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FM 1-514 DISTRIBUTION RESTRICTION: Approved for public release; distribution is unlimited. HEADQUARTERS, DEPARTMENT OF THE ARMY FUNDAMENTALS OF ROTOR AND POWER TRAIN MAINTENANCE— TECHNIQUES AND PROCEDURES
FM 1-514 CHAPTER 1 PRINCIPLES OF HELICOPTER FLIGHT • • Basic flight theory and aerodynamics are considered in full detail when an aircraft is designed. The rotor repairer must understand these principles in order to maintain aircraft safely and to make repairs that are structurally sound and aerodynamically smooth. AERODYNAMICS Aerodynamics deals with the motion of air and with the forces acting on objects moving through air or remaining stationary in a current of air. The same principles of aerodynamics apply to both rotary-wing and fixed-wing aircraft. Four forces that affect an aircraft at all times are weight, lift, thrust, and drag: Weight is the force exerted on an aircraft by gravity. The pull of gravity acts through the aircraft’s center of gravity, which is the point at which an aircraft would balance if suspended. The magnitude of this force changes only with a change in aircraft weight. Lift is produced by air passing over the wing of an airplane or over the rotor blades of a helicopter. Lift is the force that overcomes the weight of an aircraft so that it can rise in the air. Thrust is the force that moves an aircraft through the air. In a conventional fixed-wing aircraft, thrust provided by the propeller moves the plane forward while the wings supply the lift. In a helicopter both thrust and lift are supplied by the main rotor blades. Drag is the force of resistance by the air to the passage of an aircraft through it. Thrust force sets an aircraft in motion and keeps it in motion against drag force. Any device designed to produce lift or thrust when passed through air is an airfoil. Airplane wings, propeller blades, and helicopter main and tail rotor blades are all airfoils (Figure 1-1). Chord is the distance, or imaginary line, between the leading and the trailing edge of an airfoil. The amount of curve, or departure of the airfoil surface from the chord line, is known as the camber. Upper camber refers to the upper surface; lower camber refers to the lower surface. If the surface is flat, the camber is zero. The camber is positive if the surface is convex (curves outward from the chord line). The camber is negative if the surface is concave (curves inward toward the chord line). The upper surface of an airfoil always has positive camber, but the lower surface may have positive, negative, or zero camber (Figure 1-2). • • 1-1 FM 1-514 BERNOULLI’S PRINCIPLE • Bernoulli, an eighteenth century physicist, discovered that air moving over a surface decreases air pressure on the surface (Figure 1-3). As air speed increases, surface air pressure decreases accordingly. This is directly related to the flight of an aircraft. As an airfoil starts moving through the air, it divides the mass of air molecules at its leading edge. The distance across the curved top surface is greater than that across the relatively flat bottom surface. Air molecules that pass over the top must therefore move faster than those passing under the bottom in order to meet at the same time along the trailing edge. The faster airflow across the top surface creates a lowpressure area above the airfoil. Air pressure below the airfoil is greater than the pressure above it and tends to push the airfoil up into the area of lower pressure. As long as air passes over the airfoil, this condition will exist. It is the difference in pressure that causes lift. When air movement is fast enough over a wing or rotor blade, the lift produced matches the weight of the airfoil and its attached parts. This lift is able to support the entire aircraft. As airspeed across the wing or rotor increases further, the lift exceeds the weight of the aircraft and the aircraft rises. Not all of the air met by an airfoil is used in lift. Some of it creates resistance, or drag, that hinders forward motion. Lift and drag increase and decrease together. They are therefore affected by the airfoil’s angle of attack into the air, the speed of airflow, the air density, and the shape of the airfoil or wing. • • • LIFT AND THRUST The amount of lift that an airfoil can develop depends on five major factors: Area (size or surface area of the airfoil). www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：直升机基本原理及维护
