On a Thursday morning (December 17, 1903) at Kitty Hawk beach in North Carolina, USA, Orville and Wilbur Wright made the first powered, heavier-than-air flight. To date, everyone remembers them every time we talk about airplanes. But unfortunately, nobody talks about Charlie Taylor, the Wrights’ mechanic who made the beach experience possible by building an internal combustion engine from aluminum castings, thereby creating a light and powerful device propelling the plane. The thought often then crosses the mind as to whether airplanes have gears like other vehicles too?

Yes, airplanes do have gears. Gears help airplane propellers run efficiently by changing or mating the engine speed to the propeller speed.

In this article, aircraft gears will be thoroughly examined to see how important their role is in the aircraft engine. To chart the course, let’s examine the types of gearboxes we have.

Here, the words, “gear” and “gearbox” are used interchangeably because gears are a component of the gearbox.

Types of Airplane Gearboxes

There are different types of gearboxes that can be found in the engine of an airplane. These gearboxes serve peculiar purposes. Types of gearboxes may include accessory, reduction, actuation, tail rotor, auxiliary power unit (APU), transfer, and angular gearboxes. For the sake of this article, we shall consider only two types of gearboxes.

Accessory Gearbox

The accessory gearbox is a gearbox within airplane engines that drives accessories that are essential for the operation of the engine or aircraft on which it is mounted. The accessory drive gets its power from a central shaft linking the turbine and compressor stages of the airplane engine. 

The accessory drive gearbox provides shaft power for airplane accessories (fuel control unit, starter/generator, lubrication oil pump, tachometer sensor drives, and an alternator). Meanwhile, the accessory gearbox and a transfer gearbox share a similarity in that they are both driven from the high-pressure spool and can handle about 500 horsepower.

Reduction Gearbox

The reduction gear assembly, also known as the reduction gearbox, consists of a set of rotating gears connected to a power source. The number of gears used in the reduction gearbox depends on the output speed requirement.

Why Use a Reduction Gearbox?

The purpose of using a reduction gearbox in airplanes cannot be overemphasized. Some of its functions are hereby enumerated below:

• To achieve the required specific power (the number of horsepower per pound of aircraft weight). This is the second most important property of an aircraft engine after reliability.
• To achieve efficient operation by limiting the propeller rotation speed.
• To achieve efficient propeller speed in small displacement engines. These types of engines generate more power by turning at very high RPM.
• To achieve efficient speed which would have been impossible when the speed of the propeller blade tip reaches the tip speed/velocity  (this is the helical velocity of the propeller, which is the vector sum of rotational and translational velocities) of sound.

Consideration of Gear Types in Aircraft Engines

Every gear type has its own merit and demerit. Therefore, the choice of gear depends on what the gear designer has in mind to achieve. Offset helical and spur gear reduction drives have always been used in piston engines where transmission ratios are lesser than 7:2. 

Helical and Spur Gears in Piston Engines

Spur gears have shown their reliability over the years by their wide range of use in several popular aircraft engines that operate uncountable hours. Also, the accessory drives of many supercharged engines had spur gears especially in the application of supercharger drive units, having step-up ratios up to 12:1. These relatively small gear drives carry several 100 horsepower driving the blowers and are subjected to several accelerated loads.

In addition to the widespread use of spur gears in piston engine applications, they are also used in many propeller reduction units of turboprop engines. However, the main problem with spur gears is energy density.

Helical gears have two main disadvantages when compared to the spur gears. They have poorer efficiency (high helix angles cause overheating). Secondly, helical gears cannot be fully trusted with axial thrust. However, double helical gear drives provide the answer to this thrust control problem. But, because of its smaller face width, double helical gearing also has its own problems with low torque (power).

In general, a piston engine produces a very uneven output because of the occasional overloads and shock due to turbulence. The number of cylinders, the crankshaft geometry, and the firing order are vital considerations to cater to the degree of unevenness. A piston (also known as reciprocating) engine generates sway moments, horizontal, vertical, and torsional vibrations. The torsional component of the output loads the propeller speed reduction unit(PSRU) gears. 

All these challenges with the piston engine prompted Frank Whittle to come up with a radical, new design approach that ushered in the jet engine age. 

Gears in Jet Engines

The jet engine has since evolved into the turbofan which has grown larger over the years to take bigger and better fans. Most modern planes are now powered by jet engines. A jet engine in simple language converts energy-rich, liquid fuel into a pushing force called thrust. Turboshafts, turboprops, turbofans, ramjets, and scramjets are types of jet engines we have today. 

In 2016, the geared turbofan became available on Airbus, Europe’s biggest aerospace group, with a new product called A320neo. A geared turbofan uses a planetary reduction gearbox thereby increasing efficiency and reduced weight. However, some energy will be lost as heat in the gear mechanism.

Other types of gears may include the: 

• Herringbone gear (similar to the double-helical gear but only that they do not have a gap separating the two helical faces).
• Bevel gear (transmits power between shafts that intersects at right angles).
• Worm gear (transmits power through right angles on non-intersecting shafts) and hypoid gears. 

Next, we will consider gears as propellers.

Gears As Propellers

Gears as propellers help to secure the most efficient power absorption to transmit power and rotary motion from the crankshaft to another shaft. Gears help an aircraft to generate thrust after partially overcoming its weight and the drag acting on it by the aid of its propeller.

We will consider two kinds of propellers for aircraft; a constant-speed propeller and a fixed-pitch propeller.

Constant-speed Propeller Vs Fixed-pitch Propeller

A constant-speed propeller (CSP) is a variable pitch propeller that can maintain a constant RPM by automatically changing its blade pitch, regardless of the airspeed or altitude of flight or the engine torque being produced. 

The advantage of the constant-speed propeller over the fixed-pitch propeller is that from takeoff to landing, the former can deliver optimal performance at each of the flight phases. This is because fixed pitch propellers must make an inevitable bargain between high takeoff and high cruise settings, in the pitch 

Additionally, Constant-speed propellers offer improved fuel efficiency,  better deceleration smoothness, lesser noise when fully powered, and reduced strain on engines. 

A constant-speed propeller by its partial rotation along the longest axis of the blade is able to take a larger bite of air to maintain the most efficient orientation to the airflow around it. This operation better suits modern jet engines and high-performance, propeller-driven aircraft. 

Conversely, a fixed-pitch propeller (FPP) has its blade pitch fixed. For smaller engines, FPPs are a better choice because the extra weight in CSPs diminishes the load-carrying capacity. Plus, FPPs are cheaper to maintain and have lighter weight owing to having fewer parts.

Process of Manufacturing Aircraft Gears

In manufacturing gears, the flow of lubricants and weight reduction (without compromising durability) are of the utmost importance. The key to the production of accurate gears is minimizing deformation so that the complex, delicate shapes of the gears can be achieved.

The manufacturing process entails the following:

• The gear is first machined from what is called a gear blank. To get a high precision gear, manufacturers use a high-quality gear blank.
• Then the gear steel undergoes a heat treatment process by absorbing carbon while the steel changes state in the presence of carbon monoxide. This process is called carburization. The quenching process follows for surface hardening.
• The gear is then worked to an accuracy of one micrometer.
• Lastly, the gear is inspected for finishing accuracy using special equipment to ascertain no defects in hidden areas go undetected. Gear meshing and cleanliness are also thoroughly checked during inspection before assembling the gearbox.

In recent years, the aviation industry has been looking at achieving a 60-minute “LOL (loss of lubrication)” operation. This means aircraft should be able to continue flying after the loss of lubrication in the transmission, particularly for commercial aircraft and aircraft on special field missions. The key to achieving such duration (60 minutes) is efficient cooling.

Materials Used in The Manufacturing of Gears

Carburised steel is the material of choice for gears in aircraft. This choice is made because gears in aircraft have to perform maximally under higher speeds, higher loads, and higher surface temperatures than many other gears used in other vehicle engines.

However, AISI 9310 steel has gained more prominence in the manufacture of gears for aircrafts today.

Conclusion

In aviation, perpetual efforts are being made to reduce fuel consumption and carbon emissions. To achieve this, there must be progress on the optimization and efficiency of gears. 

Gradually, we are drawing closer to the emergence of advanced geared turbofans and open rotor engines. When such inventions become prevalent, gears will still remain at the core of aircraft technology. In truth, without gears, there may be no next-generation engines!