The landing gear is a very critical subcomponent of every aircraft. Weighing approximately 3% (in some cases up to 6%) of the plane’s Maximum Takeoff Weight (MTOW), landing gears are heavy structures in their own right. Though many of us are used to the retractable landing gears we find in commercial aircraft, not all landing gears come in that form. While it is interesting to note that suspension systems in other vehicles also have landing gears, we’ll be considering only landing gears in aircraft in this article.
A landing gear, also known as an undercarriage, is a complex system of different components that are used by an aircraft for the purpose of takeoff and landing. A typical landing gear consists of various structural members, hydraulics, brakes, wheels, tires, energy absorption components, etc. More often than not, landing gears always contain 2 primary wheels — the nosewheel/tailwheel and the main wheels.
In an aircraft, the landing gear performs quite a number of functions. First of all, it supports or props up the plane while it’s on the ground. It also allows the airplane to take off and land successfully. During takeoff and landing, the plane must taxi along the runway for a certain period of time. The landing gear facilitates this process.
In this article, we’ll be answering some interesting questions about how landing gears work and discussing other fascinating topics that surround aircraft landing gears.
Table of Contents
Evolution of Landing Gears
When landing gears made their way into aviation, they were originally called the alighting gear by many manufacturers such as Glenn L. Martin Company. Landing gears have been around for as long as airplanes existed. The Wright Brothers’ Flyer from 1903 had a landing gear — a fixed wooden landing gear (skids).
Wright Brothers’ Flyer
The retractable landing gear that we have now become accustomed to was first made for Glenn Curtiss’ Triad airplane in 1911. The fact that the plane was amphibious meant that the Triad needed to be able to take off and land on water, hence the need for the retractable wheels.
Glenn Curtiss’ Triad airplane
The very first landing gear design
It may not be very well known but it is true that the first landing gear design was created by the renowned inventor Leonardo da Vinci. Even more remarkable is the fact that his sketch of the Ornithopter (a human-powered flying machine) contained a retractable landing gear (shown below).
Though they made their debut in 1911, retractable landing gears didn’t become a common currency of flight until about 2 decades later in the mid-1930s. In fact, the Douglas SBD-6 Dauntless — a fighter jet that the United States cruised to victory during World War II’s Battle of Midway — used landing gears that retracted inwards. But before then, most WW I’s planes had fixed wheels on a common axle that were held by struts.
Douglas SBD-6 Dauntless
As regards commercial use, Boeing and Lockheed were the pioneers in this field, fielding commercial aircraft with retractable landing gears as early as 1930. To be more specific, the Boeing Monomail and the Lockheed Orion were the first planes that proved their practicality. Since then, many aircraft designers have implemented many performance improvements. To date, a good number of private light aircraft still have a fixed-type landing gear.
Types of Landing Gear
Landing gears are mainly classified according to the arrangement of their wheels. According to this classification, we have 3 major types of landing gears namely:
- The tailwheel-type landing gear;
- The tandem landing gear and;
- The tricycle-type landing gear.
Tailwheel-Type Landing Gear
Tailwheel-type landing gear
This arrangement of wheels on a landing gear is also known as the conventional landing gear because it was commonly used in many early aircraft. The tailwheel-type landing gear has two main wheels forward of the plane’s center of gravity and a small wheel behind to support the tail. The small wheel behind can sometimes be replaced with a skid. A very good example of this arrangement is the Cessna 150.
This type of landing gear arrangement results in a certain angular indent of the fuselage. This resulting slope allows the use of a longer propeller that compensates for older and less efficient engine designs.
Though an older solution, the tail wheel configuration still offers a couple of advantages over the now common tricycle landing gear. The tail wheel, thanks to its much further position from the plane’s center of gravity, supports a lesser portion of the aircraft’s weight. Consequently, this allows it to be both smaller and lighter than a nosewheel.
Also, a tailwheel-type aircraft has an advantage when flying in and out of unpaved runways due to the increased clearance of the fore-part of the fuselage. This is one of the major reasons why conventional landing gears are still used to date.
In the event of a tail wheel failure, the damage of the aircraft is minimal. Tailwheel aircraft are also easier to fit into and maneuver inside some hangars. But they generally experience poorer forward visibility while on the ground.
Tandem Landing Gear
Tandem Landing Gear
The tandem landing gear arrangement can best be interpreted as a bicycle layout. In this arrangement, the aircraft is fitted with two main wheels behind a single nose wheel under the fuselage and a smaller wheel near the tip of each wing. This is the rarest landing gear configuration in aircraft. They are mostly found on sailplanes.
Military bombers like the B-47, the B-52, and the U2 spy plane make use of the tandem gear. Martin also tried out the tandem layout on its first jet bomber, the Martin XB-48. The configuration produced such remarkable results, especially as regards maneuverability, that it ended up on the B-47 Stratojet.
Tricycle-Type Landing Gear
Tricycle-Type Landing Gear
The tricycle-type landing gear is quite similar to the conventional landing gear except that it has a nose wheel instead of the tail wheel. This arrangement is the most common type you’ll find in modern aircraft. Tricycle gear aircraft outshine the other two types in that they are the easiest to take off, land, and taxi.
Unlike the tail wheel, the nose wheel is level with the other two main wheels, meaning the pilot has a visibility advantage when on the ground. Tricycle gear aircraft are also easier to land because the altitude required to land on the main gear is the same as that required in the flare. They are now common in large and small aircraft alike.
The nose gear of a very few aircraft with tricycle-type landing gear is practically uncontrollable. In such an aircraft, differential braking is used to steer the plane while it is taxiing. It is, however, important to note that almost all aircraft have steerable nose gear. Multiple wheels are also used to spread the weight of the aircraft over a larger area. They provide a safety margin should one tire fail.
Landing gears can be further classified into two categories: fixed and retractable landing gears. The difference between these two types is that the former retains its exposed position even when the airplane is in the air. These design changes impact flight in different ways and we’ll get to talk more about this later in the article.
How Does a Landing Gear Work?
The most fascinating elements of how landing gears work are their ability to help bring a fast-moving descending airplane to a state of rest, and also retract and extend in the process (in most cases). In essence, they achieve these by making use of fluids to act as shock absorbers and hydraulic or electrical systems to make them retractable.
When a plane lands, the main landing gears touch down first. The pilot then eases the nose gear unto the runway and uses it to steer the plane. Each landing gear has 3 main parts, which are the struts, the wheels, and the brakes. The most important design requirement of the strut is structural strength. They are made of very high strength materials such as titanium and steel.
The landing gear uses a hydraulic mechanism that can either deploy or retract for takeoff and landing respectively. It also has ultra-high-efficiency shock absorbers to pad the impact of a fast-moving plane hitting the ground.
Each landing gear has several wheels. The number of wheels and the size of each wheel depend on the size and type of aircraft. A typical medium-sized commercial airliner has two wheels on each landing gear. Bigger aircraft can house thrice that number on each landing gear.
Why Do Airplanes Have Retractable Landing Gear?
Retractable Landing Gears
The major reason why most planes have retractable landing gear is in order to reduce the amount of drag acting on the aircraft during flight. In the 1920s, aircraft designers already recognized the need to reduce the drag on an airplane in order to improve its speed, fuel efficiency, and maneuverability. What they did not really understand was the exact cause of drag on planes. While many factors contributed to drag, they needed to figure out which of them were most responsible so as to address them.
In 1927, the American National Advisory Committee for Aeronautics (NACA) opened its new Propeller Research Tunnel (PRT) at Langley Memorial Aeronautical Laboratory in Virginia, USA. With a diameter of about 20 feet (6.1 meters), the PRT was a really big wind tunnel, especially for the time. Unlike the existing structures of the time that could only test plane parts or scale models, it was able to test an entire fuselage with the engine and propeller attached.
The PRT test results revealed that the landing gear was responsible for about 40 percent of the drag on the fuselage. This was a shocking revelation. The engineers and researchers then concluded that reducing the drag caused by the landing gear would in turn impact the plane’s flight performance positively.
There were quite a number of ways to go about reducing the landing gear’s drag. The most obvious ones were to either create a new landing gear design that reduced drag or retract the gear completely into the aircraft. In the subsequent years, especially in the 1930s, many designers opted for the retractable gear. They were willing to accept the other drawbacks of retractable landing gear simply to achieve better performance.
How Do Landing Gears Retract?
Retractable Landing Gear Mechanism
Retractable landing gear can be powered in one of two ways: either electrically or hydraulically. Most large aircraft are powered by hydraulic systems. On the other hand, light aircraft are fitted with different types of retractable landing gears. The simplest contains a lever in the flight deck mechanically linked to the gear. Through mechanical advantage, the pilot extends and retracts the landing gear by operating the lever. The use of a roller chain, sprockets, and a hand crank to decrease the required force is common.
Electrically operated landing gear systems
Electrically operated landing gear systems are also found in light aircraft. All-electric gear systems work by using an electric motor and gear reduction to move the landing gear. The gear is actuated by converting the rotary motion of the motor to linear motion. This type of mechanism is practical only with gears of smaller aircraft because of the gear’s light weight.
In general, the electric motor works through a combination of gears, shafts, adapters, a torque tube, and an actuator screw. The whole gear system is initialized using a switch in the cockpit. The switch has two positions: UP and DOWN. When the switch is in the UP position, force is transmitted to the drag strut linkages. This way, the gear is retracted and locked. This also activates the struts that open and close the gear doors.
Conversely, when the switch is in the DOWN position, the motor’s motion is reversed. As a result, the gear moves down and locks. Once the switch is depressed, the gear motor continues to operate until the moment an up or down limit switch on the motor’s gearbox is triggered.
Hydraulic landing gear retraction system
In order to power a hydraulic landing gear retraction system, pressurized hydraulic fluid is needed to activate linkages which are responsible for raising and lowering the gear. This type of retraction system is also linked to a switch in the cockpit. When the switch is moved to the UP position, the pressurized fluid is sent up into the gear. This fluid then flows through a sequence of connected valves and down-locks the cylinders that actuate the gear. A similar process, only in the opposite direction, is used to extend the gear. Limit switches are used to regulate the amount of hydraulic pressure applied in each case. Each gear has two limit switches — one dedicated to extension and the other to retraction.
What Materials Are Landing Gears Made of?
Landing Gear Material
Due to the intense levels of pressure they encounter when in use, every component of the landing gear must be made of high-quality materials. The common metallic materials used in landing gear structures include steel, aluminum, and titanium alloys.
Components that make use of steel include pistons, braces, links, switch brackets, plugs, axles, shafts, springs, clamps, sleeves, arms (tubes), etc. The main landing gear structure is made from titanium. Most modern brake discs are carbon composite discs. Carbon discs are better than steel at resisting the high-temperature changes in aircraft braking systems. Also, the treads of aircraft tires are made of highly durable rubber.
Can Planes Land Without Landing Gear? (Belly Landings)
As very unlikely as it may seem, airplanes can land without their landing gear extended. But it is almost impossible for this to happen without considerable damage to the aircraft. Of course, there are a few exceptions. When a plane lands without its landing gear, this is commonly called belly landing or gear-up landing.
In more specific terms, gear-up landing refers to situations when the pilot forgets to extend the landing gear. On the other hand, belly landing refers to incidents where a mechanical malfunction prevents the pilot from extending the landing gear.
Belly Landing
Belly landing always results in extensive damage to the aircraft. During belly landing, the plane is at risk of flipping over, disintegrating or even exploding if it lands too quickly or too hard. This type of landing often requires great accuracy and precision from the pilot as the plane needs to land as level as possible while maintaining enough airspeed to stay controlled. Take a look at a good example.
Some common factors that significantly increase the level of risk involved in a belly landing are low visibility, pre-existing damage to the airplane, strong crosswinds, and unresponsive instruments and controls. In general, belly landings make up a rather high percentage of aircraft accidents. Fortunately, they are rarely fatal, especially if executed carefully.
Gear Up Landings
In the case of gear-up landings, even the most careful pilots are at risk, because they may be distracted and forget to perform the checklist or might become interrupted while caring for other duties, such as collision avoidance or other emergencies.
It is required of all aircraft with retractable landing gear to use a system that indicates the status of the landing gear’s position. This is usually a set of lights that change colors from red to amber to green depending on whether the gears are up, in transit, or down respectively. However, there have been a number of cases where the pilot is distracted and forgets to check these lights. This is why aircraft designers now include additional safety systems that limit the possibility of human error.
Mechanical Failure
Another cause of belly landings is mechanical failure. Whether a landing gear is operated electrically or hydraulically, it can still be activated and powered through a number of sources. This is necessary in the event of a mechanical failure. If a landing gear’s power system fails, there’s always an emergency extension system in place. In most cases, this can either be a manual pump or crank, or a simple free-fall mechanism that disengages the uplocks and causes the landing gear to descend and lock under the influence of gravity and/or airflow.
There can however be circumstances when only one landing gear leg fails to extend. In such cases, the pilot may decide to retract the whole gear completely and opt for a belly landing. This choice is based on the logic that it may be much easier to steer the aircraft during rollout without a gear at all than with only one gear missing.
Manufacturing and Assembly of Landing Gears
The aircraft landing gear market is a very vibrant one generating billions of dollars in revenue annually. Some of the major manufacturers driving the market include Heroux-Devtek, CIRCOR Aerospace, Liebherr Group, Safran Landing Systems, Triumph Group, UTC Aerospace Systems, Merill Technologies Group, Magellan Aerospace, and Eaton Corporation.
The process of manufacturing aircraft landing gear involves the development of many closed die forgings, machined components from ultra-high-strength steels, titanium, and aluminum alloys. Precision tolerances are necessary for gear components such as the actuator cylinder, piston, shock absorber parts, and axle.
Heat treatment of parts is also done after rough machining and painting. Product reliability is enhanced through very stringent quality assurance requirements. Other design and manufacturing processes include concept design, detailed design, stress & fatigue analysis, and reliability & maintainability analysis.
Deciding Factors in Landing Gear Design and Development
The major factors that determine the direction of landing gear development include weight, volume, performance, lifespan, lifetime cost, and development time. These factors pose the greatest challenges for landing gear development alongside regulatory safety requirements.
As earlier mentioned, landing gears are significantly heavy structures. Fixed type landing gears have a weight of about 3% of aircraft weight while the retractable ones can weigh as much as 6% of the aircraft’s weight. The major goal of landing gear designers is to reduce the weight of the landing gear without compromising on its functional, operational, performance, and safety maintenance. This is nearly always achieved by the use of higher-strength, corrosion-resistant, lightweight metallic alloys.
Volume or space is also a great design challenge. Retractable landing gears occupy more space than the fixed type. This is due to the fact that they contain more components and mechanisms. By choosing the right materials and geometry, the requirement of the minimum retracted volume is always met.
High performance of the landing gear — especially of the shock absorbers — is required to maintain minimal ground load transmission to the airframe. Through the use of passive orifice damping coupled with a proper metering pin or valve system, landing energy absorption efficiencies can now go as high as 85% to 90%.
Qualification Testing and Airworthiness Certification
Qualification Testing and Airworthiness Certification
The qualification testing of landing gears involves functional tests, structural strength tests, stiffness and fatigue life tests, and environmental tests. Drop tests are conducted on rigs with load cell platform, wheel spinning facilities, and lift simulation devices to ensure adequate shock absorber performance. For structural strength tests, loads are applied using loading actuators in required directions. Strain data is then acquired through strain gauging.
Block-wise loading is used to conduct fatigue tests like impulse fatigue tests on actuators. This type of testing is done with adequate instrumentation needed to acquire all the required data. Endurance cycling tests are carried out in specially designed rigs. Environmental tests which include vibration, acceleration, temperature, salt spray, sand, and dust tests are also performed.
After complete installation on the aircraft, the landing gear undergoes final integration tests accompanied by taxi tests, braking, and steering tests. Fine-tuning of certain design parameters is done during this phase. Landing gear airworthiness certification is regulated differently for aerobatic, civil transport, military airplanes.
Maintenance of Landing Gears
No doubt, landing gears are expensive. A set of landing gears for the Airbus 320 will set you back around 1.8 million USD. These figures increase to about 5.0 million USD for the Airbus 330/340 and you can expect to pay higher to the tune of about 7.0 million USD for the B777. But these gears justify their costs by having an average lifespan of 60,000 hours (up to 20 years) and 20,000 hours between overhauls.
These high costs however still place a high maintenance demand on landing gears. Though they appear very robust and rugged, landing gears are highly vulnerable to corrosion and stress due to the impacts they endure.
Overhauls are a very expensive process. As simple as it may seem, the greatest way to substantially increase the time between overhauls is lubrication. Proper lubrication is crucial for the smooth functioning of the gear parts. It also ensures less friction and wear, and efficient force transfer throughout the gear during use. Lubrication also helps to protect the gear from water, de-icing fluid, and other substances that cause corrosion.
Another great landing gear maintenance practice is to protect it from paint strippers and other equally corrosive materials. This is mostly because of the chemical interaction that occurs between hydrogen and the high tensile steel that is used in making many gear components. Once such acids come in contact with the gear, the hydrogen embrittlement that follows can result in cracks. These cracks are generally very expensive to fix. In the worst scenarios, the cracks may even be large enough to require complete scrapping or replacement of the part. This is if it does not fail during use before then.
While in use, landing gears undergo different kinds of testing called in-service evaluations. They include the appraisal of different types of ambient and airfield conditions. After these tests, reliability and maintainability feedback is collected and used to improve system data generation.
Future of Landing Gears
In landing gear designs and development of the future, there will be extensive use of higher-strength materials, active damping systems, and accurate load estimation techniques. Active damping systems are needed to reduce taxi loads, which are known to be a very significant contributor to fatigue failure. Approximately 50% of the total damages in commercial aircraft were found to be taxi damages.
It is also projected that there will be an increase in the use of composite materials in many landing gear components. Actuator designs will similarly gravitate more toward “more electric” or “all-electric” designs in a bid to replace hydraulic actuation completely.
Summary
In conclusion, landing gears are a highly important part of the airplane. They are sturdy, reliable and have seen many decades of innovation and improvement. In spite of the various challenges, we can expect to see more landing gears that are completely powered electrically.
Recommended Course!
