Showing papers in "Aeronautical Journal in 2009"

Macro-fiber composite actuators for a swept wing unmanned aircraft

Abstract: The purpose of the research presented here is to exploit actuation via smart materials to perform shape control of an aerofoil on a small aircraft and to determine the feasibility and advantages of smooth control surface deformations. A type of piezoceramic composite actuator known as Macro-Fiber Composite (MFC) is used for changing the camber of the wings. The MFC actuators were implemented on a 30° swept wing, 0·76m wingspan aircraft. The experimental vehicle was flown using two MFC patches in an elevator/aileron (elevon) configuration. Preliminary flight and wind-tunnel testing has demonstrated the stability and control of the concept. Flight tests were performed to quantify roll control using the MFC actuators. Lift and drag coefficients along with pitch and roll moment coefficients were measured in a low-speed, open-section wind tunnel. A vortex-lattice analysis complemented the database of aerodynamic derivatives used to analyse control response. The research, for the first time, successfully demonstrated that piezoceramic devices requiring high voltages can be effectively employed in small air vehicles without compromising the weight of the overall system.

Aircraft parameter estimation using a new filtering technique based upon a neural network and Gauss-Newton method

Abstract: A new parameter estimation method based upon neural network is proposed. The method proposed here uses feed forward neural networks to establish a neural model that could be used to predict subsequent time histories given the suitable measured initial conditions. The proposed neural model would not represent a generic flight dynamic model. The neural model in this case develops point to point fitting of the input and the output data. Thus, it could at best be referred to as flight dynamic model in restricted sense. Gauss-Newton method is then used to obtain optimal values of the aerodynamic parameters by minimising a suitable defined error cost function. The method has been validated using longitudinal and lateral-directional flight data of various test aircraft. The results thus obtained were compared with those obtained through wind tunnel test, or those obtained using Maximum likelihood and/or Filter error methods. Unlike, most of the parameter estimation methods, the proposed method does not require a prior description of the model. It also bypasses the requirement of solving equations of motion. This feature of the proposed method may have special significance in handling flight data of an unstable aircraft.

Supersonic Boundary Layer Interactions with Various Micro-Vortex Generator Geometries

Abstract: Various types of micro-vortex generators (μVGs) are investigated for control of a supersonic turbulent boundary layer subject to an oblique shock impingement, which causes flow separation. The micro-vortex generators are embedded in the boundary layer to avoid excessive wave drag while still creating strong streamwise vortices to energize the boundary layer. Several different types of μVGs were considered including micro-ramps and microvanes. These were investigated computationally in a supersonic boundary layer at Mach 3 using Monotone Integrated Large Eddy Simulations (MILES). The results showed that vortices generated from μVGs can partially eliminate shock induced flow separation and can continue to entrain high momentum flux for boundary layer recovery downstream. The micro-ramps resulted in thinner downstream displacement thickness in comparison to the micro-vanes. However, the strength of the streamwise vorticity for the micro-ramps decayed faster due to dissipation especially after the shock interaction. In addition, the close spanwise distance between each vortex for the ramp geometry causes the vortex cores to move upwards from the wall due to induced upwash effects. Micro-vanes, on the other hand, yielded an increased spanwise spacing of the streamwise vortices at the point of formation. This resulted in streamwise vortices staying closer to the floor with less circulation decay, and the reduction in overall flow separation is attributed to these effects. Two hybrid concepts, named “thick-vane” and “split-ramp”, were also studied where the former is a vane with side supports and the latter has a uniform spacing along the centerline of the baseline ramp. These geometries behaved similar to the micro-vanes in terms of the streamwise vorticity and the ability to reduce flow separation, but are more physically more robust than the thin vanes.

Simulation of wake vortex effects for UAVs in close formation flight

Abstract: This paper addresses the development of multiple UAV deployment simulation models that include representative aerodynamic cross-coupling effects. Applications may include simulations of autonomous aerial refuelling and formation flying scenarios. A novel wake vortex model has been developed and successfully integrated within a Matlab/Simulink simulation environment. The wake vortex model is both sufficiently representative to support studies of aerodynamic interaction between multiple air vehicles, and straightforward enough to be used within real time or near real time air-to-air simulations. The model integration process is described, and computational results of a two-vehicle-formation flight are presented.

Effects of boundary-layers bleeding on unstart/restart characteristics of hypersonic inlets

Abstract: A series of mixed-compression hypersonic inlets at different bleeding rates were simulated at different freestream conditions in this paper. The unstart/restart characteristics of hypersonic inlets were analysed and the reasons why the unstart/restart phenomenon is in existence is presented. The unstart/restart characteristics of hypersonic inlets at different bleeding rates were given. The effects of boundary-layer bleeding on the performance parameter (mass-captured coefficient, total-pressure recovery coefficient), starting and restarting Mach number of hypersonic inlets were discussed. In conclusion, boundary-layer bleeding can improve the performance parameter of hypersonic inlets, and can reduce the starting and restarting Mach number, and can broad the operation range of the hypersonic inlet.

On a cellular division method for aircraft structural design

Abstract: This work concerns the development of a biologically inspired methodology for topology optimisation of aircraft structures. The methodology is based on the map L systems modelling of cellular division to generate the structural topology. The topology thus generated is analysed using the finite element method, and the rules that indirectly develop the topology are then evolved using a genetic algorithm for multi-objective optimisation. The methodology is demonstrated in the design of the wing box of a generic aircraft fighter. The results clearly show the suitability of the proposed method for the topology optimisation of aircraft structures.

Material state changes as a basis for prognosis in aeronautical structures

Abstract: The long-term performance of aeronautical structures is typically discussed in terms of concepts such as structural integrity, durability, damage tolerance, fracture toughness, etc. These familiar concepts are usually addressed by considering balance equations, crack growth relationships, and constitutive equations with constant material properties, and constant or cyclically applied load conditions. Loading histories are represented by changing stress (or strain) states, only. But for many situations, especially associated with high performance aircraft, the local state of the material may also change during service, so that the properties used in those equations are functions of time and history of applied conditions. For example, local values of stiffness, strength, and conductivity are altered by material degradation to create ‘property fields’ that replace the global constants, and introduce time and history into the governing equations. The present paper will examine a small set of such problems and offer a construct for using related solutions to estimate future performance based on history of use and current material state, a concept typically called prognosis.

A methodology for optimisation design and analysis of stratosphere airship

Abstract: This paper presents a methodology for studying the feasibility of stratosphere airship for high altitude long endurance missions and arriving at the baseline specifications of conventional configuration of stratosphere airship, given the performance and operational requirements. Based on this methodology, the AODAP platform (Airship Optimisation Design and Analysis Platform) was developed. Some innovative concepts used in AODAP that are different from previous methods and codes are presented. The shape optimisation of airship was introduced into the design process, and several optimum objectives can be selected including minimum drag, minimum weight and composite objective based on MDO (Multidisciplinary Design Optimisation). The methodology was validated for other design concepts previously developed for similar missions and also was compared to a low altitude vehicle. The baseline specifications of stratosphere airships designed for various shapes using this methodology are presented. The results of sensitivity analyses for a specified airship are discussed, and the sensitivity of airship length with some critical parameters including area density of envelope fabric, area density of solar cell, efficiency of solar cell and efficiency of fuel cell for the specified shape is also provided.

In-situ impact-induced damage assessment of woven composite laminates through a fibre Bragg grating sensor network

Abstract: Woven composite specimens with embedded fibre Bragg grating (FBG) sensor networks were impacted at low velocities, while global measurements of contact forces and dissipated energies were obtained from drop tower measurements, and local residual, post-impact strain values were obtained from the FBG sensors. Critical damage events were identified in the global data for these specimens and damage signatures in the residual strain data corresponding to these critical damage events were correlated. The results indicate that the full spectral scan information from the sensor network, although obtainable at a lower scan rate, provide more reliable residual lifetime information than average residual strains.

Effect of aft wall slope on cavity pressure oscillations in supersonic flows

Abstract: An experimental study of supersonic flow over wall mounted cavities with different aft wall angles is carried out. Unsteady pressure measurements were made on the walls and floor of the cavity. Data analysis was performed on the experimental results using statistical methods. In the case of higher angled cavities, the presence of an upstream traveling acoustic wave could be confirmed. For lower angled cavities (60 degrees and less) where the acoustic wave could not be identified, the flow inside the cavity was more or less stable. Mode switching occurring in higher angled cavities was confirmed by spectrogram studies.

The leading-edge vortex and aerodynamics of insect-based flapping-wing micro air vehicles

Abstract: The aerodynamics of insect-like flapping are dominated by the production of a large, stable, and lift-enhancing leading-edge vortex (LEV) above the wing. In this paper the phenomenology behind the LEV is explored, the reasons for its stability are investigated, and the effects on the LEV of changing Reynolds number or angle-of-attack are studied. A predominantly-computational method has been used, validated against both existing and new experimental data. It is concluded that the LEV is stable over the entire range of Reynolds numbers investigated here and that changes in angle-of-attack do not affect the LEV’s stability. The primary motivation of the current work is to ascertain whether insect-like flapping can be successfully ‘scaled up’ to produce a flapping-wing micro air vehicle (FMAV) and the results presented here suggest that this should be the case.

A CFD assessment of classifications for hypersonic inlet start/unstart phenomena

Abstract: Inlet start/unstart detection is one of the most important issues of hypersonic inlets and is also the foundation of protection controls of scramjets. In ground and flight tests, it is inevitably to introduce the sensor noises to the measurement system. How to overcome or weaken the influence of the sensor noises and the outer disturbances is an important issue to the control system of the engine. To solve this problem, the 2D inner steady flow of hypersonic inlets was numerically simulated in different freestream conditions and backpressures, and two different inlet unstart phenomena were analysed. The membership function for hypersonic inlet start/unstart can be obtained by using probabilistic output support vector machine, and the algorithm of multiple classifiers fusion is introduced. The variations of the classification accuracy with the intensity of the sensor noises and the number of the classifier were discussed respectively. In conclusion, it is useful to introduce the algorithm of support vector machine and multiple classifiers fusion to overcome or weaken the influence of the sensor noises on the classification accuracy of hypersonic inlet start/unstart. The number of the practical fusion classifiers needs a tradeoff between the fusion classification accuracy and the complexity of the classification system.

The optimum aeroplane and beyond

Abstract: A summary of the ways in which aviation impacts the environment is presented and the ratio of the energy liberated during a flight to the revenue work done ( ETRW ) is identified as a key indicator in the assessment of environmental impact. Using the ‘Breguet’ range equation, a number of theorems relating to ETRW are derived and discussed. This is followed by an approximate analysis to produce estimates for the ETRW of aircraft currently in service. It is found that the global fleet average value for ETRW is much higher than those estimated for existing individual aircraft. An explanation of the difference is presented, with the contributions from airline operations and air traffic management identified and quantified. Consideration is then given to the potential for future reduction in ETRW through advances in materials, alternative fuels, structures, aerodynamics and propulsion technologies and the likely benefits are quantified. The improvement in ETRW that could be achieved if this parameter was minimised in the design process with the current level of technology is also considered. Finally, the likelihood of performance improvements being introduced in the short, medium and long term is briefly discussed.

Interactional aerodynamics and acoustics of a hingeless coaxial helicopter with an auxiliary propeller in forward flight

Abstract: The aerodynamics and acoustics of a generic coaxial helicopter with a stiff main rotor system and a tail-mounted propulsor are investigated using Brown's Vorticity Transport Model. In particular, the model is used to capture the aerodynamic interactions that arise between the various components of the configuration. By comparing the aerodynamics of the full configuration of the helicopter to the aerodynamics of various combinations of its sub-components, the influence of these aerodynamic interactions on the behaviour of the system can be isolated. Many of the interactions follow a simple relationship between cause and effect. For instance, ingestion of the main rotor wake produces a direct effect on the unsteadiness in the thrust produced by the propulsor. The causal relationship for other interdependencies within the system is found to be more obscure. For instance, a dependence of the acoustic signature of the aircraft on the tailplane design originates in the changes in loading on the main rotor that arise from the requirement to trim the load on the tailplane that is induced by its interaction with the main rotor wake. The traditional approach to the analysis of interactional effects on the performance of the helicopter relies on characterising the system in terms of a network of possible interactions between the separate components of its configuration. This approach, although conceptually appealing, may obscure the closed-loop nature of some of the aerodynamic interactions within the helicopter system. It is suggested that modem numerical simulation techniques may be ready to supplant any overt reliance on this reductionist type approach and hence may help to forestall future repetition of the long history of unforeseen, interaction-induced dynamic problems that have arisen in various new helicopter designs.

An aviation approach to space transportation: (A strategy for increasing space exploration within existing budget streams)

Abstract: This paper presents a strategy for developing the first orbital spaceplane soon and at low cost and risk. The paper then shows how this vehicle will introduce an aviation approach to orbital space transportation to replace the present missile paradigm, leading to far lower costs and improved safety. To illustrate the potential benefits, the paper presents preliminary sizing and cost estimates of a simple lunar base. Even including the cost of developing the spaceplanes and other vehicles required, the total cost is about ten times less than that of present plans that use large new expendable launch vehicles. Timescales need not be greatly affected.

Towards Integrated design of fluidic flight controls for a flapless aircraft

Abstract: Fluidic flight controls enable forces and moments for flight vehicle trim and manoeuvre to be produced without use of conventional moving surface controls. This paper introduces a methodology for the design of Circulation Control (CC) and Fluidic Thrust Vectoring (FTV) as fluidic controls for roll and pitch. Work was undertaken as part of the multidisciplinary FLAVIIR project, with the goal of providing full authority fluidic flight controls sufficient for a fully flapless flight of an 80kg class demonstrator aircraft known as DEMON. The design methodology considers drag, mass, volume and pneumatic power requirements as part of the overall design cost function. It is shown that the fundamental flow physics of both CC and FTV are similar, and hence there are strong similarities to the design approach of each. Flight ready CC and FTV hardware has been designed, manufactured and ground tested. The CC system was successfully wind tunnel demonstrated on an 85% scale half model of the DEMON. The design condition of a control ?CL of 0�1 was achieved with a blowing coefficient of 0�01, giving a useable control gain of 10. The FTV system was static tested using a micro gas turbine source. The control characteristic was 'N' shaped, consisting of an initial high gain response in a negative sense (gain = ?30) followed by a low gain response in a positive sense (gain = +3) at higher blowing rate. CC and FTV control hardware directly contributes to around 6% to the overall mass of the flight vehicle, however provision of pneumatic power carries a significant mass penalty unless generated as part of an integrated engine bleed system.

Identification of a MIMO state space model of an F/A-18 aircraft using a subspace method

Abstract: The aim of this paper is to determine the mathematical relationship (model) between control deflections and structural deflections of the F/A-18 modified aircraft in the active aeroelastic wing technology program. Five sets of signals from flight flutter tests corresponding to the excited sources were measured by NASA Dryden Flight Research Center. These excitation inputs are: differential ailerons, collective ailerons, collective stabilisers, differential stabilisers, and rudders. The signals to be used by the model are of two types: control deflection time histories and corresponding structural deflections on the wing and trailing-edge flaps. We choose to use the subspace identification method based on reconstructing the observability matrix in order to identify the nonlinear multi-input, linear-in-the-states, multi-output system. We identify models (input/output characteristics) by applying this method for a number of sixteen flight conditions for which the Mach number varies from 0·85 to 1·30 and the altitudes vary from 5,000ft to 25,000ft. Very good results are obtained with a fit between the estimated and the measured signals and a correlation coefficient higher than 90%.

A multidisciplinary approach to structural health monitoring and damage prognosis of aerospace hotspots

Abstract: The health monitoring and damage prognosis of aerospace hotspots is important for reducing maintenance costs and increasing in-service capacity of aging aircraft. One of the leading causes of structural failure in aerospace vehicles is fatigue damage. Based on the physical mechanism of damage nucleation and growth, a physics-based multiscale model is considered for fatigue damage assessment in metallic aircraft structures. A guided-wave based sensing approach is utilised to enable effective damage detection in a common structural hotspot: a lug joint. Finite element analysis is carried out with piezoelectric wafers bonded to the host structure and the simulated sensor signals are analysed. A damage classification strategy is developed, which integrates physically motivated time-frequency approaches with advanced stochastic modelling techniques. In particular, a variational Bayesian learning scheme is used to estimate the optimal model complexity automatically from the data, adapting the classifier for real-time use. Classification performance is studied as a function of signal-to-noise ratio and results are reported for the detection of fatigue crack damage in the lug joint. An adaptive hybrid prognosis model is proposed, which estimates the residual useful life of structural hotspots using damage condition information obtained in real-time.

Computational fluid dynamics application to aerospace science

Abstract: A brief narration on significant accomplishments in computational fluid dynamics (CFD) for basic research and aerospace application is attempted to highlight the outstanding achievements by scientists and engineers of this discipline. To traverse such a vast domain, numerous and excellent contributions to CFD will be unintentionally overlooked by the author’s limited exposure. Nevertheless it is an ardent hope that the present abridged literature review will aid to reaffirm excellence in research and to identify knowledge shortfalls both in aerodynamics and its modeling and simulation capability. The future modeling and simulation technology needs, as well as potential and fertile research areas, are humbly put forth for consideration.

Development of a VTOL mini UAV for multi-tasking missions

Abstract: Recent developments in the field of Mini-UAVs lead to successful designs in both hovering rotorcraft and fixed wing aircraft. However, a polyvalent MAV capable of stable hovering and fast forward flight is still expected. A promising candidate for such versatile missions consists of a tilt-body tail-sitter configuration. That concept is studied in this paper both from the flight mechanics and control points of view. Developments are based on an existing prototype called Vertigo. It consists of a tail sitter fixed-wing mini-UAV equipped with a contra-rotating pair of propellers in tractor configuration. A wind-tunnel campaign was carried out to extract experimental results from the Vertigo aerodynamic characteristics. A 6-component sting balance was fitted in the powered model enabling excursion in angles of attack and sideslip angles up to 90°. Thus, a detailed understanding of the transition mechanism could be obtained. An analytical model including propwash effects was derived from experimental results. The analytical model was used to compute stability modes for specific flight conditions. This allowed an appropriate design of the autopilot capable of stabilisation and control over the whole flight envelope. A gain sequencing technique was chosen to ensure stability while minimising control loop execution time. A MATLAB-based flight simulator including an analytical model for the propeller slipstream has been developed in order to test the validity of airborne control loops. Simulation results are presented in the paper including hover flight, forward flight and transitions. Flight tests lead to successful inbound and outbound transitions of the Vertigo.

Control laws for a formation of autonomous flight vehicles

Abstract: This paper investigates the performance of a flight control system, based on nonlinear dynamics inversion theory, whose aim is to maintain a given geometry of a formation of unmanned aerial vehicles. A fundamental aspect is the complete three dimensionality of the formation geometry that provides a substantial improvement over existing two-dimensional control laws. The designed control system has been implemented in a Simulink® environment and its effectiveness has been tested with a campaign of numerical simulations.

Impact damage and repair of composite structures

Abstract: This paper gives an overview of the work carried out in a GARTEUR (Group for Aeronautical Research and Technology in Europe) program, under the chairmanship of the author, to develop and validate analytical and numerical methods to characterise real impact damage in composite structures, particularly those designed to sustain load in a postbuckled state, and to study the durability of bonded repairs. GARTEUR is an inter-governmental agreement between the seven European countries with the largest direct employment in the Aerospace industry, to mobilise scientific and technical knowledge between the member countries. A number of Action Groups have been launched, since GARTEUR’s inception in the early 1970s, to address specific technical issues of interest to the participating members. The research presented in this paper was performed under Action Group 28 with partners from ONERA, EADS-CCR (France), DLR, AIRBUS-Deutschland, EADS-M (Germany), CIRA (Italy), INTA (Spain), SICOMP, Saab, (Sweden), NLR (The Netherlands), QinetiQ, BAE Systems, Imperial College London and the University of Sheffield (United Kingdom). The Action Group tasks were divided into four Work Elements (WEs): WE1-Prediction and characterisation of impact damage, WE2-Postbuckling with delamination, WE3-Repair and WE4-Fatigue. This paper outlines the main developments and achievements within each Work Element.

CLoVER: an alterntive concept for damage interrogation in structural health monitoring systems

Abstract: Structural Health Monitoring (SHM) is the component of damage prognosis systems responsible for interrogating a structure to detect, locate, and identify any damage present. Guided wave (GW) testing methods are attractive for this application due to the GW ability to travel over long distances with little attenuation and their sensitivity to different damage types. The Composite Long-range Variable-direction Emitting Radar (CLoVER) transducer is introduced as an alternative concept for efficient damage interrogation in GW SHM systems. This transducer has an overall ring geometry, but is composed of individual wedge-shaped anisotropic piezocomposite sectors that can be individually excited to interrogate the structure in a particular direction. The transducer is shown to produce actuation amplitudes larger than those of a similarly sized ring configuration for the same electric current input. The electrode pattern design used allows each sector to act as an independent actuator and sensor element, decreasing the number of separate transducers needed for inspection. The fabrication and characterisation procedures of these transducers are described, and their performance is shown to be similar to that of conventional piezocomposite transducers. Experimental studies of damage detection demonstrating the proposed interrogation approach are also presented for simulated structural defects.

Time delay comb transducers for aircraft inspection

Abstract: An ultrasonic guided wave technique based on time delay comb transducers is introduced for aircraft inspection. It is demonstrated that for isotropic plate structures the time delay comb transducers with appropriate excitation are capable of performing discontinuity detections without knowing the precise isotropic material properties of the objects being inspected. Fibre-reinforced composite plates are also considered. The wave skew effects are investigated using both the slowness curve and the Poynting vector. A composite inspection technique that takes advantage of the skew effects is proposed. Using time delay comb transducers to excite the guided wave modes with different skew angles, the proposed technique is capable of scanning the composite plate in different directions without moving or rotating the transducers. By contrast with the applications in the isotropic cases, knowledge of the material properties and other necessary information that is needed to produce dispersion curves is generally required. Experimental results are provided as a validation of the proposed techniques.

Wind tunnel testing of powered lift, all-wing STOL model

Abstract: Short take-off and landing (STOL) systems can offer significant capabilities to warfighters and, for civil operators thriving on maximizing efficiencies they can improve airspace use while containing noise within airport environments. In order to provide data for next generation systems, a wind tunnel test of an all-wing cruise efficient, short take-off and landing (CE STOL) configuration was conducted in the National Aeronautics and Space Administration (NASA) Langley Research Center (LaRC) 14- by 22-foot Subsonic Wind Tunnel. The test s purpose was to mature the aerodynamic aspects of an integrated powered lift system within an advanced mobility configuration capable of CE STOL. The full-span model made use of steady flap blowing and a lifting centerbody to achieve high lift coefficients. The test occurred during April through June of 2007 and included objectives for advancing the state-of-the-art of powered lift testing through gathering force and moment data, on-body pressure data, and off-body flow field measurements during automatically controlled blowing conditions. Data were obtained for variations in model configuration, angles of attack and sideslip, blowing coefficient, and height above ground. The database produced by this effort is being used to advance design techniques and computational tools for developing systems with integrated powered lift technologies.

Design and implementation of linear- quadratic-Gaussian stability augmentation autopilot for unmanned air vehicle

Abstract: The linear-quadratic-Gaussian (LQG) control synthesis has the advantage of dealing with the uncertain linear systems disturbed by additive white Gaussian noise while having incomplete system state information available for control-loop feedback. This paper hence explores the feasibility of designing and implementing a stability augmentation autopilot for fixed-wing unmanned air vehicles using the LQG approach. The autopilot is composed of two independently designed LQG controllers which control the longitudinal and lateral motions of the aircraft respectively. The corresponding linear models are obtained through a system identification routine which makes use of the combination of two well-established identification methods, namely the subspace method and prediction error method. The two identification methods complement each other well and this paper shows that the proposed system identification scheme is capable of attaining satisfactory state-space models. A complete autopilot design procedure is devised and it is shown that the design process is simple and effective. Resulting longitudinal and lateral controllers are successfully verified in computer simulations and actual flight tests. The flight test results are presented in the paper and they are found to be consistent with the simulation results.

Large Eddy Simulation of a complete Harrier aircraft in ground effect

Abstract: This paper aims to demonstrate the viability of using the large eddy simulation (LES) CFD methodology to model a representative, complete STOVL aircraft geometry at touch down. The flowfield beneath such a jet-borne vertical landing aircraft is inherently unsteady. Hence, it is argued in the present work that the LES technique is the most suitable tool to predict both the mean flow and unsteady fluctuations, and, with further development and validation testing, this approach could be a replacement, and certainly a complementary aid, to expensive rig programmes. The numerical method uses a compressible solver on a mixed element unstructured mesh. Examination of instantaneous flowfield predictions from these LES calculations indicate close similarity with many flow features identified from ground effect flow visualisations, which are well known to be difficult to model using RANS-based CFD. Whilst significant further work needs to be carried out, these calculations show that LES could be a practical tool to model, for example, Hot Gas Ingestion for the Joint Strike Fighter aircraft.

Analysis of Helicopter Vibratory Hub Loads Alleviation by Cyclic Trailing–Edge Blade Flap Actuation

Abstract: The aim of the present work is the investigation about the use of blade trailing-edge flaps for the reduction of vibratory loads arising at the hub of helicopter main rotors in forward flight. The alleviation of these loads is achieved through multicyclic higher harmonic actuation of the blade flaps, which is related to measured vibratory loads amplitude. The feedback control law is obtained by an optimal control process based on the minimisation of a cost function, under the constraint of compatibility with the nonlinear equations governing blade aeroelasticity. In the numerical investigation concerning a four-bladed rotor in level flight conditions, a computationally efficient local controller methodology is applied, with the attention focused on the effectiveness of the control algorithm, along with its robustness with respect to differences (existing in real applications) between the aeroelastic models used for control law synthesis and validation.

Vascular design for thermal management of heated structures

Abstract: Vascular structures are contemplated for cooling the skins and leading surfaces of future high speed aircraft. This paper evaluates the proposal to cool with a flow architecture shaped as trees (dendritic) a parallelepipedic body that is heated uniformly. The coolant enters the body through one face and exits through the opposite face. The vasculature connects the two faces, and consists of trees that alternate with upside down trees. The fields for fluid flow and heat transfer are determined numerically in three dimensions. The effect of local pressure losses at bends, junctions and entrances is documented. Designs with tree-shaped architectures having up to four levels of bifurcation are evaluated for fluid flow and heat transfer performance, and are compared with the performance of a design with a single sheet of fluid sweeping the upper surface of the body. The fluid flow conductance of the tree designs increases when the number of bifurcation levels increases. The thermal performance of tree designs can be improved by endowing the tree design with more freedom such that the bifurcations generate asymmetric daughter channels. The tree designs outperform the fluid sheet design dramatically: the global thermal resistance of the tree designs is roughly one tenth of the global thermal resistance of the fluid sheet design.