This study focuses on the co-operative salvo attack problem of multiple missiles against a stationary target under jointly connected switching topologies subject to time-varying communication delays. By carefully exploring certain features of the typical pure proportional navigation guidance law, a two-stage distributed guidance scheme is proposed without any information on time-to-go in this study to realise the simultaneous attack of multiple missiles. In the first guidance stage, a co-operative guidance law is proposed using local neighbouring communications only to achieve consensus on range-to-go and heading error to provide favourable initial conditions for the latter phase, in which switching topologies and time-varying communication delays are taken into account when obtaining sufficient conditions of consensus in terms of linear matrix inequalities. Then, missiles disconnect from each other and are guided individually by the typical pure proportional navigation guidance law with the same navigation gain to realise salvo attack in the second guidance phase. Finally, numerical simulations are carried out to clearly validate the theoretical results.

Helicopter accidents cause many fatalities, and their avoidance is a major area of work for Civil Aviation safety authorities around the World. This paper uses helicopter accident data from the United Kingdom between 1986 and 2005 for 566 accidents and from New Zealand between 1996 and 2006 for 230 accidents to analyse helicopter accidents according to five categories of causes: airworthiness failure (engine); airworthiness failure (non-engine), operational failure, maintenance failure and mixed failure (i.e. operational and airworthiness combined). Factors associated with accidents, e.g. the engine types and weights of the helicopters involved; the nature of the operations and the phase of flight of the helicopter are also analysed. Operational failures were further analysed by Human Factors Analysis and Classification Scheme (HFACS) and airworthiness failures by a logical scheme of helicopter components. The results indicate that operational failures, especially due to unsafe acts, are the major cause of accidents in both countries followed by airworthiness causes. Light single piston helicopters are by far the major group associated with accidents in both countries, with few accidents for twin turbine helicopters. The majority of accidents were in non-public operations with few in public operations and in both countries, the cruise/flight/circuit phase has the largest number of accidents. Further analyses indicated statistically significant associations: type of helicopter and the cause of accidents; type of helicopter and the phase of flight; cause of accidents and nature of flights; cause of accidents and phase of flights; training flights and inadequate supervision; landing and procedural error and cruise and attention failure.

A NACA 641-412 aerofoil with circle and star damage and also three repair configurations has been numerically investigated. Two different methods of mesh generation were employed: multi structured mesh for the star damaged aerofoil and unstructured mesh for the other aerofoils. The results show that the damage will cause a reduction in lift coefficient of the aerofoil and also a different stall angle relative to that of the undamaged aerofoil. Each kind of repair improves the aerodynamic characteristics of the aerofoil considerably. The flow Field inside the damage hole and the cavity caused by the repair sheets was also investigated. Finally, the numerical solution was qualitatively and quantitatively validated using the available experimental results.

This paper describes the development of a wind turbulence criterion for the safe operation of helicopters to offshore installations. The development of the criterion was recommended following a review of the environmental effects around offshore platform helidecks.

Currently, criteria exist for ambient temperature and for vertical wind component in the vicinity of helidecks, but a questionnaire-based survey of helicopter pilots revealed that the principal safety hazard and source of highest workload is turbulence around offshore installations. The new turbulence criterion will plug a long-standing gap in the guidance on offshore helideck design.

The paper describes how the criterion has been developed using piloted flight simulation in a research flight simulator together with data from wind tunnel tests on offshore platforms. Initial validation has been successfully performed, and extended to include correlation with the large database of helicopter operational flight data records being collected through the UK North Sea Helicopter Operations Monitoring Programme (HOMP).

The turbulence criterion will be used, together with existing criteria on vertical wind component and temperature, in the assessment of new offshore installation designs, or proposed modifications to existing designs, to determine wind conditions where turbulence is likely to be excessive for safe helicopter operations. These will be used to estimate helideck operability and thereby inform the installation topsides design process, and will provide input to the setting and maintenance of helicopter operational limitations for individual installations.

The work will lead to improved safety through better prediction of safe operating envelopes and helideck operability at the design stage. In addition, development of the work is expected to enable the wind environment around offshore installations to be mapped and monitored in-service using helicopter flight data records.

The new turbulence criterion has been included in updated guidance on helideck design, and offshore installation designers are now required to inform helicopter operators about wind conditions which result in violations of the turbulence criterion on their offshore installations (as is currently the case for the temperature and vertical wind component criteria).

Collision-avoidance is a safety-critical requirement to operate UAVs in non-segregated airspaces. In case of communication problems between a UAV and the corresponding pilot-in-command, a technology is required onboard the UAV to implement a capability to detect and avoid collision-hazards even autonomously. After an introduction to the problem of developing a so-called sense-and-avoid system and its avoid-function, this work presents a solution in terms of algorithms to implement the above capability. To detect and resolve potential mid-air conflicts, a geometric deterministic approach has been utilised: an intruder is modeled trough a moving-ellipsoid and a four-dimensional approach in the time-space domain provides the solution. The approach makes use of kinematics information to detect potential conflicts and to provide actions for conflict resolution, such as speed-changes in intensity and/or direction. The proposed solution also enables the UAV to meet the applicable vertical and horizontal minima of separation and to comply with real-time constraints.

Space debris represents a significant risk to satellite operations, due to the possibility of damaging or catastrophic collisions. Consequently, many satellite operators screen the orbiting population for close approaches with their on-orbit assets and a public conjunction assessment service, Satellite Orbital Conjunction Reports Assessing Threatening Encounters in Space (SOCRATES), generates close approach predictions on a daily basis for all satellite payloads in the catalogue. These screening capabilities are used to inform operational decisions relating to risk mitigation but it is anticipated that the demands placed on these services will increase as debris becomes more prolific. This hypothesis is explored in a preliminary analysis of conjunction data for the years 2004 to 2009 and a new ‘Business as usual’ study using the Debris Analysis and Monitoring Architecture for the Geosynchronous Environment (DAMAGE) model. The results suggest a 50% increase in the number of close approaches reported by SOCRATES (or its equivalent) within the next ten years. By 2059, daily conjunction reports could contain over 50,000 close approaches below 5km, affecting the demands placed on tracking facilities and satellite resources.

The design of a 6U CubeSat including spacecraft systems and imaging payload is described for an Earth observation mission. From a Sun synchronous orbit at an altitude of 600km the design enables imaging with a 6.5m GSD, an optical MTF (on axis) of >59% at half Nyquist and >35% at Nyquist, a 26km swath, 12 bit digitisation and SNR of 120-200:one in five spectral bands; blue, green, red, red edge and near infrared. Data can be downlinked at the rate of 14 Mbps to a 3.7m S band ground station. This design allows an 8kg CubeSat to perform Earth observation missions equivalent to those of current 50-150kg microsatellites, with a corresponding reduction in cost.

The present paper focuses on a three-dimensional unsteady turbulent synthetic jet to assess the accuracy of a compressible simulation and some important parameters including the simulations of the actuator, cavity height and Reynolds number. The two-equation SST/k − ω turbulence model is used to predict the flow behaviour. Results show that the compressible simulation case is more accurate than the incompressible one and the dynamic mesh exhibits more reliable results than the mass flow inlet boundary in the compressible simulation. The compressible case displays a delay in the phase of instantaneous velocity for all three Reynolds numbers. Also, the maximum of mean velocity is less than the incompressible case. Moreover, an increase in the Reynolds number leads to an amplification of the peak of mean velocity magnitude. Finally, results demonstrate that a reduction in the cavity height regarding the compressible simulation case causes a reduction in the phase delay and rise in peak of instantaneous velocity magnitude.

Robust control of non-linear systems is a challenging task, notably in the presence of external disturbances and uncertain parameters. The main focus of this paper is to solve the trajectory tracking problem of an unconventional quadrotor with rotating arms (also known as a foldable drone), while overcoming some of the challenges associated with this type of vehicle. Therefore, in a first step, the model of this vehicle is presented, taking into account the change of the inertia, the centre of gravity, and the control matrix. The theoretical foundations of backstepping control, based on the finite time Lyapunov stability theory and enhanced by a Super-Twisting algorithm, are then discussed. Numerical simulations are performed to demonstrate the efficiency of the suggested control approach. Finally, a qualitative and quantitative comparative study of the proposed controller with the conventional backstepping controller is performed. Overall, the obtained results show that the proposed control strategy outperforms in terms of accuracy and resilience.

Kite design has recently seen a revival, with sports such as kite buggying and kite surfing becoming significant businesses in some countries. Since the aerodynamics of kites has not been explored in much depth this paper sets out to explain a major behaviour of kites, predicting where they will settle during flight. The analysis is confined to a vertical plane containing the kite string. It examines the basic forces for single-line or two-line kites and shows how the line azimuth angle(s) and equilibrium settling point(s) can be derived from the aerodynamic properties of the kite, from the kite mass and from the bridle lengths. It is noted that while several equilibrium points are predicted not all are stable equilibrium points. Wind tunnel tests with a specially built rigid model are described and these are shown to confirm theoretical predictions.

The dynamic behaviour of gas turbines is of great practical importance, but unfortunately it cannot be investigated experimentally until late in the development programme when the mechanical integrity of the engine has been established. In the case of aero engines there is a requirement for fast engine accelerations and all engine transients must be achieved with complete safety. Thrust response rates must be guaranteed by the engine manufacturer at a very early stage and clearly it would be of great value to study engine transient behaviour at the design stage; this should lead to a significant saving in development time and cost by decreasing the amount of test bed running required.

Simulated hailstones were made to impact on the rotating spinner and fan assembly of a Williams FJ44 engine. The mass distribution of ice behind the fan was determined by use of a suction tube technique. Suction was added to ensure that the tube did not affect the flow through the fan assembly. The strong air flow behind the fan meant that the ice caught by the tube melted and evaporated. This made it difficult to accurately determine the mass distribution of ice. As a result, the simulated hailstones were made from a water-salt solution so that the weight of salt residue could be measured after the water had evaporated, and hence the amount of ice caught was determined. A parametric study into the hail ingestion characteristics of the fan assembly was carried out. The parameters studied included the radial position of the impact point, the rotational speed of the fan and the position of the splitter between the core engine and bypass duct. The results showed that the impact position had a major effect on the overall ice distribution, which was determined by the combination of blade geometry at the impact point and the rotational speed. The splitter position was shown to have a significant effect on the amount of ice passing into the bypass duct.

The effect of yawing a wing in high-speed flow is to delay the onset of shock waves and to increase the critical Mach number. This is because shock waves can only develop along the isobars (running parallel to the edges of the wing, which is assumed untapered and infinitely long), and therefore only the velocity component normal to the edges is significant. To some extent the same is true for a finite sheared or swept-back wing, though the problem is made more complicated by the various additional factors, such as finite aspect ratio (tip effect), plan form (e.g. taper effect) and, what is perhaps the most important, the central kink effect. The experimental technique is extremely cumbersome because of the many shape parameters involved (see Figs. 1, 2, 3), and since analytical solutions present fundamental difficulties and pitfalls, designers tend to favour rule of thumb methods. The present paper contains a general review of the theoretical work done at The Royal Aircraft Establishment, which was limited to the case of zero incidence. This limitation is not a severe one, as flight at high speeds often implies low incidence. Incidences which are not negligible involve the solution of another fundamental problem (that of the lift distribution).

An investigation has been made to determine the positions and directions of initiation and the directions of propagation of fatigue cracks and to examine the correlation between these positions and directions and the planes on which maximum tensile and maximum shear stresses are generated.

To afford as wide a range as possible of the ratio of maximum shear stress to maximum tensile stress, tests have been made under combinations of alternating bending and torsion; and in order to separate partially the regions of high shear stress from those of high direct stress, the tests have been made on pieces of square section with the plane of bending parallel to one diagonal of the section. Two series of tests have been made; one a preliminary series on pieces having no parallel portion and the other on pieces having a parallel portion about three times the length of the side of the square section. The positions and directions of initiation and the directions of propagation of fatigue cracks have been observed and compared with the positions and directions of the maximum tensile and shear stresses.

Fatigue cracks may be initiated as a result of either high shear stress or high tensile stress and in the present series of tests on mild steel, cracking in tension has occurred in preference to cracking in shear when the ratio of the tensile stress range to the shear stress range has exceeded about 1.6; for values of this ratio less than 1.6, the cracks started in shear (and vice versa); propagation along the plane of maximum shear appears to be preferred up to a slightly greater value of the tensile/shear ratio (about 1.7 possibly). The general direction of a crack formed as a result of high tension usually follows the plane of maximum tension and that of a crack formed as a result of shear usually follows the plane of maximum shear. In detail both types of crack—in this mild steel—deviate quite widely from their general directions but this deviation bears no obvious relation to the microstructure of the material. Cracks propagating along one plane of maximum shear occasionally show a marked tendency to branch along the associated plane of maximum shear; but this tendency is not always observed and in other cases no tendency to branch has been noted.

The application of the matrix displacement method to large displacements and strains, as formulated in refs. 1, 2, 3, is based on the idea of natural modes. This method was significantly generalised to triangular and tetrahedronal curvilinear elements in TNs 17 and 18 (ref. 4), where the important new concept of the local sub-element was first presented. A further obvious extension of the approach should cover curved beams in space, and, what is much more important, shells. However, the realisation of a theory which includes such elements is not straightforward and requires some subtle considerations on the natural modes of a local sub-beam. In order to give a clear presentation of the underlying physical assumptions, we first discuss in this note the natural mode technique on a generalised basis and apply it then to a curved beam in space. The idea of the local sub-element emerges thereby in all its significance. The examples investigated here cover an arbitrary curved beam in space with initial curvature and twist and the special case of a circular arch. To limit the length of the note we have to exclude the effects of shear deformability in bending, and warping in torsion. Moreover, to simplify the formulae we assume that bend ing is defined about the principal axes. The beams in question may be classified as slender and are part of the ASKA library (CUBA for curved beam in space and CIRCA for circular aroh). Part II of the note will include the effect of large displacements and show how the ideas of ref. 4 may be adapted to curved beams and similar structural elements.

By consideration of a series of simple models the static aeroelasticity of conventional aircraft is first discussed and then extended to the integrated configuration combining fuselage, wing and tail. It is shown that, on the basis of linear aerodynamics, a maximum speed exists at which control is possible. Overall deformations and control forces become large at speeds approaching this critical condition. Part II, to be published, contains calculations on slender plate aircraft including non-linear aerodynamics.

A model of the viscous flow past a flexible sail section operating near the ideal incidence is described. Viscous effects are calculated via weak viscous-inviscid interaction of a panel method and an integral boundary layer method, and a new model for the leading edge separation bubble is introduced. The flexible section is allowed to deform in response to the pressure and shear stresses acting on it. Results are presented for the effect of incidence, excess length and Reynolds number on the development of the boundary layers on each side of the section and the consequences for the lift and drag of the section are described. The numerical model is compared with experimental results, giving in general good agreement and shedding light on the physics of the viscous flow past flexible membranes.

The aerodynamic performance of a multi-element high-lift system has a critical influence on the direct operating cost of a subsonic civil transport aircraft. A thorough understanding of the aerodynamic characteristics of these multi-element aerofoils and wings allows aircraft companies to design and build more competitive aircraft with high-lift systems that are less complex and lighter for given high-lift performance or that have improved lift and drag characteristics for given system complexity and weight. Flight experiments on NASA Langley's B737-100 aircraft have been conducted to further enhance the understanding of the complex flows about multi-element high-lift systems at full-scale flight conditions. In this paper, an overview of the flight program is provided, followed by highlights of experimental results and computational analysis. Measurements included surface pressures on the slats, main element and flap elements using flush pressure ports and pressure belts, surface shear stresses using Preston tubes, off-surface velocity distributions using boundary layer/wake rakes, aeroelastic deformations of the flap elements using an optical positioning system, and boundary layer transition detection using hot-film anemometers and an infrared imaging system. Boundary layer transition measurements on the slat using hot-film sensors are correlated with the flow visualisation results from an infrared imaging technique. Extensive application of several computational techniques and comparisons with flight measurements are shown for a limited number of cases. This program has generated an extensive set of data, much of which are still being analysed.

Three basic problems encountered in the use of propellers on hovercraft are erosion, corrosion and noise. The tip velocity of a 9 ft diameter hovercraft propeller can exceed Mach 0·8, giving rise to fairly high noise levels. When operating over shallow water the propeller may run in a fine suspension of sand and sea water, which at these high speeds is akin to operating in a highly corrosive vapour blast cabinet.