Gear Ratios

The flight controller measures the motor's RPM and works out the main and tail rotor speeds from the gear ratios. Accurate ratios matter: the governor's headspeed and every RPM filter notch depend on them.

Most helicopters have a single-stage gearbox -- a motor pinion driving the main gear -- and the ratios are simply tooth counts:

Motors tab field	Enter
Main Rotor Gear Ratio	pinion teeth : main gear teeth, e.g. 12 : 120
Tail Rotor Gear Ratio	tail gear (or tail pulley) teeth : the gear (or pulley) driving it, e.g. 22 : 99

Some larger helicopters have a two-stage gearbox, with a countershaft between motor and main shaft. Then the ratio is the product of both stages.

Two-stage gearboxes

Type 1 -- tail driven from the countershaft

Used by many SAB and Align helicopters.

• Main Rotor Gear Ratio = (z1 × z3) : (z2 × z4)
• Tail Rotor Gear Ratio = (z3 × z6) : (z4 × z5)

The defaults are an SAB Goblin 700 with a 21T motor pinion: main 378 : 3808, tail 468 : 2312.

Type 2 -- tail driven from the main shaft

• Main Rotor Gear Ratio = (z1 × z3) : (z2 × z4)
• Tail Rotor Gear Ratio = z6 : z5

The defaults are a KDS Agile A5 with a 21T pinion: main 357 : 3564, tail 14 : 57.

Any other gearbox: count the turns

For anything else, or to check your numbers:

• Tail ratio -- mark a main blade grip and a tail blade grip, turn the main rotor exactly 10 turns by hand, and count the tail rotor turns. Enter 10 : <tail turns>.
• Main ratio -- the same with the motor and main rotor, if you can see the motor turn. Or work it out stage by stage: multiply all the driving gears' teeth together for the first number, and all the driven gears' teeth together for the second.

Then compare the Rotor Speed shown on the Motors tab with an optical tachometer.