Recent Advances in Air-Vehicle Design Synthesis and Optimisation

24TH INTERNATIONAL CONGRESS OF THE AERONAUTICAL SCIENCES RECENT ADVANCES IN AIR-VEHICLE DESIGN SYNTHESIS AND OPTIMISATION David A Lovell*, Catherine A Crawford** and Kevin E Restrick** *Applied Technology Consultants, **QinetiQ Farnborough Keywords: Design synthesis, optimisation, concept design Abstract aircraft concepts that have been performed with these programs over the past 30 years has Over the past 30 years conceptual design and confirmed the effectiveness of the basic process. optimisation programs have been developed and The rapid growth in computing power per unit applied by QinetiQ and its predecessor cost in this period has enabled more complex organisations to study the effects on aircraft studies to be completed more rapidly. concepts of changes in requirements and advances in technology. Recent work is described that extends design synthesis and 1.2 Current environment optimisation to cover a far wider range of air- Design synthesis and optimisation (DSO) vehicle concepts, and links concept design methods provide the outline concept description optimisation to multi-disciplinary analysis and which is used as the starting point for more optimisation. The design and assessment comprehensive studies by technology environment being evolved at QinetiQ is specialists: configuration design in industry or reducing the time required for a design / detailed assessment by government agencies. assessment cycle and providing greater Both classes of customer place high value on flexibility in the study of concepts. the rapid turn-round of this work. At QinetiQ the two phases of work are being integrated [7] into a generic process for air-vehicle concept 1 Introduction design and assessment. Efficient definition, transfer and 1.1 Historical background enrichment of the initial concept geometry are critical to the overall process. Ref. 7 describes Work at the Royal Aerospace the integration of Computer Aided Design Establishment in the period 1970-80 coupled (CAD) into the process, and the aerodynamic aircraft synthesis, performance analysis, and shape optimisation of a military aircraft concept non-linear constrained optimisation to form generated by the QinetiQ design synthesis conceptual design optimisation methods. method. Subsequent work at QinetiQ [8] has Separate codes were written for civil [1] and added finite element structural analysis to the military [2] aircraft applications. To ensure generic process, to assess the impact of cruise acceptable execution times on the computers speed on high-speed civil transport aircraft then available, these codes were highly tailored concepts. Thus an important requirement for to specific types of aircraft. The two programs DSO methods is the availability of simple, formed the basis for subsequent code generic, geometry parameterisation schemes development at RAE, its successor that are compatible with the CAD tools used for organisations (DRA, DERA and QinetiQ), and detailed design and assessment. Airbus, e.g. refs [3,4,5]. The considerable In considering new concept shapes CAD number of studies (e.g. ref [6]) of the effects of tools are also valuable for exploring the options requirements and advances in technology on for packaging the contents of the air vehicle, © Copyright QinetiQ Limited 2004 1 LOVELL, CRAWFORD and RESTRICK and thence for defining the geometric transport aircraft, covers section shapes that constraints that need to be included in the have the body slope matched to that of the wing design synthesis. root at the body side. The second (discrete), Recent years have seen a major expansion applicable to most other civil and military air in the variety of concept shapes that are being vehicles, has a slope discontinuity at the body considered for air vehicles. Whereas in the past side. Geometry variables are defined to describe DSO could be based on the characteristics of sides that may be curved or sloped. Fig 2 existing, related air vehicles, today relevant data indicates the wide range of sections that may be do not exist for many of the concepts under modelled. consideration. Reliance now has to be placed on Five of the variables determining the the definition of a virtual air vehicle, shaped by geometry of each cross section are treated as detail design tools and analysed by state-of-the- optimisation variables, and constraints are art tools (e.g. CFD and FE structural analyses), defined to ensure internal items can be to create the required data. At QinetiQ response contained within the section. Fig 3 and Table 1 surface methods are used [7] to permit the data show the results from minimising the cross- from the performance analyses at the detailed sectional area of two blended cross-sections level (aerodynamics, mass etc) to be distilled for while containing a large payload bay between use at the concept design level. Fig 1 them. The execution times quoted are for a 1-2 summarises the generic process. GHz PC processor. Because of the needs to cover a wider 2.1.2 Longitudinal sections range of concept shapes, and to respond rapidly A simple parameterisation of the to evolving military requirements or to respond longitudinal cross section of the body, in terms to changes in the market needs for transport of thickness and camber, has been developed aircraft, QinetiQ has begun to construct DSO that can be used to model a wide variety of programs based on common modules. shapes. Applications of this parameterisation to The remainder of the paper describes the body centre line section of a flying wing, a aspects of design synthesis that have been UAV and a missile are shown in Fig 4. addressed recently and will feature as common Ten of the variables determining the modules in future programs. section geometry are treated as optimisation variables. Position variables and constraints are 2 Geometry Parameterisation also defined for internal items to ensure that they can be contained within the section. Fig 5 Geometry parameterisation schemes have and Table 1 show the results from minimising been developed for the major components of air the centre-line sectional area of a flying wing vehicles that are sufficiently generic to represent concept which contains two large packages. a wide range of concepts. The degree of detail employed enables the internal packaging of the 2.1.3 Complete body component to be modelled, and the volume and The parameterisation for cross sections has surface area to be estimated to the level of been combined with that for longitudinal accuracy appropriate for initial concept design. sections to provide a geometry description for a complete body. Fig 6 and Table 1 show the 2.1 Centre body results from an application of this geometry in which the values of the geometric variables for 2.1.1 Cross sections the centre-line section and three cross sections The geometry parameterisation developed have been optimised to minimise the volume of covers two classes of body cross section. The the body to contain two boxes. In addition to first (blended), applicable to low-observable those controlling the body shape, variables military air vehicles and blended wing-body define the fore-and-aft, and vertical positions of the packaged items. Associated constraints 2 RECENT ADVANCES IN AIR-VEHICLE DESIGN SYNTHESIS AND OPTIMISATION ensure these packages do not overlap and that Three internal items are considered: the they are contained within the body. passenger cabin, the landing gear, and the cargo hold. These items have the greatest effect on 2.1.4 Application to blended wing transport the performance and external shape of the aircraft aircraft. The layout within the passenger cabin Recent work has been undertaken by is particularly important because the QinetiQ as part of the REUTAC research unconventional shape of the aircraft means that programme (Rapid Evaluation of passenger comfort levels, and the means of Unconventional Transport Aircraft Concepts access (including emergency exit) can have a [9]), funded by Airbus and the UK Department major impact on the aerodynamic and structural of Trade and Industry. Here the centre body of a aspects of the design. For example, for a given blended-wing-body (BWB) aircraft has been cabin width, as the leading edge sweep of the modelled using the techniques described above aircraft body is increased, the overall length of (Fig.7), and implemented in a DSO program. the aircraft will increase to seat the required It is important to model accurately the number of passengers. The increased length will layout of this type of aircraft configuration change the structural and aerodynamic during the conceptual design phase. The novel properties, and change the position of the cargo design allows greater flexibility in the and the loading on the landing gear. This may positioning of items such as the passenger be offset by an increase in the space available cabin, cargo, and the undercarriage, compared for exits, access for ground handling equipment, with a conventional civil transport aircraft. and passenger movement. Factors that need to be considered early in the The undercarriage is a large package that design process are passenger acceptability, can have a major effect on the external surfaces, evacuation requirements, and accessibility for and on the position and quantity of cargo that ground handling equipment. may be carried. The position of the The external geometry and the internal undercarriage is controlled by the optimiser, packages are handled independently and are with constraints applied to ensure a sensible sized and positioned using appropriate design design (e.g. to meet centre of gravity rules. The code is modular so that alternative requirements). descriptions of the geometry, or more The cargo hold consists of up to seven comprehensive design relationships, may be separate cargo areas, depending on the size of used depending on the study requirements. aircraft. The size and position of the cargo areas The surface of the centre body is modelled are allowed to vary and constraints are applied by longitudinal sections across the span of the to ensure that the required clearance envelopes body.

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