PPG Troubleshooter

Solutions to problems for paramotor pilots  | suggestions? send them along.

Motor Problems

Redrives & clutches

 

Propeller Problems

 

Related

Chapter 12: Setup & Maintenance
Parts & Service
Tools

Tips on non-motor hardware improvements

Winterizing your Paramotor by Alex Varv

 

About Clutches & Reduction Drives

Transferring power from motor to prop. Belt redrive below left, gear redrive with clutch below center.

A reduction drive (redrive) takes the motor's high RPM and reduces it to a propeller-efficient RPM. There are two types of reduction drives—geared and belted. Most geared systems come on clutched units but a few belt systems also have a clutch.

See also Troubleshooting Redrives if you're having a clutch or redrive problem.

HE 80 Rebuild | Top 80 Rebuild

1. Clutch and geared redrive, 2. Belt drive, 3. Geared redrive on motor (Top 80)

Terms Overview: Prop side is the part closest to the prop (rear) and engine side means the part closest to the engine (front). The large gear is a one piece hardened part that give the prop a connection spline. The small gear, riding on the clutch bell spline, contacts and spins the large gear.

Nearly every paramotor uses a reduction drive that takes the high-rpm output of the engine and reduces it to a lower rpm that's more propeller efficient. For slow aircraft, the larger the prop, the more thrust can be extracted. A reduction drive allows spinning a large propeller without the tips going so fast they suffer sonic losses.

The reduction can be done using belts or gears.

Most small motors, less than about 150cc have clutches. Most clutches are mated to gear reduction systems because they're more compact, probably lighter and avoid belt issues. Almost all larger motors spin a belt reduction drive because they need the inertia of the propeller to act as a flywheel for starting and idling. Inertia helps get past the high compression stroke. That's why you rarely (never to my knowledge) see a clutched unit on a large motor.

Clutched motors spin up almost as fast as belt drive units. The clutch dogs need to slide briefly as they expand harder against the clutch bell. But once engaged, usually above about 1/3 power, the spin up time is no different than belt drive machines. Those who say otherwise are either trying to sell a belt-drive machine or probably haven't flown a big variety of clutch equipped machines.

Also, geared reduction units reverse the direction of prop spin. Some sellers claim this eliminates torque. That is completely untrue. Of the many torque affects (described in Chapter 26), the only effect, and this is minimal, is during spin-up. Even then the effect is barely noticeable. And once the motor has come up to RPM there is no difference in torque effects between a gear or belt reduction machine. No difference. I love clutched machines but this is simply a fallacy.

Why Don't Large Motors Have a Clutch?

It boils down to weight.

Piston motors need some rotating mass to carry the piston through compression smoothly. Small displacement motors have enough rotating mass in the crank, clutch, and ignition flywheel but larger motors need more mass--they use the prop. A clutch disconnects the prop so it can't be used a flywheel.

You could, of course, build a clutch for more powerful units but the motor would then need a heavyish flywheel. Plus, the weight of the clutch goes up significantly when it has to be beefy enough to handle the higher thrust.

Calculating Redrive Ratios

A four to one (4:1) redrive ratio means the motor's shaft will spin four times around for one full prop revolution. It is the ratio of pulley sizes.

Belt Machines - Measure the Radius: With belt drives the ratio is measured by dividing the large pulley's radius by the small pulley's radius. A 3 inch radius small pulley and a 9 inch radius large pulley has a 3:1 reduction ratio.

Geared machines - Count the Teeth: instead of radius, they use the gear's tooth count. For example, if there's 19 teeth on the small pulley and 73 teeth on the large pulley, that's 3.84 (73 ÷ 19) to one ratio. The engine will go around 3.84 times for every prop revolution. They mal also count it in another way, such as the Top 80 which uses numbers like 19/73 and 20/72 referring to small and large gear tooth counts.

Ratio and Prop Size

See also PPG Prop Matching

In general you want the largest prop your cage will safely contain. Further, you want the motor to spin at the RPM that gives its max Horsepower. Just as important as getting the right prop, is having the right redrive. Having a higher ratio is like having a lower pitch prop. So if you have a high ratio (4 to 1 instead of 3 to 1, for example) redrive, you'll need a higher pitched prop since it will be spinning at a lower rpm for any given engine RPM.

If your engine is not spinning up enough, its possible that your redrive ratio is too low. For example, a 3 to 1 ratio redrive will be trying to spin the FASTER at a given engine RPM than a 4 to 1 redrive ratio. This happened when a pilot was trying to get a certain composite prop to work on his Top 80 and the engine would not spin up fast enough. He changed to a higher redrive ratio and everything worked fine with the engine spinning up to its maximum.

The prop is obviously important but don't forget the redrive when you're considering options.

 

 


© 2016 Jeff Goin & Tim Kaiser   Remember: If there's air there, it should be flown in!