R i c h a r d Margason and R i c h a r d K u h nA m e s R e s e a r c h C e n t e r This paper critically reviews available prediction methods and provides an assessment of their strengths and weaknesses. The methods are applied to selected problems which represent the major aero/propulsion interactions for short takeoff and vertical landing (STOVL) aircraft of current interest. The first two problems deal with aerodynamic performance effects during hover: a) out-of-ground effect, and b) in-ground effect. The first problem can be evaluated for some multijet cases; however, the second problem is very difficult to evaluate for multijets. The ground-environment effects due to wall jets and fountain flows directly affect hover performance. gas ingestion affects the engine operation. Both of these problems as well as jet noise affect the ability of people to work near the aircraft and the ability of the aircraft to operate near the ground. Additional problems are d) the power-augmented lift due to jet-flap effects (both in-and out-of-ground effects) and e) the direct jet-lift effects during short takeoff and landing (STOL) operations. The final problem is f ) the aerodynamic/propulsion interactions in transition between hover and wing-borne flight. Areas where modern computational fluid dynamics (CFD) methods can provide improvements to the current computational capabilities are identified.
In a related problem c) hot-FOR OVER THIRTY YEARS, vertical or short takeoff and landing (VSTOL) aircraft have been developed primarily from experimental investigations because there are many unique aerodynamic/ propulsion-induced effects which occur with these vehicles. significant at low speeds from hover to wingThese problems are especially borne flight. requires an accounting of these complex phenomena. Unfortunately, the flows involved are not amenable to purely theoretical predictions at this time. needed to guide experimental programs and to reduce the testing requirements in the future, as well as to provide an understanding of the flow physics. One motivation of this paper is to identify areas where modern computational fluid dynamics (CFD) jet with a uniform nozzle exit dynamic pressure profile and low turbulence is sketched in Fig, 1. In the jet there are two regions of flow: a) the short potential-core region (up to six nozzle diameters long) which has a conical shape and a uniform velocity profile, and b) the fully turbulent region. In an early program intended to evaluate base losses during hover, a NASA investigation ( 1 7 ) evaluated the effect of the character of the jet on induced lift loss. The results showed that a relation exists between the lift loss and the rate of decay of nozzle dynamic pressure.Both of these parameters are functions of the amount of air entrained into the jet and the proximity of the entrainment to the plate. correlation between the slope of the lift-loss curve and a parameter indicative of the dynamicpressure decay was developed. As indicated in Fig. 2, this parameter is the ...
