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.
Flight without power.
Classification of flight methods.
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:
Flight methods and usage.
Some examples of usage are shown in the following table:
Lighter than air.
Lighter than air flight is only used by man. An unpowered, lighter than air craft is called a balloon.
Balloons.
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).
Drifting.
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. A sufficiently light object can make use of updrafts and drift on the wind in this way for long periods of time.
Many mould and bacterial spores, even live bacteria, are small enough to drift for 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.
Parachuting is essentially falling or drifting 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.
Small creatures and seeds that have evolved parachutes can be blown on the wind for long distances. Among the plants, Dandelion, milkweed and poplar) seeds have hairs that act as parachutes. Some spiders cast parachutes of thread. Although mostly done by small spiderlings, adults weighing over 100 mg and with a body size of up to 14 mm have been observed casting parachutes a meter across into a strong updraft.
Parachuting is also used by larger creatures and seeds to travel shorter distances. Maple, pine and sycamore seeds have one or two wings that act like parachutes to aid in seed dispersal. Flying frogs use their webbed feet as parachutes.
Gliding.
Gliding flight requires an initial launch giving the object enough energy to fly.
Aerodynamic lift.
The principles of aerodynamic lift are shared by both nature and man-made aircraft. As the aeronaut falls, outspread wings are angled to the oncoming air to create a fast forward flow of air over the wing. This flow generates aerodynamic lift which slows the rate of descent. The result is gliding flight as opposed to a simple descent like a parachute.
If the air is rising faster than the object is descending, it will be carried upwards. In this way a gliding object can gain additional potential energy from sources such as thermals and ridge lift.
Glider aircraft.
Glider aircraft include sailplanes, hang gliders and paragliders. They must gain their initial energy of motion from a launch process. The launch may be by pulling the aircraft into the air with a tow-line, with a ground-based winch or vehicle, or with a powered "tug" aircraft. For foot-launched aircraft, there is also the option of merely stepping off a high location. Once the glider is released, it flies freely.
Gliding animals.
Creatures able to launch themselves into the air and glide short distances include:
Soaring.
Unpowered flights of longer duration and distance are possible if rising air is used to gain energy. This can further reduce the rate of descent or even increase height, which is known as "soaring".
Soaring is where the object/animal obtains additional energy from rising air without exerting any power to remain airborne. An example is the albatross, which is a large seabird renowned for its ability to stay aloft by soaring above the waves for days at a time. Many other birds such as raptors and storks also deliberately soar to extend their time aloft. Insects are often caught by rising air and so can be dispersed by it.
Many types of glider aircraft are designed to exploit rising air and can therefore also soar.
Kiting.
An airflow over a tethered object can gain it height up to a maximum determined partly by the length of the tether, and then enable it to maintain height while there is sufficient airflow. Such a tethered flying object is called a kite.
If there is a wind, the tether may be attached to a fixed point. The motion of an object such as a speedboat can also be used to create an airflow or to augment the wind. The mobile object can even be another kite.
Manned kites have been flown for a variety of purposes.