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2.5 The operational requirements There are requirements related to main and emergency exits and entrances, devices for emergency escape, convenience for the crew, control automation, cockpit visibility, passengers comfort, mechanization of loading and unloading, maintainability and ease of repair, ease of detachment and replacement of parts and equipment, possibility for autonomous maintenance, airplane life-span (in flight hours), required airfield class.
32 2.6 The technical and economic requirements These are efficiency parameters of airplane manufacturing and operation: the airplane anticipated cost, traffic handling cost, fuel efficiency ratio, the cost of a flight hour and etc. 2.7 The other requirements These are airplane class according to durability standards, the anticipated airplane market, environmental requirements.
33 3 CHOOSING THE AIRPLANE SCHEME The airplane scheme defines a number, shape and relative position of its main units - the wing, the empennage, the fuselage, the take-off and landing equipment, and also the number and placement of engines and their air intakes. The airplane scheme has an influence on airplane properties and characteristics, and, finally, it defines the total efficiency of an airplane. The scheme of each airplane is stipulated by its type, operation conditions and the main requirements for airplane design. The key problem that should be solved while choosing an airplane scheme is to satisfy PR in the best way and provide the minimum airframe and takeoff weight, the highest lift-to-drag ratio and the maximum airplane efficiency. The aerodynamic scheme depends on airplane layout. Thus questions of placing the crew, payload, fuel and engines should be considered at the time of airplane scheme develop. According to a number of units there are following airplane schemes available: monoplane or biplane, with one or two fuselages, with one or more surfaces of horizontal and vertical empennage, with two-, three- or multi-support landing gear, with different number of engines. The airplane scheme main features are defined by a number and relative position of lifting surfaces, that are divided into main surfaces that create the most part of lift (wings), and additional surfaces (empennage) that provide the balancing, the control and the stability of an airplane. There are the following airplane balancing schemes according to relative positional of the empennage and the wing: "normal" - the horizontal empennage (tail) is behind a wing; "tailless" - there is no horizontal empennage; "canard" - the horizontal empennage is in front of the wing;
34 "combined" - the "normal" scheme plus the additional horizontal empennage that is in front of the wing; this scheme has some layout advantages due to the wing displaced backwards; additional front empennage also improves stability and control of maneuverable airplanes.