CoA. PhD, EngD, MPhil & MSc by research theses

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https://dspace.lib.cranfield.ac.uk/handle/1826/18365

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Open Access
The aerodynamics of slender aircraft forebodies at high angle of attack.
(2001-08) Petterson, Kristian; Poll, D. I. A.; Sillén, M.; Arlinger, B.
High angle-of-attack aerodynamics has been the focus of much attention as a result of the drive to increase combat aircraft manoeuverability and thereby improve survivability. A key factor in this regime is the behaviour of the complex vortical wake generated by the forebody. Even at zero yaw, the shear layers and vortex pairs which are symmetric at moderate angles of attack (alpha) can become highly asymmetric as alpha is increased. This can lead to large sideforces and yawing moments which may exceed available control power. Computationally simulating high-alpha forebody flowfields is known to be a challenging problem. This thesis details the evaluation and enhancement of the CFD code NSMB with the objective of improving qualitative and quantitive predictions of the flowfield around fighter-type forebodies throughout the angle-of-attack range. Results on a tangent-ogive body confirmed that computing asymmetric flow required the introduction of a space- and time-fixed surface excrescence or the use of a non-symmetric solution algorithm to simulate flow instabilities via transient numerical error. Although solution stability problems were encountered, results with the non-symmetric algorithm showed promise. Suspected turbulence modelling issues were addressed by implementing the k-w family. Having established a methodology, solutions were obtained for the forebody of a current fighter aircraft, the Saab JAS-39 Gripen. The computed data shows excellent experimental agreement for 0° ≤ a ≤50° over the clean geometry but the inclusion of a nose pitot probe was seen to destabilise the calculation and prevent convergence. Finally, a single vertical nose strake or ‘rhino horn’ was added. When undeflected, this stabilised the flowfield, reduced solution oscillation and negated sideforce. Deflecting the hom produced a stable flowfield with non-zero sideforce. Similar devices may be used, together with engine thrust-vectoring, in the next generation of combat aircraft and may also be added to existing airframes as a MLU.
Open Access
The aeroelastic behaviour and response to turbulence of a class of high performance tailless sailplanes.
(1978-09) Banerjee, J. R.; Griffin, K. H.
The symmetric flutter and response characteristics of a class of tailless .sailplanes-:are investigated combining rigid--body dynamics with elastic modes of deformation. The methods used are based on the normal mode approach through the use of finite element techniques and unsteady aerodynamic theories in two dimensional flow. The flutter speed and. aeroelastic modes are critically examined and the results are compared with those of a tailed sailplane. The flutter speed of the tailless sailplane is found to be low in comparison with those of orthodox layout and a parametric study is undertaken to locate-the reason for this; recommendations are made for improvements. A method is proposed for the analysis of the. short period oscillation characteristics of flexible aircraft allowing for the-effects of unsteady aerodynamics. - It is then applied to the type of tailless sailplane under investigation. The response to gusts and turbulence is evaluated using both statistical and discrete gust methods. Comments are made from the calculated results about the aeroelastic behaviour of tailless sailplanes in general. Computer programs developed for detailed aero-elastic calculations are presented.
Open Access
Analysis and prediction of the low speed flow over a highly swept wing
(2000-11) Shires, Andrew; Garry, Kevin P.; Fulker, J. L.
A combined experimental and theoretical study is described of the low speed flow over a highly swept and cambered wing that simulates the flow features of a transonic manoeuvre condition. The thesis is divided into two parts: Part I examines the research objectives from a customer perspective, with background information on the project history and funding sources. Since the research is aimed at improving the aerodynamic performance of low observable configurations, stealth technologies are discussed and their implications for combat aircraft wing flows. The management chapter of the thesis then discusses the influences affecting the decision making process for the acquisition of weapon systems in the UK. Part II describes the design of a highly swept and cambered wing that generates strong adverse pressure gradients near the trailing edge, leading to three-dimensional separations in this region. Using surface flow visualisation the nature of these flows is defined, indicating how the position of a separated streamline moves forward with increasing angle of incidence. These observations are confirmed by flow predictions using the SAUNA Computational Fluid Dynamics (CFD) method that solves the Reynolds Averaged Navier-Stokes equations, employing a two-equation turbulence model. The mechanism of the flow separation is also predicted using CFD, indicating that a separated stream surface reattaches at the wing trailing edge, forming a ‘tunnel’ of separated flow. To the authors knowledge this represents the first time that the main physical features of such a complex three-dimensional separated flow has been modelled using a CFD method. From an evaluation of the CFD methods employed, a design process has been proposed by which a wing designer can determine if wing flows over similar configurations remain attached. Additionally, the velocity magnitudes within parts of the separated shear layers and the wake are obtained using an optical non-intrusive measurement technique and give good agreement with the theory. -
Open Access
The application of computational techniques to the design of thermal ice protection systems
(1990-03) Leah, C. W.; Eshelby, M. E.
A bibliography citing 1350 references related to all aspects of aircraft icing is presented. An ice adhesion database has been compiled, and a survey of ice adhesion test rig designs is presented. An ice shear adhesion test rig is designed, manufactured and operated. The results are added to and compared with those in the database. The adhesion of ice accreted on metal and non-metallic composite aircraft propeller blades in flight is assessed. A one-dimensional thermodynamic study of ice accreted on a non-metallic composite propeller blade with ’built-in1 electrothermal de-icing system is effected and compared with a solid metal blade. A two-dimensional aerofoil icing prediction program is modified and executed in series with additionally created software to predict icing on aircraft engine air intake duct lips in flight. Hence a new de-icing system design method is developed to replace and improve upon an existing one. Validation of the new design method is initiated and a programme for its completion is presented. Icing predictions produced using the new method correlate well with expected values derived by several methods. The new design method is incorporated in the sponsoring companies business with an appropriate computer hardware/software system and employed, in parallel with the ’old’ approach (till validation completion), on commercial projects. Market response indicates a definite preference for the new design method. The ’non-technical’ and other related practical topics of implementing the new de-icing system design method, including specification and installation of computer hardware and software are discussed. Marketing, financial and other commercially oriented subjects are addressed in justifying the research and the implications of introducing the new design method into the commercial business of the sponsoring organisation are considered. This research programme forms a part of a large, long term icing research commitment. Continued research subjects and investigative routes including the integration of ice adhesion mechanical characteristics into the de-icing system design method are discussed together with their relationship to this and the overall icing research plan.
Open Access
The application of neural networks to spacecraft control
(1994-08) Cooper, A.; Lewis, D. J. G.
This thesis investigates how two neural network-based control techniques can be applied to a specific spacecraft control problem. The neural networks used are simple backpropagation networks, consisting of one or more tansigmoidal neurons (neurons with tanh transfer functions) in a hidden layer, and a linear neuron in the output layer. The neural network control techniques investigated here are Direct Model Inversion and Indirect Model Inversion. The spacecraft control problem is that of reducing the vibrations of a spacecraft payload. The source of the vibrations is a mass imbalance in one of the reaction wheels of the spacecraft. Four components are represented in the spacecraft model. These are rigid body inertia, solar array flexure, fuel slosh and payload vibration. A simple sinusoidal signal is used to model the disturbance torque produced by the reaction wheel mass imbalance. The complete model is broadly based on the Solar Heliospheric Observatory (SOHO) that is due for launch in 1995. Each of the neural network control techniques used is shown to be successful in reducing the effects of the disturbance torques on the spacecraft payload. However, in each case, a simple positional feedback gain term provides more effective and reliable control.
Open Access
Application of nonlinear control theory in weapon guidance and control
(2001-03) Brundle, D.; Thomasson, P. G.
This thesis considers the application of nonlinear control theory in two subjects pertinent to weapon applications. Initially, Section 2 considers the development of a simple nonlinear autopilot for a Laser Guided Bomb (LGB). Later a nonlinear autopilot design is developed using a Pulse-Width Modulated (PWM) controller derived from the method developed by Bemelli-Zazzera et al4. This is applied to an LGB utilising a “bang-bang” actuator, enabling the control surfaces to achieve a pseudo-proportional response. The PWM design stems from an equivalent Pulse Amplitude Modulated controller, which required a design technique to be developed for a linear autopilot and, in addition, simulation of an electro-mechanical actuator. Simulation demonstrated that the PWM controller can achieve the desired response but the design must incorporate actuator dynamics. Section 3 considers the use of nonlinear control theory to examine the nonlinear intercept equations using a Proportional Navigation (PN) guidance law. Using a simple heuristic example, PN is introduced and vector algebra used to develop a simple model of the intercept. The model is then used to illustrate the importance of the kinematic gain. Using the method pioneered by Ha et al16, Lyapunov theory is used to demonstrate that PN is a robust guidance law. Although generally derived assuming the target maintains rectilinear flight, Lyapunov theory is used to demonstrate interception is always possible provided the pursuer has sufficient manoeuvre advantage over the target. Noting that many missiles incorporate a 1 directional warhead, Lyapunov theory is used to design a time-varying rate bias that controls the direction of approach to the target. Simulation demonstrates that the guidance requirements are indeed achieved by this law but additional effort is required by the control system. In Section 3 it is demonstrated that the PN guidance law will always ensure an intercept, i.e. it does not by itself generate miss-distance. In the final part of Section 3, using adjoint software designed by Zarchan42, it is demonstrated that miss-distance develops in practical systems as the result of sub-system dynamics
Open Access
The application of relative navigation to civil air traffic management
(2000-08) Sangpetchsong, K; Allerton, David J.
This thesis addresses navigation and guidance which will be required for air traffic management in Future Air Navigation Systems (FANS) and Free Flight. In particular, the thesis covers the issues of data fusion and integrity monitoring, to provide an adequate level of aircraft separation assurance, based on relative navigation (RELNAV). The evolution of air navigation systems is described. The principles of Kalman filtering and Joint Tactical Information Distribution System (JTIDS) RELNAV are covered. Sensor models of strapdown Inertial Navigation System (INS), Global Positioning System (GPS), and Automatic Dependent Surveillance-Broadcast (ADS-B) are developed in Matlab and integrated to form a hybrid navigation system. RELNAV algorithms for centralised and decentralised Kalman filtering are formulated, and their respective performances are analysed using Monte Carlo simulations for an airspace containing several aircraft. It is shown that RELNAV, based on the integration of INS and ADS-B, can enable aircraft to maintain safe separation independent of GPS, where it is assumed that an ADS-B datalink provides accurate time synchronisation. An alternative approach that integrates INS, GPS, and ADS-B is developed and analysed. It is shown that this approach is more applicable to civil aviation because it eliminates the needs to establish and manage several navigation communities simultaneously, in effect, exploiting GPS as the navigation controller. The source selection functions used for RELNAV are also developed, and the stability and performance of this technique is evaluated from simulation studies. A failure detection algorithm that monitors the residuals of a Kalman filter is derived and evaluated using Monte Carlo simulations of GPS failures. It is shown that this algorithm combines the use of likelihood functions and chi-squared tests, allowing both a false alarm rate to be selected and a failed sensor to be identified. Finally, an algorithm is developed for separation assurance to determine the probability that aircraft are closer than a pre-determined distance, taking into account flight path prediction errors. It is shown that this algorithm simplifies aircraft conflict detection in three dimensions and allows the conflict probability at a particular time to be determined. This approach is validated using Monte Carlo simulations of aircraft trajectories which include near-misses.
Open Access
An approach to configuration design synthesis of subsonic transport aircraft using artificial intelligence techniques
(1991-12) Pasaribu, H. M.; Fielding, John
This thesis outlines a computer system developed to tackle the configuration design synthesis of subsonic transport aircraft. The system provides an interactive design environment which combines the reasoning process of the aircraft configuration definition and the numerical analyses that lie behind the reasoning. The system applies Artificial Intelligence (AI) techniques to the configuration analysis. This gives the system the capability to reason with the configuration choices. The system is built in modules, with each module is clearly separated to deal with a complete analysis in a specific aspect of design. The integration of the modules is done through an intelligent interface and a common database. The interface also incorporates AI techniques in deciding the sequence of execution of the modules and the processing of the input/output data for a particular module. The interface permits the sharing of information among the modules and supports modularity and flexibility of the system for future development. Since each module is independent, it can be easily modified or replaced without disturbing the balance of the system. An extensive library of application programs is included in the engineering analysis module which enable detailed analysis to be performed. The system is equipped with a simplified database management and a special purpose graphics module which is extensively used for the presentation of the output. The system has been validated and tested. It can handle both propeller driven and jet engined aircraft. An example of the case studies is presented. The improvement of the system for future development is also considered. These include the extension of the knowledge base for dealing with problems in other modules, the enhancement of the application programs in the engineering module, and a possible interface with an established Computer Aided Drafting (CAD) system.
Open Access
Buckling of corrugated core sandwich panels
(1969-09-19) Patel, N. G.; Griffin, K. H.
A computer program is developed to determine the buckling stress and deflections of symmetric corrugated core sandwich panels. In the program freedom for lateral deflections at core to face-plate junction is allowed for. Provision is also made to study the effect of variation of core bend radius. A range of test specimens using four basic core configurations is designed to assess the effect of core bend radius on the buckling stress of the panel. The computer program indicates that above a certain value of core bend radius there is a marked drop in the value of critical buckling stress and a change in buckling Mode. The values of deflections at core to face-plate junctions at low buckling wave-lengths are not reliable* Due to the limited range of the experimental work, it is not possible to draw any conclusions on the effect of core bend radius on the buckling stress. The method used for determining the experimental buckling load is somewhat subjective in application, and its accuracy is difficult to assess. In general, the experimental values of buckling stresses are 15% higher than those predicted by the computer program. These discrepancies are not large when dimensional and material property variations are considered and indicate that the computer results are giving the correct trend and are conservative. Recommendations are made for : (i) Investigation of the buckling deflections at low values of buckling wave-lengths for specimens with high face-plateto-core thickness ratio. (ii) A test programme covering a wider range of specimens than that covered by the test programme in this study. and (iii) Trying out the other two methods of determining the buckling load.
Open Access
A class of upwind methods for conservation laws
(1994-06) Billett, S. J.; Toro, E. F.
Various new methods for the solution of hyperbolic systems of conservation laws in one, two and three space dimensions are developed. All are explicit, conservative timemarching methods that are second order accurate in space and time in regions of smooth flow and make use of local Riemann problems at intercell boundaries. In one space dimension, the Weighted Average Flux (w af ) approach of Toro is extended to generate a scheme that is stable with timesteps twice as large as those allowed by the stability conditions of the original scheme. A Riemann problem based extension of the Warming-Beam scheme is considered. Total Variation Diminishing (t v d ) conditions are enforced for both schemes. Numerical results for the Euler Equations of Gas Dynamics are presented. In two and three space dimensions, finite volume versions of the waf scheme on Cartesian grids are derived for the linear advection equation. Two two dimensional schemes are found that are second order accurate in space and time. One of these is extended for the solution of nonlinear systems of hyperbolic conservation laws in two separate ways. The resulting schemes are tested on the Shallow Water equations. The equivalent three dimensional schemes are also discussed. The two dimensional schemes are then extended for use on structured, body-fitted grids of quadrilaterals and one of these extensions is used to demonstrate the phenomena of Mach reflection of shallow water bores.
Open Access
Closed loop stepping motor application in prosthetics
(1969-09) Williams, K. F.; Thomason, R.
In recent years much work has been done in many countries on the development of powered artificial limbs for use by adults and children with congenital deficiencies i.e. present at birth, and by amputees. The established methods of actuating these powered limbs are either by pneumatic motors, operated from compressed carbon dioxide or by electric motors. Some designs have also used hydraulic and electro-hydraulic actuators. In the development of the complete powered limb the following problems have also to be considered: (a). The design of the mechanical components and the harness for attaching the appliance to the body. (b). Control input units by means of which signals from the wearer operate the powered limb. Control input signals maybe either mechanical (bone movement) or electrical. (c). The control system mechanies, involving the type of feedback which should be used. This can be either visual feedback or sensory feedback from the forces exerted by the appliance.
Open Access
The design and analysis of a reconfigurable flight control system for advanced civil aircraft
(1994-04) Oliva, A. P.; Cook, M. V.
This work is concerned with the design of a pitch-rate-commandattitude-hold Command and Stability Augmentation System in order that the augmented aircraft meets the Gibson dropback criterion, the Gibson phase-rate criterion and MIL-F—8785C requirements. The work shows two methods of design, pole-placement and optimal control, and discusses the design procedures, the advantages and disadvantages of each method. The work is also concerned with the redundancy aspect of the control law design, and so not only a sensor based design but also an observer-based design are investigated. In order to design the observer-based control law, a Doyle-Stein observer was implemented. Two methods showing how to design the observer are discussed and presented, and the special characteristics of this kind of observer are also considered. The performance of the observer-based control law was compared with that of the sensor-based control law. The failure transients and characteristics of the control law are also studied and presented. Finally an evaluation of the control law was carried out with a non-linear model of the B-747 aircraft, and a simple altitude-hold autopilot was designed to work together with the stability augmentation control law.
Open Access
The design and development of high speed externally pressurized air bearings for an internal grinder
(1969-09-20) Ghosh, K.; Leete, D. L.
Briefly, the basic theory of externally pressurized air bearings which provides shaft support, with very low friction in high speed devices, has been discussed in this paper. Some considerations have been given to the various factors such as axial flow, circumferential flow, dispersion effect, compressibility,. effect and hydrodynamic effect. The main objective was to produce an air bearing system capable of running between 45,000 to 80,000 rev/min., driven by a belt, with the radial and axial stiffnesses of 250,000 lb/in. and 150,000 lb/in. respectively. Moreover, the air consumption rate had to be as low as possible, at a maximum supply pressure of 80 lb/in2 • (gauge), while at the same time the manufacturing method had to be as simple as possible. With the aim of satisfying these requirements, an experimental internal grinding head was designed and manufactured. Throughout the manufacturing stages, the most conventional production techniques were used; even the radial clearance of 0.0005:ins. between the shaft and the journal bearing was achieved by grinding and lapping, and also the smallest journal orifice hole diameter of 0.004 ins. was successfully produced by using a small drill. ·The tests consisted mainly of the verification of the predicted values of stiffness, volumetric air flow and free running and the effect of the driving method. The complete tests were carried out in two test rigs one for direct drive and another for belt drive. The test results revealed a satisfactory correlation between predicted an measured values. At the supply pressure of 80 lb/in. (gauge), the stiffnesses of 235,000 lb/in. and 244,000 lb/in. at the centre position of the rear and_ the front journal bearings, and 190,500 lb/in. and 200 7 000 lb/in. for the front and the rear thrust bearings respectively, were obtained. Without any rotation, the total air consumption was 0.95 ft3/min., which was 1{-% higher than the predicted value but, at 60 7 000 rev/min. due to hydrodynamic effects, the reduction of air consumption of 7½f; of the no rotational condition was observed. An actual speed of 75,000 rev/min. was successfully achieved by the experimental head. Further increase of speed was limited by the pull and the vibration induced by the belt.
Open Access
The development and evaluation of an instrument to assess and develop the desirable attributes of RAF Airmen
(2005-03) Johansen, Dawn; Asch, Rachel
The research reported in this thesis related specifically to Royal Air Force (RAF) personnel. The primary aim of the work was to design an instrument to assess and develop the desirable attributes of airmen regardless of rank, trade-group or location. The work comprised three separate but integrated studies that culminated in the successful introduction of the tool to the Recruit Training Squadron (RTS). The first study consisted of a job analysis based on repertory grid technique with data elicited from 128 RAF personnel with experiences of line management of airmen. This stage of the research yielded 117 constructs from which twenty desirable attributes (each illustrated by observable behavioural examples) were generated. The second study was conducted to assess the content validity and generalisability of the list of twenty attributes identified from the first study. Thirty three officers and airmen, who represented all RAF ranks, considered the face validity of the list: ranked the twenty items in order of importance; and rated a ‘good5, ‘average’ and ‘poor’ airmen on each of the attribute items. The results of this study were used to develop the instrument for recruit assessment and development. The final study was designed to examine the reliability and validity of the tool for use at RTS. In this stage of the research, RTS instructors were trained in the use of the tool which they then trialled with four separate recruit groups. In trial recruits were assessed on the development tool on three occasions over seven weeks. At the end of the initial training period development scores were correlated with standard assessment measures. The correlations between development instrument scores and standard training measures were all statistically significant, with some associations being particularly strong.
Open Access
Dynamics and control of flexible articulated space manipulators with large payloads
(2002-03) Wiedemann, Simon M.; Kirk, Colin L.
This thesis studies the dynamics and control of flexible articulated space manipulators with large payloads similar to the Space Shuttle Remote Manipulator System. For dynamic response analyses an exact analytical method to compute natural frequencies and mode shapes of space manipulator systems with varying degrees of complexity is developed. Dynamic response analyses are performed comparing the results obtained using the exact mode shapes with those obtained when using assumed mode shapes for a series of different manipulator slew manoeuvres and Shuttle thruster firings. Possible methods for active vibration damping control of the manipulator are discussed, including the methods presented by other researchers. In this thesis it is proposed to use reaction wheels in a closed-loop control scheme, and its advantages and disadvantages compared to other methods are discussed. The problem of payload capturing and post-capture dynamics are addressed, as well as the dynamics following an emergency braking of the robot. For these cases, a simple method to estimate upper limits of dynamic responses is developed, and results obtained with this method for various example cases are compared with results computed by professional software. Finally the dynamic responses of a space station to various robot manoeuvres are analysed. These responses can have detrimental effects on micro-gravity and similar experiments. The analyses are performed using a simplified model of the International Space Station and its Mobile Remote Manipulator System.
Open Access
The effect of an end plate boundary layer on half delta wing flows at low Reynolds number.
(2000-08) Alkhozam, Abdullah M; Garry, Kevin P.
An experimental investigation has been carried out to study and understand the influence of an end plate boundary layer on half delta wing models at low Reynolds Number. The programme involved measurements in two facilities: a vertical water tunnel which was used for flow visualisation studies and a conventional closed working section wind tunnel for both flow visualisation and surface static pressure measurements. In both facilities dynamic and steady state or static measurements were made on half delta wing models with 55° and 70° sweep and varying thickness/chord ratio under the influence of a number of artificially generated end plate boundary layers. In both facilities, of all model configurations tested, for both dynamic and static test conditions, vortex burst was seen to move upstream, inboard and away from the wing surface as the angle of attack is increased and vortex core trajectory is seen to move towards the wing root, which is consistent with the findings of previous researchers. Vortex breakdown position is seen to move upstream, inboard toward the wing root and away from the wing surface as the end plate boundary layer thickness is increased. This is attributed to the influence of the interaction between the horseshoe vortex and the half delta wing leading edge vortex as a result of changes in the wall boundary layer thickness. In terms of vortex core trajectory, increases in end plate boundary layer thickness are seen to displace the vortex core towards the wing root. During dynamic tests an increase in wall boundary layer thickness is seen to suppress the hysteric behaviour of the vortex trajectory. Surface static pressure measurements at Reynolds Number of 479,000, during both static and dynamic tests, make it possible to see that the influence of changes in wall boundary layer thickness are small, often insignificant, at (x/c) locations greater than 0.45. This is consistent with an increase in wall boundary layer thickness promoting earlier vortex breakdown. Correlation between smoke flow visualisation (of both vortex breakdown and trajectory) and surface static pressure measurements, using the half-width of the suction peak as a parameter, was good. Differences between vortex characteristics in the water tunnel and wind tunnel were consistent with the influence of Reynolds Number.
Open Access
The effects of junction modifications on sharp-fin-induced glancing shock wave/turbulent boundary layer interaction
(1994-06) Koide, S.; Stollery, J. L.
The effects of junction modifications on the glancing shock wave/turbulent boundary layer interaction generated by a sharp fin placed on the wall of a supersonic wind tunnel were examined experimentally at a Mach number of 2.46 and a Reynolds number of 2.59xl06/m. The interactions between a turbulent boundary layer on the wall and shock-wave systems caused by a fin with a fillet and several fins with different strakes were examined individually in order to find an effective modification technique. The flow features obtained by oil flow visualization, surface pressure measurements and liquid crystal thermography were compared with the data from an unmodified fin to evaluate the effects of each modification. The comparisons indicated that a "srake-type" modification had a weakening effect on the interaction-induced separation. To understand the flowfield structures, three-dimensional surveys using laser-light-sheet flow visualization were employed with schlieren photography and oil flow visualization. In addition to these experimental observations, an Euler CFD solver was used to help understand the inviscid flow structures which play important roles in the interaction behaviour. Based on the data experimentally and numerically obtained, a method was proposed for predicting the junction shapes needed to prevent separation.
Open Access
The employment of jet V-STOL aircraft at sea
(1981-01) Kinch, M. J.; Howe, D.
The means by which the Royal Navy will continue to operate fixed-wing aircraft at sea is by employing VTOL or· given an aid to-take-off, STOVL aircraft. The aid being ' brought into service is -the Skijump, which permits a large increase in payload over unassisted VTOL. The effectiveness of skijump increases with its exit angle up to about 40°, but other considerations of size and ungainliness set a practical lim~tation nearer to 20°. The endspeeds required for ballistic launch off a skijump could be achieved or-enhanced by the use of assistance by catapult or rocket motor. Both of these would call for the initiation of programmes of full research and development, while the skijump, capable of conferring. equivalent performance if it is long enough, already exists. The· smallest number of aircraft in an airgroup able to keep up a useable flying task is three. A vessel big enough to mount three aircraft together with the gear necessary to support and arm them would be big enough to mount a skijump as well. Its size is dictated too by the sea conditions in which it is expected to keep operational. The vessel in question should be a displacement ship, either conventional (e.g. large frigate) or unconventional (e.g. Small Waterplane Area Twin Hull). There is no role here for either hovercraft or hydrofoil. Commitment to the skijump.in the ship means commitment to vectored-thrust as a means of propulsion in the next aircraft~ When specified it must be compatible with existing skijump decks, and it should be single-engined. Its targets for Reliability and Maintainability mµst be wholly related to the Availability called for, and must be given equal prominence with performance.
Open Access
An experimental study of an air-blast atomiser
(1967-10) Bryan, R. H.; Lefebvre, A. H.; Norster, Brian
Visual observations and an optical method of measuring the mean droplet size in sprays have been used to assess the performance of a prototype airblast atomiser, particularly at the low fuel and air flow conditions corresponding to engine cranking. The results obtained from this and a shroud version of the same basic atomiser have enabled design modifications to be made which have resulted in a marked improvement in performance, both in terms of the spraying characteristics of the atomiser at low fuel and air flows, and in terms of the measured droplet size produced. The latter for the final design was very close to that predicted by Wiggs for airblast atomisers operating at the test conditions employed. An apparatus has also been designed to enable tests on the atomisation quality of airblast atomisers to be carried out at pressures up to 250 p.s.i.,, using the same optical technique to make mean droplet size measurements.
Open Access
Flame radiation & smoke emissions in gas turbine combustors
(1969-09-19) Russell, D. T.; Lefebvre, A. H.
The objective of this thesis was to investigate flame radiation and smoke emission phenomena in a combustor typical of those in use in aircraft gas turbine engines. One of the primary aspects of the investigation was to compare the performance of airblast atomisers with that of conventional swirl atomisers under as near identical conditions as possible. A brief study of combustion chamber processes and carbon formation and thermal radiation mechanisms was made, with consideration of such experimental data as exists in the literature. An airblast atomiser and combustion chamber was developed to a state whereby testing at rich mixtures at moderate combustion pressures could be performed for short periods. This enabled a preliminary comparison to be made monitoring the following parameters:- (i) Flame total radiation. (ii) Exhaust smoke level. (iii) Weak extinction. Data was collected at three different combustion pressures for various air-fuel ratios and chamber velocities. The results showed that the airblast atomiser offers significant advantages over swirl atomisers with regard to total flame radiation and exhaust smoke emissions whilst suffering from a poor weak extinction. A more detailed analysis was performed for swirl atomisers using the Schmidt Method to establish flame temperatures and emissivities. The object of this work was to assess the effects of droplet size and combustion pressure on flame radiation. Owing to the poor atomisation quality and fuel distribution of the airblast atomiser used for the preliminary work, two new injectors were designed. It was hoped that a detailed analysis for these injectors could be performed via the Schmidt Method in order to provide a comparison with the swirl atomisers. Unfortunately, manufacturing was not completed with sufficient time left for this analysis to be completed.