INTRODUCTION

Chapter 5. Design for performance

5.1. INTRODUCTION

After deciding on the general arrangement (Ch. 2), incorporating this in initial de-sign sketches, and finalizing the layout drawings of the fuselage (Ch. 3), the de-signer's next ,;tep will be to decide on the type of engine to be installed and the size of the wing. At this stage of the design the specified m1ssion and flight perform-ance will play an important part. In Chapter

air comp cr f g i land LOF MD n p res run stop

t thr to trip uc w

air maneuver in takeoff or landing compressibility effects

cruising flight fuselage; flap; fuel ground (run)

start of cruising flight landing

lift-off

minimum drag condition engine nacelle(s) payload

reserve fuel

takeoff or landing run

deceleration phase during aborted takeoff or landing

tailplane (empennage) thrust reverser(s) takeoff

trip fuel (fuel burned) undercarriage

wing

c/4 quarter-chord line

0 sea level conditions

is occasionally chosen on the basis of limited performance studies. In such a case the wing design procedure is considerably simplified but the result may be unsatis-factory if the number of suitable engines is too restricted. If several engine types are available, a systematic study is re-quired to find optimum combinations of wing design and types of engine. In this Chapter the basic relationships for the performance of such studies will be developed and ele-4 it was concluded that engine performance mentary examples will be given. A complete is affected by many parameters, such as cycle evaluation of the effect of the design temperatures and pressure ratios, bypass

ratio, et~. The most important properties of the wing are wing sections and area, as-pect ratio and high-lift devices. In a well-balanced design the various parameters and shape factors are combined so as to minimize both the initial costs and opera-ting costs, while meeopera-ting all performance requirements.

In view of the limited availability of en-gines, the type of engine to be installed

choices on the complete aircraft is virtu-ally equivalent to carrying out as many de-sign studies as there are parametric varia-tions and many details must therefore re-main undiscussed.

Simplified illustrations are presented to cover the more complex computational system and the reader may refine all relationships insofar as he thinks fit. The procedure is divided into three major parts:

a. In1tial estimation of the empty weight, fuel weight, all-up weight and drag polars

(Sections 5.2 and 5.3).

b. Derivation of boundary values of the de-sign parameters, based on ''reversed'' per-formance calculations (Section 5.4).

c. Some examples of performance optimiz-ations (Section 5.5).

The actual choice of the type of engine and wing shape is based not only on performance calculations but also on other factors to be considered in Chapters 6 and 7.

In recent years, low noise production has become a requirement of major importance in aircraft.design. Although the methods for achieving a low-noise design have not yet been settled, some aspects will be mentioned in this chapter.

It is emphasized that most of the methods presented here are intended as illustrative examples, applicable to an early stage of the design; they should be refined and

im-attempt to alter design weights at a later stage. The weight data issued by the pre-liminary design department serve as goals for other engineering departments.

Weight prediction is necessary not only for st"ress and performance computations, but also for design optimization, as reflected in the following categories of estimating methodology:

1. Pre-configuration selection methods.

2. Configuration selection methods.

3. Post-configuration selection methods.

The general requirements to be satisfied by estimating methods are given below.

a. Pre-configuration selection methods.

"Weight guesstimates" are used during the period in the design where the mission re-quirements are practically the only objec-proved as soon as more data become available. tives which are definitely known. The air-Chapter 8 and the appendices to this book plane size and structural arrangement can-contain more detailed data and many refer- not be finalized until systematic studies gnces to the relevant literature. have been made to determine the optimum

de-sign. The prediction methods to be used should be elementary in nature, and the use 5.2. INITIAL WEIGHT PREDICTION of statistics is appropriate when it

pro-duces rapid answers. Some examples are giver 5.2.1. Stages in the estimation of airplane in Section 5.2.2. and in References 5-6 to

weight 5-10.

In the early days of aviation, the design b. Configuration selection methods.

engineer was responsible for the design, The methods normally employed are semi-em-stress analysis and weight control for the pirical and seek to account for weight var-components of the aircraft. As design prob- iations due to changes in major design lems became more complex, specializedfields rameters. Such methods are valuable in pa-of engineering were developed. One pa-of these rametric studies aimed at finding out which fields is weight determination, control and combination of parameters yields the best coordination. Today, weight prediction and compromise of all the input variables. Not control are part and parcel of every phase only must the methods used provide accurate in the design and development of every type answers in an absolute sense, but the

pre-of aircraft. dieted effect of the variation of each

pa-Te be effective, weight prediction and con- rameter should be equally correct. The com-trol must be carried out during the early putation methodology is iterative in nature, stages of the preliminary design, before a i.e. starting values for the design weights design configuration becomes "frozen". As must be assumed or based on previous esti-soon as weight figures are distributed to mates, and after completion of the weight the various departments of a design office breakdown the new values must be used to as a starting point for further design e- start a further computation cycle, contin-valuation, they will strongly resist a~y uing in this way until the process has

con-144

verged to within the required accuracy. Ex-amples of these types of methods will be presented in Chapter 8.

c. Post-configuration selection methods.

In this phase a baseline configuration has been selected and it is necessary to ana-lyse the loads and weight in greater detail.

(5-3)

This equation will now be evaluated for sev-eral airplane categories.

Major elements of such a method are a de- a. Light aircraft with piston engines.

tailed weight breakdown and the use of pre- One important contribution is the engine liminary stress analysis to determine the weight, which is frequently a known factor.

amount of structural material required to Alternatively, it can be estimated fromFig.

resist the applied loads and provide ade- 4-12. We then write:

quate stiffness. Weights must be added for joints, cut-outs, splices and other features wto which complicate the structural arrangement.

Design specifications of all systems must be

1 w

-+ w

12 w var en9: wf (5-4) wto - wto

available to estimate their configuration, power requirements and weight. Due to the very complex character of the type of work involved, comprehensive methods coveringall conceivable aircraft types are not readily availaille. However, many examples of ra-tional methods applicable to detail weights can be found in the literature. The Society of Allied Weight Engineers (SAWE) has pub-lished many of these and a selection of pa-pers appears in the list of references to Chapter 8.

5.2.2. Examples of weight "guesstimates"

The takeoff weight is the sum of operating empty weight, payload and fuel weight:

(5-1)

The empty weight can be considered as the sum of a fixed weight and a .variable weight:

(5-2~

The actual subdivision can be adapted to the case under consideration. By way of ex-ample, if the engine to be used is known at this stage, it w.ill be considered as a fixed weight. Like the fuel load, the variable weight can be considered as a fraction of the takeoff weight, resulting in:

The following averages were found from data relating to some 100 light aircraft:

w _var

wto .45

.47

.so

-fixed gear lmal nor-retractable

cate-gear gory

utility (S-S) category .55 - acrobatic

category

constant 1000 R r uc A-.S + .035 (5-6)

where the constant is .31 when R is in n.m.

or .17 when R in km.

The factor rue accounts for undercarriage drag (see Section 5.3.2.).

b. Turbojet and turbopropeller aircraft.

In principle, cruise fuel can be determined with the well-known Br~guet range equation.

However, this is complicated by the fact that extra fuel is required for takeoff, climb, descent, reserves, etc. To avoid lengthy computations an estimate can be made as follows:

Turboprop aircraft: the total fuel quantity is obtained from Fig. 5-1.

Turbojet aircraft: the fuel is split up in-to trip fuel and reserve fuel. Trip fuel is given by Fig. S-2, explained in Section 5.4.2., and reserve fuel is given by eq.

5-46 or 5-47.

LB.NM

0 100 200 300 HP/H 400

.4,---~---,----~--~~--~--~~~~~~~

I

PROPELLER AIRPLANES

I

.3

.2

HIGH-SPEED CRUISE 0 LONG-RANGE CRUISE

.1

RESERVE FUEL

OL----L----L----L----L---~----L---~

0 100 200 CpR/vA 300 ~~i~G

Fig. 5-l. Estimation of fuel weight fraction for turboprop aircraft (small and transport category)

.5 Explanation:

.3

.2 • HIGH-SPEED CRUISE

{ 0 LONG-RANGE CRUISE 1 BUSINESS JETS

4

_I!~~ _•ov'O

MJA

1 +.068pM ~~~ 2 W10

p and M

R

ambient pressure and

I

^cruise

Mach number

condi-corrected s. f. c. tions max. width, height and length resp. of fuselage

range

speed of sound at SL, ISA mean skin friction coefficient, based on wetted area

Typical values:

CF = .0030 - large, long-range trans-ports

.0035 - small, short-range transports

.0040 - business and executive jets

Fig. 5-2. Estimation of trip fuel weight fraction for jet airliners and executive aircraft

146

10⁵ 3 4 5 6 7 8 9 103 2 9

8

llWe =We- Weng -.2Wto- Wfix 6 Wfix

=

1100 LB (500 KG) 5

10⁵

4 9

<.:> 8

""

_I 3 7

~., 6

<I 5

2

4

"'

..J

3 _I

~.,

<I

2

6

5 o JET AIRPLANES

4 6 PROP AIRPLANES 104

9 8 ---- STRETCHING 7 3

6 5 4

30 40 60 so 100 200 300 400 500M 2 Fig. 5-3. Chart to estimate the empty weight of transport and executive airplanes 1₁ .E.t+ht_

2

The empty weight of light aircraft (Wto ' 12,500 lb, 5670 kg) is roughly 60% of the takeoff weight, hence:

(5-7)

For transport aircraft with Wto > 12,500 lb (5,670 kg), the accuracy of the empty weight prediction can be improved by splitting it up into:

- the weight of the dry engines,

- a fixed weight of approximately 1,100 lb (500 kg),

- a constant fraction of the takeoff weight, mainly associated with wing and undercar-riage structure,

- a weight group dependent upon the fuse-lage size.

A justification for this approach is given in Fig. 5-3. The 20% fraction of Wto is derived from data relating to several developed aircraft types, leading to take-off weight growth, without changes in lage dimensions. The importance of the fuse-lage size is obvious not only from the di-rect contribution of the fuselage structure, but also from the fact that the weight of such items as sound insulation, wall trim, floor covering and the airconditioning sys-tem, etc. is closely related to the fuse-lage dimensions as well.

The result of this approach is:

(5-8)

where liWe is derived from Fig. 5-3 andWfix is equal to 1,100 lb (500 kg).

At this stage, the fuselage dimensions tf, bf and hf will generally be known from a layout drawing, or may be estimated from statistical data (e.g. from Figs. 3-12 and 3-13). The fuel weight fraction can be es-timated from comparable aircraft types or from Fig. 5-1 for turbopropeller aircraft

1.5

1.0

and Fig. 5-2 for turbojet aircraft. The en- .5 gine weight will be known once the engine

is chosen. Otherwise 5 to 6% of the takeoff weight may be assumed as a typical value.