Showing papers in "Aeronautical Journal in 2000"

The NASA Tetrahedral Unstructured Software System (TETRUSS)

Abstract: The NASA Tetrahedral Unstructured Software System (TetrUSS) was developed during the 1990's to provide a rapid aerodynamic analysis and design capability to applied aerodynamicists. The system is comprised of loosely integrated, user-friendly software that enables the application of advanced Euler and Navier-Stokes tetrahedral finite volume technology to complex aerodynamic problems. TetrUSS has matured well because of the generous feedback from many willing users representing a broad cross-section of background and skill levels. This paper presents an overview of the current capabilities of the TetrUSS system along with some representative results from selected applications.

Active inlet flow control technology demonstration

Abstract: This paper presents results from a joint Lockheed Martin/NASA Glenn effort to design and verify an ultra-compact, highly-survivable engine inlet subsonic duct based on the emerging technology of active inlet flow control (AIFC). In the AIFC concept, micro-scale actuation (∼mm in size) is used in an approach denoted ‘secondary flow control’ to intelligently alter a serpentine duct's inherent secondary flow characteristics with the goal of simultaneously improving the critical system-level performance metrics of total pressure recovery, spatial distortion, and RMS turbulence. In this approach, separation control is a secondary benefit, not a design requirement. The baseline concept for this study was a 4:1 aspect ratio ultra-compact (LID = 2·5) serpentine duct that fully obscured line-of-sight view of the engine face. At relevant flow conditions, this type of duct exhibits excessive pressure loss and distortion because of extreme wall curvature. Two sets of flow control effectors were designed with the intent of establishing high performance levels to the baseline duct. The first set used two arrays of 36 co-rotating microvane vortex generators (VGs); the second set used two arrays of 36 micro air-jet (microjet) VGs, which were designed to produce the same ‘vorticity signature’ as the microvanes. Optimisation of the microvane array was accomplished using a design of experiments (DOE) methodology to guide selection of parameters used in multiple Computational Fluid Dynamics (CFD) flow solutions. A verification test conducted in the NASA Glenn W1B test facility indicated low pressure recovery and high distortion for the baseline duct without flow control. With microvane flow control, at a throat Mach number of 0·60, pressure recovery was increased 5%, and both spatial distortion and turbulence were decreased approximately 50%. Microjet effectors also provided significantly improved performance over the baseline configuration.

Challenges in the Better, Faster, Cheaper Era of Aeronautical Design, Engineering and Manufacturing

Abstract: 'Better, faster, cheaper’ (BFC) emerged in the 1990s as a new paradigm for aerospace products. In this paper, we examine some of the underlying reasons for BFC and offer some thoughts to help frame the thinking and action of aerospace industry professionals in this new era. Examination of literature on industrial innovation indicates that aeronautical products have evolved to a ‘dominant design’ and entered the ‘specific phase’ of their product life cycle. Innovation in this phase centers on: incremental product improvement, especially for productivity and quality; process technology; technological innovations that offer superior substitutes. The first two of these are aligned with BFC objectives. The concepts of ‘value’ and ‘best lifecycle value’ are introduced as conceptual frameworks. Value is offered as a metric for BFC. Risk management is intimately tied to achieving value and needs to be integrated into aeronautical processes. The process technology area is addressed by considering ‘lean’ practices for design, engineering and manufacturing. Illustrative results of process improvements from the seven-year Lean Aerospace Initiative research programme at MIT indicate opportunities to achieve BFC. Concluding remarks offer some challenges to industry, government and academics in aeronautical design, engineering and manufacturing.

2020 vision : the prospects for large civil aircraft propulsion

Abstract: This paper will examine the prospects for propulsion systems for large civil transport aircraft over the next two decades, to the year 2020. This period is likely to see the market drivers for future propulsion system development change from the more traditional ones of fuel consumption and weight, to also include those that affect the impact of civil aviation on the environment, and the continuing pressure to reduce the cost of ownership of civil engines, namely, product unit cost, maintenance costs and reliability. Fifty years of civil aero-engine development are reviewed and trends showing the likely limits to the main engine performance parameters are provided. The paper concludes with consideration of a number of new civil aircraft and engine concepts that may emerge in the next 20 years

Future Challenges and Opportunities in Aerodynamics

Abstract: Investments in aeronautics research and technology have declined substantially over the last decade, in part due to the perception that technologies required in aircraft design are fairly mature and readily available. This perception is being driven by the fact that aircraft configurations, particularly the transport aircraft, have evolved only incrementally over last several decades. If, however, one considers that the growth in air travel is expected to triple in the next 20 years, it becomes quickly obvious that the evolutionary development of technologies is not going to meet the increased demands for safety, environmental compatibility, capacity, and economic viability. Instead breakthrough technologies will be required both in traditional disciplines of aerodynamics, propulsion, structures, materials, controls, and avionics as well as in the multidisciplinary integration of these technologies into the design of future aerospace vehicle concepts. The paper discusses challenges and opportunities in the field of aerodynamics over the next decade. Future technology advancements in aerodynamics will hinge on our ability to understand, model, and control complex, three-dimensional, unsteady viscous flow across the speed range. This understanding is critical for developing innovative flow and noise control technologies and advanced design tools that will revolutionize future aerospace vehicle systems and concepts. Specifically, the paper focuses on advanced vehicle concepts, flow and noise control technologies, and advanced design and analysis tools.

Effect of aircraft surface smoothness requirements on cost

Abstract: Reduction of aircraft manufacturing cost benefits aircraft direct operating cost (DOC). The degree of stringency in specifying aircraft smoothness influences cost, i.e. the tighter the tolerance, the higher is the manufacturing cost. Discrete surface roughness arising from manufacturing tolerance at the wetted surface may be seen as an ‘aerodynamic’ defect. Features such as steps, gaps, waviness and fastener flushness (termed excrescence), seen as defects, contribute to aircraft parasitic drag. The study is conducted on an isolated nacelle which is considered to be representative of an entire aircraft. Eleven key manufacturing features at the wetted surface of a generic long duct nacelle are identified, each associated with surface roughness. The influence of tolerance allocation at each of the key features is investigated to establish a relationship between aircraft aerodynamics and associated costs. The initial results offer considerable insight to a relatively complex problem in a multi-disciplinary environment. Excrescence drag arising out of these ‘aerodynamic’ defects is assessed by using CFD and semi-empirical methods. Cost versus tolerance relationships are established through in-house methods using industrial data. The aircraft unit price typically contributes from two to four times more than the fuel burn to aircraft direct operating costs. A trade-off study between manufacturing cost and aircraft drag indicates that, in general, there is scope for some relaxation of present-day tolerance allocation, to reduce aircraft acquisition cost, which would in turn reduce direct operating costs.

Investigation of the upstream end effect on the flow characteristics of yawed circular cylinders

Abstract: The flow over a yawed circular cylinders is a research area of current interest in aerodynamics due to the inherent similarities with that of the swept wing configuration. These models however, must be carefully mounted in order to suppress a phenomenon known as attachment line contamination. This contamination occurs when models span the entire test section width, exposing the flow to the highly disruptive test section wall turbulent boundary layers. One way of suppressing this phenomenon is to mount the upstream end of the model off the test section wall, whereby eliminating the interaction between the boundary layer and the model. Mounting the model with the upstream end free of the wall however, introduces end effects which are transported inboard from the downwash over the free end.

Flight simulation : past, present and future

Abstract: This paper outlines the development of flight simulators used in civil training, military training and in engineering simulation. It describes the evolution of flight simulators and the typical architecture of a modern flight simulator. The technical innovations, which have occurred in modelling, motion systems and visual systems are reviewed. The paper also reviews the transfer of training in flight simulation, the application of simulation to engineering design and outlines problems which are encountered in flight simulation. The paper concludes by reviewing the current trends in flight simulation.

The influence of mid-chord battle damage on the aerodynamic characterstics of two-dimensional wings

Abstract: This paper briefly considers the method of simulating gunfire damage to a wing and outlines the key basic assumptions used in modelling. The results of qualitative and quantitative investigations into the aerodynamic characteristics of a wing damaged at quarter chord are then presented. The results are discussed in terms of flow mechanisms, changes to surface pressure distributions and increments in lift, drag and pitching moment coefficients. For the damaged wing, the influence on force and moment coefficients was attributed to flow through the damage. This through flow was driven by the pressure differential between the upper and lower wing surfaces, and took one of two forms. The first form was a ‘weak-jet’ which formed an attached wake and resulted in small changes in force and moment coefficients. The second form resulted from either increased incidence, or damage size. This was the ‘strong-jet’, where through flow penetrated into the freestream flow, resulting in separation of the oncoming surface flow, and the development of a larger separated wake with reverse flow. The effect on force and moment coefficients was significant. The paper also compares the structure of the damage through flow with previously published results for jets in crossflows. Many similarities in the flow features were identified, although there were significant differences in the surface pressure distributions for the two cases.

Analysis of the stability modes of the non-rigid airship

Abstract: This paper describes and compares various analyses leading to the development of approximate models for the linear stability modes of the non-rigid airship. The progress of the analyses was variously dependent on assumptions made from the detailed scrutiny of linear numerical models for three airships. For each airship studied, the linear models were obtained from non-linear simulation models by linearising about a number of chosen trim speeds representative of a typical speed envelope. The decoupled linear models comprised the longitudinal and lateral-directional state equations of the neutrally buoyant airship, for speeds from the hover to 30m/sec. Since the fidelity of the earliest airship models was not known, the principal purpose of this paper is to re-visit the original analysis using a later airship model of known excellent fidelity. The longitudinal modes of the airship comprise the surge mode, the heave-pitch subsidence mode and the oscillatory pitch-incidence mode. The lateral-directional modes of the airship comprise the sideslip subsidence mode, the yaw subsidence mode and the oscillatory roll pendulum mode. Approximate models for these modes are derived and expressed in terms of concise aerodynamic stability derivatives. The mode characteristics are discussed, and the approximate models are compared with the actual airship modes over the typical airspeed envelope.

Acfd applications to store separation - status report

Abstract: : During the past decade, the Navy has considerably improved its capabilities in aircraft/weapon integration. In 1989 it took more than 400 hours of wind tunnel testing, which cost 1,500,000, and 20 flights to clear the JSOW from the F-18 to Mach 0.95. This year the MK-83 JDAM was cleared after only 60 hours of wind tunnel testing and five flights to the full F-18 aircraft envelope of Mach 1.3. This reduction occurred because the Navy not only learned to test smarter, but also developed an integrated approach to Modeling & Simulation (MS any further gains will have to come not by improving existing techniques, but by bringing new resources into the process. Several years ago the Office of the Secretary of Defense (OSD), under the Central Test and Evaluation Investment Program (CTEIP) funded a tri-service research project termed Applied Computational Fluid Dynamics (ACFD) for store separation. This project is meant to provide analysis tools that effectively use Computational Fluid Dynamics (CFD) for store certification analysis. ACFD will provide the needed tools that will reduce DOD dependence on wind tunnel and flight-testing. ACFD is not intended to replace the wind tunnel in the near future; rather it will be used to determine the critical regions of the flight envelope to help structure the wind tunnel test, and to explain any wind tunnel anomalies and help structure the flight test program. The objective of the program is to provide upgraded analysis tools that will support store certification requirements at less cost and in less time.

Life extending control of aircraft: trade-off between flight performance and structural durability

Abstract: The goal of life extending control (LEC) is to enhance the service life of complex mechanical systems, such as aircraft, spacecraft, and energy conversion devices, without any significant loss of performance, and can be achieved by making a trade-off between dynamic performance and structural durability. This paper presents the concept and a design methodology for robust life extending control of aircraft structures that are typically subjected to cyclic mechanical stresses. The controller design procedure relies on the specifications of flight performance and allowable fatigue crack damage at critical points of aircraft structures that serve as indicators of the effective service life. As an example, an aeroelastic model of the aircraft wings has been formulated and is incorporated into a nonlinear rigid-body model of the flight-dynamics. The H∞-based structured singular value (μ) synthesis method has been used to design robust life extending controllers based on a linearised model of the aircraft and a (nonlinear) state-space model of fatigue crack growth. The results of simulation experiments show significant savings in fatigue life of the wings while retaining the dynamic performance of the aircraft.

Vortices, sound and flames — a damaging combination

Abstract: The interaction between vortices, sound and combustion can lead to self-excited oscillations of such large amplitudes that structural damage is done. These occur because any small unsteadiness in the rate of combustion is a source of sound, generating pressure and velocity fluctuations. However, the velocity fluctuations perturb the flame, thereby altering the instantaneous rate of heat release. Instability is then possible because while acoustic waves perturb the combustion, the unsteady combustion generates yet more sound! Combustion oscillations can occur in afterburners and at idle in conventional aeroengine combustors. Lean premixed, prevapourised technology has tremendous potential to reduce NOx emissions, but is proving highly susceptible to self-excited oscillations. An overview of the physics of the interaction between vortices, sound and flames is presented, and illustrated by examples of instability in generic premixed ducted flames and in aeroengine combustors. The potential for both passive and active control is discussed.

NASA subsonic jet transport noise reduction research

Abstract: Although new jet transport airplanes in today s fleet are considerably quieter than the first jet transports introduced about 40 years ago, airport community noise continues to be an important environmental issue. NASA s Advanced Subsonic Transport (AST) Noise Reduction program was begun in 1994 as a seven-year effort to develop technology to reduce jet transport noise 10 dB relative to 1992 technology. This program provides for reductions in engine source noise, improvements in nacelle acoustic treatments, reductions in the noise generated by the airframe, and improvements in the way airplanes are operated in the airport environs. These noise reduction efforts will terminate at the end of 2001 and it appears that the objective will be met. However, because of an anticipated 3-8% growth in passenger and cargo operations well into the 21st Century and the slow introduction of new the noise reduction technology into the fleet, world aircraft noise impact will remain essentially constant until about 2020 to 2030 and thereafter begin to rise. Therefore NASA has begun planning with the Federal Aviation Administration, industry, universities and environmental interest groups in the USA for a new noise reduction initiative to provide technology for significant further reductions.

The truth about global warming

Abstract: God said to Noah, “Make yourself an ark with ribs of cypress; cover it with reeds and coat it inside and out with pitch. The length shall be three hundred cubits, its breadth fifty cubits and its height thirty cubits. You shall make a roof, giving it a fall of one cubit when complete; and put a door in the side of the ark and build three decks. I intend to bring the waters of the flood over the earth to destroy every human being under heaven.“ Genesis Ch. 6 God and Noah were not only the earliest recorded partnership in the history of naval architecture but they were also the most successful. God prepared the requirement and outline specification; Noah responded with a fully engineered design and construction which was not only to survive the worst floods ever seen but was also completed on time. Of course, we know nothing about cost overruns, but I doubt that cost was uppermost in Noah's mind and labour - he used his sons - was in any case cheap!

Elastic buckling design curves for isotropic rectangular plates with continuity or elastic edge restraint against rotation

Abstract: Elastic buckling design curves are presented for flat isotropic rectangular plates, which are either isolated or form part of a larger continuous plate structure. The design curves illustrate the effect of introducing combinations of elastic rotational restraints to the edges of simply supported isolated plates, which are subject to uniform in-plane compression or shear. Results for infinitely long and/or wide plates, with rectangular bays, are superimposed for comparison.

Use of bifurcation diagrams in piloted test procedures

Abstract: Abstract This paper investigates the potential benefits of utilising bifurcation analysis results in the planning and evaluation of piloted simulator trials of manoeuvrable aircraft, particularly when encountering nonlinear phenomena such as departure. Comparisons of bifurcation diagram predictions with piloted simulations are performed in order to validate the technique for application to realistic flight conditions. A close relationship is found between the theoretical and the experimental responses, and potential improvements in effectiveness of piloted test programmes are identified. Proposals for enhancing the predictive capability of bifurcation methods are made.

Experimental investigation of axisymmetric bodies with negative pressure gradients

Abstract: Using an exact solution of the Euler equations, the pressure distributions over some axisymmetric bodies with a negative pressure gradient over the majority of the surface (from 99.7% to 88% of the total body length) have been calculated. The results of wind tunnel tests for these bodies are presented.

The United Kingdom's contributions to the development of aeronautics. Part 1. From antiquity to the era of the Wrights

Abstract: The Millennium now being upon us, it is perhaps appropriate to pause a while and take stock before leaping into an ever more problematic future. But why pause? Indeed, why bother with the past? Surely our newest, ever more sophisticated computer packages provide all the necessary paths to that next commercial edge, that future, unassailable military superiority? To this the German polymath, Gottfried Wilhelm von Leibniz, provides an apt, timely and, indeed, timeless corrective: “It is an extremely useful thing to have knowledge of the true origins of memorable discoveries, especially those that have been found not by accident but by dint of meditation. It is not so much that thereby history may attribute to each man his own discoveries and others should be encouraged to earn like commendation, as that the art of making discoveries should be extended by considering noteworthy examples of it.”

The United Kingdom's contributions to the development of aeronautics: Part 2. The development of the practical aeroplane (1900-1920)

Abstract: The first part of this survey dealt mainly with the development of the earliest ideas on air resistance and their application to flight. That part of the survey paused at the achievement of powered flight during the early years of the twentieth century. Even then, the main problem remained as that of aerodynamics, a subject only vaguely understood by those aviation pioneers endeavouring to get off the ground. For them theoretical guidelines, as yet, could offer little help. One of the earliest of those very few analytically based results to achieve general acceptance was the statement that resistance is proportional to the product of fluid density, a characteristic body area and the square of the flow's speed. This result had appeared during the seventeenth century in Newton's work and had received some experimental confirmation. Newton's concurrent ‘rare medium’ concept, viewing fluid flow as a stream of disconnected particles, gained credence at that time since its results agreed with the above resistance proportionality. Yet this concept became mistakenly applied to airflow so as to predict plate resistances proportional to the square of the sine of the plates' incidence angles, a misleading result which caused confusion for at least the next century.

Recent experience in flight testing for pilot induced oscillations (PIO) on transport aircraft

Abstract: Though the subject of pilot induced oscillations (PIO) is not new, it has garnered significant attention in the past several years. Since the mid-1990s, Boeing Commercial Airplanes has been conducting specific flight tests of its products in order to evaluate PIO tendencies. Beginning with the 777-200, a generic suite of test manoeuvres has been used to evaluate PIO tendencies on each product. The testing has been conducted on a window of opportunity' basis with the intent to gather data and evaluate each model. To date, specific evaluations have been carried out on six different aircraft models spanning a wide range of aircraft sizes, inertial characteristics, and control system implementations. Each manoeuvre in the generic suite is discussed in detail. In addition, along the way, a large number of other manoeuvres have been used at various times to evaluate PIO tendencies. These are briefly described. No single test manoeuvre or technique has been identified which provides effective discrimination of PIO tendencies. Finally, the subject of the pilot in the loop is discussed with regard to achieving consistency in conducting PIO evaluations

The accuracy of flat plate, turbulent skin friction at supersonic speeds

Abstract: We present a comparison of turbulent skin-friction coefficients measured by floating element balances in zero pressure gradient conditions for a wide range of Mach numbers and Reynolds numbers. From these comparisons it is clear that although a large number of measurements have been made it is impossible to use these measurements to find the skin-friction coefficient for a given Mach number and Reynolds number to an accuracy of better than ±2 to 3%. This estimate of accuracy applies only to the range of Mach numbers and Reynolds within which there are a reasonable number of measurements to compare. Outside this range, say for Reynolds numbers based on momentum thickness greater than 40,000 and Mach numbers greater than 2 the uncertainty is greater

The aerodynamic interaction at the junction between a forward-swept wing and a plate

Abstract: A study has been carried out of the aerodynamic interference flow arising at the junction of a cambered, swept-forward wing and a flat plate on which a fully-developed turbulent boundary layer approached the junction. CFD predictions of the pressure field in the junction region were carried out. Flow visualisation tests and surface pressure measurements over a wind-tunnel model were conducted at incidences from -3° to +9°. With the wing at zero incidence, a single-tube yawmeter was used to explore the flow around the leading edge of the junction and an X-wire anemometer to examine the mean velocity and turbulence fields in the streamwise corners and at the trailing edge. The Reynolds number of the tests, based on the streamwise chord and a free-stream velocity of 30ms -1 , was 1.03 × 10 6 . At low incidence, a very'weak separation occurred in the plate boundary layer, a very short distance upstream of the junction. However the oncoming stream converges into the junction, appearing to confine any vortical motion at the leading edge to within a very thin layer below the closest point of measurement to the plate. Rudimentary vortical flow developed slightly downstream of the leading edge, but dissipated further downstream

The use of fuzzy logic in adaptive flight control systems

Abstract: A two step procedure is proposed for the design of nonlinear aircraft control systems. A classical design is first defined, based on a linearised aircraft model, and easily optimised by means of standard approaches. An outer-loop nonlinear controller is then used to enhance the whole control system. This latter controller is based on fuzzy logic rules and is of fixed structure. Its behaviour is based on the choice of a set of parameters that may be tuned by means a genetic algorithm based routine. The whole methodology is simple to handle and may be effectively used to give quick and efficient responses to the designer. The procedure has been verified with a couple of examples: the obtained results clearly show important improvements with respect to a classical methodology

The Cranfield aircraft handling qualities rating scale : a multidimensional approach to the assessment of aircraft handling qualities

Abstract: The Cooper-Harper scale is currently the only well-established scale for assessing aircraft handling qualities. However, as a result of having a unidimensional format the Cooper-Harper scale lacks diagnostic power and has also been criticised for exhibiting poor reliability. The Cranfield Aircraft Handling Qualities, Rating Scale (CAHQRS) is a new, multidimensional rating scale using concepts from two established scales, the NASA-TLX workload scale and the Cooper-Harper scale. This paper provides an overview of the development of the CAHQRS and the results from a series of validation trials in an engineering flight simulator. An in-flight refuelling task and an approach and landing task using a range of control laws were used in these evaluations. The results showed that the CAHQRS had high positive correlations with the Cooper-Harper scale. However, the CAHQRS also demonstrated greater validity, diagnostic power and higher test/re-test reliability than the previous scale.

Elastic–plastic buckling behaviour of shafts in torsion

Abstract: The development of more powerful and efficient aero-engines requires ways of increasing the torque transmitted by shafts, whilst also restricting their dimensions and weight. Thin-walled designs can assist this objective, but their use is limited by their torsional collapse behaviour. Of particular interest are conditions leading to buckling instability. The paper investigates the factors influencing this behaviour in order to provide the basis for an improved analysis method applicable to typical gas turbine aero-engine components. The Riks finite element algorithm has been successfully applied to both plain shafts and shafts with holes. In the former case, it is shown that the perfect cylindrical geometry must be given an initial perturbation in order to give accurate predictions. The perturbation imposed is obtained by scaling the mode shape from an eigenvalue solution so that the maximum radial deformation is a percentage of the wall thickness. The predictions for both plain and holed shafts have been validated experimentally.

Propulsion airframe integration design, analysis and challenges going into the 21 st century

Abstract: Propulsion airframe integration design and analysis has many challenges as we move into the 21st century. Along with the conventional challenges of integrating a propulsion system on an aircraft, new technologies and new business drivers dictate innovative approaches to the design process be developed and applied. On the technical side, new innovations in jet noise suppression and new aircraft concepts such as the blended wing body (BWB) will give rise to many challenges in propulsion system performance, operability, and meeting system requirements such as thrust reverse capability. Aircraft engine and aircraft manufacturers must have the appropriate design and analysis tools in place which provide the ability to react quickly to inevitable design changes, driven by constantly changing requirements, during the product development cycle. On the business side, the rapid globalisation of the business dictates that the latest electronic technology be utilised to enable speed in communication with global customers as well as revenue sharing partners. More than ever, cost and schedules dictate the use of analytical methods to minimise the amount of qualification testing. Design and analysis software must be flexible and capable of integrating CAD/CAM and CAE tools while maintaining configuration control of the product. The following paper describes some of the new technical challenges facing the industry. Innovative methods of addressing those challenges are described.