Landing gear system is one of the critical subsystems of an aircraft and is often configured along with the aircraft structure because of its substantial influence on the aircraft structural configuration itself. Landing gear detail design is taken up early in the aircraft design cycle due to its long product development cycle time. The need to design landing gear with minimum weight, minimum volume, reduced life cycle cost and short development cycle time, poses many challenges to landing gear designers and practitioners. These challenges have to be met by employing advanced technologies, materials, analysis methods, processes and production methods. Various design and analysis tools have been developed over the years and new ones are still being developed.
Overviewing landing gear design & development
The landing gear design and integration process encompasses knowledge of many engineering disciplines, such as structures, dynamics, kinematics, fluid mechanics and runway flotation. The geometry, flotation requirements, mission requirements and operational requirements of the aircraft govern the landing gear configuration. The configuration design includes choice of number of wheels, tyre sizes, pressures, type of shock absorbers, landing gear layout, retraction kinematics and bay geometry design. Airworthiness regulations play a crucial role in arriving at the landing gear configuration, such as sink rate, allowable load factors and ground maneuvering conditions, stipulated in the applicable airworthiness regulations.
A brief summary of various life cycle stages of landing gear design and development are described ahead:
Concept design: The concept design starts with a study of all design specifications and airworthiness regulations. A concept is then evolved, while meeting the functional and regulatory requirements. Major design drivers are performance, safety, cost, time frame, technology and resources. The landing gear location is arrived at and type of landing gear is selected.
Preliminary design: In the preliminary design phase, dynamic simulations are carried out for landing, take off and retraction kinematics to arrive at data required for sizing of components and material selection. Preliminary design of components is performed and weight estimates are arrived at.
Detailed design: In this phase, the detailed design of all the landing gear components is performed and an integrated landing gear system is defined with all interfaces and associated systems. Component loads are estimated and material selection and sizing are done in this phase.
Stress & fatigue analysis: Finite element modeling and analysis and conventional hand calculation methods are used for landing gear stress analysis. Landing gear is designed as a safe life structure and fatigue analysis methods are used for prediction of life.
Reliability & maintainability analysis: Proper failure mode and effect and criticality analysis (FMECA) is performed to assess reliability. Data on failure modes and failure rate are collected from previous designs to conduct this analysis and reliability is predicted before the design freeze. The design aims at increased mean time between failures (MTBF) and reduced mean time to repair (MTTR).
Manufacturing & assembly: Landing gear manufacturing involves development of many closed die forgings, machined components from ultra-high strength steels, titanium and aluminium alloys. Precision tolerances are required for components like actuator cylinder, piston, shock absorber parts and axle.
Qualification testing: The qualification testing of landing gears involves functional tests, structural tests for strength, stiffness and fatigue life tests, and environmental tests. Platform drop tests are conducted on rigs with load cell platform, wheel spinning facility and lift simulation devices to verify shock absorber performance. Fatigue tests including impulse fatigue tests on actuators, are conducted by block wise loading with sufficient instrumentation for data acquisition. Endurance cycling tests are conducted in special rigs. Environmental tests including vibration, acceleration, temperature, altitude, salt spray, sand and dust etc. are performed.
On- aircraft testing: The final integration tests of the landing gear are carried out after installation on the aircraft followed by taxi tests, braking and steering tests. Fine tuning of certain design parameters are done during this phase. This is followed by flight testing phase where the capability of the landing gear is evaluated.
In-service evaluation: This includes evaluation in various types of airfield conditions and ambient conditions. Feedbacks on reliability and maintainability results are taken for further improvements in the system and data generation.
Challenges in design & development
The need to design landing gear with minimum weight, minimum volume, high performance, improved life and reduced life cycle cost poses many challenges. These challenges are met, while adhering to all regulatory requirements of safety, by employing advanced technologies, materials, processes, analysis and production methods.
Weight: Landing gear varies its weight from 3% of aircraft all-up weight for a fixed type to about 6% for a retractable type landing gear. The challenge is to reduce the weight of the landing gear without compromising on its functional, operational, performance, safety and maintenance requirements. This is made possible by using materials of higher strength, fracture toughness, fatigue properties and by making correct choice for each application.
Volume: Space is one of the most important constraints within which an aircraft component needs to be designed, especially in a military aircraft. A retractable landing gear contains more components and mechanisms than a fixed landing gear.
Performance: High performance of the landing gear is expected in order to reduce the ground loads transmitted to the airframe. This is ensured by accurate dynamic analysis and simulation to arrive at key performance characteristics like orifice sizing, air and oil volumes. Efficiencies as high as 85% to 90% are achievable in landing energy absorption with passive orifice damping with proper metering pin or valve system.
Life: Long life and minimum maintenance requirements are vital for reduction in operating and maintenance costs, while minimising the overall life cycle cost. This dictates the choice of materials, its corrosion properties and fatigue properties.
Development time: The landing gear design is iterative involving trade-off studies between various configurations and their impact on weight and cost benefits. This usually takes substantial time and effort. It is essential to reduce this product development cycle time by automating the design process using CAD/CAE/CAM tools.
Life cycle cost: Use of advanced technologies like health management systems and maintenance philosophies helps in reducing the life cycle costs. Design for condition based maintenance instead of scheduled maintenance is one such trend and is compatible with health management systems.
Landing gear technologies
Landing gear technologies are continuously evolving to meet the challenges of functional and non-functional requirements. Some of these important technologies are as follows:
Steering system: Steering control systems are moving towards electronic control systems replacing hydro-mechanical systems. The main advantage with electronic control system is its accuracy and its ability to incorporate changes in design parameters like steering rate and steering ratio with ease.
Actuation system: In actuation systems, more electric or all electric systems are replacing the conventional hydraulic systems. The electric systems offered today have become weight competitive with use of brushless high power motors.
Brake system: Electronically controlled anti-skid brake management systems are replacing old mechanical or electric anti-skid systems. Electronic systems are more efficient and trouble free.
Tyre: Radial tyre is one of the advanced technologies employed in aircraft for the past 25 years. Landing gear radial tyres offer lighter tyres with longer life compared to bias ply tyres.
Up-locks: Hydro-mechanical locking systems and proximity switches are replacing mechanical locks and micro-switches. They have higher reliability.
Materials: Composites are being used in some components of landing gear because of their superior specific strength and stiffness properties. The choice of material for a landing gear component is decided depending on its application and this requires trade off studies of strength, stiffness and cost to arrive at the optimal choice.
Corrosion protection: Good corrosion protection is important for the landing gear components as they are susceptible for easy environment attack. Apart from normal electrolytic finishes like cadmium plating, hard chromium plating, HVOF etc. epoxy or polyurethane primer and polyurethane top coats are applied for the exposed landing gear parts.
CAX technologies: Many commercially available CAD/CAM/CAE/CFD and dynamic simulation software tools are used in the design and development of landing gear. These tools have helped in virtual product development of landing gear before actual prototype is being fabricated.
Knowledge Based Engineering (KBE): Many KBE tools and information intelligence tools are being developed and used by landing gear designers to automate many engineering processes, while retaining company specific knowledge.
Dynamic simulation: Dynamic simulation helps to predict the performance of a component or assembly. The results of these simulations will be more accurate compared to hand calculations. These simulations help in handling large number of studies in short time.
Health monitoring: Landing gear is a maintenance intensive system of the aircraft next to only engine. Health monitoring of landing gear is gaining importance as suitable sensors and processing units are available today. Wireless sensor network and RFID technologies are being employed in health management of aircraft systems and structures including landing gear.
The future landing gear design for aircraft poses many new challenges in configuration design, use of materials, design and analysis methods. These challenges can be met, while adhering to all regulatory requirements of safety, by employing advanced technologies, materials, analysis methods, processes and production methods. By applying functional simulation and developing design tools, the development time and cost are reduced considerably. Use of higher strength materials, composites, and technologies like active damping control, electric systems, along with CAX, KBE and health monitoring technologies will steer the landing gear design in the days to come.
The article is reproduced with courtesy to Infosys