6 Incompressible Flow Over Finite Wings EAS 4101 S11

# 6 Incompressible Flow Over Finite Wings EAS 4101 S11 - EAS...

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EAS 4101, S11 University of Florida 2/23/11 1/25 Section 6, Incompressible Flow Over Finite Wings 6 Flow Over Finite Wings In the period 1912-1918, Ludwig Prandtl showed that the aerodynamics of wings could be split into two parts: o The study of the section of a wing – an airfoil (infinite wing, no tips) We just completed this section “Introduction Thin Airfoil Theory” o The modification of such airfoil properties to account for the complete, finite wing We continue our discussion focusing now on finite effects of wings and how they modify airfoil performance properties. 6.1 Wingtip Vortices Experimentally, researchers have noticed that the flow over finite wings greatly differs from that of flow over an airfoil (i.e., infinite span). The pressure difference between the top and bottom of the wing (required for lift) generates wing tip vortices that modify the flow. In our treatment of finite wings, we will determine the effects of these wing tip vortices and their relevant aerodynamic properties, but we first need a physical picture of what is occurring.

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EAS 4101, S11 University of Florida 2/23/11 2/25 Section 6, Incompressible Flow Over Finite Wings 6.1.1 Physics The pressure imbalance drives the flow on the bottom of the wing to curl around to the top generating a wing tip vortex . The pressure imbalance also causes span wise flow on the top and bottom of the wings. On the top of an unswept wing, the streamlines gradually bend towards the root. On the bottom, they bend towards the tip. The wingtip vortices induce a downward velocity component of velocity component behind the wing. This is called downwash, () wy . 6.1.2 Effects Downwash in turn, modifies the incoming freestream velocity to produce a local relative wind. The modification of the local relative wind effectively decreases the angle of attack and induces drag 6.1.3 Practical Issues and Problems Wake Turbulence is a term pilots use for the hazardous effects of wing tip vorticies. Planes taking off (or landing) after another plane has taken off (or landed) may experience extreme induced roll or yaw rates. They limit the minimum time between planes taking off (landing) at airports Here is a website that discusses these effects from a pilot’s perspective http://www.pilotfriend.com/safe/safety/wake_turb.htm Here is a cool picture of a C17 that released some flares that ended up being wonderful flow visualization tools for the wing-tip vorticies
EAS 4101, S11 University of Florida 2/23/11 3/25 Section 6, Incompressible Flow Over Finite Wings http://antwrp.gsfc.nasa.gov/apod/ap060822.html Vortex drag - even in ideal 3 I flow, drag exists for finite wings (d’Alembert does not hold) 6.2 Downwash Downwash modifies the freestream velocity at the airfoil Occurs whenever 1 M (due to particle “communication” of downstream motion, which occurs at sonic speed)

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EAS 4101, S11 University of Florida 2/23/11 4/25 Section 6, Incompressible Flow Over Finite Wings Nomenclature o = geometric AOA o i
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## This note was uploaded on 09/05/2011 for the course EAS 4101 taught by Professor Sheplak during the Spring '08 term at University of Florida.

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6 Incompressible Flow Over Finite Wings EAS 4101 S11 - EAS...

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