Lecture 3

Lecture 3 - Section 3 Aircraft Drag D.Toohey Photo Courtesy...

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Section 3 Aircraft Drag Photo Courtesy USAF 73 D.Toohey
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Two types of drag All types of drag can be included in these two groups Skin friction drag Results from the viscous shearing forces tangential to body’s surface Influenced by Reynold’s number and surface roughness Laminar flow leads to lower skin friction coefficients than turbulent flow Pressure Drag Results from pressure distribution normal to body’s surface Dependent on Reynold’s number, projected frontal area. Laminar flow can lead to an adverse pressure gradient that increases drag In general, both skin friction and form drag are difficult to predict. Usually experimental wind tunnel tests are required to obtain accurate predictions w f f qS D C = 74 D.Toohey
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Laminar Airfoils Laminar airfoils are shaped to prevent or delay the onset of turbulent flow in order to reduce skin friction Laminar airflow is difficult to produce. Airfoil must be very smooth to promote laminar flow For laminar airfoils, favorable pressure conditions only exist for a narrow C L and AoA range, leading to a “drag bucket” on the drag polar. 75 D.Toohey
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Turbulators Although laminar flow leads to reduced skin friction drag, laminar flow has a tendency to separate easier than turbulent flow. Flow separation is usually a lot worse than skin friction drag. Some aircraft intentionally trip the flow to initiate turbulent flow. This thickens the boundary layer and helps delay or prevent flow separation. 76 D.Toohey
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Designing for Drag Reduction Reducing Skin Friction Laminar airfoils can be used, but not always very realistic depending on the materials and construction tolerances being used. Wetted area should be minimized, usually by having the fuselage taper in the back. However, useful volume of fuselage is sacrificed. Can consider optimum fineness ratio for fuselage. However, this usually isn’t very realistic either. Most aircraft fuselages are longer to provide sufficient moment arms for tail control surfaces. Lancair 360 Photo by Omoo 77 D.Toohey
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Designing for Drag Reduction Reducing form drag Make everything as streamlined as possible Retractable landing gear (added complexity) Shorter, thinner wings (leads to increased induced drag) Avoid flow separation Transonic Area Rule At higher speeds, shock waves form on the
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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.

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Lecture 3 - Section 3 Aircraft Drag D.Toohey Photo Courtesy...

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