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Project Final Report – CUDBF April 30 th , 2009 ASEN 4028: Aerospace Senior Projects skin thickness was calculated for each of the three materials. Using the minimum thickness, wing area, and the material density, a wing skin weight was estimated. The estimated wing skin weights for each material are provided within Table 17. Table 17: Wing Skin Material Comparison Property EPS Foam Balsa Fiberglass Carbon Fiber (w/ the grain) (E-Fiber) (with epoxy) Tensile Strength 16 psi 566 psi 250,200 psi 602,000 psi Young’s Modulus 435 psi 261 ksi 1,595 ksi 14,500 ksi Density 0.0006 lb/in³ 0.0032 lb/in³ 0.0939 lb/in³ 0.065 lb/in³ Weight per wing 0.280 lb 0.0834 lb 0.7737 lb 0.5356 lb It should be pointed out that the Young’s modulus for both fiber-glass and carbon fiber are orders of magnitude higher than balsa, resulting in a minimum required thickness far thinner than what could actually be applied with epoxy. Both fiber-glass and carbon fiber thickness were then estimated as the thinnest possible for the team to apply, approximately 0.06 inches. After computing the skin weights from these thicknesses, it was determined that the much higher density of both composite materials far outweighed the balsa. These thicknesses were the minimum required to meet the strength requirements, and due to physical practicalities the composite materials are over designed for this aircraft. Since wing weight is a significant design driver, balsa sheeting was selected because it satisfies the strength requirements and weighs the least of the three materials. 8.4.3 Wing Stress Analysis Using the Young’s Modulus for the balsa skin, the maximum deflection of the wing is 0.62 inches. This value was then compared later to a COSMOSWorks FEM model in order to validate a proper COSMOSWorks study. The COSMOSWorks simulations were simulated using the half wing span from the Solidworks models [25] . This model varies from the hand calculations due to the balsa sheeting and foam core analysis (rather than just the balsa). Other changes include the removal of control surfaces and the addition of the propulsion motor reinforcement due to its large size. The restraint for the study was defined as fixed at the root and the load was defined by the parabolic 3.5 g lift distribution shown in Equation 19. The COSMOSWorks results for URES displacement is 0.4957 inches. The discrepancy between the hand calculation and COSMOSWorks result is 25%. This error can be attributed to extra stiffness in the wing due to the addition of the foam core. Equation 20 below shows the displacement distribution results for the entire wing half. 86
Project Final Report – CUDBF April 30 th , 2009 ASEN 4028: Aerospace Senior Projects Figure 58: Wing Displacement Distribution Upon acceptance that the run case in COSMOSWorks produced accurate results, stress distributions can be used to determine the maximum stress acting on the wing and if unexpected stress concentration are developed. Figure 59 below shows the Von Mises stress distribution for the wing half. Figure 59: Von Mises Stress Distribution 87
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University of Colorado Design/Build
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