Automated Trailing-Edge Flap for Airfoil Drag Reduction over a Large Lift-Coefficient Range

Abstract

A small trailing-edge flap, often referred to as a “cruise flap” or camber-changing flap, 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 important step in achieving the automation is to arrive at a simple approach for determination of the optimum flap angle for a given airfoil lift coefficient. This optimum flap angle can then be used in a closedloop control system to automatically set the flap. The first part of the paper presents 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 bucket 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 in the paper using computational and wind-tunnel results for two NASA laminar flow airfoils. In the second part of the paper, 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. In the third part of the paper, an aircraft performance simulation approach is used to analyze the potential aircraft performance benefits while addressing trim drag considerations.