# Mini Glider Design and Implementation with Wing-Folding Mechanism
## 4 Experimental Work
### Magnet Effects  
First of all, the potential energy stored must be found in the spring by calculating the spring coefficient, k. Then, the appropriate magnet sizes can be determined by calculating the forces depending on the angles that can be seen in the first wing-folding configuration, which is the only spring that is used. Ten different experiments are done to determine k by using the experimental data and curve fitting plot is also given with %1.42 error in Figure 11[31]. In this figure, k is obtained by measuring the spring extension according to ten different weights on the same spring. A precision balance (TRYTE TECHNOLOGY WB3003N, Hunan, China) is used for weight measurement. The difference between the initial and ending positions of the spring was recorded with an optical microscope (Olympus SZX-7, Olympus Corporation, Tokyo, Japan and PointGrey GS3-U3, FLIR Integrated Imaging Solutions Inc., Richmond, BC, Canada). According to the obtained results, the points are combined with the error of 1.42%, so that k can be calculated by the slope of the plot. By using the curve fitting method, the equation of this plot could be derived as \(150.9x+0.2\) by MATLAB(r) and \(k\) is calculated as \(k=150.9\) (N/m). Repulsive force, \(F_{rep}\), can be determined \(F_{rep}=0.6941\) (N) when the elongation of the spring is \(\delta_{x}=4.6\) mm for \(19^{\circ}\) wing angle. \(F_{rep}\) is the maximum force that has to be damped by the folding mechanism with additional neodymium magnets. The attraction forces of these magnets, which will be used to absorb the potential energy and repulsive forces generated during the folding of the wing, should be calculated. This force must be greater than the forces that oscillate the wing during folding. Magnetic attraction force of two block magnets can be seen in Figure 12. In this figure, the magnetic flux is the net number of field lines that are passing through that surface. Because the magnets are conjoined, the flux density could be seen more in the corner regions. Figure 10: Helical path is followed by glider that shows a stable flight during descent. The attraction force between these magnets varies depending on the distance \(\delta_{Z}\), which is between 0 mm and 10 mm. Depending on the distance between the magnets, the magnetic attraction force also decreases. According to the parametric analysis, the force calculation is done when the step size is 0.01 mm. The attraction force graph of magnets can be seen in Figure 13. In this figure, the graph shows that the force is affected by the distance between magnets as an exponential function as expected. Such a consequence is due to the nature of nonlinear behavior in the magnetic field. For this reason, it is very difficult to separate the magnets when they are attached to each other. Figure 11: Experimental results (red dots) of k shows, coefficient of spring with respect to measure masses that are known. Curve fitting (blue line) method k is obtained as an equation to determine average k.  
Figure 12: Two block type magnets, which are acting as a damping element; analysis can be seen. The aim is to determine magnetic attraction force by a COMSOL® Magnetic Fields Module. Figure 13: Magnetic force calculation is obtained by COMSOL® in AC/DC Module. Two neodymium magnets (N48 grade) are positioned in the same center line and parametric analysis is performed. Depending on the distance between the magnets, an exponential force curve is obtained as expected. The maximum magnetic force is calculated as 11.46 N when the magnets are attached. When the wings become parallel to the glider's body after separating from the container, the magnets will also become parallel and attached. In this case, it is necessary to apply the force, which should be greater than maximum magnetic force, in order to have oscillation wings. However, \(F_{m}=11.46\) N is almost 15 times greater than the repulsive force \(F_{rep}\); thus, it is guaranteed that wings can be deployed without repulsive force and oscillation.