Formation flying
Formation flying is the flight of multiple objects in coordination. Formation flying occurs in nature among flying and gliding animals, and is also conducted in human aviation, often in military aviation and air shows. A multitude of studies have been performed on the performance benefits of aircraft flying in formation. History Birds have been known to receive performance benefits from formation flight for over a century, through aerodynamic theory of Wieselsberger in 1914. Formation flight in human aviation originated in World War I, when fighter aircraft were assigned to escort reconnaissance aircraft. It was found that pairs of aircraft were more combat effective than single aircraft, and therefore, military aircraft would always fly in formations of at least two. By World War II, pilots had discovered other strategic advantages to formation flight such as enhanced stability and optimal visibility. Mechanism of drag reduction It is a common misunderstanding to relate the reduction of drag in organized flight to the reduction of drag in drafting. However, they are quite different mechanistically. The drag reduction occurred in the drafting is due to a reduction in flow speed in the wake of a leading vehicle, reducing the amount the flow needs to accelerate to move around the body, reducing pressure in front of the trailing vehicle. This leads to a lesser pressure differential between the frontal and rear projected surfaces of the body, and hence, less drag. This can also be understood somewhat tautologically through the common drag equation for a body , where is the experimentally obtained unitless number, is the density of the fluid medium through which the object travels, is the cross-sectional area normal to the direction mean flow, and is the speed of the mean flow. It can be seen by inspection, that a decrease in mean velocity will generate less drag force, as is the case with drafting. In juxtaposition, the drag reduction felt by trailing agents in formation flight may be thought more of as the trailing agents "surfing" on the vortices shed by wings of leading agents, reducing the amount of force needed to stay in the air. This force is known as lift and acts perpendicular to the freestream flow direction and drag. These vortices are known as wingtip vortices and are formed by fluid flowing around the wingtips from the high-pressure region that is the bottom of the wing to the low-pressure region that is the top of the wing. The flow becomes separated from the airfoil and rotates about a low pressure wake that forms the core of the vortex. This vortex acts to change the direction of the flow for trailing aircraft, increasing the lift over a segment of the wing and allowing for a reduction in induced drag by lowering its angle of attack. This can also be shown by the drag and analogous lift equation, . The difference now is that and vary linearly with angle of attack , which is the angle formed by the neutral axis of the