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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.
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
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.
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