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Do planes have gears
Nov 11, 2022

It is often wondered if airplanes also have gears like other vehicles? I have always been interested in this issue, and I have consulted the relevant information and shared it with you:


Yes, planes do have gears. Gears help aircraft propellers operate efficiently by changing or matching engine speed to propeller speed.


In this article, aircraft gears will be thoroughly examined to understand their role in aircraft engines. To draw a roadmap, let's examine the type of gearbox we have.


Here, the terms "gear" and "gearbox" are used interchangeably because gears are an integral part of a gearbox.

I. Types Of Aircraft Gearboxes

Do planes have gears(图1)

Transmissions in GE turboshaft engines


Different types of gearboxes can be found in the engines of aircraft. These gearboxes have a special purpose. Types of gearboxes may include attachment, reduction, drive, tail rotor, auxiliary power unit (APU), transmission and angle gearboxes. We will only consider two types of gearboxes.


1. Accessory Gearbox


An accessory gearbox is a gearbox within an aircraft engine that drives accessories that are critical to the operation of the engine or the aircraft on which it is installed. The accessory drive derives its power from a central shaft connecting the turbine and compressor stages of the aircraft engine.


The accessory drive gearbox provides shaft power to aircraft accessories (fuel control unit, starter/generator, lube oil pump, tachometer sensor driver and alternator). Meanwhile, the accessory gearbox and transfer gearbox are similar in that they are both driven by a high-pressure spool and can handle around 500 horsepower.


2. Reducer


A reduction gear assembly, also known as a 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 requirements.


Why use a reducer?


The purpose of using a gearbox in an airplane cannot be overemphasized. Some of its functions are listed below:


To achieve the desired specific power (horsepower per pound of aircraft weight). This is the second most important characteristic of an aircraft engine after reliability.

Efficient operation is achieved by increasing the propeller speed.

Achieve higher propeller speeds in small displacement engines. These types of engines produce more power by turning at very high revs.

Ⅱ.Aircraft Engine Gear Type Considerations

Do planes have gears(图2)

Each gear type has its advantages and disadvantages. Therefore, the choice of gear depends on what the gear designer wants to achieve. Offset helical and spur gear reduction drives have been used in piston engines with ratios less than 7:2.

III. Helical and Spur Gears In Piston Engines


Over the years, spur gears have shown their reliability through their widespread use in several popular aircraft engines operating for countless hours. In addition, many supercharged engine accessory drives have spur gears, especially in supercharger drive unit applications, with boost ratios as high as 12:1. These relatively small gear drives can carry 100s of horsepower to drive the blower and take several acceleration loads.


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


Compared to spur gears, helical gears have two major disadvantages. They are less efficient (high helix angles can cause overheating). Second, helical gears cannot completely eliminate axial thrust. However, the double helical gear drive provides the answer to this thrust control problem. However, double helical gears also suffer from low torque (power) problems due to their small flank width.


In general, piston engines produce very uneven output due to occasional overload and turbulence-induced shock. The number of cylinders, crankshaft geometry, and firing order are important considerations for meeting the degree of non-uniformity. Piston (also known as reciprocating) engines generate roll moment, horizontal, vertical, and torsional vibrations. The torsional component of the output loads the propeller reduction unit (PSRU) gears.


All these challenges faced by piston engines prompted Frank Whittle to propose a radical new design approach that ushered in the era of the jet engine.

IV. Gears In A Jet Engine


Since then, jet engines have evolved into turbofans, which have grown larger over the years to accommodate bigger and better fans. Most modern aircraft are powered by jet engines. Simply put, jet engines convert energy-rich liquid fuel into thrust. Turboshafts, turboprops, turbofans, ramjets, and scramjets are the types of jet engines we have today.


In 2016, Europe's largest aerospace group, Airbus, launched a new product called the A320neo with a geared turbofan engine. Geared turbofans use a planetary reduction gearbox, which increases efficiency and reduces weight. However, some energy is lost as heat in the gear mechanism.


Other types of gears may include:


Herringbone gears (similar to double helical gears, except they don't have the gap separating the two helical faces).

Bevel gears (transmitting power between shafts that intersect at right angles).

Worm gears (transmitting power through right angles on non-intersecting shafts) and hypoid gears.

Next, we consider the gears as propellers.

Ⅴ.Gear As Propeller

Do planes have gears(图3)

The gears that act as propellers help ensure the most efficient power absorption to transfer power and rotational motion from the crankshaft to the other shaft. The gears help the aircraft generate thrust after partially overcoming its weight and drag on it with the help of its propellers.


We will consider two types of aircraft propellers; fixed speed propellers and fixed pitch propellers.

VI. Constant-speed Propellers And Fixed-pitch Propellers


A constant speed propeller (CSP) is a variable pitch propeller that maintains a constant rotational speed by automatically changing its pitch, regardless of airspeed or altitude or engine torque produced.


The advantage of constant speed propellers over fixed pitch propellers is that constant speed propellers provide optimum performance in every phase of flight, from takeoff to landing. This is because fixed-pitch propellers have to make the inevitable compromise between high takeoff and high cruise settings.


In addition, constant speed propellers provide greater fuel efficiency, better deceleration smoothness, less noise at full power, and less stress on the engine.


A constant speed propeller is able to draw in more air by rotating the portion of it along the longest axis of the blade to maintain the most efficient direction of airflow around it. This operation is more suitable for modern jet engines and high performance propeller driven aircraft.


In contrast, a fixed pitch propeller (FPP) has a fixed blade pitch. For smaller engines, FPP is a better choice because the extra weight in the CSP reduces load carrying capacity. Also, with fewer parts, FPPs are less expensive to maintain and lighter in weight.

VII. The Process Of Making Aircraft Gears


Lubricant flow and weight reduction (without compromising durability) are critical when manufacturing gears. The key to producing precision gears is to minimize distortion in order to obtain complex, delicate gear shapes.


The manufacturing process requires the following:


The gears are first machined from so-called gear blanks. To obtain high-precision gears, manufacturers use high-quality gear blanks.

The gear steel then undergoes a heat treatment process by absorbing carbon while the steel changes state in the presence of carbon monoxide. This process is called carburizing. The quenching process is then used for case hardening.

The gears are then machined to an accuracy of one micron.

Finally, the finishing accuracy of the gears is checked using special equipment to determine that there are no undiscovered defects in hidden areas. Gear meshing and cleanliness are also thoroughly checked during the inspection process prior to assembling the gearbox.

In recent years, the aviation industry has sought to achieve a 60-minute "LOL (Loss of Lubrication)" operation. This means that the aircraft should be able to continue flying after the gearbox loses lubrication, especially for commercial aircraft and aircraft performing special field missions. The key to achieving this duration (60 minutes) is efficient cooling.

VIII. Material Used To Make Gears


Carburized steel is the material of choice for aircraft gears. This choice was made because gears in aircraft must perform their best at higher speeds, higher loads and higher surface temperatures than many other gears used in other vehicle engines.

However, AISI 9310 steel is gaining more attention in aircraft gear manufacturing today.

IX. Conclusion


In aviation, efforts have been made to reduce fuel consumption and carbon emissions. To achieve this, progress must be made in the optimization and efficiency of gears.


Gradually, we are getting closer to the advent of advanced geared turbofans and open rotary engines. When such inventions prevail, gears will remain at the heart of aircraft technology. In fact, without gears, there may be no next-generation engine!

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