## Generation of Aerodynamic Lift

When an object moves through a fluid, forces are generated by the relative motion between the fluid and the surfaces of the object. Aerodynamics is the study of these forces. At low speeds, there is much in common between the behavior of gases and liquids in this context. In fact, many scientific studies of aircraft models are carried out in water tunnels, and many studies of submarines are carried out in wind tunnels! The equivalent of "aerodynamics" for liquids is called "hydrodynamics". There are also fields known as "aerostatics" and "hydrostatics". These typically deal with the variations in fluid properties through the atmosphere and through the ocean depths, and with the forces of gravity and buoyancy acting on objects in air and water respectively.

The generation of aerodynamic lift can be viewed quite simply from Newton's laws of motion. The momentum of a given mass is the product of the mass and its velocity. When flowing air is induced to turn (i.e., change direction), there is a rate of change of momentum along the direction perpendicular to the original direction. The net force due to this change in direction causes an equal and opposite reaction on the object which turns the flow. This reaction is the force acting on the object. It can be resolved into two components—one perpendicular to the oncoming flow (the freestream) and one parallel to it. The perpendicular component is called the aerodynamic "lift". The parallel component forms the "drag". How the flow is induced to turn is the subject of much of this book.

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