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AE 1350 Lecture Notes #8

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We have looked at.. Airfoil Nomenclature Lift and Drag forces Lift, Drag and Pressure Coefficients The Three Sources of Drag: –skin friction drag in laminar and turbulent flow –form drag –wave drag

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Airfoil Drag Polar C d vs. C l Rough airfoils have turbulent flow over them, high drag. Smooth airfoils have laminar flow over at least a portion of the surface. Low Drag.

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Form Drag Source: http://www.allstar.fiu.edu/aerojava/flight46.htm Form drag may be reduced by proper design, and streamlining the shape.

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Supersonic wave Drag For a given airfoil or wing or aircraft, as the Mach number is increased, the drag begins to increase above a freestream Mach number of 0.8 or so due to shock waves that form around the configuration.

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Shock waves

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How can shock waves be minimized? Use wing sweep. Use supercritical airfoils, which keep the flow velocity over the airfoil and the local Mach number from exceeding Mach 1.1 or so. Use area rule- the practice of making the aircraft cross section area (from nose to tail, including the wing) vary as smoothly as possible.

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How can shock waves be minimized? Use sweep. M= 0.8 0.8cos30 30 sweep

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In your design... The Maximum Mach number is 0.85 Wings for supersonic fighters are designed to reduce wave drag up to 80% of the Maximum speed. In our case, 80% of 0.85 is 0.68. If we use a wing leading edge sweep angle of 30 degrees or so, the Mach number normal to the leading edge is 0.68 cos 30° ~ 0.6

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Effect of Thickness and Sweep on Wave Drag Source: http://www.hq.nasa.gov/office/pao/History/SP-468/ch10-4.htm

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Supercritical Airfoils Their shape is modified to keep the Mach number on the airfoils from exceeding 1.1 or so, under cruise conditions.

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Conventional vs. Supercritical Airfoils

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Wing Drag Since a wing is made up of airfoils, it has –skin friction drag –profile drag –wave drag at high speeds, and –Induced drag due to tip vortices

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TIP VORTICES

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Effect of Tip Vortices Downwash

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Induced Drag Induced drag is caused by the downward rotation of the freestream velocity, which causes a clockwise rotation of the lift force. From AE 2020 theory, e= Oswald efficiency factor

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Variation of Drag with Speed Induced drag decreases as V increases, because we need less values of C L at high speeds. Other drag forces (form, skin friction, interference) increase. Result: Drag first drops, then rises.

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At High Values of Wings Stall We need high C L to take-off and land at low speeds. http://www.zenithair.com/stolch801/design/design.html

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Achieving High Lift

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One form of flaps, called Fowler flaps increase the chord length as the flap is deployed.

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High energy air from the bottom side of the airfoil flows through the gap to the upper side, energizes slow speed molecules, and keeps the flow from stalling. How do slats and flaps help? 1. They increase the camber as and when needed- during take-off and landing.

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Leading Edge Slats Help avoid stall near the leading edge

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High Lift also Causes High Drag

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