A propeller is a device which provides thrust for propulsion of a vehicle such as an airplane, ship, or submarine though a fluid such as water or air, by rotating two or more twisted blades about a central shaft, in a manner analogous to rotating a screw though a solid. However, the blades of a propeller don't push directly on the fluid, but rather act as rotating wings, and produce force via Bernoulli's principle, generating a difference in pressure from the forward and rear surfaces of the airfoil-shaped blades.
Aircraft propellers (airscrews)Edit
A propeller's efficiency is determined by (thrust × axial speed)/(resistance torque × rotational speed). Changes to a propeller's efficiency are produced by a number of factors, notably adjustments to the helix angle, the angle between the resultant relative velocity and the blade rotation direction, and to blade pitch. Very small pitch and helix angles give a good performance against resistance but provide little thrust, while larger angles have the opposite effect. The best helix angle is as if the blade were a wing producing much more lift than drag, roughly 45° in practice. However due to the shape of the propeller, only part of the blade can actually be operating at peak efficiency. The outer part of the blade produces the most thrust and so the blade is positioned at a pitch that gives optimum angle to that portion. Since a large portion of the blade is therefore at an inefficient angle, the inboard ends of the blade are subsumed into a streamlined spinner to reduce the resistance torque that would otherwise be created.
Very high efficiency propellers are similar in aerofoil section to a low drag wing and as such are poor in operation when at other than their optimum angle of attack. Advanced control systems and better section profiling are required to counter the need for accurate matching of pitch to flight speed and engine speed to power so as to make this type of propellers usable.
However with a propeller at a pitch angle of 45° at low flight speeds the angle of attack will be high, possibly high enough to stall the airfoil. Since this is an extremely inefficient regime in which to operate the propeller, it means that most propellers are fitted with mechanisms to allow variable pitch – coarse pitch for high speed flight and fine pitch for climbing or accelerating at lower speeds. Early pitch control settings were pilot operated and so limited to only three or so settings, later systems were automatic. Variable pitch was replaced with the constant speed mechanism. In some aircraft (e.g., the C-130 Hercules), the pilot can manually override the constant speed mechanism to reverse the blade pitch angle, and thus the thrust of the engine. This allows the aircraft to back up on its own, at unimproved airfields when aircraft tractors are unavailable.
Constant-speed propellers automatically adjust the blade pitch angle to alter resistance torque in response to sensed changes in rotational speed. Initially in a rather crude fashion with the pilot altering the setting via control of the propeller governor, in more advanced aircraft the mechanism is linked into the entire engine management system for very fine control. The system is termed constant-speed because aero engines produce maximum power at high revolutions and changing engine speed increases fuel consumption. It is, therefore, beneficial to run an engine at an optimum constant independent of flight speed, setting separate requirements for high power situations and cruising and controlling speed within these bands without changing RPM.
A further consideration is the number and the shape of the blades used. Increasing the aspect ratio of the blades reduces drag but the amount of thrust produced depends on blade area, so using high aspect blades can lead to the need for a propeller diameter which is unusable. A further balance is that using a smaller number of blades reduces interference effects between the blades, but to have sufficient blade area to transmit the available power within a set diameter means a compromise is needed. Increasing the number of blades also decreases the amount of work each blade is required to perform, limiting the local Mach number - a significant performance limit on propellers.
Contra-rotating propellers involve a second propeller rotating in the opposite direction immediately 'downstream' of the main propeller so as to recover energy lost in the swirling motion of the air in the propeller slipstream. Contra-rotation also increases power without increasing propeller diameter and provides a counter to the torque of high-power piston engines' gyroscopic precession effects, and of the slipstream swirl. However on small aircraft the added cost, complexity, weight and noise of the system rarely make it worthwhile.
The propeller is usually attached to the crankshaft of the engine, either directly or through a gearbox. Light aircraft sometimes forego the weight, complexity and cost of gearing but on larger aircraft and with turboprop engines it is essential.
As mentioned, a propeller's performance suffers as the blade speed exceeds the speed of sound. As the relative air speed at the blade is rotation speed plus axial speed, a propeller blade tip will reach sonic speed sometime before the rest of the aircraft (with a theoretical blade the maximum aircraft speed is about 845 km/h (Mach 0.7) at sea-level, in reality it is rather lower). When a blade tip becomes supersonic, drag and torque resistance increase suddenly and shock waves form creating a sharp increase in noise. Aircraft with conventional propellers, therefore, do not usually fly faster than Mach 0.6. There are certain propeller-driven aircraft, usually military, which do operate at Mach 0.8 or higher, although there is considerable fall off in efficiency.
Naturally there have been efforts to develop propellers for aircraft at high subsonic speeds. The 'fix' is similar to that of transonic wing design. The maximum relative velocity is kept as low as possible by careful control of pitch to allow the blades to have large helix angles; thin blade sections are used and the blades are swept back in a scimitar shape; a large number of blades are used to reduce work per blade and so circulation strength; contra-rotation is used. The propellers designed are more efficient than turbo-fans and their cruising speed (Mach 0.7-0.85) is suitable for airliners, but the noise generated is tremendous.
A fan is a propeller with a large number of blades. A fan therefore produces a lot of thrust for a given diameter but the closeness of the blades means that each strongly affects the flow around the others. If the flow is supersonic, this interference can be beneficial if the flow can be compressed through a series of shock waves rather than one. By placing the fan within a shaped duct – a ducted fan – specific flow patterns can be created depending on flight speed and engine performance. As air enters the duct, its speed is reduced and pressure and temperature increase. If the aircraft is at a high subsonic speed this creates two advantages – the air enters the fan at a lower Mach speed and the higher temperature increases the local speed of sound. While there is a loss in efficiency as the fan is drawing on a smaller area of the free stream and so using less air, this is balanced by the ducted fan retaining efficiency at higher speeds where conventional propeller efficiency would be poor. A ducted fan or propeller also has certain benefits at lower speeds but the duct needs to be shaped in a different manner to one for higher speed flight. More air is taken in and the fan therefore operates at an efficiency equivalent to a larger un-ducted propeller. Noise is also reduced by the ducting and should a blade become detached the duct would contain the damage. However the duct adds weight, cost, complexity and (to a certain degree) drag.
See also Airscrew wind generator.
Ship/Submarine propellers (screws)Edit
James Watt of Birmingham, England is generally credited with applying the very first screw propeller to an engine, an early steam engine, so the idea of engine propulsion utilising a hydrodynamic screw was born. If we ignore the mechanics of sailing and rowing, mechanical ship propulsion properly began with the steam ship. The issue of which was the first successful ship of this type is a matter of debate; candidate inventors of the 18th century include William Symington, the Marquis de Jouffroy, John Fitch and many others. The American Robert Fulton is the most widely credited. Fulton's choice of paddle-wheels as the main motive source became standard on many of the following vessels (see Paddle steamer). Robert Fulton had tested, and rejected, the screw-propeller.
The screw-propeller (as opposed to paddle-wheels) was introduced in the latter half of the 18th century. David Bushnell's invention of the submarine (the Turtle) in 1775 utilized hand-powered screws for vertical and horizontal propulsion. Josef Ressel designed and patented a screw propeller in 1827. Francis Petit Smith tested a screw propeller similar to Ressel's in 1836. In 1839, John Ericsson introduced the screw-propeller design onto a ship which then sailed over the Atlantic Ocean in 40 days. Mixed paddle and propeller designs were still being used at this time (vide the 1858 SS Great Eastern).
In 1848 the British Admiralty held a tug of war contest between a propeller driven ship the Rattler and a paddle wheel ship the Alecto. The propeller won, towing the Alecto stern first at a speed of 2.8 knots, but it was not until the early 20th century that paddle propelled vessels were entirely superseded. The screw propeller replaced the paddles owing to its greater efficiency, compactness, less complex power transmission system and reduced susceptibility to damage.
Initial designs owed much to the ordinary screw from which their name derived - early propellers consisted of only two blades and matched in profile the length of a single screw rotation. This design was common, but inventors endlessly experimented with different profiles and greater numbers of blades. The propeller screw design stabilized by the 1880s.
In the early days of steam power for ships, when both paddle wheels and screws were in use, ships were often characterized by their type of propellers, leading to terms like screw steamer or screw sloop. Submariners call the propellers on submarines "screws".
Propellers are referred to as "lift" devices, while paddles are "drag" devices.
Cavitation can occur if an attempt is made to transmit too much power through the screw. The cohesion of the water is broken and the water thread 'strips' with explosive force. It wastes energy, makes excessive noise and most seriously, erodes of the screw's surface. (See also fluid dynamics).
Apart from propelling a vessel ahead or astern, a propeller will also exert a lateral force on a boat owing to an effect known as propeller walk. This effect is most noticeable when beginning to run astern. With a normal single, right-hand screw, beginning to run the engine astern tends to throw the stern to starboard. This means that, when coming alongside a hard in a space between moored boats, other considerations allowing, it is best to come alongside starboard side to, so that the stern can be tucked in by running the engine astern fairly abruptly while taking the way off the boat. It seems to arise from the paddle effect of the lower blades in the more dense, less aerated water.
The first screw propeller to be added to an engine was installed by James Watt in Birmingham, England. He used it with his Steam engine although the screw propeller in itself can be traced to the time of the Egyptians by way of Leonardo da Vinci.
The first screw propeller to be powered by a petrol engine, fitted to a small boat (now known as Powerboat was installed by Frederick William Lanchester also from Birmingham, England. This was tested in Oxford.
The airfoil (aerofoil) shape of modern aircraft propellers was pioneered by the Wright brothers. Realizing that a propeller was a rotating wing, they applied what they learned from wing design to the cross-section of wooden propellers for their Flyer.
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