Aircraft design refers to the process of designing an aircraft. It is generally divided into three distinct phases: the conceptual design, the preliminary design, and the detail design. Each phase brings its own unique characteristics and influence unto the final product. All these phases involve aerodynamic, propulsion, and structural design, as well as the design of the aircraft control and supplemental systems.
The conceptual design phase is characterized by comparative evaluation of numerous alternative design concepts potentially reaching the design requirements. The conceptual design phase is iterative in nature. A large quantity of design concepts are thus evaluated, compared to the requirements, revised, reevaluated, and so on until the convergence to one or more satisfactory concepts is achieved. During this process, the greatest flaws for reaching the requirements are exposed, so that the products of conceptual design frequently include a set of revised requirements.
During the preliminary design phase, one or more promising concepts from the conceptual design phase are subjected to more rigorous analysis and evaluation in order to define and validate the design that best meets the requirements. With extensive experimental efforts, including wind-tunnel testing and the evaluation of any unique materials or structural concepts, are conducted during preliminary design. The end product of preliminary design is a complete aircraft design description including all aircraft systems and subsystems.
During the detail design phase, the selected aircraft design is translated into the detailed engineering data required to support tooling and manufacturing activities.
Aerodynamic design is looked into in each phase, thus allowing the elimination of errors.
The initial aerodynamic design centers on defining the external geometry and general aerodynamic configuration of the new aircraft. To begin the building a aircraft concept model to start with, the main aerodynamic forces that will take effect on the model need to be looked into. These main aerodynamic forces that determine the aircraft performance and capabilities are drag and lift. The basic, low-speed drag level of the aircraft is usually expressed as a term at zero lift composed of friction and pressure drag forces plus a term associated with the generation of lift, the drag due to lift or the induced drag. Since wings generally operate at a positive angle to the relative wind (called angle of attack) in order to generate the necessary life forces, the wing lift vector is tilted aft, resulting in a component of the lift vector in the drag direction.
Propulsion design is looked into in each phase, thus allowing the elimination of errors. Generally, propulsion design comprises the selection of an engine from among the available models and the design of the engine's installation on or in the aircraft.
Propulsion design at the conceptual design phase primarily involves the selection of the best propulsion concept involves choosing from among a wide variety of types ranging from reciprocating engine-propeller power plants through turboprops, turbojets, turbofans, and ducted and unducted fan engine developments. The selection process involves propulsionperformance analyses comparing the flight performance of various candidate engines when installed into the aircraft. Once the engine has been selected, the additional propulsion engineering tasks are the design of the air inlet for the engine, as well as the exhaust nozzle and/or the engine nacelles with the rest of the airframe. All elements are tested to assure the their satisfactory physical and aerodynamic integration. Major parameters to be chosen include the throat area, the diffuser length and shape, and the relative bluntness of the inlet lips.
Structural design is looked into in each phase, thus allowing the elimination of errors. Generally, the structural design is started when the first complete, integrated aerodynamic and propulsion concept is formulated.
The structural design at the conceptual design phase starts off with preliminary estimates of design airloads and inertial loads (loads due to the mass of the aircraft being accelerated during maneuvers).Then, the structural design effort centers on a first-order structural arrangement which defines major structural components and establishes the most direct load paths through the structure that are possible within the constraints of the aerodynamic configuration. An initial determination of the structural and material concepts to be used is made at this time. This includes, for example, the deciding on whether the wing should be constructed from built-up sheet metal details, by using machined skins with integral stiffeners, or by fiber-reinforced composite materials.
The structural design at the preliminary design phase starts off by taking into consideration the dynamic loads, airframe life, and structural integrity. The dynamic loading conditions arise from many sources: landing impact, flight through turbulence, taxiing over rough runways, and so forth. The airframe life requirements are usually stated in terms of desired total flight hours or total flight cycles. To the structural designer these requirements are used as guidelines to determine the requirements for airframe fatigue life. Airframe fatigue life measures the ability of an airframe to withstand repeated loadings without failure. Aircraft design for high airframe fatigue life involves the selection of materials and the design of structural components that minimize concentrated stresses. The structural integrity is then looked into and imposes the requirements for damage tolerance, the ability of the structure to continue to support design loads after specified component failures, ... The structural integrity is approached from the same angle as the design for aiframe fatigue resistance: the avoidance of stress concentrations and the spreading of loads out over multiple supporting structural members.
Aircraft systems design is looked into in each phase, thus allowing the elimination of errors. Generally, the aircraft systems include all of those systems and subsystems required for the aircraft to operate. The major aircraft systems are power systems, flight-control systems, navigation systems, communication systems, crew systems, the landing-gear system, and the fuel systems. So-called mission systems are those additional systems and subsystems peculiar to the role of military combat aircraft.
The design of aircraft systems must begin relatively early because they represent large dimensional and volume requirements which can influence overall aircraft size and shape or because they interact directly with the aerodynamic concept (as in the case of flight-control systems) or propulsion selection (as in the case of power systems).
During the preliminary design phase, the aircraft system definition is completed to include additional subsystems. Installation of the many aircraft system components and the routing of tubing and wiring through the aircraft are complex tasks which are often aided by the construction of partial or complete aircraft mock-ups. These mock-ups are full scale models of the aircraft, made of inexpensive materials, which aid in the locating of structural and system components.
The design requirements are the specifics to which the design, as ordered by the customer must adhere to. They requirements are often stated at the beginning of the aircraft design and are used as a guideline in each phase of the project. The requirements begin with the national and/or international requirements eg regarding safety, ... These requirements are expanded with the specific wishes of the customer, eg technical requirements as the capability of reaching a certain speed, range, payload, and so forth and/or economic requirements eg regarding costs, maintenance characteristics, and so forth.