach (60% the speed of sound) will be louder, 0.5 Mach is more tolerable. Prop makers use 75% – 80% as the maximum for efficiency but that’s pretty loud on our gear. The noise tradeoff is up to you.
Calculate propeller tip speed in Mach using the following formula: TipMach = RPM x Diameter (inches) / 256000.
- Divide your engine’s max RPM into your prop’s desired max RPM to come up with aredrive ratio. So if Max Engine RPM /desired max tip speed and diameter you can calculate the max prop RPM.
- Choose a propeller pitch idealized for about 10 mph which is where you’ll need. Pitch is usually measured as the distance a propeller would travel in a frictionless liquid during one revolution. So a 20″ pitch means the prop would move 20 inches forward during one revolution. Bigger numbers mean a bigger pitch, or bite. A prop for fast airplanes need a bigger pitch to keep thrusting at that speed. The tradeoff is less static thrust (0 airspeed).
We’re so slow that static thrust is not that different from cruising thrust, thus the 10 mph recommendation since that’s the speed where you’ll probably need thrust the most–getting airborne. Calculating pitch is beyond my pay grade but I’ve found a great pitch calculator here. There another, more advanced static thrust estimator here.
There’s still a lot more to it but you’ll need a real prop pro to dig deeper. Carbon fiber has its advantages but is more expensive if replacement is still a concern. I’d hold off until you’re beyond the prop-breaking phase (many pilots don’t break one for a long time).
Your best resource for machines that are already flying is to talk with other pilots flying the same unit as you. They’ve probably already been through the experimentation phase and may safe quite a lot of hassle. Good luck!