Chapter 5. Design for performance
5.5. AIRCRAFT SYNTHESIS AND OPTIMIZATION
From the previous chapters it will have become clear that the performance aims may be realized iD various ways, by choosing suitable combinations of wing loading, as-pect ratio and thrust loading. A frequently used systematic approach to this problemis the parametric investigation. A parametric
mentioned in the introduction to Section 5.4. Engine cycle parameters like design Turbine Entry Temperature, Overall Pres-sure Ratio and bypass ratio, may be sub-jected to parametric investigations in or-der to gain an insight into the most suit-able type of engine to be installed. As there are many types of parametric design studies, depending on the accuracy requirec and the phase of the design, some simpli-fied examples to demonstrate the principle will be given below. The list of references relating to this chapter contains a number of more complete studies.
5.5.1. Purpose of parametric studies Parametric studies can be useful in many types of design problems, provided the de-sign criteria can be quantified in terms of minimum weight, cost and/or~noise fig-ures. However, in airplane design they re-quire a considerable amount of computation-al work, as computation-all variations will have far-reaching consequences. For this reason, computerized design studies are increasing-ly becoming a prerequisite for advancedand complex aircraft. However, the system must be devised, monitored and utilized by ex-perienced designers in order to define the design problems and the interfaces between various technical disciplines in the design an essential for avoiding unrealistic re-sults.
Although the optimum choice of aircraftde-sign parameters has been a major deaircraftde-sign problem since the early days of aviation, the generalized approaches described in the published literature are scanty and have a very limited validity due to the necessary simplifications and lack of flex-ibility. The following design goals can be achieved with a computerized system:
171
INPUT ~ INITIAL ESTIMATE OF
~"~EMPTY AND TAKE-OFF WEIGHT CHANGE WEIGHT. WING & ENGINE SIZE
I
MISSION AND PERFORMANCE CRITERIA
PAYLOAD WING SIZING
•
FIELD PERFORMANCERANGE
CRUISE ALTITUDE CRUISE SPEED
NO. OF ENGINES ENGINE CONFIGURATION AND SIZE
UNDERCARRIAGE DESIGN re' TAKE-OFF FIELD LENGTH
LANDING FIELD LENGTH COMMUNITY NOISE TAKE-OFF FIELD LENGTH t
LANDING FIELD LENGTH OR
APPROACH SPEED LAYOUT DESIGN t
CLIMB REQUIREMENTS CONFIGURATION GEOMETRY AND DATA TECHNOLOGY DATA
GENERAL ARRANGEMENT GEOMETRY PARAMETERS EXCEPT EMPENNAGE
•
AERODYNAMICS
PROPULSION WEIGHT AND BALANCE STABILITY AND CONTROL
AIRFRAME AND SYSTEMS WEIGHT-DATA
GROUP WEIGHTS
WING LOCATION EVALUATION AND OUTPUT LOADING C.G. LIMITS
HORIZONTAL TAIL SIZE AERODYNAMIC C .G. LIMITS VERTICAL TAIL SIZE
THREE-VIEW DRAWINGS WEIGHT-BALANCE DIAGRAM DRAG POLARS, LIFT CURVES OFF-DESIGN PERFORMANCE WEIGHT STATEMENT OPERATING COST t
MISSION PERFORMANCE
!
CHANGEJ
. CRUISE SPEED . PAYLOAD-RANGE WEIGHTFig. 5-18. Example of a generalized design procedure (Ref. 5-45, modified)
a. Determination of combinations of param-eters, characterizing designs that satisfy specified operational requirements.
b. Calculation of values for the configu-ration parameters already mentioned, re-sulting in the most favorable objective function, e.g. takeoff weight or operating costs.
c. Sensitivity studies to assess the ef-fects of minor changes in the shape or ge-ometry, material properties, drag coeffi-cients, etc.
d. Mission/performance analysis and trade-off studies to investigate the effect of variations in the performance requirements.
e. The effect of certain technological constraints in terms of weight and cost penalties. It should be realized that the validity of computerized studies is deter-mined entirely by the accuracy of the in-put data and design methods available. In that respect they are no better than manu-al computations. However, improved data can 172
easily be incorporated in a computerized system. On the other hand, the ~esign of the program and evaluation of the output may be time-consuming and costly.
5.5.2. Basic rules
Variation of design parameters in effect means designing many aircraft layouts. It is therefore necessary to lay.down basic rules governing the design process foreach variant. A generalized procedure as de-picted in Fig. 5-18 will be explained. In-put data are performance criteria and tech-nological constraints. Performance data are generally mission data (payloa1 and range) and performance constraints (field length, approach speed) • Cruise performance may be considered as a constraint ~r as a fixed mission requirement, depending on the ap-plication.
The first design step will be to generate initial configuration characteristics such
as takeoff weight, wing loading andkhrust loading, on the basis of semi-sta;tistical formulas of the type presented in this chapter. ConfigUration and layout design activities generally refer to the work on the drawing board; for computerized design this must be translated into a mathematical procedure for defining the external geo-metry of the major aircraft components.
This is a difficult part of the program and in many cases it will, for example, be ac-ceptable to use the geometry of a fuselage design generated outside the numerical program.
The next step is to calculate groupweights and the empty weight, desirable loading center of gravity limits and empennage size*. Sufficient data are then available to calculate the maximum range for the de-sign payload and a conclusion can be drawn about whether the fuel weight is suffi-cient or not. In the first attempt, this will not be the case and the airplane is not balanced with respect to the all-up weight. The fuel and takeoff weights must be changed and the procedure repeated, un-til the design characteristics have con-verged sufficiently. Field performance and noise characteristics may now be calculated and if there is some deficiency, the wing size must be changed. Except in the case of a fixed engine, some scaling of the en-gine size may be feasible as well. The de-sign procedure is repeated again, until all performance requirements are met. The out-put is presented in the form of drawings, diagrams and characteristic data. Excellent examples of such a program are describedin Refs. 5-40, 5-43 and 5-45.
Two comments must be made on the type of studies discussed.
1. The prediction methods for weight and aerodynamic characteristics must not only be accurate in absolute values, but must also predict the effect of design changes accurately. More accurate data than those presented in this chapter are generally
*These subjects will be dealt with in Chapter 8
required.
2. The evaluation of computer output is a time-consuming problem. The computer graphics facility is a useful tool here
(cf. Ref. 5-41).
5.5.3. Sizing the wing of a long-range passenger transport
This section is concerned with investiga-ting the effect of wing area variation on the takeoff weight (Fig. 5-19) of a large, long-range passenger transport, equipped with turbofan engines with a total thrust of 176,000 lb (80, 000 kg) at St.S (ISA)I.~
The airplane will be designed to fly a payload of 126,000 lb (57,000 kg) over a range of 4,000 n.m. (7,400 km); M = .85 at 35,000 ft (10,700 m). Fuel reserves must be available for two hours holding at 95%
of (L/D)max' plus 5% tolerance on the total fuel weight. The field length is limited to 3,000 m (10,000 ft) and 2,000 m (6,500 ft) for takeoff and landing respectively. For simplicity, climb performance requirements are summarized in. the condition that at 85% of the ISA-SLS thrust the second seg-ment climb gradient (FAR 25.121 b) must be achieved.
The all-up weight at takeoff is initially estimated at 705,000 lb (320,000 kg), with a wing area of 5,900 sq.ft (550m2). A sub-division is made as follows:
a. A fixed weight of 390,000 lb (177,000 kg) for the payload and payload service items, the fuselage group, the propulsion group, the undercarriage, fixed equipment and systems.
b. A variable weight affected by the wing and tailplane size and fuel quantity re-quired.
The wing plus tailplane structure weight is estimated after several iterations by a simple formula given in Fig. 5-19. Re-serve fuel for 2 hours holding is estimated as follows:
(5-99)
while
...
:a:
"
iii
s:
3
2
174 .4
.3
4 ENGINES 80,000 KGF THRUSTSLS
400
MAX. TAKEOFF WEIGHT 95000 KG
600
WING AREA - M 2
450
1000
TAKEOFF
Fig. 5-19. Effect of wing area on the weight breakdown of a large passenger transport (project)
Fig. 5-20. Wing loading vs. thrust loading dia-500 / 550 KG/M 2 gram of the three-engine
Wto $ airliner in Fig. 11-1
(5-100)
It is noted that