# Selection Methodology of an Electric Actuator for
## 1 Introduction  
Landing gear system is one of the essential subsystem for aircraft operations that dampens the impact force during the landing phase of an aircraft by absorbing and dissipating kinetic energy, and it acts as a suspension system during taxi and takeoff mission segments [1]. It ensures that structural loads that are transferred to the air-frame of an aircraft are within allowable limits. The landing gear system also houses braking and steering systems for on ground speed reduction and directionalcontrol. Aside from the essential necessity of landing gear system, they are heavy on aerodynamics performance. They not only produce significant amounts of aerodynamic drag, but also add plenty of weight to the aircraft, which affects the aircraft performance parameters. The design of landing gear systems are constantly evolving not only to reduce aerodynamic drag, but also mitigate their overall negative impacts. Starting from a simple fixed-gear configurations in pre-WWI aircraft to the installation of fairings and, later, to retractable landing gears, their designs have revolutionized aircraft performance. The landing gear system design process is a complex process in which the designer has to overcome many constraints, including minimum volume, minimum weight, short development cycle, and reduced life cycle cost. In addition, various requirements of ground clearance, stability, stiffness, control, and damping of impact force under all possible landing conditions of aircraft also demand significant attention [2]. From an aerodynamics perspective, landing gear design constraints are weight and drag reduction during the design process. The retraction design leads to a significant aerodynamic drag reduction, while the fairings alone design feature also promise large savings on drag, but they add to landing gear's weight and complexity [3]. Small aircraft, mainly unmanned aerial vehicles (UAVs), are opted for fixed landing gears system in order to avoid the potential design complexity and weight penalties that can severally effect the payload capabilities. However, recent research has shown that landing gear drag can be as high as 15-28% of a UAV's total parasite drag. Consequently, the designers now have to weigh the potential benefits of retraction system by avoiding a huge chunk of drag versus the additional weight, which is approximately 10% of aircraft structural weight [4]. Designers are looking for alternate systems to the conventional hydraulic retraction mechanism for reducing the associated weight penalty. The rapid advancement during recent years in the field of electric drives has provided new opportunities in improving the performance of an aircraft at all levels from production, serviceability, maintainability to environmental footprint, fuel burn, and safety [5]. Generally, conventional actuation systems for critical safety applications, like landing gears, are dependent on hydraulic power and need distribution system along with centralized hydraulic power, which make the system very complex. In this situation, electric drives offers a very effective solution to remove the conventional actuation drawbacks by offering new opportunity for performance improvement [5]. Moreover, recent researchers have concluded that electric actuation systems also contribute in weight savings significantly. All of these factors contribute to the reduction of operational cost in comparison with existing aircraft. Electrically driven actuation systems have already been extensively tested and researched for the purpose of actuation in aircraft critical subsystems, like flight control, and are being used on \"Airbus A380\" and \"Boeing 787\" as backup flight control actuators. The Boeing 787 and many other aircraft are also using electro-mechanical actuators for brakes and landing gear locks.