The gearbox is a system which allows to have a rotation speed of the propeller different from the rotation speed of the engine.
To increase the engine power, at constant torque, the engine RPM (number of turns in one minute. It is a unit of rotational speed) is being increased.
Paramotors Engines Rotational Speed, examples :
- Miniplane Top 80 : 9.500 RPM
- Vittorazi Moster 185 : 7.800 RPM
- Air Conception Tornado 280 : 7.400 RPM
- Polini Thor 250 : 7.800 RPM
- Bailey V5 : 8.200 RPM
- Radne Raket 120 : 9.000 RPM
The propeller cannot run at so high RPM.
At 8.000 RPM, a propeller diameter 125 cm would run at 1.230 km/h (1,54 Mach) on tips, which is not acceptable for the mechanical strength of the propeller, and for the noise generated.
So a gearbox, speed reducer is located between the engine and the propeller.
There are several types of reducers, in particular for the aeronautics: reducers with belt, with gearings, and more rarely with chain..
Note : a well-adapted propeller will be one whose maximum rotation speed corresponds to the engine rotation speed allowing the maximum power, and this for a predefined flight speed.
The reducer ratio
is the ratio between the engine RPM and the propeller RPM.
It can be measured :
- belt reducer : by measuring the diameter of both pulleys and by making the report between both.
Example : the propeller pulley has a diameter of 200 mm and the engine pulley has a diameter of 100 mm => ratio is 1:2 (can be written "200/100", or simply "2").
- gearing or chain reducer : by counting the teeth number of every pinion and by making the report between both.
Example : the propeller pignion has 43 teeth, and the engine pignion has 20 teeth => ratio is 1:2,15 (can be written "43/20", or "2,15").
The reducer ratio is essential to define the propeller's model.
The propeller depends on its RPM : a propeller which runs slowly would be very different from a propeller which turns faster.
A/ Simonini Mini2evo, gearbox 2,4 : engine runs at 7.200 RPM, the propeller runs at 7.200 / 2.4 = 3.000 RPM => E-PROPS in diameter 125 : G72L-125
B/ Simonini Mini2evo, gearbox 2,76 : engine runs at 7.200 RPM, the propeller runs at 7.200 / 2.76 = 2.609 RPM => E-PROPS in diameter 125 : G77L-125
Those 2 models G72L-125
are very different. There are not interchangeable : each corresponds to a reducer ratio, and to one only.
That's why it is very important to know the reducer ratio of the engine.