In aeronautics, an aircraft propeller, also called an airscrew,Beaumont, R.A.; Aeronautical Engineering, Odhams, 1942, Chapter 13, "Airscrews". is an aircraft propulsion device that converts rotary motion from an engine or other power source into a swirling slipstream that generates thrust. A typical propeller engine comprises the motor driving a rotating shaft (driveshaft), to which several radial airfoil-section blades are attached, so that the whole blade assembly (i.e. the propeller) rotates around an axis of rotation longitudinal to the direction of travel. The propeller blades, which can be made from wood, metal or composite materials, attaches to the driveshaft either directly or through reduction gearing, and the blade pitch may be fixed, manually variable to a few set positions, or of the automatically variable "constant-speed" type.
Propeller-driven fixed-wing aircraft are the earliest type of powered heavier-than-air aircraft (i.e. aerodynes), with the Wright Flyer (which used two push propellers powered by a chain drive gasoline engine) being the first-ever manned fixed-wing aircraft to achieve sustained flight. In addition to conventional shaft engine propeller aircraft, almost all human- and solar-powered aircraft, as well as some lighter-than-air aircraft (i.e. powered airships), also use propellers to sustain and/or to steer flight. Tiltrotor/tiltwing aircraft can use a rotatable nacelle/wing mechanism to turn a rotorwing (which generates lift for vertical takeoff and landing) from a vertical axis to a horizontal one, which converts the rotorwing into a functional propeller — sometimes referred to as a proprotor. Propeller engines are typically only capable of subsonic airspeeds, generally below about , although a transonic speed of Mach 1.01 was recorded in a dive by the McDonnell XF-88B experimental propeller-equipped aircraft, with a propeller efficiency of 78%.Flight Investigation Of A Supersonic Propeller On A Propeller Research Vehicle At Mach Numbers To 1.01, Hammack et al., Research Memorandum RM L57E20, June 10, 1957, National Advisory Committee For Aeronautics, Washington
Propeller aircraft, since their debut in 1903, played a dominant role in aviation of the first half of the 20th century and in the aerial theaters of both World Wars, until the advent of the much faster and heavier-payload jet aircraft began supplanting them after the end of the Second World War. Nevertheless, propeller engines remain popular among light/ultralight aircraft in recreational and utility general aviation, while gas turbine-powered turboprop engines are used in commercial aviation for regional air travel, freight and aeromedical services, as well as by the military for low-intensity aerial tasks that warrant low-airspeed/altitude loitering or simply do not require fast airspeeds, such as tactical airlift, early warning and control, maritime patrol/anti-submarine warfare, electronic warfare and even gunship/close air support operations.
History
Early designs
The earliest references for vertical flight came from China. Since around 400 BC,Leishman, J. Gordon. Principles of Helicopter Aerodynamics. Cambridge aerospace series, 18. Cambridge: Cambridge University Press, 2006. . Web extract Chinese children have played with bamboo flying toys. "Early Helicopter History." Aerospaceweb.org. Retrieved: 12 December 2010Goebel, Greg. Vectorsite.net. Retrieved: 11 November 2008 This bamboo-copter is spun by rolling a stick attached to a rotor between one's hands. The spinning creates lift, and the toy flies when released. The 4th-century AD Daoist book Baopuzi by Ge Hong (抱朴子 "Master who Embraces Simplicity") reportedly describes some of the ideas inherent to rotary wing aircraft.Fay, John. "Helicopter Pioneers – Evolution of Rotary Wing Aircraft." Helicopter History Site. Retrieved: 28 November 2007
Designs similar to the Chinese helicopter toy appeared in Renaissance paintings and other works.Donald F. Lach. (1977). Asia in the making of Europe. Volume II, A Century of Wonder. p. 403
It was not until the early 1480s, when Leonardo da Vinci created a design for a machine that could be described as an "aerial screw", that any recorded advancement was made towards vertical flight. His notes suggested that he built small flying models, but there were no indications for any provision to stop the rotor from making the craft rotate.Rumerman, Judy. "Early Helicopter Technology." Centennial of Flight Commission, 2003. Retrieved 12 December 2010Pilotfriend.com "Leonardo da Vinci's Helical Air Screw." Pilotfriend.com. Retrieved 12 December 2010 As scientific knowledge increased and became more accepted, man continued to pursue the idea of vertical flight. Many of these later models and machines would more closely resemble the ancient bamboo flying top with spinning wings, rather than Leonardo's screw.
In July 1754, Russian Mikhail Lomonosov had developed a small coaxial modeled after the Chinese top but powered by a wound-up spring device Leishman, J. Gordon (2006). Principles of Helicopter Aerodynamics. Cambridge University Press. p. 8. and demonstrated it to the Russian Academy of Sciences. It was powered by a spring, and was suggested as a method to lift meteorological instruments. In 1783, Christian de Launoy, and his mechanic, Bienvenu, used a coaxial version of the Chinese top in a model consisting of contrarotating turkey flight feathers as rotor blades, and in 1784, demonstrated it to the French Academy of Sciences. A dirigible airship was described by Jean Baptiste Marie Meusnier presented in 1783. The drawings depict a streamlined envelope with internal ballonets that could be used for regulating lift. The airship was designed to be driven by three propellers. In 1784 Jean-Pierre Blanchard fitted a hand-powered propeller to a balloon, the first recorded means of propulsion carried aloft.Winter & Degner (1933), pp. 26–27. Sir George Cayley, influenced by a childhood fascination with the Chinese flying top, developed a model of feathers, similar to that of Launoy and Bienvenu, but powered by rubber bands. By the end of the century, he had progressed to using sheets of tin for rotor blades and springs for power. His writings on his experiments and models would become influential on future aviation pioneers.
William Bland sent designs for his "Atmotic Airship" to the Great Exhibition held in London in 1851, where a model was displayed. This was an elongated balloon with a steam engine driving twin propellers suspended underneath.Airship honours for Australia – Bland's remarkable invention more than 70 years ago. The Argus, September 13, 1924 Alphonse Pénaud developed coaxial rotor model helicopter toys in 1870, also powered by rubber bands. In 1872 Dupuy de Lome launched a large navigable balloon, which was driven by a large propeller turned by eight men.Brooks, Peter, W., Zeppelin: Rigid Airships 1893–1940, Washington, Smithsonian Institution Press, 1992, p. 19. Hiram Maxim built a craft that weighed , with a wingspan that was powered by two steam engines driving two propellers. In 1894, his machine was tested with overhead rails to prevent it from rising. The test showed that it had enough lift to take off.Beril, Becker (1967). Dreams and Realities of the Conquest of the Skies. New York: Atheneum. pp. 124–125 One of Pénaud's toys, given as a gift by their father, inspired the Wright brothers to pursue the dream of flight.
First modern propellers
The twisted airfoil (aerofoil) shape of an aircraft propeller was pioneered by the Wright brothers. While some earlier engineers had attempted to model air propellers on marine propellers, the Wrights realized that a propeller is essentially the same as a wing, and were able to use data from their earlier wind tunnel experiments on wings, introducing a twist along the length of the blades. This was necessary to maintain a more uniform angle of attack of the blade along its length. Their original propeller blades had an efficiency of about 82%,Ash, Robert L., Colin P. Britcher and Kenneth W. Hyde. "Wrights: How two brothers from Dayton added a new twist to airplane propulsion." Mechanical Engineering: 100 years of Flight, 3 July 2007. compared to 90% for a modern (2010) small general aviation propeller, the 3-blade McCauley used on a Beechcraft Bonanza aircraft.Rogers, David F. "Propeller Efficiency ", Figure 3. NAR, 2010. Accessed: 28 August 2014. Roper quotes 90% for a propeller for a human-powered aircraft.
Mahogany was the wood preferred for propellers through World War I, but wartime shortages encouraged use of walnut, oak, cherry and ash. Alberto Santos Dumont was another early pioneer, having designed propellers before the Wright BrothersHenri R. Palmer Jr. "The birdcage parasol", Flying Magazine Oct. 1960 p. 51 for his airships. He applied the knowledge he gained from experiences with airships to make a propeller with a steel shaft and aluminium blades for his 14 bis biplane in 1906. Some of his designs used a bent aluminium sheet for blades, thus creating an airfoil shape. They were heavily undercambered, and this plus the absence of lengthwise twist made them less efficient than the Wright propellers.Physical propeller theory was at the time restricted to the Rankine–Froude theory, also known as the "actuator disc theory" or the axial momentum theory. That theory, however adequate, does not give indication on the shape that should be given to the propeller. This would be solved regarding that theory only in the 1920s by complement of the Betz law (Goldstein, Betz, Prandtl and Lanchester): William Graebel, Engineering Fluid Mechanics, p. 144, , John Carlton, Marine Propellers and Propulsion, p. 169, . The Wright brothers however were equating the propeller blade to an airfoil instead, which for they previously had already determined the aerodynamic behavioural patterns: John David Anderson, A History of Aerodynamics: And Its Impact on Flying Machines, Even so, this was perhaps the first use of aluminium in the construction of an airscrew. Originally, a rotating airfoil behind the aircraft, which pushes it, was called a propeller, while one which pulled from the front was a tractor.Encyclopædia Britannica, 1910 edition, volume 30 (1922 supplement), in the article "Aeronautics" p. 20. "Airscrews have been described as 'tractors' and 'propellers', according as the airscrew shaft is placed in tension or in compression by the thrust, and corresponding aeroplanes are usually called by the same names. The first biplanes, those of the Wrights and the Farmans, were of the propeller type, colloquially 'pushers'; almost all monoplanes were 'tractors.' Later the term 'pusher' became adopted for the rear-mounted device in contrast to the tractor configuration and both became referred to as 'propellers' or 'airscrews'. The understanding of low speed propeller aerodynamics was fairly complete by the 1920s, but later requirements to handle more power in a smaller diameter have made the problem more complex.
Propeller research for National Advisory Committee for Aeronautics (NACA) was directed by William F. Durand from 1916. Parameters measured included propeller efficiency, thrust developed, and power absorbed. While a propeller may be tested in a wind tunnel, its performance in free-flight might differ. At the Langley Memorial Aeronautical Laboratory, E. P. Leslie used Vought VE-7s with Wright E-4 engines for data on free-flight, while Durand used reduced size, with similar shape, for wind tunnel data. Their results were published in 1926 as NACA report #220.William Durand & E. P. Leslie (1926) Comparison of tests on air propellers in flight with wind tunnel model tests on similar forms, National Advisory Committee for Aeronautics # 220
Theory and design