Short Takeoff And Vertical Landing Aircraft Research Articles

Experiments on free and impinging supersonic microjets

The fluid dynamics of microflows has recently commanded considerable attention because of their potential applications. Until now, with a few exceptions, most of the studies have been limited to low speed flows. This experimental study examines supersonic microjets of 100–1,000 μm in size with exit velocities in the range of 300–500 m/s. Such microjets are presently being used to actively control larger supersonic impinging jets, which occur in STOVL (short takeoff and vertical landing) aircraft, cavity flows, and flow separation. Flow properties of free as well as impinging supersonic microjets have been experimentally investigated over a range of geometric and flow parameters. The flowfield is visualized using a micro-schlieren system with a high magnification. These schlieren images clearly show the characteristic shock cell structure typically observed in larger supersonic jets. Quantitative measurements of the jet decay and spreading rates as well as shock cell spacing are obtained using micro-pitot probe surveys. In general, the mean flow features of free microjets are similar to larger supersonic jets operating at higher Reynolds numbers. However, some differences are also observed, most likely due to pronounced viscous effects associated with jets at these small scales. Limited studies of impinging microjets were also conducted. They reveal that, similar to the behavior of free microjets, the flow structure of impinging microjets strongly resembles that of larger supersonic impinging jets.

Control of Supersonic Impinging Jet Flows Using Supersonic Microjets

Supersonic impinging jets, such as those occurring in the next generation of short takeoff and vertical landing aircraft, generate a highly oscillatory e ow with very high unsteady loads on the nearby aircraft structures and the landing surfaces. These high-pressure and acoustic loads are also accompanied by a dramatic loss in lift during hover.Previousstudies of supersonic impinging jets suggestthatthehighly unsteady behavioroftheimpinging jets is due to a feedback loop between the e uid and acoustic e elds, which leads to these adverse effects. A unique active control technique was attempted with the aim of disrupting the feedback loop, diminishing the e ow unsteadiness, and ultimatelyreducing theadverseeffectsofthise ow.Flowcontrolwasimplementedbyplacingacirculararray of 400-πm-diamsupersonicmicrojetsaroundtheperipheryofthemainjet.Thiscontrolapproachwasverysuccessful in disrupting the feedback loop in that the activation of the microjets led to dramatic reductions in the lift loss (40%), unsteady pressure loads (11 dB), and near-e eld noise (8 dB). This relatively simple and highly effective control technique makes it a suitable candidate for implementation in practical aircraft systems. NUNDERSTANDINGoftheimpingingjete owe eld isnecessary for the design of efe cient short takeoff and vertical landing (STOVL) aircraft. When such STOVL aircraft are operating in hovermode,thatis,in closeproximityto theground,thedownwardpointing lift jets produce high-speed, hot e ow that impinges on the landing surface and generates the direct lift force. It is well known that in this cone guration several e ow-induced effects can emerge, which substantially diminish the performance of the aircraft. In particular, a signie cant lift loss can be induced due to e ow entrainment bytheliftingjetsfromtheambientenvironmentinthe vicinityofthe airframe. Other adverse phenomena include severe ground erosion on the landing surface and hot gas ingestion into the engine inlets. In addition, the impinging e owe eld usually generates signie cantly highernoiselevelsrelativetothatofafreejetoperatingundersimilar conditions. Increased overall sound pressure levels (OASPL) associated with the high-speed impinging jets can pose an environment pollution problem and adversely affect the integrity of structural elements in the vicinity of the nozzle exhaust due to acoustic loading. Moreover, the noise and the highly unsteady pressure e eld are frequently dominated by high-amplitude discrete tones, which may match the resonant frequencies of the aircraft panels, thus further exacerbating the sonic fatigue problem. These problems become more pronounced when the impinging jets are supersonic, the operating regime of the STOVL version of the future joint strike e ghter. In addition, the presence of multiple impinging jets can potentially further aggravate these effects due to the strong coupling between the jets and the emergence of an upward-moving fountain e ow e owing opposite to the lift jets. 1 A

Numerical studies of twin‐jet impingement for STOVL flow application

The behaviour of turbulent jet impingement is of considerable importance to the performance of STOVL (Short Take‐Off and Vertical Landing) aircraft. Theoretical analyses of the flow field produced by jet impingement on a ground surface are formulated in terms of solving the full Reynolds‐Averaged Navier‐Stokes (RANS) equations. This paper presents predictions of a twin‐jet impingement on ground plane flow using the standard two‐equation k‐ϵ turbulence model. Predictions were performed for two impingement heights and two jet spacings. An examination of the effect of switching from first‐order (Hybrid) to second‐order (Quick) convection discretization was also studied. The results presented are sensibly independent of numerical influence and represent a true assessment of the performance of the turbulence model. The predictions were compared with LDV (Laser Doppler Velocimetry) experimental results. The main features of the flow field were correctly predicted. The fountain formation region was qualitatively predicted but quantitative under‐prediction of fountain development characteristics was observed to be around 50%. This is probably due to fountain unsteadiness, which is not included in the steady state CFD predictions presented here.

Stability and control of STOVL aircraft: The design of longitudinal flight control laws

AbstractOver the past three decades, the UK aerospace industry has carried out significant research into the development of short take-off and vertical landing (STOVL) technology, to enhance the performance and operation of the Harrier aircraft, and for possible application to future aircraft such as those being developed under the Joint Strike Fighter (JSF) programme. Some of this research has focused on aircraft handling and flight control for the transition between wing-borne and jet-borne flight. Following on from internal research at British Aircraft Corporation/British Aerospace (now part of BAE Systems) in the mid to late 1970s, further development work has been carried out in the 1980s and 90s in support of the UK’s Vectored thrust Advanced Aircraft flight Control (VAAC) Harrier and Integrated Flight and Propulsion Control System (IFPCS) programmes. This paper contains a short review of STOVL aircraft longitudinal flight control law design, and how basic feedback control schemes can be used to influence the aircraft’s response and hence its handling qualities.

Experimental data for CFD validation of the intake ingestion process in STOVL aircraft

Intake ingestion can cause many major problems (e.g. compressor surge) for a Short Take-Off and Vertical Landing (STOVL) aircraft in ground effect. As part of a program of experimental work on ingestion problems, a generic jet discharge/intake model was designed, constructed and tested in a water tunnel specially designed for STOVL flow applications. Laser Doppler Velocimetry (LDV) surveys were conducted to identify the mean velocity and turbulence structure of twin-jet and twin-jet plus intake configurations. The LDV measurements confirmed that the flow pattern produced with the impingement height and velocity ratio parameters selected from this study were typical of practically occurring re-ingestion flow fields. Sufficient profiles of velocities and turbulence quantities have been measured for this data set to serve as benchmark validation data for time-averaged turbulence-model-based RANS CFD predictions.

Flight control and handling research with the VAAC Harrier aircraft

Future short take-off and vertical landing (STOVL) concepts present unique design challenges. The flight control system designer must consider several issues including the control response type, the appropriate cockpit displays, and the control strategy for the flight and propulsion controls. Advanced STOVL concepts will have many force and moment generators. To be operationally effective such configurations need to have precise handling and control properties such that the pilot is not faced with an unacceptably high workload. The vectored thrust aircraft advanced flight control (VAAC) is a research programme which is addressing these issues. Its primary objective is to develop flight control and handling concepts which, when applied to future STOVL configurations, will provide good handling qualities with a low pilot workload. Four potential methods of controlling future STOVL aircraft are described and assessed with manned simulation and with the VAAC Harrier aircraft.

Application of nonlinear inverse methods to the control of powered-lift aircraft over the low-speed flight envelope

This paper presents a summary of experience in the design of velocity vector control systems for operation of short take-off and landing (STOL) and short take-off and vertical landing (STOVL) aircraft over their low-speed flight envelopes. A nonlinear inverse control method developed at NASA Ames Research Center was employed to deal with the highly nonlinear, multi-dimensional aerodynamic and propulsion system characteristics that are inherent in these aircraft. The method partitions the control system into a command element that is defined solely by the dynamic response to achieve good flying qualities, and a nonlinear inverse element that computes the aerodynamic and propulsion control effector positions required to satisfy the pilot's commands and to regulate against disturbances. Experience gained at flight evaluations with a powered-lift STOL aircraft and in piloted simulation with a STOVL aircraft design indicates that this system design approach produces the desired flying qualities over the low-sp...

Design and piloted simulation of integrated flight/propulsion controls for STOVL aircraft

An integrated flight/propulsion control system has been designed for operation of short takeoff and vertical landing (STOVL) fighter aircraft over the low-speed, powered-lift flight envelope. The control system employs command modes for attitude, flight path angle, and flight path acceleration during transition, and translational velocity command for hover and vertical landing. In this paper, only the longitudinal modes of control are discussed. Piloted evaluations of the control system have been conducted on Ames Research Center's Vertical Motion Simulator. Results indicate that level 1 flying qualities are achieved during transition and vertical landing over a wide range of wind, atmospheric turbulence, and visibility conditions.

STOVL Hot Gas Ingestion Control Technology

A comprehensive wind tunnel test program was conducted to evaluate control of Hot Gas Ingestion (HGI) on a 9.2 percent scale model of the McDonnell Aircraft Company model 279-3C advanced Short Takeoff and Vertical Landing (STOVL) configuration. The test was conducted in the NASA-Lewis Research Center 9 ft by 15 ft Low Speed Wind Tunnel during the summer of 1987. Initial tests defined baseline HGI levels as determined by engine face temperature rise and temperature distortion. Subsequent testing was conducted to evaluate HGI control parametrically using Lift Improvement Devices (LIDs), forward nozzle splay angle, a combination of LIDs and forward nozzle splay angle, and main inlet blocking. The results from this test program demonstrate that HGI can be effectively controlled and that HGI is not a barrier to STOVL aircraft development.