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Project Final Report – CUDBF April 30 th , 2009 ASEN 4028: Aerospace Senior Projects Static Margin 0.2 0.15 0.1 0.05 0 -0.05 -0.1 -0.15 Static Margin as a function of Taper Ratio and Sweep Angle Sweep Angle of 25 o Sweep Angle of 20 o Sweep Angle of 15 o Sweep Angle of 10 o Sweep Angle of 5 o -0.2 Sweep Angle of 0 o -0.25 -0.3 1 2 3 4 5 6 7 8 9 10 Taper Ratio * 10 -1 Figure 12: Determining Wing Geometry The leading edge sweep angle was narrowed down to between 15 degrees and 25 degrees such that a positive static margin was assured. The choice of configuration is discussed in the Mechanical Design Elements section for aerodynamics. 5.1.2 Airfoil Selection In order to choose the best suited airfoil for a flying wing configuration, several hundred airfoils on the UIUC database as well as the DBF Osborne databases were analyzed. All the airfoils were plotted in the airfoil analysis tool XFOIL [8] . The three best airfoils were chosen based on their drag polars and variance in coefficient of moment with angle of attack. The main driver for the airfoil selection was the coefficient of moment as a function of angle of attack. Since the configuration chosen for this aircraft is a flying wing design, the longitudinal static stability of the aircraft is very important. Flying wings have the tendency to be unstable in pitch; therefore, selecting an airfoil with optimal lift characteristics as well as a coefficient of moment very close to zero was extremely important. The variation in moment coefficient with angle of attack and the variation of the lift coefficient with angle of attack for the three best airfoils are shown in Figure 13. The top three airfoils shown are the HS520, Eppler216 and the HS602. 32
Project Final Report – CUDBF April 30 th , 2009 ASEN 4028: Aerospace Senior Projects Moment Coefficient and Lift Coefficient as a Function of Angle of Attack 2 Re: 844,493 Mach: .0912 Moment Coefficient and Lift Coefficient C m and C l , non-dimensional 1.5 1 0.5 0 hs520 e216 hs602 -0.5 -5 0 5 10 15 20 Angle of Attack, α, degrees Figure 13: Moment Coefficient and Lift Coefficient as a Function of Angle of Attack Figure 14 shows the drag polars of the top three airfoils. The HS520 and HS602 have very similar drag polars. The eppler216 airfoil has a greater increase in drag for the most increase in lift. Table 5 summarizes the airfoil characteristics of the top three airfoils. 2 Re: 844,493 Mach: .0912 Drag Polar Coefficient of Lift, C l , non-dimensional 1.5 1 0.5 0 hs520 e216 hs602 -0.5 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 Coefficient of Drag, C d , non-dimensional Figure 14: Drag Polars for the Top Three Airfoils 33
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