Use of a Water Basin for Ground Effect Testing of VTOL Models

Abstract

RELIMINARY design analysis encompasses varieties of configurations for which aerodynamic characteristics must be established. Some of the major characteristics of interest for VTOL aircraft pertain to lift, rolling moment, pitching moment, and to reingestion during hover in ground effect. The state-of-the-art for estimating these VTOL characteristics analytically is inadequate, and empirical data must be obtained. In order to acquire such data inexpensively for a variety of configurations, a test facility was established! which provides data with a quality suitable for preliminary assessment. Contents Description of Facility The test facility consists mainly of a water basin about 1.5 m in diameter and 45 cm deep, containing 30 cm of water in which a small scale model is suspended. Aircraft jet engines are represented by tubes through which water is pumped using a flow system illustrated in Fig. 1. The basin floor can act as the ground surface for ground effect testing. A typical model installation is shown schematically in Fig. 2. The model of approximately 25 cm span (in this case a flat plate) is suspended in the water by means of vertical rods from scales which, in turn, are attached to a support frame above the water basin. The scales measure vertical forces on the model excluding the forces on the water tubes themselves, thus indicating the interference forces of jet exhausts on the model. The water tubes extend through openings in the model so that the tube ends are flush with the bottom surface of the model. The tubes can move independently in a vertical direction. A small-diameter Teflon ring is attached to the tubes, outside near the exits, to minimize sliding friction with the model and, in effect, to seal the gaps between the tubes and the model by leaving only a small clearance. The vertical thrust of each of the water jets is measured separately by additional scales. The model distance to the ground can be altered by moving the entire model support frame upward or downward with respect to the water basin. Simulation of bank and pitch near the ground is provided by installing an inclined plane on the bottom of the basin beneath the model. Engine failure effects can be investigated by shutting off one of the water supply lines. Intake flow effects can be investigated by installing suction pipes. Flow visualization and reingestion measurements are made easily using pellets in the exhaust flow with a density equal to water.