Refinement and analysis of concept

Refinement and Analysis 

of Design Concept 

Moving into the Stage of Preliminary Design

1. Specification 

2. Configuration A 

3. Flight regime / Powerplant 

4. Fuselage layout 

5. Wing configuration 1 

6. Lift, drag, mass 

7. performance 

8. Stage one optimization:T/Mg, mg/S 

8. Stage two optimization: referee design 

8. Configuration comparison 

9. Concept analysis 

Road Map 

航空宇航学院 

Configuration B Configuration C 

Wing configuration 2 Wing configuration 3 

Aircraft Conceptual Design

Introduction 


• We just completed the conceptual design synthesis 

• Now moving on to the stage of preliminary design 

– Refinements 

• More detail parameters of the configuration 

• More detail and precise arrangements of the layout 

– Analysis 

• Verifying the design concept using more precise models 

– Optimization 

• Searching a optimum design with more detail parameters of the 

configuration using more precise models 


Refinements 

• Powerplant 

• Aerodynamic Configuration 

• Structure Layout 

• Landing Gear 

• Fuselage Layout


Refinements of Powerplant 

• More accurate engine data 

– Mass data: 

– Size data: 

– Performance data: Variation of thrust and fuel consumption as 

function of flight speed, altitude and engine setting 

• How to get the data 

– From the engine manufactures 

– Using the more precise model of the engine based the engine 

design method 

• Powerplant Location 

– The development of the design concept now enables the question 

of powerplant location to be considered more precisely. 


Powerplant Location 

• Versatility and access: 


– As a general rule engines mounted in pods or nacelles on the 

outside of the aircraft are more versatile than those buried within 

the airframe. 

– It is usually possible to accommodate different engines without 

too much difficulty, thereby more readily handling upgrades or 

specific customer requirements. 

– Podded and nacelle-mounted engines are also preferable for 

maintenance since access is generally more direct and need not 

involve significant structural penalties. 

– Also air intakes and exhausts are usually shorter and can be 

more efficient. 



• Installation clearance 


– As the engine dimensions are defined it is necessary to check 

installation clearances, such as propeller tip spacing from adjacent 

structure or the ground and under-wing nacelle ground clearance on 

a low wing aircraft. 


• Safety 



– The powerplants must be positioned so that in emergency 

situations they do not pose a threat to the occupants of the 

aircraft or other safety critical components. 

– The major concerns are the consequences of an engine 

becoming detached during an emergency alighting situation 

or the failure of rotating components such as fan and turbine 

blades or discs and propeller blades. 

– The engines must be located such that these component 

failures do not present a hazard to other engines, primary 

structures or vital systems. 



• Acoustic considerations 


– The further aft on an aircraft the engine is located the less is the 

noise impact upon the occupants, but this is not usually a major 

consideration. 

– Some engine positions, for example those above the wing or 

fuselage, reduce the noise as perceived at the ground but at the 

expense of powerplant efficiency. 

– Acoustic fatigue damage to airframe components located near the 

exhaust is likely to be a more important consideration. 

– It is especially significant for engines which employ reheat when the 

exhaust outlets should be behind all major airframe components. 



• Stability and control 


– Lateral stability and control requirements for a multi-engined 

aircraft may well be dominated by the need to maintain the 

path of the aircraft subsequent to the failure of the most 

critical engine. 

– From this point of view wing-located powerplants should be 

as far inboard as allowed by other considerations. 



• Powerplant/airframe effciency 


– This often means a compromise between engine requirements 

and those of the overall aerodynamics. 

– For example: 

• it may be desirable on a short take-off propeller driven aircraft to space the 

engines across the wing to enhance slipstream effect upon low speed lift. 

• Ground clearance may suggest that large diameter power units should be 

mounted over the top of the wing but this can lead to a considerable loss of 

efficiency of lift development. 

• In the case of buried engines the intake arrangements may have a significant 

effect upon powerplant efficiency and must be given careful consideration, 


• Introduction 

Refinement of wing 


– The analysis of the lifting characteristics of the wing demands a 

more specific consideration of both the basic aerofoil section and 

the geometry of the high lift devices than was possible at the 

conceptual design stage 

• Airfoil selection and design 

– The output of the parametric analysis includes some aerofoil detail, 

such as the root thickness to chord ratio and the desired critical 

Mach number characteristics. 

– This information may be used in the selection of a suitable aerofoil. 

– The airfoil design and optimization will be introduced later. 


• High lift device 


– Leading edge high lift devices 


• While leading edge high lift devices may be located across the greater part 

of the wing span, it may be necessary to accept some interruption, as for 

example in the region of wing-located powerplants. 

• Such effects should be minimised and carefully considered as the details of 

the design are established. 



– The trailing edge high lift devices 


• Generally the spanwise extent of the trailing edge high lift devices is limited 

by the need to provide ailerons for roll control although in some special 

circumstances these may be "drooped" to augment the lift. 

• Outboard ailerons are usually needed for low speed roll control and typically 

occupy 20-25% of the wing semi-span. 



Refinement of aerodynamics design 

• Control/stabilizer surface 

– Outer ailerons on high aspect ratio sweptback wings may 

introduce aeroelastic difficulties when used at high speed, and to 

overcome this problem alternative high speed roll control may be 

provided in the form of inboard ailerons or differential longitudinal 

controls such as tailerons. 

– In the case of inboard ailerons there will be an effect upon the 

extent of the trailing edge flaps although this may be minimised 

by locating the controls immediately behind powerplants. 

– Within some limits the effectiveness of trailing edge devices may 

be adjusted by variation of their chordwise extent. 



Refinement of control and stabiliser surfaces 

• The design of the control and stabiliser surfaces 

requires ensuring that there is: 

– Ability to provide the forces/moments needed to maintain the 

trim of the aircraft in all flight conditions. 

– Ability to provide control forces/moments to meet prescribed 

handling criteria, including situations in which a partial failure 

has occurred. 

– Stability in both the static and dynamic sense, or alternatively 

acceptable artificial damping or stability in all critical modes. 



Refinement of control and stabiliser surfaces 

• The design of the control and stabiliser surfaces is 

complex because of the number of separate conditions 

which have to be met. 

• Some simplifications may be made to facilitate the 

process during initial design work 

• We have presented the design methods for the 

longitudinal and lateral requirements respectively. 

• The output from these methods may then be used for 

a full analysis. 


Structure 


• The overall layout of the aircraft must be compatible with 

an efficient structure 

• The aerodynamic configuration must not be such as to 

imply an unacceptable high structure weight. 

• Wing structure layout 

– Referring to the course “ Design of Aircraft Structure” 

• Fuselage structure layout 

– Referring to the course “ Design of Aircraft Structure” 


Wing structure layout 


• Sometimes the spanwise bending and torsion requirements may 

be met by a single beam or spar located near to the maximum 

chordwise depth of the aerofoil and associated with a shear 

carrying leading edge, or 'D' nose, terminating at the spar. 

• More usually there are two or more spars located across the 

chord, and together with the upper and lower covers between 

their chordwise extremities they form a box beam which also 

meets the torsion requirement. 

• The forward and aft extremities of this beam are determined by 

the auxiliary lifting surfaces and ailerons. 

• The rearmost spar is often located at 60-70% of the chord, 

leaving room for the trailing edge surfaces 


Wing structure layout 


• The box beam also conveniently acts as integral tankage for the fuel 

and access panels are required for inspection purposes. 

• For moderate to high aspect ratio wings the box beam should be 

continuous across the whole span. 

• Large cutouts for such items as landing gear stowage should be 

outside its boundaries. 

• When the powerplant is buried within the fuselage and the wing is of 

relatively low aspect ratio, it is often necessary to use several spanwise 

spars and to pass the bending loads round the fuselage by means of a 

series of ring frames to which the spars are connected. 



An example of structure layout 



Refinement of Landing Gear 

• The main points to be investigated are 

– Overall layout geometry, including the number of main gear units 

– Number and size of the wheels/tyres 

– Location of suitable stowage volume 

– Location of suitable load attachment points. 

• Clearly these latter two issues are closely related to the 

layout of the structural members. 

See Addendum 1 for more detail 


Fuselage Layout 


• A more precise and detailed layout of the fuselage may 

now be undertaken. 

– Influence of the wing junction with the fuselage 

– Landing gear attachment and stowage 

– Layout of the cockpit region, windscreen geometry, etc. 

– Detail of the powerplant installation 

– Payload detail 

• Doors,windows 

• Freight loading access and doors 

– Special detail considerations 

With an aid of CAD software 


Now, we embark on analysis of the more 

detail design using more precise models 

• Mass prediction 

• Aerodynamics analysis 

• Structure analysis 

• Performance analysis 

• Stability and control analysis 

• Cost


Mass Prediction Techniques 

• Empirical comparisons 

– Predictions by direct comparison with other similar design 

• Empirical formulae 

– Derived by a statistical analysis of available information from 

many aircraft (see chapter 6 ) 

• Theoretically derived formulae 

– Based on a simplified design of the component 

– See Addendum 4 

• Detail estimation method 

– Need information of the detailed component 


Aerodynamic Analysis 


• The different levels of the analysis models 

– Theoretical method 

• Aerodynamic theory + empirical data (past experience) 

– Numerical methods 

• Vortex lattice method 

• Panel Method 

• Computational Fluid Dynamics (CFD) 

– Experimental method 

• Wind tunnel tests 


Aerodynamic Analysis 

Theoretical + empirical method 

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Structure Analysis 

• Beam theory 

– Refer to the course “Structure Mechanics” 

• Finite Element Method (FEM ) 



Performance Analysis 


• Predicate a precise performance using the 

following upgraded information 

– Engine data 

– More precise lift and drag data 

– More precise mass data 

Performance 

Powerplant aerodynamics Mass 


Stability and Control Analysis 

• Theoretical + empirical method 

– Referring to textbook on “Flight Dynamics” 

– DatCom software 

– AAA software 

• Numerical method 

– Vortex lattice 

– Panel method 

– CFD 

• Wind tunnel test 



Cost Analysis 


Cost estimation should base on a 

comparative rather than absolute value 



Different Cost Meanings 

• Selling cost 

– driven by market forces 

• First cost 

• Operating costs 

• Lift cycle costs 


• What is first cost 

First cost 


– The sum of the actual cost of making the aircraft and the 

share of the development costs allocated to it. 

• First cost is difficult to be predicted at the aircraft 

conceptual design 

– Simple cost models are usually based on aircraft volume 

and/or mass and derived empirically from past experience. 

– First cost is not a very suitable criterion for project 

optimization. 


Operating costs 

• Operating costs are split into two categories 

– Indirect costs 


• Independent of the characteristics of a given aircraft and 

represent the overall costs of the operation of the airline. 

– Direct Operating Costs (DOC) 

• The initial purchase price 

• Insurance 

• Crew costs 

• Engineering replacement items 

• Maintenance 

• Operational charges 

• Fuel and other expendable items 



Direct Operating Costs (DOC) 

• DOC is a better criterion for optimization 

than first cost. 

• DOC is difficult to be calculated. 

• There is a close relationship between 

direct operating costs and aircraft mass. 


Life cycle costs 

• Life Cycle Costs (LCC) covers: 

– Design and development of the type 

– Procurement of the aircraft 

– Cost of operations 

– Final cost disposal of the aircraft 


• LCC is a relevant criterion for optimization, but difficult to be 

estimated 

– Recourse may have to made to comparisons on relative basis. 

– LCC of military aircrafts is more or less directly proportional to mass 

and maximum operating speed for Mach 1.3 or below. 


Introduction to AAA software 

A software for analysis of aircraft preliminary design



Finally, we use optimization to 

search a better design 

Introduction to Multidisciplinary Design Optimization (MDO) 

A advanced method for aircraft preliminary design optimization

(MDO) Framework 

气动(CFD) 

外形(CAD) 

隐身 

结构(FEM) 

协调(MDO方法) 

中心数据库 

设计过程监控 

性能 

重量重心 

操稳 

发动机特性 



Output of the Preliminary Design

A detail configuration 

728JET 三维外形 



Fwd FA Seat 

(View Fwd) 


A detail Fuselage Layout 

728JET and 928JET Galleys and Wardrobes 

Forward Galley 

(View Fwd) 

Forward Wardrobe 

(View Aft) 

Forward Lavatory 

(View Aft) 

Rear Galley 

(View Aft) 

Flexible Design to Meet Highest Customer Requirements. 

Rear Lavatory 

(View Aft) 


A overall layout (F-16) 



A overall layout (AD-400)

Refinement and analysis of concept

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