# Selection Methodology of an Electric Actuator for
## Proposed Methodology  
The retraction mechanisms of landing gear system is composed of three parts [13]  
* a hardware that contains all mechanical components and landing gears structure;
* an electrical control system including control software; and,
* pilot interface. Designing the retraction mechanism for landing gear system is a highly tedious job and it requires a lot of focus. The kinematics, along with geometry parameters of the landing gear system, are dependent on aircraft design features and, therefore, is specific to every aircraft. In design methodology designer has to follow the constraints that are imposed by the aircraft design in terms of weight, space, location, and mission requirements, etc. Some of the constraints that are offered by the aircraft design in this work are listed below in Table 1. Moreover, the landing gear should be structurally capable of withstanding impact loading at touchdown and remaining in extended position. The loads acting on the landing gear are divided into the following categories: [14]  
* aerodynamic forces;
* externally applied forces i.e., torques;
* mass forces i.e., gravity; and,
* impact loads  
In this work, a comprehensive selection methodology of landing gear actuator has been presented, as per the FAR 23 standards. Various certification requirements for the landing retraction cycle have been discussed in detail and time dependent analytical equations have been derived. The analytical equations are then verified after retraction analysis on MSC ADAMS. The final form of these analytical equations can be utilized in order to calculate various parameters of retraction cycle for any landing gear with similar mechanism and identify the energy efficient actuator that consumes less power. Moreover a methodology has been presented for computational evaluation of impact force after modelling oleo-pneumatic strut on MSC ADAMS. The calculation of impact force is a very critical analysis in designing any retractable or fixed landing gear mechanism. In a retractable landing gear system, it is even more important to calculate the exact force acting on the actuator and its locking mechanism during landing, so that landing gear might not collapse or retract back to fuselage during impact. A kinematic design has been proposed for the retraction mechanisms of a light weight aircraft and analytical equations were developed for the retraction/extension cycle to calculate the stroke and force requirements, as shown in Figure 2. While deriving the analytical equations, the translational velocity of landing gear was modelled as a quadratic function, which means that the lading gear will slowly start to retract and reach its maximum velocity in the first half of the retraction cycle, and then its velocity will start to decrease in the next half. If the retraction cycle was designed in such a way that the velocity increases gradually, a lot of stresses would have been introduced at the strut-actuator due to a sudden jerk at the start of retraction that is not usually considered to be a safe practice during design. The results were then also compared with the MSC ADAMS(r). There is very less possibility that the analytical and computational method can yield exactly the same results, so, usually, a less than 5 percent error can be considered to be acceptable in verifying the authenticity of analytical and computational method [15]. Once the retraction mechanism was developed, spring characteristic, and damping constants of oleo pneumatic shock absorber were calculated in o