Experimental and computational investigation of cross-flow fan propulsion for lightweight VTOL aircraft

Abstract

Cross-flow fan propulsion has not been seriously considered for aircraft use since an Vought Systems Division (VSD) study for the U.S. Navy in 1975. A recent conceptual design study of light-weight, single seat VTOL aircraft suggest that rotary-engine powered cross-flow fans may constitute a promising alternative to the conventional lift-fan vertical thrust augmentation systems for VTOL aircraft. The cross-flow fan performance data obtained by VSD supported the hypothesis that they could be improved to the point where their thrust augmentation could be used in a VTOL aircraft. In this paper we report results of a NASA Glenn supported experimental and computational cross-flow fan investigation which is currently in progress and we provide an assessment of the potential suitability of cross-flow fans for VTOL aircraft propulsion. The tests are carried out in the Turbopropulsion Laboratory of the Naval Postgraduate School, using an existing Turbine Test Rig as a power source to drive the cross- flow fan. A 0.305 m (12-inch) diameter, 38.1 mm (1.5-inch) span cross-flow fan test article was constructed to duplicate as closely as possible the VSD fan so that baseline comparison performance data could be obtained. Performance measurements were taken over a speed range of 1,000 to 7,000 RPM and results comparable to those measured by Vought Systems Division were obtained. At 3,000 RPM a 2:1 thrust-to-power ratio was measured which dropped to one as the speed was increased to 6,000 RPM. Performance maps were experimentally determined for the baseline configuration as well as one with both cavities blanked off, for the speed range from 2,000 to 6,000 rpm. Using Flo++, a commercial PC-based computational fluid dynamics software package by Softflo, 2-D numerical simulations of the flow through the cross-flow fan were also obtained. Based on the performance measurements it was concluded that the optimum speed range for this rotor configuration was in the 3,000 to 5,000 rpm range. The lower speed producing the best thrust-to-power ratio and the upper speed range producing the highest efficiency over sizeable throttling range.