What is an Aircraft Gearbox?

Piston engine aircraft are often compared to automobiles in their basic construction, both having shared similar engine types and piston powered operations over the years. Like automobiles, piston aircraft also feature gears, allowing for the engine speed to be changed or synchronized to the propeller speed. Gears are a part of a gearbox, and there are many types of gearboxes that may be found on a typical piston aircraft. In this blog, we will provide a brief overview of the most common gearbox set types, allowing you to best understand their importance and implementation in piston powered aircraft.

Within the engine of a piston aircraft, the most common gearbox set types include accessory, reduction, actuation, tail rotor, auxiliary power unit (APU), transfer, and angular gearboxes. While all are important parts of an engine, the accessory gearbox and reduction gearbox in particular are often essential for proper operations. The accessory gearbox is one within aircraft engines that is capable of driving the various accessories that are essential for proper operations.

With their shaft power, such drives are used for the fuel control unit, lubrication oil pump, starter, generator, tachometer sensor drives, and alternators. As such, engines would not be capable of properly starting, would have inefficient cooling, and lack the ability to produce electricity for standard operations without such parts.

The reduction gearbox assembly, or reduction gear assembly, is a set that is composed of rotating gears which are attached to a power source. Depending upon the output speed requirement of a particular aircraft, the number of gears present within the reduction gearbox set may vary. While there are numerous roles that a typical reduction gearbox plays for an aircraft, there are a few functions that are extremely important.

For one, reduction gearboxes make specific power settings achievable, only coming in second in importance to reliability. Additionally, such gearbox sets also function to limit propeller rotation speed for efficient operations, often generating power by turning at a very high RPM for small displacement engines. Lastly, the reduction gearbox assembly makes it possible to reach an efficient speed when the propeller blade tip reaches its velocity.

When procuring gearbox sets, there are a number of gear types that one may use. As each type features its own advantages and disadvantages, the choice will typically fall on the developer and how they want to execute their design. Helical and spur gears have long been used for piston engines, capable of providing high reliability for many hours of operation. When comparing the two together, however, helical gears tend to have lower efficiency and are less capable of handling axial thrust.

When manufacturing such gears for a gearbox and drive, gearbox antenna, gearbox compressor, or another set, such components are first machined from a gear blank and then are subject to a carburization heat treatment process. After, the gear is then worked to a high degree of accuracy before being inspected for any defects. Once inspection is complete, the gearbox set may be assembled.

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