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Project Final Report – CUDBF April 30 th , 2009 ASEN 4028: Aerospace Senior Projects Table 5: Characteristics of the Top Three Airfoils Analyzed Airfoil % t/c C l max C d (at Cl mx) C m (at cruise) Stall AOA (deg) Eppler216 10.40 1.78 0.0446 -0.1925 13.27 HS520 8.84 1.42 0.0306 -0.0059 13.70 HS602 10.21 1.38 0.0339 -0.101 12.60 5.2 Missions Subsystem Design Alternatives The design alternatives for the Missions subsystem was broken into three separate elements: the wing mounted stores, the centerline store, and the aircraft container. Each of these were vital to the mission of the aircraft and its performance at competition. The design process included a complete understanding of the score sensitivities and the importance of store load times, an analysis of the loads acting on these stores, consideration of multiple design alternatives, testing of those alternatives, an educated selection based on the test results, and finally a successful implementation of the design choice. Based on established requirements, it was imperative the stores load quickly, release reliably, and do not release in flight. 5.2.1 Wing Store Release Mechanism From the sensitivity analysis, the loading of these stores has a large effect on the aircraft’s score at competition. This loading time factors into both the System Complexity Factor score of the aircraft through its effect on assembly time, and wing store loading time is measured directly for the score in Mission 3 (the heaviest weighted mission). It was therefore important that the release mechanisms for these stores have the ability to load quickly, and this is a key design driver. It was also important the store release on the ground and not while in flight. To this end, the release mechanism must release 95% of the time while being able to constrain the store under flight conditions and loads. These flight conditions include a 3 g force in the aircraft’s Z-axis and a 1.73 g lateral force. These design-to loads were determined from Figure 15. Figure 15: FBD and Summary of Equations Calculating Centripetal Force on Wing Stores 34
Project Final Report – CUDBF April 30 th , 2009 ASEN 4028: Aerospace Senior Projects This was important information for the brainstorming phase when several conceivable ideas for releasing the stores were generated. These included a sliding pin design, a tab-spring design, a magnetic design, a rotating platform design, and a sliding trigger design. The sensitivity analysis was applied, and the pin and rotating platform designs were eliminated because of their projected high load times. The magnetic, spring-tab, and sliding trigger designs were found to be the two best designs because they could be loaded the fastest and both stood a high probability of success of being able to keep the store secure. 5.2.1.1 Magnetic Design The concept of using magnets to attach the store had several drivers. The first and largest is that this design had the potential for putting a large amount of the release mechanism weight into the store itself. The store must weigh 1.5 lbs. Therefore, much of the release mechanism could be directly integrated into the store itself (rather than into the aircraft). With the magnetic system, this works well as the magnets are the heaviest portion of this release mechanism and will be mounted to the store. Another advantage of this system is that competition rules stipulate that the release mechanism have no moving parts within the store. Magnets are an effective solution for this because they are solid state. A concept of this can be observed in Figure 16. Figure 16: Preliminary Design of Magnetic Release Mechanism Another driver was the amount of free play present in the release mechanism. Free-play would be disastrous to the stability of the aircraft as additional forces and vibrations in flight would affect all other systems. It was predicted that the magnetic design had the potential of keeping the store rigidly attached in all directions. The magnetic attraction to the underside of the wing keeps the store restrained in the aircraft’s Z direction. Because magnetic attraction is significantly weaker in the shear direction, restraining tabs will be placed on the underside of the wing that keeps the store from sliding off by restraining it in the X and Y directions. 5.2.1.2 Tab-Spring Payload Design The tab-spring payload release mechanism was designed in order to deploy two rockets using one servo while still allowing fast loading. In order to decrease load time, it was decided that a spring system would need to restore a beveled aluminum strip. Therefore, the pin could retract 35
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