WHAT IS AERODYNAMICS? Aerodynamics is the branch of dynamics that treats the motion of air (and other gaseous fluids) and the resulting forces acting on solids moving relative to such fluid. Aerodynamic results will fall into different categories of behaviour depending on velocity range (slow speed, high speed, supersonic, hypersonic), depending on size and shape of the object (large, small, complex 3D solid) and the physical properties of the fluid (dense, rarefied, viscous, inviscid). Many different aerodynamic situations can be analysed using a range of available theories. The important steps of flow field definition, calculation of velocity field around the object, calculation of flow pressure and shear distribution, integration of these distributions on the surface of the body are the tools used for most theoretical aerodynamic prediction. The aim is to be able to predict the lift, drag, thrust and moments acting on objects or vehicles in motion. WHAT IS LIFT? Lift is the aerodynamic force acting at right angles to the direction of motion of the object. It is produced by the interaction of the moving object and the fluid. This interaction typically leads to a pressure differential being set up between upper surface and lower surface of the object. The nett effect of high pressure below and low pressure above will produce a force which sustains the object against descent due to gravity. The physical mechanisms in the fluid/body interaction that create lift are very complex. The laws of conservation of mass and momentum (including the effect of fluid rotation) result in fluid flow paths, velocity and pressure distributions which can significantly change the magnitude of lift due to small changes in flow angle or surface curvature. It is hoped that by studying the following chapters on the theoretical basis of fluid flow, students will begin to understand these mechanisms. WHAT IS DRAG? Drag is the aerodynamic force resisting the motion of the object through the fluid. It is produced by front/rear pressure differences, shearing between fluid and solid surface, compression of the gas at high speed and residual lift components induced by 3D flow rotation. WHAT IS THRUST? Thrust is the aerodynamic force produced in the direction of motion and is required to overcome drag and thus sustain the forward flight of the vehicle. It is produced by mechanical means (an engine) which effectivily transfers energy into the flow, in the form of increased fluid momentum. Thrust is the forward reaction to this fluid momentum change. WHAT IS MOMENT? Moment is the aerodynamic torque produced by out of balance forces. An object or vehicle has no solid structure to support it in the air. A balance is required and all forces must act through the same point (typically the center of gravity of the object). Any variation of the point of application for aerodynamic forces will produce a couple, leading to a moment which will cause the vehicle to start to rotate. The study of these moments and the effect they have on the stability of motion of the vehicle is covered in more detail in the Flight Mechanics courses. The primary system of units to be used in this text is the SI System. Contents Page
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