A wind tunnel study of icing effects on a natural laminar flow airfoil

Abstract

Tests were conducted to investigate the aerodynamic effects of aircraft icing on a twodimensional, NLF-0414 airfoil. Unique to this series of tests was the use of both three-dimensional ice castings and two-dimensional ice shapes to study the effects of natural ice on aerodynamic performance over a range of Reynolds numbers from 3 to 10 million and a range of Mach numbers from 0.12 to 0.29. The threedimensional ice castings were made from molds of ice generated in an icing wind tunnel. The two-dimensional ice shapes were made from numerically smoothed tracings of the original ice. Results of the tests showed that, in general, maximum lift coefficient remained unchanged over the range of Reynolds numbers tested for a particular simulated ice shape. When Mach number was increased, the lift coefficient decreased slightly for a particular ice shape. Stall angles changed little when either Mach or Reynolds numbers were varied for a particular ice shape. For the ice shapes tested, the three-dimensional ice castings showed a larger effect on maximum lift and stall angle than the corresponding two-dimensional ice shapes. Introduction While aerodynamic performance data pertaining to clean airfoils are widely available; a relatively small amount of aerodynamic performance data is available for airfoils with realistic in-flight ice accretions. Ice accretions from aircraft icing encounters can only be formed in flight or in icing tunnels. These ice accretions are difficult and expensive to record and replicate accurately. Because of these complications, many efforts to evaluate the aerodynamics of ice contaminated airfoils have used only rough approximations to realistic aircraft ice accretions. These rough approximations have included handmade plaster ice shapes and two-dimensional, machined ice shapes. Handmade plaster ice shapes have been primarily artistic renditions of natural ice. The two-dimensional, machined ice shapes were typically based on ice profiles generated either by a computer icing code or from ice shape traced in an icing tunnel. Aerodynamic performance measurements are not often made in icing tunnels for several reasons. One is the difficulty in exposing the entire model to a uniform icing cloud in an icing tunnel, since the uniform cloud is smaller than the cross sectional area of the tunnel. This results in ice shapes that taper off at either one end or both ends of the model depending upon whether the * Copyright 1999 by the American Institute of Aeronautics and Astronautics, Inc. No copyright is asserted in the United States under Title 17, U.S. Code. The U.S. Government has a royalty-free license to exercise all rights under the copyright claimed herein for Governmental Purposes. All other rights are reserved by the copyright owner. 1 American Institute of Aeronautics and Astronautics (c)2000 American Institute of Aeronautics & Astronautics or published with permission of author(s) and/or author(s)’ sponsoring organization. model is semi-span or full-span. Because force balance measurements of aerodynamic performance are made over the entire model, they are likely to be affected by the tapered ice shapes at the ends of the model. Furthermore, model pressure measurements are all but impossible in an icing tunnel due to ice and water fouling the pressure taps. Finally, icing tunnels typically have higher turbulence values than aerodynamics-only tunnels. Elevated turbulence levels can alter the boundary layer flow around the airfoil, thereby changing the aerodynamic performance of the model. To address these issues, the Icing Branch at NASA Glenn conducted a test program consisting of icing tests in Glenn’s Icing Research Tunnel (IRT) and aerodynamic tests in NASA Langley’s Low Turbulence Pressure Tunnel (LTPT). Records were made of the ice shapes accreted in the IRT using traces and molds. Castings were made of selected ice shapes and attached to a model built for the LTPT. Castings are currently the most accurate method available to record and replicate the complex features of an ice shape. These castings were applied to the entire span of this model, eliminating the tapered ice shape complication present in the IFT. Finally, the model and ice shapes were fully instrumented with pressure taps to permit aerodynamic performance calculations based on model pressures.