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minated and the surface allowed to cool and re-ice before another application of heat would be necessary. Although cyclic de-icing previously had been studied for propellers and jet engine guide vanes, the results of these investigations did not apply to the airfoils that were found on the wings and empennages of jet aircraft. In order to obtain infor- mation on cyclic de-icing systems that could be used by jet transports, the NACA launched a major investigation of the various types of heaters and methods of heating in the early 1950s. The investigation began with an examination of the characteristics and require- ments of de-icing a NACA 62 (2)-216 airfoil by using an external electric heater. Researchers Lewis and Bowden mounted the airfoil, which had an 8-foot chord and a 6-foot span, vertically in the test section of the IRT. Technicians installed an external electric heater on the forward section of the model, extending chordwise a distance of 14.1 percent chord on the upper surface and 23.4 percent cord on the lower surface. The heater consisted of 0.125-inch-wide Nichrome resistance strips, each 0.001-inch thick and spaced 0.0313 of an inch apart. The heating ribbons, enclosed between two layers of neoprene, were connected to a variable cycle timer that allowed researchers to control the heat-on and heat-off periods. A recording wattmeter measured total power input to each heater element. The airfoil also featured a parting strip to facilitate ice removal. The parting strip consisted of a 1-inch-wide spanwise area located near the airfoil stagnation region that was continuously heated. During icing tests, researchers found that quick and complete ice removal, when cyclic heat was applied to the airfoil, could only be achieved with the assistance of the parting strip. In conducting the icing tests, Lewis and Bowden used a variety of air temperatures, airspeeds, angles of attack, droplet sizes, and liquid water content. Their aim was to determine the minimum power output that was required for complete and consistent ice removal. They found that the most important variables in determining power require- ments were temperature and heat-on time. Heat-off time, droplet size, and liquid water content had only a secondary effect, while angle of attack had no appreciable effect. High local application of power and short—less than 15 seconds—heating periods provided the most efficient removal of ice, with a maximum total energy output of only 490,000 Btu per hour.18 While an electric de-icing system accomplished the task of removing ice with low energy outputs, it had numerous disadvantages. It added to the weight of the aircraft, was susceptible to failure by damage to the heating circuits, was costly to maintain, and posed A Golden Age 18 Lewis and Bowden, “Preliminary Investigation of Cyclic De-Icing of an Airfoil Using an External Electric Heater,” NACA RM E51J30 (1952). 51PDF Image | History of NASA Icing Research Tunnel
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