f21_fall

# f21_fall - Fluids Lecture 21 Notes 1 Airfoil Polar...

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Fluids Lecture 21 Notes 1. Airfoil Polar Relations Airfoil Polar Relations Wing Loading For an aircraft in steady level ﬂight, we have that lift equals weight. 1 L = ρ V 2 S C L = mg 2 1 ρ V 2 = mg 1 2 S C L The ratio mg/S is called the wing loading , and has the units of force/area, or pressure. The lift coeﬃcient C L can be interpreted here as the constant of proportionality between the wing loading and the dynamic pressure. The airspeed is given explicitly by mg 2 V = S ρ C L An important performance measure of many airplanes is their speed range, or their max/min speed ratio. From the above equations we see that V is maximum when C L is minimum, and vice versa. mg 1 1 2 = ρ V min 2 S C L max We can therefore form the ratio V max C L min = C V min C L max so a large speed ratio requires a very large C L ratio. Eﬃcient ﬂight at any particular C L , whether large or small, requires that the corresponding C D is acceptably low. The acceptable L range can be discerned on a

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f21_fall - Fluids Lecture 21 Notes 1 Airfoil Polar...

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