Short Takeoff And Vertical Landing Research Articles

Propulsive jet aerodynamics and aeroacoustics

AbstractComprehensive understanding of propulsive jet aerodynamics and aeroacoustics is key to engine design for reduced jet noise and infra-red signature in civil and military aerospace, respectively. Illustrated examples are provided of other aerodynamic/aeroacoustic problems involving jet development, including chevron nozzles, increased jet/wing/flap interference (as fan diameter increases), high acoustic environment (and potentially damaging screech) of supersonic jets on carrier decks and the strongly Three-Dimensional (3D) unsteady flow during the in-ground effect operation of Short Take-Off and Vertical Landing (STOVL) aircraft. To date, laboratory/rig test measurements have primarily been used to identify design solutions; increased use of Computational Fluid Dynamics (CFD) would help achieve cost/time reductions, but Reynolds-Average Navier–Stokes (RANS) CFD with statistical turbulence modelling has proven inadequate for such flows. The scenarios described are far removed from flows used to calibrate model constants, and predictive accuracy demands detailed insight into unsteady flow. Large-Eddy Simulation (LES), whilst computationally more demanding, offers a potential solution. Research undertaken to assess LES capability to address the challenges described is reviewed here. This demonstrates that tremendous progress has been made, indicating that LES can provide sufficiently accurate predictions, representing high value for engineering design. A series of validation studies of increasing realism to practical engineering systems is presented to underpin this conclusion. Finally, areas for further work are suggested to support the combined application of RANS and LES that is probably the optimum way forward.

Research on Guidance and Control Law Design of Decelerating Transition and Vertical Landing for a STOVL UAV

In this paper, an integrated design method for guidance and control laws for a short takeoff and vertical landing (STOVL) unmanned aerial vehicle (UAV) during decelerating transition and vertical landing phases under thrust vector control is studied. Based on the principle of time-scale separation, the outer-loop guidance law adopts an implicit dynamic inverse method to provide attainable overload vector control instructions for decelerating transition and vertical landing phases. Meanwhile, the inner-loop control law adopts an improved eigen-structure assignment method, which tracks guidance instructions and maintains a stable attitude. The attitude nozzles are joined to the attitude control with the dropping of dynamic pressure, thereby assisting aerodynamic surfaces to stabilize attitudes. Subsequently, the feasibility and effect of the control methods for a STOVL UAV during decelerating transition and vertical landing phases are verified by a 6-DOF flight simulation platform. The simulation results demonstrate that the integrated design method is beneficial for a quick evaluation of the overall scheme.

High-performance aircraft short-takeoff and vertical-landing noise measurements on an aircraft carrier

The noise levels caused by high-performance aircraft are relatively high in the close proximity experienced by crew on board aircraft carriers, which can interfere with communications and may pose a risk for hearing loss. This paper reports on preliminary results of noise measurements of the operations of F-35B aircraft performing short-takeoff and vertical-landing (STOVL) operations on the flight deck of an LHA aircraft carrier. This noise measurement campaign was performed in late 2016, by scientists from the Air Force Research Laboratory (AFRL) in collaboration with the Naval Air Systems Command (NAVAIR) and the F-35 Integrated Task Force (ITF). The measurements were taken using hand-held noise recorder systems, and the recording engineers shadowed actual locations of crew. These data will be used to validate STOVL models of crew noise exposures on deck. [Work supported by F-35 JPO.]

The International Politics of the F-35 Joint Strike Fighter

There is a good reason why the F-35 Joint Strike Fighter program is so often described as the arms deal of the century. In a report published on the last day of 2010, the Pentagon estimated lifetime operating and sustainment costs for the US F-35 Aeet-then projected at 2,443 units, not counting the prototypes-at US$1.45 trillion.1 Cost analyses of this type are always much-debated: How many units will be sold in total? How does one define lifetime? How reliable will the system be once it enters service? What will be the nature of its deployment? And so on. Beyond dispute is that the F-35 constitutes one of the largest, if not the largest, weapons programs in modem history.The story of the Joint Strike Fighter (JSF) began in 1994, when the United States Congress decided to merge two separate Department of Defense fighter jet research projects. The idea was to produce a fifth generation fighter aircraft for the joint use of the US Air Force (USAF), Navy, and Marine Corps. Preliminary research contracts were divided mainly among McDonnell Douglas, Boeing, Northrop Gmmman, and Lockheed Martin, which subsequently formed teams that also involved other contractors, like Dassault (partnered with Boeing) and BAE (forming a threesome with McDonnell Douglas and Northrop Gmmman). The JSF program was purposely internationalized: a memorandum of understanding was signed by the US and British governments in December 1995, specifying a 90-10 split for co-funding the aircraft's demonstration phase, or, as it was officially known, the Weapons System Concept Demonstration (WSCD) phase.4 In November 1996 the Pentagon selected Lockheed Martin and Boeing as lead contractors; in October 2001 a long and fierce competition to carry out System Development and Demonstration (SDD) was won by the former. The Lockheed demonstrator X-35 turned into the F-35 Lightning II, which carried out its maiden flight in 2006; the two Boeing X-32 prototypes ended up in museums in Ohio and Maryland.The JSF was conceived from the beginning as a multi-role aircraft that would be capable of replacing four or five separate aircraft in use by the US military. To that end, the program gave birth to three different warplanes: the F-35A or the Conventional Take-Off Landing (CTOL) version, which is intended to replace the F-16 Fighting Falcon and A-10 Thunderbolt; the F-35B Short Take-Off and Vertical Landing (STOVL), replacing the AV-8B Harrier II; and the F-35C Carrier Version (CV), taking over from the F/A18 Hornet and possibly the Super Hornet. All three versions completed the critical design reviews and are now in the SDD phase. Low Rate Initial Production (LRIP) aircraft are rolling out from Lockheed Martin's assembly plant in Forth Worth, Texas, and the final Production, Sustainment, and Follow-on Development (PSFD) phase appears to be on the horizon. The US Marine Corps have established an operational squadron of F-35S at their Yuma, Arizona airbase, while instructor pilot training on the initial F-35S set to roll off the assembly line is progressing at a USAF base in Florida.5Globalized industrial partnerships propelled the JSF from the outset, but the multinational collaborative project took shape gradually. Most US allies were not briefed on the JSF until the beginning of the SDD phase, and it is unclear how the US government-specifically the nascent JSF office-decided which governments to engage.4 Canada joined in 1997, turning the US-UK collaboration into a multinational one. At that point, the JSF specified the conditions for different levels or tiers of partnership. Each level was mainly determined by the size of the partner's contribution. Level 1 partners contribute approximately 10 percent to the development of the aircraft. The UK is the only Level 1 partner and has committed $2.2 billion to the program. Level 2 partners, such as Italy and the Netherlands, contribute approximately $1 billion. Level 3 partners-Australia, Canada, Denmark, Norway, and Turkey-contribute $ioo-$2oo million. …

Supersonic moist air jet impingements on flat surface

Pronounced aeroacoustic resonances are exhibited in the flowfield where a jet emerges from an orifice or a nozzle and impinges on a solid surface. One instance where such resonances are produced is in a high speed jet impingement, such as in the space launch vehicle systems, jet-engine exhaust impingement, and in the short take-off and vertical landing (STOVL) aircraft, etc. A highly unsteady flowfield leading to a drastic increase of noise level with very high dynamic pressure and thermal loads are noticed on nearby surfaces results dramatic lift loss, severe ground erosion and hot gas ingestion to the inlet in the jet engines. This highly unsteady behavior of the impinging jets is due to a feedback loop between the fluid and acoustic fields. In actual jet flow, the working gas may contain condensable gas such as steam or moist air. In these cases, the non-equilibrium condensation may occur at the region between nozzle exit and an object. The jet flow with non-equilibrium condensation may be quite different from that without condensation. Therefore, in this study, the effect of the non-equilibrium condensation of moist air on the axisymmetric under-expanded supersonic impinging jet on a vertical flat plate was investigated numerically.

Experimental study of physical mechanisms in the control of supersonic impinging jets using microjets

Supersonic impinging jet(s) inherently produce a highly unsteady flow field. The occurrence of such flows leads to many adverse effects for short take-off and vertical landing (STOVL) aircraft such as: a significant increase in the noise level, very high unsteady loads on nearby structures and an appreciable loss in lift during hover. In prior studies, we have demonstrated that arrays of microjets, appropriately placed near the nozzle exit, effectively disrupt the feedback loop inherent in impinging jet flows. In these studies, the effectiveness of the control was found to be strongly dependent on a number of geometric and flow parameters, such as the impingement plane distance, microjet orientation and jet operating conditions. In this paper, the effects of some of these parameters that appear to determine control efficiency are examined and some of the fundamental mechanisms behind this control approach are explored. Through comprehensive two- and three-component velocity (and vorticity) field measurements it has been clearly demonstrated that the activation of microjets leads to a local thickening of the jet shear layer, near the nozzle exit, making it more stable and less receptive to disturbances. Furthermore, microjets generate strong streamwise vorticity in the form of well-organized, counter-rotating vortex pairs. This increase in streamwise vorticity is concomitant with a reduction in the azimuthal vorticity of the primary jet. Based on these results and a simplified analysis of vorticity transport, it is suggested that the generation of these streamwise vortices is mainly a result of the redirection of the azimuthal vorticity by vorticity tilting and stretching mechanisms. The emergence of these longitudinal structures weakens the large-scale axisymmetric structures in the jet shear layer while introducing substantial three-dimensionality into the flow. Together, these factors lead to the attenuation of the feedback loop and a significant reduction of flow unsteadiness.

Characteristics of Supersonic Impinging Moist Air Jets

Supersonic impinging Jets, such as those occurring in the space launch vehicle systems, multi-stage rocket separation, jet-engine exhaust impingement, terrestrial rocket launch, and in the next generation short take off and vertical landing (STOL) aircraft, generate a highly oscillatory flow with very high unsteady loads on the near by structures and the landing surfaces. These high-pressure and acoustic loads are also accompanied by a dramatic loss in lift during aircraft hover. Previous studies of supersonic impinging jets suggest that the highly unsteady behavior of the impinging jets is due to a feedback loop between the fluid And acoustic fields, which leads to these adverse effects, In actual supersonic jet flow, usually the working gas nu be condensable gas such as steam or moist air. In these cases, the non-equilibrium condensation may occur in the region between nozzle exit and an object. The jet flow with non-equilibrium condensation may be quite different from that without condensation, In the present study, characteristics of under-expanded axisymmetric supersonic impinging moist air jets on a vertical flat plate were investigated numerically.

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.

Joint Strike Fighter Dual-Cycle Propulsion System

The Joint Strike Fighter (JSF) is a family of aircraft that will be built in conventional, naval, and short-takeoffand-vertical-landing (STOVL) variants. The key to the development of this family of aircraft is a new dual-cycle propulsion system, which is used to convert some of the jet thrust to shaft horsepower in order to power a lift fan in the STOVL variant. The theoretical basis for the dual-cycle operation of this engine will be presented. Some results of the engine test and development program conducted by Pratt and Whitney and Rolls Royce and the JSF STOVL flight-test program will also be discussed. Potential future applications of this dual-cycle propulsion concept to STOVL transport aircraft, compound rotorcraft, and for takeoff noise reduction will be described. I. Introduction T HE F-35 Joint Strike Fighter will combine the supersonic performance of the F-16C Falcon with the short-takeoff-andvertical-landing (STOVL) capabilities of the AV-8B Harrier, while providing greater range and increased survivability. There will be three variants: a conventional takeoff-and-landing variant for the U.S. Air Force, a short-takeoff-and-vertical-landing variant for the U.S. Marine Corps and United Kingdom, and a carrier-based variant for the U.S. Navy. These three variants are shown in Fig. 1. The naval variant has a somewhat larger wing, in order to reduce landing speeds for carrier operations. This also gives it somewhat greater range, both by reducing the induced drag and by providing additional volume for fuel. The STOVL variant has a shorter canopy and a slight bulge behind the cockpit. These accommodate a lift fan installed in a bay between the inlet ducts. As shown in Fig. 2, the lift fan is driven by a drive shaft extending from the front of the cruise engine. This lift fan provides 18,000 lbs of thrust, almost half of the total lift in hover. A thrust-vectoring nozzle at the rear of the aircraft deflects the core thrust of the cruise engine, which provides another 17,000 lbs of thrust. A roll control nozzle in each of the wings is fed by fan air diverted from the cruise engine. These provide approximately 2500 lbs of thrust apiece. The two main lift nozzles and the two auxiliary roll control nozzles constitute a two plus two hover lift and control system. The cruise engine is a conventional mixed-flow turbofan, providing more than 25,000 lbs of dry thrust. The lift fan is not connected to the engine during cruise. For STOVL operations, the engine operating point is changed so that its turbine section extracts additional energy from the exhaust jet and converts it to shaft horsepower. This power is delivered to the lift fan by engaging a clutch at the end of the drive shaft extending from the front of the engine. The airplane is controlled in pitch by shifting power between the lift fan and the cruise engine nozzle. The cruise engine nozzle rotates from side to side in order to control the aircraft in yaw. To control the aircraft in roll, the area of the auxiliary nozzle on one side is opened, and the nozzle on the other side is closed. The total thrust remains constant while the thrust of the nozzle pairs are varied to provide control forces.

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

Experimental and Computational Investigation of Supersonic Impinging Jets

The results of an experimental and computational study of a moderately underexpanded axisymmetric supersonicjetissuingfroma convergingnozzleand impingingonagroundplanearepresented.Thegoalofthisworkisto develop a better understanding oftheimpinging jet e owe eld, which is of signie cantpracticalinterestbecause of its presence in short takeoff and vertical landing (STOVL)aircraft during hover as well as in other aerospace-related and industrial applications. Theexperimental measurementsinclude e ow visualization, surface-pressuredistributions,and velocitye elddataobtained using particleimagevelocimetry (PIV).Theexperimentaldata,especially the velocity e eld measurements, were used to verify theaccuracy of computational predictions. Computational results obtained using two differentturbulencemodels produced almost identical results. Comparisons with experimental results reveal that both models capture the signie cant features of this complex e ow and were in remarkably good agreement with the experimental data for the primary test case. The experiments and computations both revealed the presence of the impingement zone stagnation bubble, which contains low velocity recirculating e ow. Other features, including the complex shock structure and the high-speed radial wall jet, were also found to be very similar. The ability to measure and predictaccurately theimpinging jetbehavior, especially neartheground plane, is critical because these are regions with very high mean shear, thermal loads, and unsteady pressure forces, which contribute directly to the problem of ground erosion in STOVL applications.

Experimental and computational investigation of supersonic impinging jets

The results of an experimental and computational study of a moderately underexpanded axisymmetric supersonic jet issuing from a converging nozzle and impinging on a ground plane are presented. The goal of this work is to develop a better understanding of the impinging jet flowfield, which is of significant practical interest because of its presence in short takeoff and vertical landing (STOVL) aircraft during hover as well as in other aerospace-related and industrial applications. The experimental measurements include flow visualization, surface-pressure distributions, and velocity field data obtained using particle image velocimetry (PIV). The experimental data, especially the velocity field measurements, were used to verify the accuracy of computational predictions. Computational results obtained using two different turbulence models produced almost identical results

Particle Image Velocimetry for Intake Ingestion in Short Takeoff and Landing Aircraft

Exhaust gas ingestion can cause many major problems for a short takeoff and vertical landing (STOVL) aircraft operating in ground effect, for example, engine surge. As part of a program of experimental and computational work on intake ingestion problems, a generic twin jet discharge/intake model was designed, constructed, and tested in a water tunnel especially designed for STOVL flow applications. Tunnel operating conditions were chosen to focus specifically on the intake ingestion process. Particle image velocimetry (PIV) has now been developed into a well-established technique that can provide two-dimensional spatially resolved velocity measurements in complex fluid flows. This technique is used to measure the instantaneous velocity field in a vertical plane close to the intake. The flow processes important to ingestion (fountain flow, ground vortex, intake suction) are all captured. The large-scale unsteadiness of the ingestion events are studied and quantified. For the present jet-discharge/intake configuration, PIV is demonstrated as capable of producing quantitative information on instantaneous eddy structure and its temporal development, which is essential to improving our understanding of the exhaust gas ingestion phenomenon

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.

Computational fluid dynamics prediction of intake ingestion relevant to short take-off and vertical landing aircraft

Intake ingestion can cause several major problems (e.g. compressor surge and stall) for short take-off and vertical landing (STOVL) aircraft operating in ground effect. Numerical predictions of the flowfield associated with a generic twin-jet plus intake model operating under ingestion flow conditions are reported using computational fluid dynamics (CFD) techniques. The results have been compared with laser Doppler anemometry (LDA) validation measurements taken in a specially designed test case configuration. The k-ε turbulence model and both first-order and second-order (QUICK) convection discretization schemes were employed. Fine meshes and second-order accurate discretization were found essential to produce solutions close to grid independence. A reasonable prediction of the general flow pattern has been achieved. Several features of the mean velocity field were close to the experimental results; however, the k-ε model was shown to produce significant errors in the prediction of the forward penetration distance of the ground sheet flow and in the shape of velocity profiles and turbulence levels near to the intake.

Laser Doppler velocimetry measurements of twin-jet impingement flow for validation of computational models

Jets discharged traversely into a cross-flow appear as a flow element in many propulsion related systems such as short take-off and vertical landing (STOVL) aircraft. The purpose of this paper is to report on a recent experimental study of a closely spaced pair of jets which may be of the same or different jet velocities and which interact with each other, with a cross-flow and with an opposite solid wall. In this paper an experimental facility, used to gather validation data suitable for testing computational fluids dynamics (CFD) model predictions of multi-jet ground impingement flows, is described. Water is used as the working fluid and laser Doppler velocimetry (LDV) measurements of twin impinging jets are reported, both with and without a cross-flowing stream. Emphasis is placed on the presentation of the mean and r.m.s. velocity contours in the fountain formation region between the jets. The effect of jet imbalance and velocity ratio were studied. Preliminary CFD predictions of the flow using a k– ε turbulence model are presented. An overview is provided to guide users through the data and highlight several important flow features which have already emerged from the data and which are considered of importance to CFD model validation studies.

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...