I OPERATING COST I
1.4. THE INITIAL SPECIFICATION
There is certainly no need to prove that sufficient material on a subject such as
"market analysis aimed at the development of new aircraft" exists to warrant the publication of a separate volume. Thepres-ent paragraph will of necessity have to be restricted to a few general observations with civil aviation as their main back-ground. The example used will be an ini-tial design specification for a hypothet-ical short-haul airliner for 180 passengers in the all-tourist layout, referred to as
"Project M-184". A design evaluation of this project can be found in Ref. 1-64; it was intended as a highly simplified exam-ple for the purpose of illustrating the design process in a series of lectures. An apology is due for the fact that most of the considerations which follow in the present section apply to this particular design, intended for introduction into service around 1980.
In civil aviation the specification of a new aircraft type is generally drawn up by the manufacturer. Airlines are usually more content to evaluate projects offered to them for use on their own route network, though in a few cases they themselves have taken the initiative and written the spec-ification which they felt was required.
The designer will, however, realize that
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a project can only be justified when it is likely to find a worldwide market. A spec-ification issued by an airline will only be interesting provided it also appeals to competitors. It should also be rea.lized that operators do not necessarily possess the best insight into the technical capa-bilities and knowhow of the airplane manu-facturer. Nevertheless, there are examples of successful aircraft which have been de-signed to an operator's specification or a specification written with the customer's active cooperation (Viscount, Tristar and DC-10). All the same, the responsibility for the specification and the resulting project will still rest squarely on the shoulders of the aircraft manufacturer.
This procedure is quite different from the case of military aircraft, where the spec-ification will nearly always be issued by the customer: the armed forces.
The term "initial specification11 , as opposed to the more detailed type specification of a design, is used here to emphasize that there is an inter-action between the technical design work and the development of the design requirements as a result of market analysis, engine development and various assessments during the development phase.
1.4.1. The need for a new type of aircraft
The following are some good reasons for initiating a new aircraft design:
a. Existing aircraft are becoming either technically or economically obsolescent, and a new type may do the job better. New standards for equipment, maintenance, op-erational use, noise suppression, passen-ger comfort, etc. may make renovation of the operator's fleet desirable.
b. Certain developments in trafficpatterns have created a need for new types of air transport. For example, the growth of traffic may, as explained in Section 1.2, result in a new class of (larger) trans-port aircraft, or new travel habits (home to work and back) may open up the possi-bility of a new class of commuter aircraf~
Air transport may fulfil the needs of de-veloping countries, where the
infrastruq-• NUMBER OF PASSENGERS IN AN ALL-TOURIST LAYOUT (SEAT PITCH 34 IN., .87 M): 180 OR MORE.
CORRESPONDING DESIGN PAYLOAD: 20,000 KG (44,100 LB). AN UNDERFLOOR FREIGHTHOLD VOLUME OF AT LEAST 50M3 (1,762 CU.FT) WILL BE REQUIRED. STANDARD SIZE BELLY CONTAINERS ARE PREFERRED.
• RANGE, WITH ABOVE MENTIONED PAYLOAD: 2,200 KM (I, 200 NM) IN A HIGH-SPEED CRUISE, ATA DOMESTIC RESERVES. MAXIMUM RANGE (REDUCED PAYLOAD): 3,200 KM (1,726 NM) AT LONG-RANGE CRUISE TECHNIQUE.
•MAX. CRUISING SPEED AT 9,150 M (30,000 FT)
AL-TITUDE: M • .82. DESIGN LIMITS:
'\to •
.85,VMO • 704 KMH (380 KTS) EAS.
• FIELD LENGTH REQUIRED FOR TAKEOFF AND LANDING, ACCORDING TO AIRWORTHINESS RULES: 1,800 M (5,900 FT) AT SEA LEVEL, ISA + 20 °C (95 °F), AT MAXIMUM (CERTIFICATED) TAKEOFF WEIGHT.
RUNWAY LOADING: LCN • 30, RIGID PAVEMENT, 18 CM (7 IN.) THICKNESS.
• REGULATIONS: FAR PARTS 25, 36 AND 121 . THE NOISE CHARACTERISTICS MUST SHOW AN IMPROVEMENT RELA-TIVE TO THE 1969 VERSION OF FAR PART 36 OF I 0 EPNdB.
Fig. 1-5. Initial specification of a hypothetical short-haul airliner for introduction into service around 1980
ture is inadequate for surface transport.
c. A new type of aircraft is built and tested in order to give added impetus to an important new technical development, such as a V/STOL demonstrator prototype.
Since experimental aircraft nearly always lead to a financial loss, at least in the first stages, there will have to be govern-ment funding, e.g. in the form of a devel-opment contract.
Manufacturers should be wary of aiming at filling the "gap in the market". That gap may well have remained unfilled for the simple reason that the need for an air-plane of the kind was insufficient. Another danger which should be warned against is the adoption of a particular technicalnov-elty which in itself may be a very clever achievement but is unlikely to contribute to profitable operation of the aircraft.
Nevertheless, the design office will be continually involved in studies aimed at determining the potentialities of new technical developments and innovations.
Any new type designed will have to be marketed in accordance with a properly thought-out time schedule. It is important to remember that if it is offered too early the production rate will increase too
slow-ly, resulting in a productivity loss on investments which the company has put into the project. A launching delayed too long may be equally disadvantageous, either be-cause the market has meanwhile been satura-ted by competitors' products or because the production line has to expand too fast and excessive manpower has to be (temporarily) hired and additional investments made.
The initial specification shown in Fig. 1-5 was drawn up for an airliner intended to augment and replace the current class of high-subsonic short-haul passenger trans-ports: the BAC 1-11, McDonnell Douglas DC-9 and Boeing 737, and to some extent also aircraft designed for medium ranges: the Hawker Siddeley Trident, Aerospatiale Caravelle and Boeing 727. The category con-sidered does not include smaller aircraft such as the Fokker F-28 or the VFW-614. The aircraft mentioned above are powered by low-bypass turbofans and have a capacity of 80-120 passengers (short-haul) or 120-180 (medium-haul). The need for a new type stems from the following considerations:
a. The increased traffic volume requires larger-capacity aircraft.
b. The new standard of passenger comfort
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introduced by the wide-body jets will un~
doubtedly be extended to short-haul traf-fic.
c. Reduction of noise production will be a prerequisite in the eighties.
d. Technology improvements in the fields of high-speed and low-speed wing aerodynamics, new structural materials (composite struc-tures), lightweight avionics and improved flight control systems ,may be considered for application in this new aircraft cat-egory.
In view of the large volume of short-haul traffic the market seems to offer scope for a new aircraft with smaller capacity as compared to the Airbus A-300, for ex-ample.
1.4.2. Transport capacity
When a new specification is being drawn up, the first step will have to be a forecast of the traffic and.the transport demand over the route sector concerned during the period under review. A technique commonly used here is a statistical analysis of the yearly growth percentage of the total dis-tance covered by passengers in terms of passenger-miles (passenger-kilometers).· On the basis of.an extrapolation of this growth percentage, the total transport de-mand for the period considered may be es-timated. Assumptions will next have to be made regarding the frequency of the flights, the average load factor and yearly utili-zation and from these the desired produc-tivity (number of passengers times block speed) can be deduced. A rule of the thumb sometimes used states that the most favor-able time between successive flights over a particular route is about equal to the time taken to fly the route. Hence, if the block speed increases, the frequency of the service should also be stepped up. The fol-lowing are some other aspects to be con-sidered:
a. For a large capacity aircraft the oper-ating costs per aircraft-mile will be high, but those for a seat-mile will be low, since certain costs do not rise
proportion-ally to the size of the aircraft, e.g. to-tal· salaries of the flight crew and the cost of avionics and certain services, and will therefore decline with each addition-al seat.
b. A comparatively small aircraft will show a low cost· per aircraft-mile and its critical load* will be smaller than that of a large aircraft. This does not neces-sarily apply to the critical load factor
(critical load/maximum load).
Generally speaking, large aircraft are best suited to routes with high traffic density, provided the frequency of opera-tions is compatible with the market re-quirements.
In drawing up the specification for the M-184 project (Fig. 1-5), the following con-clusions were arrived at:
a. During the 1960-1970 period short-haul traffic grew considerably, both in the United States and in Europe. A yearly growth of 15 percent, resulting in a doubling in five years, was no exception and the growth was even more marked during the 1965-1970 period. Charter traffic in fact underwent an explosive expansion dur-ing that same period, with growth percent-ages as high as 25 to 30. Factors which contributed to this growth were: regular tariff decreases, a rising level of pros-perity, and the greatly improved comfort of jet aircraft compared with other means of transport.
b. A gradual decrease in the yearly growth can be.expected for the period 1975-1985 as a result of a slackening-off or decline in the economy, a certain measure of satu-ration of the transport market, and una-voidable increases in tariffs. The latter are a result of the rapidly increasing costs of fuel and the measures which have to be taken to meet the certification re-quirements regarding noise levels. Assuming an annual growth of 10 percent for the years 1973-1980 the total yearly production
*The number of passengers required to pay the cost of the flight.
on short routes will have to rise to 195 percent of the 1973 value, while during the first three years after the airplane's introduction the traffic demand will rise to about 250 percent.
c. On very busy routes the Airbus A-300 and possibly also the Trijets McDonnell Douglas DC-10 and Lockheed 1011 will take over a large share of the short/medium-haul traffic. On routes where the growth will be less progressive, however, the jump in capacity from current short-haul aircraft to the A-300 will probably be too great and there will be an opening for aircraft with a capacity some 80 to 100 percent greater than that of the DC-9, provided it offers good possibilities for further growth.
d. For the M-184 a capacity of at least 180 passengers has been chosen for an all-tourist layout with a possible later
"stretch'' to about 250 passengers, while the cargo holds require a total volume of at least 1800 cu. ft (50 m3 ). Compared to that of current airliners the passenger accommodation must show an improvement in the level of comfort, but this need not necessarily be achieved by the use of two aisles. A very close watch will have to be kept on the .economical consequences of an increased level of comfort.
1.4.3. Design cruising speed and range
The speed factor has constituted an out-standing contribution to the development of aviation; the aircraft has proved to be the only means of transport in which in-creased speed does not necessarily lead to an increase in fuel consumption. Although a fast means of transport will be attrac-tive to the passenger, the air transport companies in particular rate the speed element highly because, broadly speaking, it means that more trips can be made per day and production is increased. It is not only the cruising speed, however, that is important; equally vital is the time de-voted to taxying, takeoff, climb, descent, approach and landing, which means that the
block speed is· a better yardstick than the cruising speed. Any new type of short-haul aircraft will have to possess a consider-ably higher cruising speed than the one it is intended to replace, in order to save the time needed for an extra flight.
In the case of smaller general aviation aircraft the value of speed mainly depends on how the aircraft is used. A top execu-tive whose working hours are assumed to be extremely valuable will be prepared to pay considerably more for speed than the owner of a small utility aircraft which is used for tourism or in regions with an under-developed infrastructure where reasonable surface transport is lacking.
In drawing up the specification for theM-184 project (Fig. 1-5), it has been as-sumed that the design cruising speed must not be less than that of existing aircraft.
In the high-subsonic speed bracket, how-ever, any increase in speed will consid-erably influence the external shape (angle of sweep, airfoil shape and thickness), generally resulting in an empty weight crement, extra development costs and in-creased fuel consumption. The extent to which the economic advantages of the high-er block speeds will outweigh these losses cannot be predicted offhand; this would have to be ascertained by a tradeoff study, which could also take into account the possibilities of recent developments in high-speed wing aerodynamics.
In the case considered here a design Mach number of .82 in high-speed cruise has been chosen on the basis of conventional section shapes, while the possible gain resulting from the use of an advanced wing shape may be either the use of a thicker airfoil - and hence a lighter structure -, a larger wing span, or a higher economical cruising speed.
As regards the choice of the design range of the M-184 it was concluded from a sur-vey of route distributions that a peak occurs for traffic on ranges of about 280 nm (500 km), e.g. Los Angeles - San
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1120~--~~~3~0--~1~~~~,~960~---I~M0~--~~.*~=---~1910:'·0 YEAR
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Fig. 1-6. Trends in the takeoff performance of civil aircraft (only trend-setting types have been plotted)
cisco. Another peak, although less pro-nounced, is observed around 500 nm (900 km). An aircraft designed to fly ranges be-tween 110 and 1,200 nm (200 and 2,200 km) will cover 87 percent of the traffic mar-ket. Although a decrease to 600 nm (1,100 km) in the range for maximum payload may lead to a slight improvement in the direct operating costs at short ranges, 25 percent of the short-haul routes are longer and a considerable number of operators would not choose the aircraft. A design range of 1,200 nm (2,200 km) at high-speed cruise was decided for the M-184. In view of the specified field performance there may be an opening for a version with increased all-up weight and fuel capacity to suit operators who require a longer range ver-sion and put less emphasis on low-speed performance.
1.4.4. Low-speed characteristics and field performance
Two starting-points may be used for speci-fying the runway length for takeoff and landing:
a. The aircraft is optimized for cruising flight. The shape and dimensions of the wing, as well as the cruising altitude, are so chosen that the fuel consumption is a minimum for the design range flown at the design cruising speed. The thrust of the engines will be based either on the re-quired climb performance or on the design cruising spee0 requirement. The takeoff and landing performance will now become more or less derived values and can be influenced only to a limited extent by the design of the flaps and the wheel brakes. The contin-uous growth in aircraft weight and
conse-quent increase in wing loading (Fig. 1-6) have resulted in increased takeoff dis-tances which have demanded a steady length-ening of the runways, in some cases and certainly at the principal international airports to as much as about 13,000 feet
(4,000 m). The approach speeds for the landing have risen to 160 to 170 kts (300 to 315 km/h), although the landing distance is not critical for most long-range trans-ports.
Any further continuation of this trend would only be justified if adaptation of the aircraft to existing runways led to a considerable increase in operating costs and, moreover, the lengthening of the run-ways was environmentally acceptable. If we also take into account the 1969 require-ments regarding noise production (FAR 36) and a possible tightening up of these in the future, it would not appear very likely that future generations of transport air-craft will require any appreciable length-ening of runways which are now being used by aircraft like the DC-8, Boeing 707 and Boeing 747.
b. The runway performance of the new de-sign will be adapted to the airports from which the future customer is now operating the aircraft that the: new product will have to replace. For a new short-haul aircraft this means that the runway length should not exceed that used for the category to which the DC-9, BAC 1-11 and Boeing 737 be-long and that the design of the landing gear should be adapted to the strength of these runways. Any increase in operating costs resulting from these requirements should be carefully watched and realistic data should be available when it comes to discussing the tradeoff between shorter runways and cost increase. The design study will therefore have to include an investi-gation into the effect of field require-ments on the design characteristics, direct operating costs and noise characteristics.
that the majority of potential customers will be able to operate the aircraft from the runways now being used, provided that the runway Load Classi-fication Number at Maximum Takeoff Weight does not exceed 30 on a rigid pavement 7 inches ( 18 em}
thick*.
1.4.5. Other requirements
a. The engine constitutes an important fac-tor in the reduction of the operatingcosts and its choice should be carefully matched to the aircraft. In the case of transport aircraft the design range is particularly important, while the noise level has to satisfy exacting requirements if restric-tions are not to be applied to the use of the aircraft. Fuel consumption has to be carefully watched.
b. Much attention should be paid to an timum cabin arrangement to enable the op-erator to use different layouts. In general the distance between fixed partitions at the front and rear of the cabin should be as great as possible.
c. Equipment and instruments. The specifi-cation will state the amount of NAV/COM e-quipment to be carried and its degree of duplication. This will result from discus-sions with customers and will be based on the mode of operation of the aircraft (VFR and/or IFR flights category of landings) , and a distinction will generally be made betwean standard and optional equipment.
d. Construction, inspection and maintenance.
Apart from the airworthiness requirements (Section 1.6) the specification will gen-erally also feature special requirements such as a fail-safe or safe-life design philosophy and a service life of the struc-ture, expressed in terms of the maximum number of flight hours or flight cycles or both. The manufacturing and production processes, etc., may also be subject to With a specified runway length for the M-184 project special requirements which can have
far-(Fig. 1-5) of not more than 6,000 ft (1,800 m) at
sea level (standard atmosphere) , it is anticipated * cf. Chapter 10.2 .1.
reaching effects when certain structural parts or even main structures are adopted from types already in use. A case in point is the Boeing 707, 727 and 737 family of aircraft all of which have almost identical fuselage cross-sections.
e. Airframe services and noise level. The principal design requirements to be met by the air-conditioning and pressurization system are related to the air supply, tern-perature and degree of humidity, cabin pressure differential, etc. Noise levels, both internal and external, are also de-cided upon. Requirements may also be written into the specification with respect to the electrical, hydraulic and pneumatic systems, anti-icing equipment and possibly also the Auxiliary Power Unit (APU).
1. 5. THE "CONTINUOUS THREAD" RUNNING THROUGH THE DESIGN PROCESS
1.5.1. The iterative character of design
The creation of an airplane configuration cannot be laid down in a universal, de-tailed procedure. However, some general characteristics of the design process may be amplified with the help of Fig. 1-7,
I I
l
II r ___ J _____ ..,
l--~ CHANGED 1
I REQUIREMENT I L. _________ .J
Fig. 1-7. General design procedure
which shows the principal phases schemati-cally. This diagram deals with technical and computational elements and could apply equally well to the design of other tech-nical products, unlike Fig. 1-3 which re-fers specifically to an aircraft develop-ment.
An essential element of the design process is that it is always made up of iterations.
After a trial configuration has been sub-jected to a first analysis of its charac-teristics (weights, mass distribut.ion, per-formance, flying qualities, economy, etc.), it will be seen either that it does not meet all the requirements, or that it does comply with them but improvements in some respects are possible. Only after a number of configuration changes have been incor-porated will the designer be able to de-termine whether the final configuration satisfies the requirements in every respect and may also· be regarded as the best con-ceivable design, bearing in mind the inev-itable uncertainties which are peculiar to the preliminary design phase. The conver-gence test has been incorporated in the diagram to indicate that a situation may arise in which, ~spite all the improve-ments made in the d~sign, no configuration can be found which entirely meets all re-quirements simultaneously.
The reason may be that certain requirements in the specification and other constraints have proved to be contradictory or too ex-treme, taking into account the state of the art, or that the basic conception has not been chosen properly. For example, the de-signer may be confronted with a situation which, to ensure that the engines selected will supply the power required to keep the aircraft in the air after engine failure, would necessitate leaving a large part of the payload back at the airport. The con-vergence test in Fig. 1-7 is therefore a general indication showing whether the at-tempts to improve the design have brought it closer to the requirements or not.