PROPELLERS

A propeller converts through helical motion the energy supplied by a power source into thrust, a force that moves a vehicle forward in a fluid medium. They are used primarily for marine and aerial propulsion, but they are found on other technologies such as hovercraft and wind turbines as well. Propellers, which are essentially a series of twisted wings, or blades, connected to a central hub, are efficient energy transmission devices for those applications. The blades strike the air or water at a certain angle, called the pitch, and create an area of low pressure in front of the propeller. As a result, the blades generate thrust through either fluid or aerodynamic means by pushing forward through the low-pressure area. Slip, or the energy lost as the propeller rotates, offsets the full output of thrust. The effectiveness of the propeller is measured by propulsive efficiency, the ratio of engine power to the actual thrust produced minus slip, during one complete revolution of the propeller. Because this process resembles the twisting movement of a carpenter's screw as it advances through wood, marine propellers are often called "screws" and aerial propellers are called "airscrews."

There exists a wide range of propellers for different applications. Fixed pitch propellers, most commonly found on ships and small aircraft, are simple in operation and efficient for one operating regime. Transport aircraft use variable-pitch propellers that can alter their pitch (sometimes automatically) to perform efficiently over a variety of conditions. Counter-rotating propellers are two propellers placed in tandem and operating from the same power source. They produce thrust efficiently, but they are also very complicated in design. Feathering propellers can set their blades parallel to the movement of either water or air in case of engine failure, which increases passenger safety. Reversible pitch propellers decrease landing distances for aircraft and aid in the maneuvering of ships by changing the direction of thrust. Depending on their application and operating regime, propellers can be made from wood, metal, or composite materials.

The concept of the propeller originates from three important antecedents. First, the Greek mathematician Archimedes developed a method of transporting water uphill through helical motion during the third century b.c.e. Second, the emergence of windmills in western Europe in the twelfth century c.e. indicated an inverse understanding of the principles and capabilities of thrust. Finally, Leonardo da Vinci's adaptation of the Archimedean helical screw to the idea of aerial propulsion during the fifteenth century inspired later propeller designers. These ideas influenced the development of both marine and aerial propellers but were mainly conceptual. It was not until the nineteenth and twentieth centuries that the availability of adequate power sources such as steam and internal combustion engines would make propellers feasible power transmission devices.

MARINE PROPELLERS

Propellers for the marine environment appeared first in the eighteenth century. The French mathematician and founder of hydrodynamics, Daniel Bernoulli, proposed steam propulsion with screw propellers as early as 1752. However, the first application of the marine propeller was the hand-cranked screw on American inventor David Bushnell's submarine, Turtle in 1776. Also, many experimenters, such as steamboat inventor Robert Fulton, incorporated marine propellers into their designs.

The nineteenth century was a period of innovation for marine propellers. Reflecting the growing importance of the steam engine and the replacement of sails and side paddle wheels for propellers on ocean-going vessels, many individuals in Europe and America patented screw propeller designs. Two individuals are credited with inventing the modern marine propeller: English engineer Francis P. Smith and American engineer John Ericcson. Both placed important patents in 1836 in England that would be the point of departure for future designs. Smith's aptly named ship Archimedes featured a large Archimedean screw and demonstrated the merits of the new system. Swedish-born Ericcson employed a six-bladed design that resembled the sails on a windmill. He immigrated to the United States in 1839 and pioneered the use of screw propellers in the United States navy. The first American propeller-driven warship, the U.S.S. Princeton, began operations in 1843. Other notable screw propulsion designs included British engineer Isambard Kingdom Brunel's 1843 iron-hull steamship, the Great Britain, which was the first large vessel with screw propellers to cross the Atlantic. In 1845, the British Admiralty sponsored a historic "tug-of-war" between the paddle wheel-driven H.M.S. Alecto and screw propeller-driven H.M.S. Rattler. The victory of the Rattler indicated the superiority of propellers to both sail and paddle wheel technology. However, naval vessels did not rely exclusively on screw propulsion until the 1870s. The early twentieth century witnessed the universal adoption of screws for oceangoing ships, which include the majority of vessels from the largest battleships to the smallest merchant marine vessels.

Typical marine propellers are fixed pitch and small in diameter with very thin, but broad, blade sections. They are made from either cast metal, corrosion-resistant metal alloys such as copper, or composite materials. Marine propellers normally operate at 60 percent efficiency due to the proximity of the ship's hull, which limits the overall diameter of the propeller and disturbs the efficient flow of water through the blades. As a result, the blades have to be very wide to produce adequate thrust. Marine propeller designers use innovations such as overlapping blades and wheel vanes to offset those problems and improve efficiency.

Another important consideration for marine propeller design is cavitation, the rapid formation and then collapse of vacuum pockets on the blade surface at high speed, and its contributions to losses in propulsive efficiency. The phenomenon can cause serious damage to the propeller by eroding the blade surface and creating high frequency underwater noise. Cavitation first became a serious problem in the late nineteenth and early twentieth centuries when innovations in steam and diesel propulsion technology drove propellers at unprecedented speeds. The introduction of gearing to make propellers rotate more efficiently at high revolutions and blades designed to resist cavitation alleviated the problem. However, research in the 1950s found that cavitation was a desirable trait for many high-speed marine propeller designs such as hydrofoils.

AERIAL PROPELLERS

As early as the eighteenth century, flight enthusiasts gradually began to consider the aerial propeller a practical form of transmitting propulsion. French mathematician J. P. Paucton revived the idea of the aerial propeller in Europe in his 1768 text, "Theorie de la vis d'Archimede." His Pterophoredesign used propellers for propulsion as well as for overall lift. Pioneers of lighter-than-air flight such as the French aeronauts Jean-Pierre Blanchard and Jean Baptiste Meusnier, used propellers on their balloons and airships in the 1780s. The increased attention given to the development of a practical heavier-than-air flying machine during the late nineteenth century ensured that experimenters rejected power transmission devices such as flapping wings, oars, sails, and paddle wheels, and incorporated propellers into their designs. Even though the propeller became a common component of proto-aeroplanes such as wings, aeronautical enthusiasts did not recognize the importance of propeller efficiency.

Wilbur and Orville Wright first addressed the aerial propeller from a theoretical and overall original standpoint during the successful development of their 1903 Flyer. They conceptualized the aerial propeller as a rotary wing, or airfoil, that generated aerodynamic thrust to achieve propulsion. They determined that the same physics that allowed an airfoil to create an upward lifting force when placed in an airstream would produce horizontal aerodynamic thrust when the airfoil was positioned vertically and rotated to create airflow. As a result, the Wrights created the world's first efficient aerial propeller and the aerodynamic theory to calculate its performance that would be the basis for all propeller research and development that followed. Used in conjunction with the reciprocating internal combustion piston engine, the aerial propeller was the main form of propulsion for the first fifty years of heavier-than-air flight.

Others built upon the achievement of the Wrights by improving the overall efficiency of the propeller. American engineer William F. Durand developed the standard table of propeller design coefficients in his landmark 1917 National Advisory Committee for Aeronautics study, "Experimental Research on Air Propellers." Besides the need for an aerodynamically efficient blade shape, propellers needed to be efficient over a variety of flight regimes. Developed simultaneously by Frank W. Caldwell in America, H. S. Hele-Shaw and T. E. Beacham in Great Britain, and Archibald Turnbull in Canada in the 1920s and becoming widely adopted in the 1930s, the variable-pitch mechanism dramatically improved the performance of the airplane. It linked aeronautical innovations such as streamline design, cantilever monoplane wings, and retractable landing gear with the increase in power brought by sophisticated engines, fuels, and supercharging. The properly designed variable-pitch propeller has been critical to the economic success of commercial and military air transport operations since the 1930s.

After World War II, the variable-pitch popeller was combined with a gas turbine to create the turbine propeller, or turboprop. The use of a gas turbine to drive the propeller increased propulsive efficiency, fuel economy, and generated less noise than conventional piston engine propeller aircraft. Turboprop airliners, first developed in Great Britain, began commercial operations in the early 1950s and were considered an economical alternative to turbojet airliners. Propellers are the most efficient form of aerial propulsion because they move a larger mass of air at a lower velocity (i.e., less waste) than turbojets and rockets. That efficiency is only present at speeds up to 500 mph (800 km/h). Beyond that, the tips of the rotating propeller suffer from near-sonic shockwaves that degrade its aerodynamic efficiency to the point where it loses power and cannot go any faster. Given that limitation and the higher efficiencies of turbojets at speeds above 500 mph for long distance flights, the propeller appeared to be obsolete as a viable energy conversion device for air transport. What occurred was that each propulsion technology proved the most efficient in different applications. The high efficiency of the aerial propeller, between 85 to 90 percent, and its lower operating costs created a strong economic impetus for air carriers to encourage the improvement of propeller-driven aircraft for the rapidly expanding short haul commuter market in the 1970s.

Further attempts to improve the efficiency of airplanes resulted in advances in propulsion technology based on the aerial propeller. Introduced in the 1960s, the turbofan, a large, enclosed multiblade fan driven by a turbojet core, harnessed the efficiency of the propeller while developing the thrust of the turbojet. NASA's long-range aircraft energy efficiency program of the 1980s developed the propfan, which is an advanced turboprop that employs multiple scimitar-shaped blades with swept-back leading edges designed for high speed. Propfans are capable of operating at speeds comparable to those generated by turbofan and turbojet-powered aircraft at a 25 percent savings in fuel.

The extreme climatic and physical environments in which propellers operate test the limits of aerodynamics, mechanical engineering, and structural theory. Depending upon the size of the power source, aerial propellers can be made from wood, metal, or composite materials and feature from two to six long and slender blades. The blades must be able to withstand exposure to harsh weather and stand up to aerodynamic loading, engine vibration, and structural bending while efficiently creating thrust.

A significant characteristic of the aerial propeller since its invention has been noise. Aircraft propellers, especially those made of metal, often produce an extremely loud "slapping, beating" sound when operating at high speeds. The increased use of propeller-driven commuter aircraft in the 1970s contributed to the growing noise pollution around busy urban airports. Measures to quiet aircraft included propeller synchronization to prevent high frequency vibration on multiengine aircraft and the use of elliptical blades, thinner airfoil sections, smaller propeller diameters, and lower rotational speeds.

Technologies other than conventional aircraft use aerial propellers for propulsion. Helicopters and vertical and short takeoff and landing aircraft (V/STOL) use rotor blades shaped to produce aerodynamic lift much like a propeller produces thrust. Hovercraft, or air-cushion vehicles, use propellers designed to be efficient at slower speeds for both movement and maneuverability when mounted on swiveling pylons. The search for an alternative energy source to fossil fuels in the 1970s encouraged many experimenters to use propellers to catch the wind and generate electricity through a turbine. Wind turbines benefit from advances in propeller design to create energy rather than converting the energy of a power source into forward movement.

Jeremy R. Kinney

See also: Aircraft; Ships; Turbines, Wind.

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