Rotor Wake/Fixed Wing Interactions with Flap Deflection

The basic features in the interaction of a rotor wake with a wing in low-speed forward flight are studied using low-speed wind tunnel experiments. The configuration captures several of the aerodynamic interactions in the tiltrotor transition phase, and during wake/empennage interactions. Previous work showed that the pressure field on the wing surface below the rotor is dominated by n-per-rev "blade passage", while the velocity field is dominated by onceper-rev repetition of the vortex geometry, due to vortex interactions. The effect of deflecting trailing edge flaps is studied here. Large area SCV is used to enable velocity field acquisition at various sections and test conditions. Flap deflection modifies the spanwise flow on the wing surface, and causes an apparent lateral shift in the wake interaction. This shift influences the effectiveness of inboard vs. outboard flaps, and opens possibilities for augmenting rolling moments. The paper describes both a unique capability for scanning several cross-sections of a periodic velocity field during such interaction conditions, and the correlation of data from velocity, pressure and force measurements to synthesize the nature of the complex flowfield with its multiple periodicities. INTRODUCTION 1 During hover and transition, wake-induced download on the wings of a tiltrotor aircraft is mitigated, and lift is enhanced, by deflecting wing trailing edge flaps. Wake / lifting surface interactions are also important in predicting empennage buffeting. The experiment described here is a basic test case of rotor wake/ lifting Presented at the American Helicopter Society 55th Annual Forum, Montreal, Canada, May 25-27, 1999. Copyright 1999 by the American Helicopter Society, Inc. All rights reserved. surface interaction, where various phenomena are visualized, isolated, quantified, and modified. Figure 1 shows the relation between the tiltrotor case, and the basic full-span wingrotor configuration in the wind tunnel. The retreating blade side on the wing surface is analogous to the wing of the tiltrotor. The rotor-wing set-up in the John Harper wind tunnel at Georgia Tech is shown in Figure 2. A full-span NACA0021 wing with 0.4 m chord at a 0 degree angle of attack is centered about the rotor axis. It is mounted on a stand below a 0.914 m diameter, two-bladed teetering rotor. The rotor is mounted on the tunnel roof with its hub at 0.127 m upstream of the wing leading edge and centered at mid-span. The rotor hub is at a height of 0.4191 m above the wing centerline. Two trailing edge flaps were used in this experiment. The first was a fullspan cambered flap with a 0.125 m chord is attached to the trailing edge of the wing. The second was a flap system, consisting of 4 computer controlled NACA0012 flap segments with a 0.127 m chord. Computer control of the flaps allowed them to be independently deflected to various angles from outside the windtunnel. The rotor was run at 1050 RPM and an advance ratio of 0.075 was maintained by keeping the tunnel freestream steady at 3.77 m/s. The conceptual difficulty of transposing results to the tiltrotor, due to the presence of the wing under the advancing blade side (ABS), is balanced by the efficiency of understanding what the rotor wake does when it interacts with an easily-modeled full-span lifting surface. Likewise, the wake vortex system behavior is better understood by considering a 2-bladed, untwisted rotor even though there are no 2bladed tiltrotor craft under official consideration. Future load-modification tests, using the basic results from the present experiments, will use a half-span wing with suitable controls, in combination with a 2bladed rotor and then a 3-bladed, twisted Figure 1: Relationship between tiltrotor case and full-span wing-rotor experiment Figure 2: Experimental Setup ABS