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.
Pennycuickdivides 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:
|Short duration||Sustained free flight||Tethered|
through the air
|None||Not classified as flight||Lighter than air|
|Lighter than air|
|Parachuting||Parachuting||Not classified as flight|
Some examples of usage are shown in the following table:
|Aircraft||Animals||Plants and fungi|
|Lighter than air||Balloon||—||—|
|Drifting||—||Small insects||Spores, Orchid seeds|
|Parachuting||Parachute||Spider kite|| Rotary wings (maple, sycamore)|
|Gliding|| Glider ||Flying squirrel||Winged seeds (Alsomitra macrocarpa)|
|Kiting|| Kite |
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. 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 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 flight requires an initial launch giving the object enough energy to fly.
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 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.
Creatures able to launch themselves into the air and glide short distances include:
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.
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.
An aircraft is a vehicle that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines. Common examples of aircraft include airplanes, helicopters, airships, gliders, paramotors and hot air balloons.
Hang gliding is an air sport or recreational activity in which a pilot flies a light, non-motorised foot-launched heavier-than-air aircraft called a hang glider. Most modern hang gliders are made of an aluminium alloy or composite frame covered with synthetic sailcloth to form a wing. Typically the pilot is in a harness suspended from the airframe, and controls the aircraft by shifting body weight in opposition to a control frame.
A wing is a type of fin that produces lift, while moving through air or some other fluid. As such, wings have streamlined cross-sections that are subject to aerodynamic forces and act as airfoils. A wing's aerodynamic efficiency is expressed as its lift-to-drag ratio. The lift a wing generates at a given speed and angle of attack can be one to two orders of magnitude greater than the total drag on the wing. A high lift-to-drag ratio requires a significantly smaller thrust to propel the wings through the air at sufficient lift.
Unpowered aircraft can remain airborne for a significant period of time without onboard propulsion. They can be classified as fixed-wing gliders, lighter-than-air balloons and tethered kites. This requires a trajectory that is not merely a vertical descent such as a parachute. In the case of kites, lift is obtained by tethering to a fixed or moving object, perhaps another kite, to obtain a flow of wind over the lifting surfaces. In the case of balloons, lift is obtained through inherent buoyancy and the balloon may or may not be tethered. Free balloon flight has little directional control. Gliding aircraft include sailplanes, hang gliders, and paragliders that have full directional control in free flight.
A fixed-wing aircraft is a flying machine, such as an airplane, which is capable of flight using wings that generate lift caused by the aircraft's forward airspeed and the shape of the wings. Fixed-wing aircraft are distinct from rotary-wing aircraft, and ornithopters. The wings of a fixed-wing aircraft are not necessarily rigid; kites, hang gliders, variable-sweep wing aircraft and airplanes that use wing morphing are all examples of fixed-wing aircraft.
Paragliding is the recreational and competitive adventure sport of flying paragliders: lightweight, free-flying, foot-launched glider aircraft with no rigid primary structure. The pilot sits in a harness suspended below a fabric wing. Wing shape is maintained by the suspension lines, the pressure of air entering vents in the front of the wing, and the aerodynamic forces of the air flowing over the outside.
Flight is the process by which an object moves through an atmosphere without contact with the surface. This can be achieved by generating aerodynamic lift associated with propulsive thrust, aerostatically using buoyancy, or by ballistic movement.
A variometer – also known as a rate of climb and descent indicator (RCDI), rate-of-climb indicator, vertical speed indicator (VSI), or vertical velocity indicator (VVI) – is one of the flight instruments in an aircraft used to inform the pilot of the rate of descent or climb. It can be calibrated in metres per second, feet per minute or knots, depending on country and type of aircraft. It is typically connected to the aircraft's external static pressure source.
The Rogallo wing is a flexible type of wing. In 1948, Francis Rogallo, a NASA engineer, and his wife Gertrude Rogallo, invented a self-inflating flexible wing they called the Parawing, also known after them as the "Rogallo Wing" and flexible wing. NASA considered Rogallo's flexible wing as an alternative recovery system for the Mercury and Gemini space capsules, and for possible use in other spacecraft landings, but the idea was dropped from Gemini in 1964 in favor of conventional parachutes.
Kite types, kite mooring, and kite applications result in a wide variety of kite control systems. Contemporary manufacturers, kite athletes, kite pilots, scientists, and engineers are expanding the possibilities.
Ridge lift is created when a wind strikes an obstacle, usually a mountain ridge or cliff, that is large and steep enough to deflect the wind upward.
An aerostat is a lighter than air aircraft that gains its lift through the use of a buoyant gas. Aerostats include unpowered balloons and powered airships. A balloon may be free-flying or tethered. The average density of the craft is lower than the density of atmospheric air, because its main component is one or more gasbags, a lightweight skin containing a lifting gas to provide buoyancy, to which other components such as a gondola containing equipment or people are attached. Especially with airships, the gasbags are often protected by an outer envelope.
A windsport is any type of sport which involves wind-power, often involving a non-rigid airfoil such as a sail or a power kite. The activities can be land-based, on snow, on ice or on water. Windsport activity may be regulated in some countries by aviation/maritime authorities if they are likely to interfere with other activities. Local authorities may also regulate activity in certain areas, especially on crowded beaches and parks.
A parafoil is a nonrigid (textile) airfoil with an aerodynamic cell structure which is inflated by the wind. Ram-air inflation forces the parafoil into a classic wing cross-section. Parafoils are most commonly constructed out of ripstop nylon.
A number of animals have evolved aerial locomotion, either by powered flight or by gliding. Flying and gliding animals have evolved separately many times, without any single ancestor. Flight has evolved at least four times, in the insects, pterosaurs, birds, and bats. Gliding has evolved on many more occasions. Usually the development is to aid canopy animals in getting from tree to tree, although there are other possibilities. Gliding, in particular, has evolved among rainforest animals, especially in the rainforests in Asia where the trees are tall and widely spaced. Several species of aquatic animals, and a few amphibians and reptiles have also evolved to acquire this gliding flight ability, typically as a means of evading predators.
A walkalong glider is a lightweight, slow flying model aircraft designed to be kept aloft by controllable slope soaring in the rising air generated by the pilot who walks along with the glider as it flies, usually holding a paddle. Hands or even the forehead can also be used to create an updraft. This type of soaring differs from other types of slope soaring in that the orographic lift is following the plane as it flies in the air and thus no other wind is required.
Gliding flight is heavier-than-air flight without the use of thrust; the term volplaning also refers to this mode of flight in animals. It is employed by gliding animals and by aircraft such as gliders. This mode of flight involves flying a significant distance horizontally compared to its descent and therefore can be distinguished from a mostly straight downward descent like with a round parachute.
A glider is a fixed-wing aircraft that is supported in flight by the dynamic reaction of the air against its lifting surfaces, and whose free flight does not depend on an engine. Most gliders do not have an engine, although motor-gliders have small engines for extending their flight when necessary by sustaining the altitude with some being powerful enough to take off self-launch.
Gliding is a recreational activity and competitive air sport in which pilots fly unpowered aircraft known as gliders or sailplanes using naturally occurring currents of rising air in the atmosphere to remain airborne. The word soaring is also used for the sport.
A glider or sailplane is a type of glider aircraft used in the leisure activity and sport of gliding. This unpowered aircraft can use naturally occurring currents of rising air in the atmosphere to gain altitude. Sailplanes are aerodynamically streamlined and so can fly a significant distance forward for a small decrease in altitude.