Aircraft Design Optimizing Operators, Environmental System and Manufacturers Requirements

The aircraft industry in the coming years will produce airliners of different categories for a constantly growing market. Despite this apparently advantageous position it seems increasingly difficult to design new products and new technologies which promise the same rate of economic improvements the customers enjoyed in the past. By realizing this, the analysis of the future market driven aircraft requirements becomes increasingly important. Product strategies considering a total system approach become a more and more important factor in the aircraft design process. Competition in the global transport aircraft market forces aircraft manufacturers progressively more to take the right decisions today for investments in cost effective technologies for the products of tomorrow to stay successful, efficient and respond exactly to the variety of future customer and transportation system requirements. The present paper describes a procedural approach to global aircraft optimization considering these requirements. A special focus is laid on a description of those elements of the decision chain, that are usually not considered as "classical" disciplines in the early stages of aircraft design: economics of operator as well as manufacturer and environmental issues. FACTORS OF SUCCESS IN THE AIRCRAFT DESIGN PROCESS Increasing demand for air travel growing on average by more than 4% per year, supported by continuously declining ticket prices, has converted air transportation from a previous luxury good into a commodity accessible to the broad public. This development forces the airlines to focus on a permanent adjustment to the market requirements by a variety of competitive supplies and service. Factors of success in the aircraft design process are to a large extent depending on the operating environment the product will face from it's entry into service far through the decades of service life (Fig. 1). Major design drivers can be arising from anticipated fuel price developments or environmental regulations, from airport slot limitations or deregulation and segmentation. Each of those different scenarios requires a particular type of aircraft to satisfy different demands. Fig. 1: Typical life cycle of a commercial transport aircraft For the manufacturer all those often conflicting market trends have to be monitored carefully, evaluated and converted into aircraft design requirements, technology development and finally design initiatives. A new product definition process starts after market analysis indicating a window of opportunity. Intensive scenario based studies for envisaged markets and regions follow to observe program viability under a number of different assumed developments of future economy, infrastructure, environmental legislation and further elements in the entire air transport system. A task not too easy, given the principal unpredictability of the future. However, careful observation and analysis of trends can be done. From this, technical requirements for the aircraft must be identified. Then, in most cases, technologies must be identified, which respond best to the task of filling the gap between status and requirements. Finally those technologies that best serve (c)2000 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. the purpose, will be selected to make their way into the aircraft. A continuous monitoring and controlling of the process makes several iteration loops during the product definition phase necessary. In this context, to be considered for future application, any design proposal or technology development should have positive effects on the aircraft in the eyes of the airlines, for the aircraft's manufacturer and with increasing importance also to provide significant benefits for the entire air transport system and the broad public. The primary, desired effects of a design change are most of the time accompanied by secondary effects which tend to diminish the primary benefits. Beside this, every design change has an impact on the cost and weight situation of the aircraft under consideration. Each of these characteristics of the design change influences the attractiveness of the change for either the airline or the manufacturer to some extent. Therefore, the evaluation of the overall attractiveness of a design for both, manufacturer and customer, is a prerequisite for an enhanced global optimization approach and forms the key for success. FROM PROJECT REQUIREMENTS TO TARGET DEFINITION Fig. 2 sketches a conventional process chain, starting with the initial design change triggering various consequences in terms of the aircraft's properties. The initial design change as well as the resulting aircraft's properties are influencing the factors of success mentioned above and thus lead to the characteristics of the decision criteria, on base of which the design change proposal will be adopted or not. Only a coarse, summarizing view is given here. Trying to further detail this sketch down to physics-based items would lead beyond the intention of this paper by far. Principles of this decompositioning approach are discussed in [1]. The symbolic cross-couplings shown in this figure may indicate the complexity of the process actually lying behind each design decision. design change classical, straightforward process alternative design targeting process evaluation of results Fig. 2: Sketch of the overall process of design changes During this process, the fulfillment of several design requirements has to be assured and, simultaneously, the optimization for the decision triggering criteria must be performed. The distinction between requirements to be fulfilled on one hand and decision criteria which have to be met on the other, is somewhat arbitrary, but this just mirrors the actual decision chain in the development phase of aircraft, where requirements are traditionally fulfilled via engineering work and decision criteria are evaluated by managing boards. In Fig. 3, a typical selection of requirements to be fulfilled and examples for decision criteria are shown. Unfortunately, the means to enhance the fulfillment of one requirement may act contradictory on another one and therefore, the "table cloth syndrome" is often used to characterize this. Introducing double slotted flaps in order to enhance the field performance characteristics, for instance, increases maintenance cost, shown on the opposite side of the "table". Another example: Increasing the environmental compatibility in most cases diminishes operational efficiency. Having balanced all requirements through thoroughly performed design loops (a typical engineering task), the overall goodness of the design change under consideration is evaluated by reflecting the impact the design change has on the value of, for instance, the internal rate of return (IRR), as one example of a decision criteria. In case of positive impact on IRR and other decision criteria, a positive decision will be the result. 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