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Aircraft Design

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Aircraft Design

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Aircraft Design

  1. 1. Aircraft Design - The Design Process For more detailed notes please refer to www.rmcs.cranfield.ac.uk/aeroxtra
  2. 2. Recommended Further Reading <ul><li>D.Howe – Aircraft Conceptual Design Synthesis </li></ul><ul><li>D.Raymer – Aircraft Design, A Conceptual Approach </li></ul><ul><li>J.Roskam – Airplane Design, Parts 1-8 </li></ul><ul><li>E.Torenbeek – Synthesis of Airplane Design </li></ul><ul><li>L.Jenkinson, P.Simpkin & D.Rhodes – Civil Jet Aircraft Design </li></ul><ul><li>D.Stinton – The Design of the Aeroplane </li></ul><ul><li>S.Brandt, J.Stiles & R.Whitford – Introduction to Aeronautics – A Design Perspective </li></ul>
  3. 3. Design Process - Overview <ul><li>Basic & general requirements. </li></ul><ul><li>Feasibility study. </li></ul><ul><li>Detail requirements & specification. </li></ul><ul><li>Design phases – Roskam/Raymer models </li></ul><ul><li>Project synthesis process (Howe model). </li></ul><ul><ul><li>Configuration, flight regime & powerplant, fuselage layout, wing configuration, lift, drag & mass representations, performance representation, parametric analysis & optimization </li></ul></ul><ul><li>Analysis of detailed design. </li></ul><ul><li>Detail design phase. </li></ul><ul><li>Testing, certification & project life cycle. </li></ul>
  4. 4. Basic Requirements <ul><li>New design launched when perceived requirement arises for aircraft beyond capability of those existing. </li></ul><ul><li>Usually due to: </li></ul><ul><ul><li>aircraft approaching end of its useful life. </li></ul></ul><ul><ul><li>design overtaken by technological developments. </li></ul></ul><ul><li>Identification of need may originate from: </li></ul><ul><ul><li>manufacturing organization (especially if civil). </li></ul></ul><ul><ul><li>potential operator (especially if military). </li></ul></ul>
  5. 5. Basic Requirements (Cont.) <ul><li>Initial basic requirements statement often brief, including class of aircraft and major performance characteristics. </li></ul><ul><li>Initial statement usually refined after consultations with appropriate operators and major manufacturers. </li></ul>
  6. 6. General Requirements <ul><li>Result of many years of previous experience applicable to various classes of a/c. </li></ul><ul><li>Act as: </li></ul><ul><ul><li>guide to designers. </li></ul></ul><ul><ul><li>basis for eventual clearance of a/c for intended operators. </li></ul></ul><ul><li>Most important for civil/general aviation are: </li></ul><ul><ul><li>FAR 25/23 (US), JAR 25/23 (Europe) </li></ul></ul><ul><ul><li>(Federal or Joint Airworthiness Requirements) </li></ul></ul>
  7. 7. General Requirements (Cont.) <ul><li>FAR and JAR written in identical format with only a few subtle differences – eventual aim is for commonality. </li></ul><ul><li>For military a/c use: </li></ul><ul><ul><li>DEF STAN 00-970 (UK), MIL SPECS (US) </li></ul></ul><ul><ul><li>MIL SPECS being replaced with requirements defined by individual manufacturers (Lockheed Martin, Boeing). </li></ul></ul>
  8. 8. Feasibility Study <ul><li>Follows basic requirement to assess whether need can be met with existing technology or not. </li></ul><ul><li>|Needed due to complexity of aeronautical projects. </li></ul>
  9. 9. Feasibility Study (Cont.) <ul><li>Also used for other purposes: </li></ul><ul><ul><li>how best to meet basic requirement (adaptation of existing a/c, major modification of existing a/c, completely new design (highest risk & cost)). </li></ul></ul><ul><ul><li>concept/configuration comparison studies also undertaken. </li></ul></ul><ul><ul><li>review and revision of basic requirement performance characteristics. </li></ul></ul><ul><ul><li>likely output is definition of detailed set of requirements (specification). </li></ul></ul><ul><ul><li>initial cost estimation. </li></ul></ul>
  10. 10. Detail Requirements / Specification <ul><li>Covers many aspects, though not all significant for project synthesis process phase. </li></ul><ul><li>Performance </li></ul><ul><li>Range with basic payload mass. </li></ul><ul><li>Alternative range/payload combinations (+ reserves). </li></ul><ul><li>Max (or max normal) operating speed. </li></ul><ul><li>Take-off & landing field length limitations. </li></ul><ul><li>Climb performance (time to height, ceiling, etc.). </li></ul><ul><li>Manoeuvre & acceleration requirements. </li></ul>
  11. 11. Detail Requirements / Specification (Cont.) <ul><li>Operations </li></ul><ul><li>Size & mass limitations (runway loading). </li></ul><ul><li>Crew complement. </li></ul><ul><li>Occupant environment (pressure, temperature). </li></ul><ul><li>Navigation/communications equipment. </li></ul><ul><li>Payload variation & associated equipment. </li></ul><ul><li>Maintenance targets. </li></ul><ul><li>Stealth aspects (military a/c). </li></ul><ul><li>Extended engine failed allowance (ETOPS) – civil. </li></ul>
  12. 12. Detail Requirements / Specification (Cont.) <ul><li>General </li></ul><ul><li>Growth potential. </li></ul><ul><li>Cost targets, availability. </li></ul><ul><li>Airframe life. </li></ul><ul><li>Airworthiness requirements (JAR 25, etc.). </li></ul>
  13. 13. Detail Requirements Example <ul><li>C-5 Specific Operational Requirement – June 1963 (Selected Items) </li></ul><ul><li>Basic design mission: 100,000 to 130,000 lb for 4000 nm </li></ul><ul><li>Alternate mission: 50,000 lb for 5500 nm </li></ul><ul><li>Load factor: 2.5 </li></ul><ul><li>Maximum design payload: 130,000 – 150,000 lb </li></ul><ul><li>Cruise speed: > 440 kts (TAS) </li></ul><ul><li>Cruise ceiling: > 30,000 ft </li></ul><ul><li>Take-off at max TOW: < 8000 ft </li></ul><ul><li>Take-off at 4000 nm weight: < 4000 ft </li></ul><ul><li>Landing with 100,000 lb & fuel reserves for 4000 nm: < 4000 ft </li></ul>
  14. 14. Detail Requirements Example <ul><li>C-5 Specific Operational Requirement – June 1963 (Selected Items) – (Cont.) </li></ul><ul><li>Cargo compartment: length 100 – 110 ft, width 16 – 17.5 ft, height 13.5 ft. </li></ul><ul><li>Cargo landing: straight through, one full section, one 9x10ft, truck bed floor height desirable. </li></ul><ul><li>Powerplant: 6 x turbofans. </li></ul><ul><li>Reliability: 95% probability of completing 10 hr mission. </li></ul><ul><li>Availability: June 1970. </li></ul>
  15. 15. Aircraft Design Phases (Raymer/Roskam Models) <ul><li>Conceptual Design </li></ul><ul><li>All major questions asked and answered. </li></ul><ul><ul><li>will it work? </li></ul></ul><ul><ul><li>what does it look like? </li></ul></ul><ul><ul><li>what requirements drive the design? </li></ul></ul><ul><ul><li>what trade-offs should be considered? </li></ul></ul><ul><ul><li>what should it weigh and cost? </li></ul></ul>
  16. 16. Aircraft Design Phases (Raymer/Roskam Models) <ul><li>Conceptual Design (Cont.) </li></ul><ul><li>No correct solution and process involves great deal of compromise, iteration and trade-offs. </li></ul><ul><li>Illustrated when different teams are requested to submit designs based upon an initial basic requirement or specification – all will be different and the customer can then choose accordingly. </li></ul>
  17. 17. JSF Conceptual Designs (a) Lockheed-Martin X-35 – successful (b) Boeing – rejected after demonstrator flights (c) McDonnell-Douglas – rejected after concept design phase (a) (c) (b)
  18. 18. Aircraft Design Phases (Raymer/Roskam Models) <ul><li>Conceptual Design (Cont.) </li></ul><ul><li>Various activities to be covered include: </li></ul><ul><ul><li>configuration possibilities </li></ul></ul><ul><ul><li>preliminary sizing (weight) </li></ul></ul><ul><ul><li>drag polar equation estimation </li></ul></ul><ul><ul><li>performance sizing & matching using W/S and T/W relationships – to optimally fix wing size and engine thrust power </li></ul></ul><ul><ul><li>wing layout and high-lift devices </li></ul></ul>
  19. 19. Aircraft Design Phases (Raymer/Roskam Models) <ul><li>Conceptual Design (Cont.) </li></ul><ul><li>Followed by: </li></ul><ul><ul><li>confirmation of configuration </li></ul></ul><ul><ul><li>fuselage sizing </li></ul></ul><ul><ul><li>propulsion selection & integration </li></ul></ul><ul><ul><li>empennage sizing </li></ul></ul><ul><ul><li>weight and balance analysis </li></ul></ul><ul><ul><li>stability analysis </li></ul></ul>
  20. 20. Aircraft Design Phases (Raymer/Roskam Model) <ul><li>Preliminary Design </li></ul><ul><li>Begins when major design changes are over. </li></ul><ul><ul><li>configuration and major characteristics “frozen”. </li></ul></ul><ul><ul><li>“ lofting” developed. </li></ul></ul><ul><ul><li>testing and development tools developed. </li></ul></ul><ul><ul><li>major items designed. </li></ul></ul><ul><ul><li>cost estimates refined. </li></ul></ul><ul><li>Followed by detail design, production, testing and certification phases. </li></ul>
  21. 21. Project Synthesis Process (Howe Model) <ul><li>Considered as an extension of feasibility study. </li></ul><ul><li>Though a different aim – to produce reasonably well-defined design to be offered to potential customers. </li></ul><ul><li>Requires considerably more thorough and detailed studies than in feasibility work. </li></ul><ul><li>Forms bulk of undergraduate group project work. </li></ul><ul><li>Involves parallel working of many inter-related disciplines with numerous trade-offs and optimization procedures. </li></ul><ul><li>Equivalent to Raymer/Roskam “ Conceptual Design ” phase. </li></ul>
  22. 22. Project Synthesis Process
  23. 23. Project Synthesis Process <ul><li>Configuration Selection </li></ul><ul><li>First task is selection of one or more configurations. </li></ul><ul><li>Unconventional layouts only adopted if unusually dominant requirement. </li></ul><ul><li>Usually well-established conventional layout for given class of a/c. </li></ul><ul><li>Technological advances may render some concepts as unsuitable for future (e.g. impact of flight control systems and thrust vectoring on stability/control surfaces). </li></ul><ul><li>Optimum solution often not adopted due to lack of experience, uncertain design data, customer reticence, etc. </li></ul>
  24. 24. Project Synthesis Process <ul><li>Flight Regime & Powerplant Selection </li></ul><ul><li>Set of operating conditions (Mach number, altitude) usually defined in specification. </li></ul><ul><ul><li>if only given in general terms then have to be assumed in greater detail for synthesis process. </li></ul></ul><ul><li>Flight regime directly defines powerplant type to be used: </li></ul><ul><ul><li>piston-prop, turbo-prop, turbofan, low bypass turbofan, propfan, turbojet, ramjet, rocket, etc. </li></ul></ul><ul><li>Powerplant selection also influences configuration. </li></ul>
  25. 25. Project Synthesis Process <ul><li>Fuselage Layout </li></ul><ul><li>Good starting point for synthesis process. </li></ul><ul><li>Often established independently of lifting surfaces. </li></ul><ul><li>Payload definition main driver and often specified. </li></ul><ul><li>Also crew provision affects forward fuselage design and often known at outset. </li></ul><ul><li>Only overall dimensions required to make first prediction of aircraft mass. </li></ul><ul><li>Geometry and size primarily derived with little use of analytical methods so no single solution. </li></ul>
  26. 26. Project Synthesis Process <ul><li>Wing Configuration </li></ul><ul><li>Fundamental to aircraft performance. </li></ul><ul><li>Complex with large number of parameters to be considered and refined during optimization process. </li></ul><ul><li>Major impact on lift, drag & mass of a/c design - all should be considered when initially selecting layout. </li></ul><ul><li>Initial aim to produce layout with minimum number of parameters for use in initial synthesis. </li></ul><ul><li>Soon leads to wing loading estimation and then wing area once initial mass prediction is known. </li></ul>
  27. 27. Project Synthesis Process <ul><li>Lift, Drag & Mass Estimations </li></ul><ul><li>These are the primary characteristics determining a/c performance for given powerplant & flight regime. </li></ul><ul><li>Can sometimes be estimated using typical values from previous similar a/c (if information is available). </li></ul><ul><li>But preferable to use simple analytical expressions to formulate initial values for use on first optimization. </li></ul><ul><li>More comprehensive methods necessary eventually. </li></ul><ul><li>High degree of interdependence with wing configuration. </li></ul>
  28. 28. Project Synthesis Process <ul><li>Performance Representation </li></ul><ul><li>Vital part of synthesis process – done by expressing various flight stages using equations. </li></ul><ul><li>Flight phases include: </li></ul><ul><ul><li>take-off & initial climb, climb to operating altitude, ceilings, cruise, operating/maximum speed, manoeuvres, descent, approach & landing, baulked landing & missed approach. </li></ul></ul><ul><li>Recommended equations are specific to design process: </li></ul><ul><ul><li>theoretically derived but modified with empirical data. </li></ul></ul><ul><ul><li>used to give early optimum values of wing loading and thrust/weight ratio. </li></ul></ul>
  29. 29. Project Synthesis Process <ul><li>Parametric Analysis – 1 st Stage </li></ul><ul><li>Brings together results of all previous tasks. </li></ul><ul><li>Combines wing and fuselage dimensions into overall a/c layout. </li></ul><ul><li>Lift, drag and powerplant representations used in performance equations to produce variations of wing loading (W/S) and thrust/weight ratio (T/W) for each performance requirement. </li></ul><ul><li>Comparison produces design space to meet all requirements. </li></ul><ul><li>Suitable values for W/S (low) and T/W (high) selected. </li></ul>
  30. 30. Project Synthesis Process <ul><li>Parametric Analysis – 2 nd Stage </li></ul><ul><li>Selected values of wing loading and thrust/weight ratio used to calculate aircraft mass. </li></ul><ul><li>Various combinations used to determine minimum (i.e. optimum) mass value. </li></ul><ul><li>Yields “ referee design ”, which is then used as basis for more detailed analysis and evaluation. </li></ul><ul><li>Revised wing size follows directly from procedure, along with initial notional representations of empennage and landing gear. </li></ul>
  31. 31. Project Synthesis Process <ul><li>Optimization </li></ul><ul><li>Essential feature of project process. </li></ul><ul><li>Target criterion imposed – most usually mass but sometimes cost . </li></ul><ul><li>Mass Optimization </li></ul><ul><li>Size & mass closely related. </li></ul><ul><li>Unusual for size constraints to drive design (exceptions – a/c operating from ships, large airliners with airport gate restrictions). </li></ul><ul><li>Generally, lightest a/c is most efficient with greatest development potential so useful optimisation criterion. </li></ul>
  32. 32. Project Synthesis Process <ul><li>Cost Optimization </li></ul><ul><li>Several possible aspects: </li></ul><ul><ul><li>first cost </li></ul></ul><ul><ul><li>operating costs </li></ul></ul><ul><ul><li>life cycle costs </li></ul></ul><ul><li>More difficult to obtain accurate cost predictions than mass predictions. </li></ul>
  33. 33. Project Synthesis Process <ul><li>Analysis of Derived (Referee) Design </li></ul><ul><li>Involves use of better analytical tools, including: </li></ul><ul><ul><li>size prediction for stability and control surfaces. </li></ul></ul><ul><ul><li>completion of landing gear layout. </li></ul></ul><ul><ul><li>improved estimation of lift, drag and mass characteristics. </li></ul></ul><ul><ul><li>revised performance calculations using improved input data and more elaborate estimation methods. </li></ul></ul><ul><ul><li>reconsideration of stability & control requirements. </li></ul></ul><ul><ul><li>repetition of process until mass convergence. </li></ul></ul><ul><li>Sensitivity studies involving variation of certain parameters to identify critical design areas. </li></ul>
  34. 34. Project Synthesis Process <ul><li>Optimization Procedures </li></ul><ul><li>Graphical Techniques </li></ul><ul><li>Parametric study results plotted onto graphs and superimposed, leading to “ design space ” which meets various performance requirements. </li></ul><ul><li>Limited to number of parameters conveniently handled. </li></ul><ul><li>Mathematical Techniques </li></ul><ul><li>Can handle many parameters simultaneously, e.g. using the multi-variable optimization (MVO) method. </li></ul><ul><li>Needs powerful computational packages. </li></ul>
  35. 35. Other Activities <ul><li>Many other activities often undertaken in typical undergraduate group project, depending on a/c type but typically: </li></ul><ul><ul><li>Structural layout – wing, fuselage, empennage. </li></ul></ul><ul><ul><li>Stress & structural analysis and materials selection. </li></ul></ul><ul><ul><li>Intake/exhaust design. </li></ul></ul><ul><ul><li>flight deck & avionics suite, weapons selection/integration. </li></ul></ul><ul><ul><li>passenger/payload compartment. </li></ul></ul><ul><ul><li>reliability & maintainability. </li></ul></ul><ul><ul><li>survivability & stealth, defensive aids suite. </li></ul></ul><ul><ul><li>hydraulics, pneumatics, electrics, ice protection, fire detection/suppression, etc. </li></ul></ul>
  36. 36. Detail Design Phase <ul><li>Most extensive phase of whole process. </li></ul><ul><li>Purpose is to verify earlier assumptions and produce data needed for hardware manufacture. </li></ul><ul><li>Requires generation of many drawings (by computer aided design nowadays). </li></ul><ul><li>Best solution required for performance, manufacturing costs and operations. </li></ul>
  37. 37. Testing <ul><li>Ground and flight test hardware manufactured from detail design phase. </li></ul><ul><li>Ground Testing </li></ul><ul><li>Includes wind tunnel tests, structural specimens and systems rigs. </li></ul><ul><li>Flight Tests </li></ul><ul><li>To verify performance and flight characteristics of actual aircraft. </li></ul><ul><li>Expensive – so must be completed quickly. </li></ul>
  38. 38. Certification <ul><li>Operational flight clearance issued when calculations, ground and flight testing of design demonstrate to satisfaction of appropriate airworthiness authority that all relevant requirements are met. </li></ul><ul><li>Customer also requires demonstration of performance capabilities. </li></ul>
  39. 39. Project Life Cycle <ul><li>Design phase leading to certification may take up to a decade. </li></ul><ul><li>Development costs rise with time taken to achieve certification. </li></ul><ul><li>Manufacturer continues to support aircraft throughout operational life – can last 50 years+ for a successful design. </li></ul>

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