AA311.Lecture13.Slides

AA311.Lecture13.Slides - AA311 Atmospheric Flight Mechanics...

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Unformatted text preview: AA311: Atmospheric Flight Mechanics Lecture 13 Dr. Laszlo Techy University of Washington Department of Aeronautics & Astronautics November 02, 2011 Dr. Laszlo Techy AA311: Atmospheric Flight Mechanics Critical pressure coefficient Dr. Laszlo Techy AA311: Atmospheric Flight Mechanics Thin VS Thick Airfoils Thin: The expansion over the top surface is mild; the velocity increases only slightly; the pressure decreases only a relatively small amount in comparison with thicker airfoil profiles. Therefore M cr is higher for thin airfoils. Dr. Laszlo Techy AA311: Atmospheric Flight Mechanics Critical pressure coefficient Clarifying critical pressure coefficient. Recall, as the Mach number is increased, the pressure coefficient, C p will become increasingly negative. The specific value of C p , where the flow becomes sonic is by definition, the critical pressure coefficient : C p , cr . Although not immediately intuitive, there exists a fundamental relationship between free-stream Mach number, and critical pressure coefficient that is rooted in pure aerodynamics, and has nothing to do with the shape, thickness, or angle of attack of the airfoil. This relationship plays a similar role to Bernoulli’s equation for incompressible flow, in that it relates pressure and velocity. To derive that relationship, we need to go back to our findings on isentropic relations. Keep in mind: We are after C p , cr = f ( M ∞ ) Dr. Laszlo Techy AA311: Atmospheric Flight Mechanics Total pressure and other thermodynamic properties Recall the principle of operation of the Pitot tube....
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AA311.Lecture13.Slides - AA311 Atmospheric Flight Mechanics...

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