Two-Stage Gear Train Ratios

Having an accurate gear ratio for the main and tail rotor is essential for Rotorflight accurate RPM Filters performance.

Therefore it is best to count the tooth on the main and tail gear/pulley reduction sets and enter the values into the configurator.

This is relatively easy in single stage drive train that are used on most helicopters.

SAB Style Two-Stage Gear Train

The SAB Goblin helis have a two-stage transmission, therefore you have double the amount of gears/pulleys, also the main tail pulley is mounted on the intermediate shaft, which make the tail ratio a little more complicated to determine.

The above pictures shows the configuration of the transmission on the SAB Raw 580, 700, Piuma, Black Thunder 700, etc.

The gears/pulleys tooth count is marked by Z, and is numbered to identify each one.

For the main ratio, Rotorflight requires you to enter the motor pinion and main gear tooth count respectively in the two fields of the Main Rotor Gear Ratio line.

Based on the above picture, enter motor pinion value as (Z1 x Z3) and main gear value as (Z2 x Z4).

For the tail ratio, Rotorflight requires you to enter the tail pulley and front pulley tooth count respectively in the two fields of the Tail Rotor Gear Ratio line.

Also based on the above picture, enter tail pulley value as (Z3 x Z6) and front pulley value as (Z4 x Z5).

The below table shows some gear/pulley tooth counts for some famous two-stage helicopters.

Heli Model	Z1 (Motor Pulley)	Z2	Z3	Z4	Z5	Z6 (Tail Pulley)
SAB Goblin 500 Sport	15 to 24 ( Std 18T )	48	18	62	28	21
SAB Black Thunder 700	18 to 25 ( Std 21T )	60	19	68	37	26
SAB Raw 580	18 to 26 ( Std 22T )	50	14	58	27	23
SAB Raw 580 Nitro	26 to 28 ( Std 26T )	50	14	58	27	23
SAB Raw 700	18 to 25 ( Std 21T )	56	18	69	34	26
SAB Raw 700 Nitro	26 to 28 ( Std 27T )	52	14	58	27	22
SAB Raw Piuma	18 to 26 ( Std 20T )	52	14	58	27	22

For example for the Raw 700 with STD 21T motor pulley, the calculation will be as follow:

• motor pinion = (Z1 x Z3) = (21 x 18) = 378.
• main gear = (Z2 x Z4) = (56 x 69) = 3864.
• tail pulley = (Z3 x Z6) = (18 x 26) = 468.
• front pulley = (Z4 x Z5) = (69 x 34) = 2346.

After entering the value the ratios in the configurator motor page should look like this.

KDS Style Two-Stage Gear Train

The KDS Agile helis also have a two-stage transmission, however the autorotation gear/pulley is mounted on the main shaft, thus the tail ratio calculation will be straightforward.

The above pictures shows the configuration of the transmission on the KDS Agile 5.5, 7.5, A5, and A7.

For the main ratio, Rotorflight requires you to enter the motor pinion and main gear tooth count respectively in the two fields of the Main Rotor Gear Ratio line.

Based on the above picture, enter motor pinion value as (Z1 x Z3) and main gear value as (Z2 x Z4).

For the tail ratio, Rotorflight requires you to enter the tail pulley and front pulley tooth count respectively in the two fields of the Tail Rotor Gear Ratio line.

Also based on the above picture, enter tail pulley value as Z5 and front pulley value as Z6.

The below table shows some gear/pulley tooth counts for these KDS helis.

Heli Model	Z1 (Motor Pulley)	Z2	Z3	Z4	Z5	Z6 (Tail Pulley)
KDS Agile A5	20 or 21 ( Std 21T )	54	17	66	54	14
KDS Agile 7.2	19 to 21 ( Std 19T )	54	20	66	57	12
KDS Agile A7	19 to 21 ( Std 20T )	54	20	66	57	12

For example for the KDS Agile A7 with STD 20T motor pulley, the calculation will be as follow:

• motor pinion = (Z1 x Z3) = (20 x 20) = 400.
• main gear = (Z2 x Z4) = (54 x 66) = 3564.
• tail pulley = Z6 = 12.
• front pulley = Z5 = 57.

After entering the value the ratios in the configurator motor page should look like this.