A favorable pressure gradient is required to maintain laminar flow. Laminar flow airfoils are designed to have long favorable pressure gradients. All airfoils must have adverse pressure gradients on their aft end. The usual definition of a laminar flow airfoil is that the favorable pressure gradient ends somewhere between 30% and 75% of chord.
The airfoil performance data from these tests are have been published in several books that are available in pdf format here. Printed copies published by SoarTech Aero can still be purchased online through a donation to our ongoing wind tunnel test program (see below).
Airfoil Aerodynamics | SpringerLink
The Laminar flow theory dealt with the development of a symmetrical airfoil section which had the same curvature on both the upper and lower surface. The design was relatively thin at the leading edge and progressively widened to a point of greatest thickness as far aft as possible. The theory in using an airfoil of this design was to maintain the adhesion of the boundary layers of airflow which are present in flight as far aft of the leading edge as possible. On normal airfoils, the boundary layer would be interrupted at high speeds and the resultant break would cause a turbulent flow over the remainder of the foil. This turbulence would be realized as drag up the point of maximum speed, at which time the control surfaces and aircraft flying characteristics would be affected. The formation of the boundary layer is a process of layers of air formed one next to the other, i.e.; the term laminar is derived from the lamination principle involved.
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Also available here online and in printed format by SoarTech Aero Publications is the book Airfoils at Low Speeds (SoarTech 8), which includes wind tunnel data on 54 airfoils tested at Reynolds numbers ranging from 60,000 to 300,000. The book contains extensive commentary and analysis. Testing was performed at Princeton University during 1987 and 1988 by Michael Selig, John Donovan and David Fraser. This work was the important precursor to the low Reynolds number airfoil research at UIUC.
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The goals of the UIUC Low-Speed Airfoil Tests (UIUC LSATs) program are to design, analyze and wind tunnel test airfoils for low Reynolds number applications. The UIUC Applied Aerodynamics group has been a leader in this research area with new airfoil designs being applied to unmanned aerial vehicles, small wind turbines, model aircraft and many other applications. Over 200 low Reynolds number airfoils, many of them new designs, have been built and validated in the UIUC Subsonic Aerodynamics Lab 3x4 ft wind tunnel with the results being documented in books, reports, theses, and journal/conference publications.
Hierarchical modeling and optimization of camber …
The use of this airfoil on the would greatly add to the drag reducing concept that was paramount in all design phases of the airplane. The few applications of this foil, prior to this time, had been handbuilt structures which were finished to exacting tolerances. An absolutely smooth surface was necessary due to the fact that any surface break or rough protrusion would interrupt the airflow and detract from the laminar flow theory. Because of the exactness required, the foil had been shelved by other manufacturers due to the clearances and tolerances which are used in mass production. The engineers at North American Aviation (NAA) approached this problem with a plan to fill and paint the wing surface to provide the necessary smoothness. The foil which was used for the Mustang had a thickness ratio of 15.1 percent at the wing root at 39 percent of the chord. The tip ratio was 11.4 percent at the 50 percent chord line. These figures provided the maximum thickness area at 40 percent from the leading edge of the wing and resulted in a small negative pressure gradient over the leading 50-60 percent of the wing surface.