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lec11 - 11 FINS AND LIFTING SURFACES Vessels traveling at...

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11 FINS AND LIFTING SURFACES Vessels traveling at significant speed typically use rudders, elevators, and other streamlined control surfaces to maneuver. Their utility arises mainly from the high lift forces they can develop, with little drag penalty. Lift is always defined to act in a direction perpendicular to the flow, and drag in the same direction as the flow. 11.1 Origin of Lift A lifting surface is nominally an extrusion of a streamlined cross-section: the cross-section has a rounded leading edge, sharp trailing edge, and a smooth surface. The theory of lifting surfaces centers on the Kutta condition, which requires that fluid particle streamlines do not wrap around the trailing edge of the surface, but instead rejoin with streamlines from the other side of the wing at the trailing edge. This fact is true for a non-stalled surface at any angle of attack. Since the separation point on the front of the section rotates with the angle of attack, it is clear that the fluid must travel faster
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