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Propulsion-Induced Effects Caused by Out-Of-Ground Effects Richard Margason

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Propulsion-Induced Effects Caused by Out-Of-Ground Effects Richard Margason . NASA Technical Memorandum 100032 Propulsion-Induced Effects Caused by Out-of-Ground Effects Richard Margason {IfASA-T8-100032) PBOPULSIOY-IJ#DDCED BPPBCTS CAUSED BY OUT-OF-GBOUID EFFECTS @AS&) 188- 140 88 30 P CSCL 03c Unclas 63/03 01.18113 December 1987 National Aeronautics and Space Administration J NASA Technical Memorandum 100032 Propulsion-Induced Effects Caused by Out-of-Ground Effects Richard Margason, Ames Research Center, Moffett Field, California December 1987 NASA National Aeronautics and Space Administration Ames Research Center Moffett Field. California 94035 ABSTRACT Extensive research has been conducted on the flow field associated with the jet in a cross- Propulsion induced effects encountered by flow. The propulsion-induced effects that moderate- to high-disk loading STOVL or VSTOL moderate-to-high-disk-loading STOVL and VSTOL aircraft out-of-ground effect during hover and aircraft encounter are caused by lifting jets transition between hover and wing-borne flight mixing with the freestream. The research of this are discussed. Descriptions of the fluid flow effect includes: 1) uniform jets, 2) nonuniform phenomena are presented along with an indication jets, 3) dual jets, 4) rectangular jets, and of the trends obtained from experimental inves- 5) jets in a body of revolution. Only example tigations. In particular, three problem areas results will be highlighted; however, a fairly are reviewed: 1) the performance losses sus- complete list of references is identified. In tained by a VSTOL aircraft hovering out-of- addition, several aspects of lateral/directional ground effect, 2) the induced aerodynamic interactions are discussed. Finally these effects encountered as a VSTOL aircraft flies on effects are considered on a complete aircraft the combination of powered and aerodynamic lifts configuration. A new aero/propulsion parameter between hover and cruise out-of-ground effect, is suggested to more completely depict the and 3) the aerodynamic characteristics caused by interaction effects. deflected thrust during maneuvering flight over a wide ranges of both angle of attack and Mach BACKGROUND number. The challenge of VSTOL aircraft design is an intriguing one which has been studied for many years. The first workable solution, the powered balloon, reached operational status in DURING HOVER, there is a base loss caused by the 1930's and is currently receiving another interactions between the lifting jets and the look by some people. The second workable solu- lower surface of the aircraft which results in a tion was the helicopter which was first demon- distribution of induced suction pressures which strated around 1940 by Igor Sikorsky and is produce a lift loss. Additional performance currently the most successful application of losses include inlet flow distortion, hot-gas VTOL aircraft. The third workable solution was ingestion, hot day conditions, control bleed, the deflected thrust aircraft which is currently internal nozzle flow, thrust vectoring, static represented by the AV-8B Harrier. This paper ground effect. There are many items related to will deal with the propulsion-induced effects the details of the aircraft design which deter- encountered by the third VSTOL group of aircraft mine the magnitude of the losses. The aerody- concepts which use moderate to high-disk-loading namic lift loss from suction forces on the propulsion devices. underside of the aircraft is described later. During the last 50 years NACA/NASA has In transition flight, there is a similar conducted research specifically on the third lift loss because of the interaction between the class of aircraft. This research has focused on efflux of the lifting propulsive device and the the development of the technology needed to aircraft's aerodynamic lifting surfaces. identify and design feasible and practical 1 aircraft concepts. During this period the major transition or maneuvering flight. For conven- advances in propulsion concepts, structures, and tional flight, the propulsion system provides flight controls has enabled the consideration of horizontal thrust. Each of these concepts intro- an overwhelming number of configuration types. duces its own set of problems. For example, As a result, a perception has developed that complicated geometries for the transfer ducts nearly every VSTOL concept has been studied and which terminate in complex devices such as been found to be impractical. slots, vanes, nozzles, wide-angle diffusers, and In reality many of the early VSTOL aircraft so forth. identified technology deficiencies which have In all cases, the lift jets issuing from since been overcome. For example, the first the aircraft mix with the external flow to tilt rotor, the XV-3, had major difficulties generate extremely complicated, three- with a rotor-induced instability. Small-scale dimensional flow phenomena. In general, the wind tunnel models were used to develop an jet-induced effects cause additional forces and understanding of the coupling between the vari- moments on the aircraft during hover and in the ous rotor-induced instabilities and the dynamic transition between hover and wing-borne character of the rotor/hub/pylon/wing assem- flight. The character and magnitude of these bly. This understanding was validated using the jet-induced effects is influenced by the flight XV-3 as a model in the ARC 40- by 80-ft wind regime being encountered as well as the specific tunnel. By careful structural and rotor design aircraft configuration. The present paper will these problems were overcome by the XV-15 and draw from several earlier surveys (1-5)* of now the final development of the operational VSTOL propulsion-induced effects as well as V-22 Osprey is underway. Another example, the recent research results. The paper will focus on P-1127 deflected thrust aircraft of the late those effects which are especially pertinent to 1950's had marginal performance. Through dedi- STOVL fighter or attack aircraft configurations. cated, persistent effort it evolved into the This paper summarizes some of the propul- present AV-8B Harrier which is competitive with sion-induced effects encountered out-of-ground the best contemporary subsonic combat aircraft effect during hover, transition, and maneuver. of the 1980's. To illustrate this progress, Descriptions of the fluid-flow phenomena are this paper will examine propulsion-induced presented along with an indication of the trends effects encountered by high-disk-loaded STOVL obtained from experimental investigations. In configurations during flight out-of-ground particular, three problem areas are reviewed; effect. 1) the performance losses sustained by a VSTOL The propulsion system of STOVL aircraft aircraft hovering out-of-ground effect, 2) the must be designed to generate forward thrust for induced aerodynamic effects encountered as a conventional flight, lifting force for hovering VSTOL aircraft flies on a combination of powered flight, and in many cases, additional force and aerodynamic lift between hover and cruise components for control purposes during hover, out-of-ground effect, and 3) the aerodynamic during transition between hover and wing-borne characteristics caused by deflected thrust flight, and during maneuvering flight over a during maneuvering flight over a wide ranges of large angle-of-attack range. This multiple both angle of attack and Mach number. Where function character of VSTOL propulsion systems there are available data, an attempt will be leads to design features which make them signif- made to identify fundamental differences among icantly different from conventional propulsion the propulsion concepts currently being studied systems. for STOVL combat aircraft. These propulsion In most aircraft configurations, the lift- concepts include; 1) vectored thrust, 2) ejec- ing force for hovering flight is provided at tors, 3) remote augmented-lift systems (RALS), two or more locations to permit moment trim and 4) tandem fanhectored thrust, and 5) lift plus control about all three axes. This requirement lift/cruise. Where possible some of the design is met either by location of propulsion units at conflicts among the requirements of the various several places on the aircraft (e.g., the propulsion concepts and the different modes of liftllift-cruise concept or remote augmented flight will be discussed, along with the present lift system (RALS)), or the addition of internal state of the art solutions to some of the flow transfer ducts (e.g., the ejector or the problems. circulation control concepts), or by use of shafts to transfer power (e.g., helicopters or tilt rotorcraft). The aircraft configurations must use the propulsion system to provide a combination of lift and horizontal thrust during Wumbers in parentheses designate references at end of paper. 2 AERO/PROPULSION-INDUCED EFFECTS IN TRANSITION nozzle-exit-velocity profile and low turbulence FLIGHT is sketched in Figure 3. In the jet there are two regions of flow: 1) the short potential The effects caused by the interaction core region (up to six nozzle diameters long) between propulsion efflux and aircraft aerody- which has a conical shape and a uniform velocity namic-induced flows have been the subject of profile, and 2) the fully turbulent region. many experimental investigations. These effects In an early program intended to evaluate are encountered in hover, during transition base losses during hover, NASA Langley Research flight between hover and transition, and during Center built a plenum chamber which would
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