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Unformatted text preview: Section 2 Lift 2D Airfoils & 3D Wings XFOIL Pressure Distribution Plot 30 D.Toohey Lift and Drag • Lift: The aerodynamic force produced by a surface in the direction normal to the velocity vector. • Drag: The aerodynamic force parallel to the velocity vector. S = reference area – wing area ρ = density of air C L = lift coefficient C D = drag coefficient q = ½ ρ V 2 , dynamic pressure 31 D.Toohey Where does lift come from? • Transverse pressure gradient forces fluid to follow prescribed path. – Pressure increases when turned by a concave surface. – Pressure decreases when turned by a convex surface. Pressure increases Pressure increases Pressure decreases 32 D.Toohey Where does lift come from? • Pressures acting on airfoil come from the airfoil shape and the angle of attack of the oncoming air. – When the angle of attack = 0, a symmetric airfoil will see 0 net lift since the pressure distribution is symmetric. – At a positive angle of attack, the pressure gradients are no longer balanced. The flow on the upper whips around the leading edge, and this leads to a more severe pressure change. α = 0 α > 0 33 D.Toohey • More correct definition of lift – Lift is created when a moving fluid is turned by a solid object. Flow is turned in one direction, and lift is generated in the opposite direction. – Both upper and lower surfaces contribute to the turning flow. – Bernouli’s equation holds  lower pressure does lead to increased velocity, but the velocity change is more an effect of lift, and not the cause. – Turns out that the velocity on the upper surface is much faster than what equal transit theory predicts. Common Misconception • Equal transit theory of lift – The upper surface is shaped to provide a longer path than the bottom surface. – Air molecules on the top must move faster to meet up with the molecules that go underneath. – According to Bernouli’s principle, the faster air on top is at a lower pressure than the air on the bottom, and this pressure differential creates lift. INCORRECT 34 D.Toohey Airfoil Definitions NACA 4digit airfoils • 1 st digit: max camber in % chord • 2 nd digit: position of max camber in 1/10 of c • Last 2 digits: max thickness in % chord • ie: NACA 4412 – 4% camber, at 40% chord, 12% thickness Leading Edge Trailing Edge Camber Line Max Thickness Max Camber Chord Length, c Chord Line 35 D.Toohey Thin Airfoil Theory – Important terms • α : angle of attack • α L=0 : angle of attack at zero lift • : vorticity distribution • : induced velocity • U ∞ : Free stream velocity • C l : 2D Lift coefficient • C l α : 2D lift curve slope • C m,LE : Moment coefficient about leading edge • C m,1/4 : Moment coefficient about ¼ chord....
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This note was uploaded on 02/02/2011 for the course MAE 154s taught by Professor Tooney during the Spring '09 term at UCLA.
 Spring '09
 Tooney

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