# Selection Methodology of an Electric Actuator for
## 1 Introduction
### Regulations on Landing Gear Retraction Mechanisms  
There are many design requirements regarding the landing gear retraction mechanism. High reliability, along with safety, minimal weight, low manufacturing cost, and proper maintainability, are all demanded. These necessities are conflicting in several factors. However, some priorities do also exist which are clearly dictated by the airworthiness authorities. The civil aviation regulations/directives that governs the design features for retractable landing gear mechanism, which the designer must comply for the certification and safety purpose states that the landing gear retraction mechanism along with its supporting structure must be designed for maximum of the flight load elements with the gear being retracted. They should be designed to deal with the combination of air loads, friction, brake torque, and inertia occurring for the whole duration of retraction at any airspeed up to 1.6 times the velocity with flaps retracted, moreover aside from the flight loads, they should be able to withstand the loads due to all yawing conditions with the touchdown gear extended. Stability is an other important issue to be addressed while designing landing gear system. The stability characteristics of a system strongly depend on the level of non-linearity and time dependence of the governing equations [10]. While designing, dynamic forces between the tires and ground during high-speed taxiing should be given importance, as they cause instability in high speed performances that can damage the landing gear of the aircraft and its fuselage [11]. It is suggested that, aside from the use of an actuator, there must to be some other mean to maintain the landing gear at extended position during landing and for an aircraft having a retractable landing gear system that cannot be set to extended position manually, there must be some secondary way to extend the landing gear in the emergency occasion due to any possible failure in the normal operation of landing gear retraction system. The aviation regulations further suggest that, if landing gear is retractable, then there should be a position indicator for landing gear in addition to essential switches or there must be use of some other approach to inform the pilot (or ground control station in case of Unmanned Aerial Vehicle) that every gear is securely in its retracted or extended position [12]. For airworthiness authorities, there is no compromise over safety in any sense during the landing gear actuation mechanism design due to the extreme outcomes of failure. From the above evaluation, it has been concluded that at least a two stage power source redundancy level must be incorporated in the system for safety, and it will be enough to satisfy the airworthiness requirements. For actuation purposes of landing gear, the subsequent fail-safe design factor must also be incorporated within the system by using a secondary motor in actuator design, so that:  
* with the help of primary motor, the time requirement for retraction must be fulfilled; and,
* in the case of primary motor failure, the secondary motor must be able to provide enough torque for extending and retracting the landing gear.