Heavier-than-air unpowered aircraft such as gliders (i.e. sailplanes), hang glidersparagliders, and other gliders usually do not employ propulsion once airborne. Take-off may be by launching forwards and downwards from a high location, or by pulling into the air on a tow-line, by a ground-based winch or vehicle, or by a powered "tug" aircraft. For a glider to maintain its forward air speed and lift, it must descend in relation to the air (but not necessarily in relation to the ground). Some gliders can 'soar'- gain height from updrafts such as thermal currents. The first practical, controllable example was designed and built by the British scientist and pioneer George Cayley, who many recognise as the first aeronautical engineer. and
Main article: Balloon
Balloons drift with the wind, though normally the pilot can control the altitude, either by heating the air or by releasing ballast, giving some directional control (since the wind direction changes with altitude). A wing-shaped hybrid balloon can glide directionally when rising or falling; but a spherically shaped balloon does not have such directional control.
Kites are aircraft that are tethered to the ground or other object (fixed or mobile) that maintains tension in the tether or kite line; they rely on virtual or real wind blowing over and under them to generate lift and drag. Kytoons are balloon kites that are shaped and tethered to obtain kiting deflections, and can be lighter-than-air, neutrally buoyant, or heavier-than air.
A propeller comprises a set of small, wing-like aerofoils set around a central hub which spins on an axis aligned in the direction of travel. Spinning the propeller creates aerodynamic lift, or thrust, in a forward direction. A contra-prop arrangement has a second propeller close behind the first one on the same axis, which rotates in the opposite direction.
A tractor design mounts the propeller in front of the power source, and a pusher design mounts it behind. Although the pusher design allows cleaner airflow over the wing, tractor configuration is more common because it allows cleaner airflow to the propeller and provides a better weight distribution.
A variation on the propeller is to use many broad blades to create a fan. Such fans are traditionally surrounded by a ring-shaped fairing or duct, as ducted fans.
Many kinds of power plant have been used to drive propellers.
The earliest designs used man power to give dirigible balloons some degree of control, and go back to Jean-Pierre Blanchard in 1784. Attempts to achieve heavier-than-air man-powered flight did not succeed until Paul MacCready's Gossamer Condor in 1977.
The first powered flight was made in a steam-powered dirigible by Henri Giffardsteam engine to a practical fixed-wing airframe did not succeed until much later, by which time the internal combustion engine was already dominant. in 1852. Attempts to marry a practical lightweight
From the first controlled powered fixed-wing aircraft flight by the Wright brothers until World War II, propellers turned by the internal combustion piston engine were virtually the only type of propulsion system in use. (See also: Aircraft engine.) The piston engine is still used in the majority of smaller aircraft produced, since it is efficient at the lower altitudes and slower speeds suited to propellers.
Turbine engines need not be used as jets (see below), but may be geared to drive a propeller in the form of a turboprop. Modern helicopters also typically use turbine engines to power the rotor. Turbines provide more power for less weight than piston engines, and are better suited to small-to-medium size aircraft or larger, slow-flying types. Some turboprop designs (see below) mount the propeller directly on an engine shaft, and are called propfans.
Since the 1940s, propellers and propfans with swept tips or curved "scimitar-shaped" blades have been studied for use in high-speed applications so as to delay the onset of shockwaves, in similar manner to wing sweepback, where the blade tips approach the speed of sound. The Airbus A400M turboprop transport aircraft is expected to provide the first production example: note that it is not a propfan because the propellers are not mounted direct on the engine shaft but are driven through reduction gearing.
Other less common power sources include:
- Electric motors, often linked to solar panels to create a solar-powered aircraft.
- Rubber bands, wound many times to store energy, are mostly used for flying models.
Air-breathing jet engines provide thrust by taking in air, burning it with fuel in a combustion chamber, and accelerating the exhaust rearwards so that it ejects at high speed. The reaction against this acceleration provides the engine thrust.
Jet engines can provide much higher thrust than propellers, and are naturally efficient at higher altitudes, being able to operate above 40,000 ft (12,000 m). They are also much more fuel-efficient at normal flight speeds than rockets. Consequently, nearly all high-speed and high-altitude aircraft use jet engines.
The early turbojet and modern turbofan use a spinning turbine to create airflow for takeoff and to provide thrust. Many, mostly in military aviation, use afterburners which inject extra fuel into the exhaust.
Use of a turbine is not absolutely necessary: other designs include the crude pulse jet, high-speed ramjet and the still-experimental supersonic-combustion ramjet or scramjet. These designs require an existing airflow to work and cannot work when stationary, so they must be launched by a catapult or rocket booster, or dropped from a mother ship.
The bypass turbofan engines of the Lockheed SR-71 were a hybrid design – the aircraft took off and landed in jet turbine configuration, and for high-speed flight the afterburner was lit and the turbine bypassed, to create a ramjet.
The motorjet was a very early design which used a piston engine in place of the combustion chamber, similar to a turbocharged piston engine except that the thrust is derived from the turbine instead of the crankshaft. It was soon superseded by the turbojet and remained a curiosity.
The rotor of a helicopter, may, like a propeller, be powered by a variety of methods such as an internal-combustion engine or jet turbine. Tip jets, fed by gases passing along hollow rotor blades from a centrally mounted engine, have been experimented with. Attempts have even been made to mount engines directly on the rotor tips.
Helicopters obtain forward propulsion by angling the rotor disc so that a proportion of its lift is directed forwards to provide thrust.
Other forms of propulsion
- Rocket-powered aircraft have occasionally been experimented with, and the Messerschmitt Komet fighter even saw action in the Second World War. Since then they have been restricted to rather specialised niches, such as the Bell X-1 which broke the sound barrier or the North American X-15 which travelled up into space where no oxygen is available for combustion (rockets carry their own oxidant). Rockets have more often been used as a supplement to the main powerplant, typically to assist takeoff of heavily loaded aircraft, but also in a few experimental designs such as the Saunders-Roe SR.53 to provide a high-speed dash capability.
- The flapping-wing ornithopter is a category of its own. These designs may have potential, but no practical device has been created beyond research prototypes, simple toys, and a model hawk used to freeze prey into stillness so that it can be captured.