The first step in designing a flight vehicle is to define why it is needed, and what it must do. A thorough analysis of what would attract customers and make the vehicle succeed in its market, and a good understanding of why existing solutions or competitor’s solutions will not meet these needs, leads to a careful definition of the actual requirements. While exceeding these requirements in the design sounds good, it may be a fatal mistake in the marketplace to exceed the requirements by a long margin, because this usually comes at some high cost.
The example of a new airliner is used in the following, because most people have seen airliners and many have flown on them. The aircraft developer company conducts discussions with the airlines, which are their prime customers, to decide where the best market opportunities may be. They also conduct their own surveys of demographics and economics, to better understand the passengers who will buy the airline tickets and travel on the aircraft. Predictions of economic growth, the availability and costs of different fuels, the opening or closing of routes, and the prospects of making sales to various airlines and other customers of the design, all enter the Requirements Definition. A few examples of questions to answer are: What should be the passenger capacity of the vehicle? What airport landing field lengths are available, and what are weight limit, noise and curfew restrictions at the various airports that are essential? What flight speed is best?
As part of the research done to define requirements, the capabilities of existing vehicles, and the projected capabilities of technology available by the time the vehicle must be built, are laid out. These data give the designer a good set of upper and lower bounds, to reduce the uncertainty in making decisions during the design process. Below we will see where the benchmarks come into play.
Based on the requirements definition, a reference mission for the vehicle is carefully developed. This again must be developed to pose the right requirements.
Conceptual design is an iterative process. We start with a guess of the payload fraction, which is the payload weight divided by the takeoff weight of the aircraft. This guess and a few other “thumb rules” are based on the benchmarking process, where the designer starts with data on what has already been proven feasible, and then projects what will be possible. Based on these decisions, the fuel load required to meet the range requirement is determined. By the time this is done, all other weights have been guessed or determined, except for the structure weight. The structure fraction, which is the structure weight of the aircraft divided by the takeoff gross weight, is then compared against the minimum structure fraction that the designer believes to be essential to build the vehicle using the technology that will be available. If the available fraction exceeds the minimum, then the design is basically feasible. Otherwise, the payload fraction must be reduced, or some other way found. Beyond determining this basic feasibility, the designer determines the parameters needed to ensure stability and controllability of the vehicle, so that small disturbances do not upset the equilibrium of its flight, and yet there is enough power available to control the vehicle through the most demanding maneuvers that are anticipated.
It is after all these are done that the designer must decide the external configuration of the aircraft, and then its internal layout. Below we take each of the stages of the design process in turn.
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