Aircraft Design

04/08/23 Dr Derek Bray, DAPS 1

Aircraft Design- The Design Process

For more detailed notes please refer to www.rmcs.cranfield.ac.uk/aeroxtra


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


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.


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).


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.


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)


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).


Feasibility Study• Follows basic requirement to assess whether

need can be met with existing technology or not.

• |Needed due to complexity of aeronautical projects.

http://www.airliners.net/open.file/034851/L/


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.


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.


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.


Detail Requirements / Specification (Cont.)

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





Detail Requirements ExampleC-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: > 440 kts (TAS)

• Cruise ceiling: > 30,000 ft

• Take-off at max TOW: < 8000 ft

• Take-off at 4000 nm weight: < 4000 ft

• Landing with 100,000 lb & fuel reserves for 4000 nm: < 4000 ft


Detail Requirements ExampleC-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.


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?



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.


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)



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



Conceptual Design (Cont.)• Followed by:

– confirmation of configuration– fuselage sizing– propulsion selection & integration– empennage sizing– weight and balance analysis– stability analysis


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.


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.


Project Synthesis Process


Project Synthesis ProcessConfiguration 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.


Project Synthesis ProcessFlight 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.


Project Synthesis ProcessFuselage 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.


Project Synthesis ProcessWing 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.


Project Synthesis ProcessLift, 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.


Project Synthesis ProcessPerformance 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.


Project Synthesis ProcessParametric 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.


Project Synthesis ProcessParametric 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.


Project Synthesis ProcessOptimization• 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.


Project Synthesis ProcessCost Optimization• Several possible aspects:

– first cost

– operating costs

– life cycle costs

• More difficult to obtain accurate cost predictions than mass predictions.


Project Synthesis ProcessAnalysis 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.


Project Synthesis ProcessOptimization ProceduresGraphical 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.


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.


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.


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.


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.


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.


Technology

Aircraft Design