Unpowered flight

Unpowered flight is the ability to stay airborne for a period of time without using any power source. There are several types of unpowered flight. Some have been exploited by nature, others by man, and some by both.

Unpowered aircraft are aerial vehicles that can fly without any propulsion mechanism.

The ability to fly short or long distances without power has evolved many times in nature. Many creatures capable of sustained wing-powered flight also soar unpowered for much of the time they are airborne.

Pennycuick divides animal flight into three types: parachuting, gliding and powered. He observes however that these have no sharp boundaries. For example at one point he sees parachutes as unpowered and as a primitive form of soaring, while soaring itself he sees as being powered by air movement (wind). Other methods, such as lighter-than-air flight, are used only by man.

This article makes the following distinctions between types or methods of unpowered flight, based on their characteristics:

These are summarised in the table:

Some examples of usage are shown in the following table:

Lighter than air flight is only used by man. An unpowered, lighter than air craft is called a balloon.

A balloon is a bag filled with a gas with a lower density than the surrounding air to provide buoyancy. The gas may be hot air, hydrogen or helium. The use of buoyant gases is unknown in the natural world.

A balloon may be tethered like a kite or drift with the wind in free flight. The pilot can control the altitude of a free-flying balloon, either by heating the gas or by releasing ballast, giving some directional control (since the wind direction changes with altitude).

A free-falling object without any adaptation to flight can only be sustained by the wind if it is very light and falls more slowly than the wind blows it upwards.

Many mould and bacterial spores, even live bacteria, are small enough to be carried long distances and to great heights on the wind.

Some plants also use the wind for seed dispersal in this way. Orchid seeds are very small and dust-like.

Parachuting is essentially falling but with an aerodynamic braking surface. The high ratio of surface area to weight reduces the rate of descent of the parachute, allowing it to stay airborne for longer periods. The aerodynamic surface may also allow a small amount of forward motion, but a parachute always falls faster than it can travel forwards. The airflow around a parachute is typically turbulent.

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Wikipedia