**2.2 Commercial helicopter transmission**

This gear transmission, studied in depth by White (1998), is sized for two engines, each with a continuous rating of 1200 kW turbine at a nominal speed of 22976 rpm. The main rotor speed is 350 rpm for an overall speed reduction ratio of nearly 66:1. Fig. 3 depicts a plan view of the gear transmission and Fig. 4 is a three-dimensional view showing the gears.

Fig. 3. Arrangement of gear trains between engines.

Fig. 4. Three-dimensional view of the gear train arrangement

The transmission is sized for input of 373 kW at a speed of 8780 rpm. As can be observed in

 First reduction stage. The first stage is a helical gear with a input pinion with 32 teeth and two output gears with 124 teeth each. The gear ratio is 3.875:1, resulting in an output speed of 2256.806 rpm. This is the stage where torque is split between the input

 High torque reduction stage. The output shaft is driven by a gear which is driven simultaneously by two spur pinions, each coaxial to the gear in the first reduction stage. The ratio between the gear teeth is 27/176, so the transmission ratio is 6.518:1, resulting

This configuration results in torque of 9017.56 Nm. being transmitted through two paths.

This gear transmission, studied in depth by White (1998), is sized for two engines, each with a continuous rating of 1200 kW turbine at a nominal speed of 22976 rpm. The main rotor speed is 350 rpm for an overall speed reduction ratio of nearly 66:1. Fig. 3 depicts a plan view of the gear transmission and Fig. 4 is a three-dimensional view showing the gears.

Fig. 2, the transmission has two stages:

pinion and the two output gears.

in an output shaft speed of 347.6 rpm.

**2.2 Commercial helicopter transmission** 

Fig. 3. Arrangement of gear trains between engines.

Fig. 4. Three-dimensional view of the gear train arrangement

Total transmission reduction is achieved by three gearing stages, clearly depicted in Fig. 3 and Fig. 4:


The between-teeth gear ratio is 23/232, so the transmission ratio is 10.087:1, resulting in an output shaft speed of 350 rpm.

This configuration uses double-helical gearing at the output stage to drive the output shaft. The helical pinions have opposing angles, which ensures equilibrium between the axial forces. When a double gear operates on the output shaft, the area of support is twice that of a simple gear. This causes a reduction in contact force, which in turn results in a reduction ratio that is twice that of the simple case, with the corresponding reduction in weight and mechanical load.

Overall, this constitutes a transmission ratio of 65.64:1, with the total torque in the output shaft exercised by each engine of 28818Nm, split between the four pinions that engage the output shaft crown. This calculation is based on estimating overall losses, with each input engine operating independently, of 12%.

One of the main problems in split torque transmission is ensuring equal torque split between the paths. To ensure correct torque split, a long, torsionally flexible shaft is used between the intermediate-stage spur gear and the output-stage helical pinions. Section 4 describes the methods most frequently used to ensure correct torque split between paths.
