Power speed reduction units for general aviation part 4: Simplified gear design for piston-powered, propeller-driven "heavy duty" aircrafts and helicopters

Bending fatigue (Strength) or surface compression/lubrication (Hertz stress) and scuffing resistance define aerospace gearing design and optimization. In addition, a correct design method must include adequate ability to resist all these types of failures. Of all the failures modes, tooth bending has the most severe consequences, whereas pitting and scuffing are durability-type failures that can be (theoretically) anticipated and corrected before final failure. However, in helicopter transmission pitting and scuffing are the main failure cause. Therefore, it is important for the designer to understand the criticalities of the different application. The third paper dealt with the general problem of designing the PSRU (Power Speed Reduction Unit) gear drives on a general aviation propeller-driven aircraft. This fourth part deals with aerobatic/racing/STOL-utility "heavy duty" aircrafts and with helicopters. In the first part of this paper, a verification method of a "general aviation PSRU" for a heavy duty aircraft is introduced. Then helicopter transmissions are discussed, starting from the most suitable gear types, the transmission architecture and the main problems. Bevel gearing are briefly introduced by defining design criteria, suitable materials properties and selection method. Then the flash temperature concept is briefly summarized along with experimental data on the most advanced steel alloys available on the market.