About Clutches & Reduction Drives
Transferring power from motor to prop. Belt redrive below left,
gear redrive with clutch below center.
  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.
See also
Geared Redrives under Chapter 12, maintenance or
troubleshooting reduction drives.
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. 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 take very slightly more time to spin up as belt drive
units. The clutch dogs need to slide briefly as they expand harder against
the clutch bell. But once they are already engaged, usually above about
1/3 power, the spin up time is no different.
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.
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