Aircraft Design - The Design Process

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Aircraft Design- The Design Process
For more detailed notes please refer to
www.rmcs.cranfield.ac.uk/aeroxtra
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Recommended Further Reading

• D.Howe Aircraft Conceptual Design Synthesis
• D.Raymer Aircraft Design, A Conceptual Approach
• J.Roskam Airplane Design, Parts 1-8
• E.Torenbeek Synthesis of Airplane Design
• L.Jenkinson, P.Simpkin D.Rhodes Civil Jet
  Aircraft Design
• D.Stinton The Design of the Aeroplane
• S.Brandt, J.Stiles R.Whitford Introduction to
  Aeronautics A Design Perspective

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Design Process - Overview

• Basic general requirements.
• Feasibility study.
• Detail requirements specification.
• Design phases Roskam/Raymer models
• Project synthesis process (Howe model).
• Configuration, flight regime powerplant,
  fuselage layout, wing configuration, lift, drag
  mass representations, performance representation,
  parametric analysis optimization
• Analysis of detailed design.
• Detail design phase.
• Testing, certification project life cycle.

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Basic Requirements

• New design launched when perceived requirement
  arises for aircraft beyond capability of those
  existing.
• Usually due to
• aircraft approaching end of its useful life.
• design overtaken by technological developments.
• Identification of need may originate from
• manufacturing organization (especially if civil).
• potential operator (especially if military).

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Basic Requirements (Cont.)

• Initial basic requirements statement often brief,
  including class of aircraft and major performance
  characteristics.
• Initial statement usually refined after
  consultations with appropriate operators and
  major manufacturers.

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General Requirements

• Result of many years of previous experience
  applicable to various classes of a/c.

• Act as
• guide to designers.
• basis for eventual clearance of a/c for intended
  operators.
• Most important for civil/general aviation are
• FAR 25/23 (US), JAR 25/23 (Europe)
• (Federal or Joint Airworthiness Requirements)

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General Requirements (Cont.)

• FAR and JAR written in identical format with only
  a few subtle differences eventual aim is for
  commonality.
• For military a/c use
• DEF STAN 00-970 (UK), MIL SPECS (US)
• MIL SPECS being replaced with requirements
  defined by individual manufacturers (Lockheed
  Martin, Boeing).

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Feasibility Study

• Follows basic requirement to assess whether need
  can be met with existing technology or not.
• Needed due to complexity of aeronautical
  projects.

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Feasibility Study (Cont.)

• Also used for other purposes
• how best to meet basic requirement (adaptation of
  existing a/c, major modification of existing a/c,
  completely new design (highest risk cost)).
• concept/configuration comparison studies also
  undertaken.
• review and revision of basic requirement
  performance characteristics.
• likely output is definition of detailed set of
  requirements (specification).
• initial cost estimation.

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Detail Requirements / Specification

• Covers many aspects, though not all significant
  for project synthesis process phase.
• Performance
• Range with basic payload mass.
• Alternative range/payload combinations (
  reserves).
• Max (or max normal) operating speed.
• Take-off landing field length limitations.
• Climb performance (time to height, ceiling,
  etc.).
• Manoeuvre acceleration requirements.

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Detail Requirements / Specification (Cont.)

• Operations
• Size mass limitations (runway loading).
• Crew complement.
• Occupant environment (pressure, temperature).
• Navigation/communications equipment.
• Payload variation associated equipment.
• Maintenance targets.
• Stealth aspects (military a/c).
• Extended engine failed allowance (ETOPS) civil.

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Detail Requirements / Specification (Cont.)

• General
• Growth potential.
• Cost targets, availability.
• Airframe life.
• Airworthiness requirements (JAR 25, etc.).

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Detail Requirements Example

• C-5 Specific Operational Requirement June 1963
  (Selected Items)
• Basic design mission 100,000 to 130,000 lb for
  4000 nm
• Alternate mission 50,000 lb for 5500 nm
• Load factor 2.5
• Maximum design payload 130,000 150,000 lb
• Cruise speed gt 440 kts (TAS)
• Cruise ceiling gt 30,000 ft
• Take-off at max TOW lt 8000 ft
• Take-off at 4000 nm weight lt 4000 ft
• Landing with 100,000 lb fuel reserves for 4000
  nm lt 4000 ft

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Detail Requirements Example

• C-5 Specific Operational Requirement June 1963
  (Selected Items) (Cont.)
• Cargo compartment length 100 110 ft, width 16
  17.5 ft, height 13.5 ft.
• Cargo landing straight through, one full
  section, one 9x10ft, truck bed floor height
  desirable.
• Powerplant 6 x turbofans.
• Reliability 95 probability of completing 10 hr
  mission.
• Availability June 1970.

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Aircraft Design Phases (Raymer/Roskam Models)

• Conceptual Design
• All major questions asked and answered.

• will it work?
• what does it look like?
• what requirements drive the design?
• what trade-offs should be considered?
• what should it weigh and cost?

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Aircraft Design Phases (Raymer/Roskam Models)

• Conceptual Design (Cont.)
• No correct solution and process involves great
  deal of compromise, iteration and trade-offs.
• 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.

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JSF Conceptual Designs
(a)
(b)
(a) Lockheed-Martin X-35 successful (b) Boeing
rejected after demonstrator flights (c)
McDonnell-Douglas rejected after concept design
phase
(c)
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Aircraft Design Phases (Raymer/Roskam Models)

• Conceptual Design (Cont.)
• Various activities to be covered include
• configuration possibilities
• preliminary sizing (weight)
• drag polar equation estimation
• performance sizing matching using W/S and T/W
  relationships to optimally fix wing size and
  engine thrust power
• wing layout and high-lift devices

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Aircraft Design Phases (Raymer/Roskam Models)

• Conceptual Design (Cont.)
• Followed by
• confirmation of configuration
• fuselage sizing
• propulsion selection integration
• empennage sizing
• weight and balance analysis
• stability analysis

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Aircraft Design Phases (Raymer/Roskam Model)

• Preliminary Design
• Begins when major design changes are over.
• configuration and major characteristics frozen.
• lofting developed.
• testing and development tools developed.
• major items designed.
• cost estimates refined.
• Followed by detail design, production, testing
  and certification phases.

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Project Synthesis Process(Howe Model)

• Considered as an extension of feasibility study.
• Though a different aim to produce reasonably
  well-defined design to be offered to potential
  customers.
• Requires considerably more thorough and detailed
  studies than in feasibility work.
• Forms bulk of undergraduate group project work.
• Involves parallel working of many inter-related
  disciplines with numerous trade-offs and
  optimization procedures.
• Equivalent to Raymer/Roskam Conceptual Design
  phase.

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Project Synthesis Process
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Project Synthesis Process

• Configuration Selection
• First task is selection of one or more
  configurations.
• Unconventional layouts only adopted if unusually
  dominant requirement.
• Usually well-established conventional layout for
  given class of a/c.
• Technological advances may render some concepts
  as unsuitable for future (e.g. impact of flight
  control systems and thrust vectoring on
  stability/control surfaces).
• Optimum solution often not adopted due to lack of
  experience, uncertain design data, customer
  reticence, etc.

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Project Synthesis Process

• Flight Regime Powerplant Selection
• Set of operating conditions (Mach number,
  altitude) usually defined in specification.
• if only given in general terms then have to be
  assumed in greater detail for synthesis process.
• Flight regime directly defines powerplant type to
  be used
• piston-prop, turbo-prop, turbofan, low bypass
  turbofan, propfan, turbojet, ramjet, rocket, etc.
• Powerplant selection also influences
  configuration.

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Project Synthesis Process

• Fuselage Layout
• Good starting point for synthesis process.
• Often established independently of lifting
  surfaces.
• Payload definition main driver and often
  specified.
• Also crew provision affects forward fuselage
  design and often known at outset.
• Only overall dimensions required to make first
  prediction of aircraft mass.
• Geometry and size primarily derived with little
  use of analytical methods so no single solution.

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Project Synthesis Process

• Wing Configuration
• Fundamental to aircraft performance.
• Complex with large number of parameters to be
  considered and refined during optimization
  process.
• Major impact on lift, drag mass of a/c design -
  all should be considered when initially selecting
  layout.
• Initial aim to produce layout with minimum number
  of parameters for use in initial synthesis.
• Soon leads to wing loading estimation and then
  wing area once initial mass prediction is known.

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Project Synthesis Process

• Lift, Drag Mass Estimations
• These are the primary characteristics determining
  a/c performance for given powerplant flight
  regime.
• Can sometimes be estimated using typical values
  from previous similar a/c (if information is
  available).
• But preferable to use simple analytical
  expressions to formulate initial values for use
  on first optimization.
• More comprehensive methods necessary eventually.
• High degree of interdependence with wing
  configuration.

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Project Synthesis Process

• Performance Representation
• Vital part of synthesis process done by
  expressing various flight stages using equations.
• Flight phases include
• take-off initial climb, climb to operating
  altitude, ceilings, cruise, operating/maximum
  speed, manoeuvres, descent, approach landing,
  baulked landing missed approach.
• Recommended equations are specific to design
  process
• theoretically derived but modified with empirical
  data.
• used to give early optimum values of wing loading
  and thrust/weight ratio.

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Project Synthesis Process

• Parametric Analysis 1st Stage
• Brings together results of all previous tasks.
• Combines wing and fuselage dimensions into
  overall a/c layout.
• 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.
• Comparison produces design space to meet all
  requirements.
• Suitable values for W/S (low) and T/W (high)
  selected.

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Project Synthesis Process

• Parametric Analysis 2nd Stage
• Selected values of wing loading and thrust/weight
  ratio used to calculate aircraft mass.
• Various combinations used to determine minimum
  (i.e. optimum) mass value.
• Yields referee design, which is then used as
  basis for more detailed analysis and evaluation.
• Revised wing size follows directly from
  procedure, along with initial notional
  representations of empennage and landing gear.

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Project Synthesis Process

• Optimization
• Essential feature of project process.
• Target criterion imposed most usually mass but
  sometimes cost.
• Mass Optimization
• Size mass closely related.
• Unusual for size constraints to drive design
  (exceptions a/c operating from ships, large
  airliners with airport gate restrictions).
• Generally, lightest a/c is most efficient with
  greatest development potential so useful
  optimisation criterion.

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Project Synthesis Process

• Cost Optimization
• Several possible aspects
• first cost
• operating costs
• life cycle costs
• More difficult to obtain accurate cost
  predictions than mass predictions.

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Project Synthesis Process

• Analysis of Derived (Referee) Design
• Involves use of better analytical tools,
  including
• size prediction for stability and control
  surfaces.
• completion of landing gear layout.
• improved estimation of lift, drag and mass
  characteristics.
• revised performance calculations using improved
  input data and more elaborate estimation methods.
• reconsideration of stability control
  requirements.
• repetition of process until mass convergence.
• Sensitivity studies involving variation of
  certain parameters to identify critical design
  areas.

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Project Synthesis Process

• Optimization Procedures
• Graphical Techniques
• Parametric study results plotted onto graphs and
  superimposed, leading to design space which
  meets various performance requirements.
• Limited to number of parameters conveniently
  handled.
• Mathematical Techniques
• Can handle many parameters simultaneously, e.g.
  using the multi-variable optimization (MVO)
  method.
• Needs powerful computational packages.

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Other Activities

• Many other activities often undertaken in typical
  undergraduate group project, depending on a/c
  type but typically
• Structural layout wing, fuselage, empennage.
• Stress structural analysis and materials
  selection.
• Intake/exhaust design.
• flight deck avionics suite, weapons
  selection/integration.
• passenger/payload compartment.
• reliability maintainability.
• survivability stealth, defensive aids suite.
• hydraulics, pneumatics, electrics, ice
  protection, fire detection/suppression, etc.

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Detail Design Phase

• Most extensive phase of whole process.
• Purpose is to verify earlier assumptions and
  produce data needed for hardware manufacture.
• Requires generation of many drawings (by computer
  aided design nowadays).
• Best solution required for performance,
  manufacturing costs and operations.

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Testing

• Ground and flight test hardware manufactured from
  detail design phase.
• Ground Testing
• Includes wind tunnel tests, structural specimens
  and systems rigs.
• Flight Tests
• To verify performance and flight characteristics
  of actual aircraft.
• Expensive so must be completed quickly.

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Certification

• 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.
• Customer also requires demonstration of
  performance capabilities.

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Project Life Cycle

• Design phase leading to certification may take up
  to a decade.
• Development costs rise with time taken to achieve
  certification.
• Manufacturer continues to support aircraft
  throughout operational life can last 50 years
  for a successful design.