(4) shear, and (5) bending. A single member of the structure is often subjected to a combination of stresses.
Fuselage usually consists of frame assemblies, bulkheads, and formers. The skin is reinforced by longitudinal members called longeron. Often, wings/tails are of full cantilever design. In general, wing construction is based on one of three fundamental designs: (1) monospar, (2) multispar, and (3) box beam. Spars are the principal structural members of the wing. They correspond to the longeron of the fuselage. Spars run parallel to the lateral axis of the aircraft, from the fuselage toward the tip of the wing, and are usually attached to the fuselage by a beam, or a truss. Generally, a wing has two spars. One spar is usually located at the maximum thickness, and the other about two-thirds of the distance toward the wing’s trailing edge (in front of flap/control-surface).
Honeycomb structured wing panels are often used in composite wings. Nacelles (i.e., pods) are streamlined enclosures used primarily to house the engine and its components. Engine mounts are also found in the nacelle. These are the assemblies to which the engine is fastened. They are usually constructed from chrome/molybdenum steel tubing in light UAV and forged chrome/nickel/molybdenum assemblies in larger UAVs. Cowling are the detachable panels covering those areas into which access must be gained regularly, such as the engine and its accessories. In the design of airframe, several factors such as ultimate load, aerodynamic loads (pressure distribution), weight loads (e.g., fuel and engine), weight distribution, gust load, load factor, propulsion loads, landing loads (e.g., brake), and aero-elasticity effects must be considered.
One of the design requirements for some military UAVs is stealth. In the concept of stealth, the three basic methods of minimizing the reflection of pulses back to a receptor are: (1) to manufacture appropriate areas of the UAV from radar-translucent material such as Kevlar or glass composite as used in radomes which house radar scanners; (2) to cover the external surfaces of the aircraft with RAM (radar absorptive material); and (3) to shape the aircraft externally to reflect radar pulses in a direction away from the transmitter. The acoustic (i.e., noise) wavelength (signature) range for detecting an air vehicle is 16 m–2 cm.
The operating flight loads limits on a UAV are usually presented in the form of a V-n diagram. Structural designers will construct this diagram with the cooperation of the flight dynamics group. The diagram will determine the structural failure areas, and area of structural damage/failure. The UAV should not be flown out of the flight envelope, since it is not safe for the structures. The UAV structural design is out of scope of this book, you may refer to references such as Megson [47] for more details.
2.4 PROPULSION SYSTEM DESIGN
A heavier-than-air craft (UAV) requires a propulsion system in order to have a sustained flight. Without a proper aero-engine or powerplant, a heavier-than-air vehicle can only glide for a short time. The contribution of a powerplant to an aircraft is to generate the most influential force in the aircraft performance; that is, the propulsive force or thrust. The secondary function of the propulsion system is to provide power/energy to other subsystems such as hydraulic system, electric system, pressure system, air conditioning system, and avionics. These subsystems rely on the engine power to operate.
Soon after the design requirements and constraints are identified and prioritized, the propulsion system designer will begin to select the type of engine. There are a number of engine types available in the market for flight operations. They include: electric (battery), solar-powered, piston-prop, turbojet, turbofan, turboprop, turboshaft, ramjet, and rocket engines.
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