Automated Cruise Flap for Airfoil Drag Reduction over a Large Lift Range

Abstract

A small trailing-edge e ap, often referred to as a cruise e ap or camber-changing e ap, can be used to extend the low-drag range of a natural-laminar-e ow airfoil. Automation of such a cruise e ap is likely to result in improved aircraft performance over a large speed range without an increase in the pilot work load. An important step in achieving the automation is to arrive at a simple approach for determination of the optimum e ap angle for a given airfoil lift coefe cient. This optimum e ap angle can then be used in a closed-loop control system to set the e ap automatically. Two pressure-based schemes are presented for determining the optimum e ap angle for any given airfoil lift coefe cient. 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 coefe cient, this nondimensionalized pressure difference is brought to a predetermined target value by dee ecting the e ap. It is shown that the drag bucket is then shifted to bracket the given lift coefe cient. This nondimensional pressure difference, therefore, can be used to determine and set the optimum e ap angle for a specie ed lift coefe cient. The two schemes differ in the method used for the nondimensionalization. The effectiveness of the two schemesisverie ed using computational and windtunnel results for two NASA laminar e ow airfoils. Finally, an aircraft performance simulation approach is used to analyze the potential aircraft performance benee ts while addressing trim drag considerations.