Analytical and Experimental Approaches to Airfoil-Aircraft Design Integration
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2002-06-07
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Abstract
The aerodynamic characteristics of the wing airfoil are critical to achieving desired aircraft performance. However, even with all of the advances in airfoil and aircraft design, there remains little guidance on how to tailor an airfoil to suit a particular aircraft. Typically a trial-and-error approach is used to select the most-suitable airfoil. An airfoil thus selected is optimized for only a narrow range of flight conditions. Some form of geometry change is needed to adapt the airfoil for other flight conditions and it is desirable to automate this geometry change to avoid an increase in pilot workload.
To make progress in these important aeronautical needs, the research described in this thesis is the result of seeking answers to two questions: (1) how does one efficiently tailor an airfoil to suit an aircraft? and (2) how can an airfoil be adapted for a wide range of flight conditions without increased pilot workload?
The first part of the thesis presents a two-pronged approach to tailoring an airfoil for an aircraft: (1) an approach in which aircraft performance simulations are used to study the effects of airfoil changes and to guide the airfoil design and (2) an analytical approach to determine expressions that provide guidance in sizing and locating the airfoil low-drag range. The analytical study shows that there is an ideal value for the lift coefficient for the lower corner of the airfoil low-drag range when the airfoil is tailored for aircraft level-flight maximum speed. Likewise there is an ideal value for the lift coefficient for the upper corner of the low-drag range when the airfoil is tailored for maximizing the aircraft range. These ideal locations are functions of the amount of laminar flow on the upper and lower surfaces of the airfoil and also depend on the geometry, drag, and power characteristics of the aircraft. Comparison of the results from the two approaches for a hypothetical general aviation aircraft are presented to validate the expressions derived in the analytical approach.
The second part of the thesis examines the use of a small trailing-edge flap, often referred to as a 'cruise flap,' that can be used to extend the low-drag range of a natural-laminar-flow airfoil. Automation of such a cruise flap is likely to result in improved aircraft performance over a large speed range without an increase in the pilot work load. An approach for the automation is presented here using two pressure-based schemes for determining the optimum flap angle for any given airfoil lift coefficient. The schemes use the pressure difference between two pressure sensors on the airfoil surface close to the leading edge. In each of the schemes, for a given lift coefficient this nondimensionalized pressure difference is brought to a predetermined target value by deflecting the flap. It is shown that the drag polar is then shifted to bracket the given lift coefficient. This non-dimensional pressure difference can, therefore, be used to determine and set the optimum flap angle for a specified lift coefficient. The two schemes differ in the method used for the nondimensionalization. The effectiveness of the two schemes are verified using computational and wind-tunnel results for two NASA laminar flow airfoils. To further validate the effectiveness of the two schemes in an automatic flap system, a closed-loop control system is developed and demonstrated for an airfoil in a wind tunnel. The control system uses a continuously-running Newton iteration to adjust the airfoil angle of attack and flap deflection. Finally, the aircraft performance-simulation approach developed in the first part of the thesis is used to analyze the potential aircraft performance benefits of an automatic cruise flap system while addressing trim drag considerations.
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Keywords
aircraft performance, airfoil, aircraft, design, newton method, optimization, laminar flow, cruise flap, automatic flap, camber, trailing-edge flap, subsonic, general aviation, thin airfoil theory, UAV, stagnation point, low speed flow, low reynolds number, wind tunnel, NLF, drag polar, drag bucket, XFOIL, low-drag range, design integration, aerospace engineering, aeronautical, airfoil model construction, pressure distributions, trim effects, trim considerations, surface pressure sensing
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Degree
MS
Discipline
Aerospace Engineering
