Tethered balloon

A tethered, moored or captive balloon is a balloon that is restrained by one or more tethers attached to the ground and so it cannot float freely. The base of the tether is wound around the drum of a winch, which may be fixed or mounted on a vehicle, and is used to raise and lower the balloon. A balloon is a form of aerostat, along with the powered free-flying airship, although the American GAO has used the term "aerostat" to describe a tethered balloon in contrast to the powered airship. Tethered balloons have been used for advertising, recreation, observation, and civil or military uses. Early balloons were simple round spheres, with a payload hung beneath. The round shape uses the minimum material to accommodate a given volume of lifting gas, making it the lightest construction. However, in any significant wind the round shape is aerodynamically unstable and will bob about, risking damage or the balloon breaking free. To avoid this problem, the kite balloon was developed. This form has an elongated shape to reduce wind resistance and some form of tail surface to stabilize it so that it always points into the wind. Like the powered airship, such balloons are often called blimps. A hybrid tethered balloon or kytoon is shaped to provide aerodynamic lift similar to a kite, as well as to reduce drag. Designed by Albert Caquot, a French engineer, in 1914, the barrage balloons of World War I and World War II were early examples of tethered balloons. Military observation balloons were also used extensively in World War I. These early types used hydrogen as their lifting gas. Tethered balloons are used for lifting cameras, radio antennas, electro-optical sensors, radio-relay equipment and advertising banners – often for long durations. Tethered balloons are also used for position marking and bird control work. Typically, they use non-flammable helium gas to provide lift. Tethered balloons are often used for advertising, either by lifting advertisement signs, or by using a balloon with advertisements written on, or attached to it.