28/12/2022
𝐏𝐫𝐨𝐩𝐞𝐥𝐥𝐞𝐫 (aeronautics)
An aircraft propeller, also called an airscrew,
converts rotary motion from an engine or other power source into a swirling slipstream which pushes the propeller forwards or backwards. It comprises a rotating power-driven hub, to which are attached several radial airfoil-section blades such that the whole assembly rotates about a longitudinal axis. The blade pitch may be fixed, manually variable to a few set positions.
Propellers are most suitable for use at subsonic airspeeds generally below about 480 mph (770 km/h), although supersonic speeds were achieved in the McDonnell XF-88B experimental propeller-equipped aircraft. Supersonic tip-speeds are used in some aircraft like the Tupolev Tu-95, which can reach 575 mph (925 km/h)
𝐓𝐡𝐞𝐨𝐫𝐲 𝐚𝐧𝐝 𝐝𝐞𝐬𝐢𝐠𝐧
for analyzing the performance of light general aviation aircraft using fixed pitch or constant speed propellers. The efficiency of the propeller is influenced by the angle of attack (α). This is defined as α = Φ - θ,[27] where θ is the helix angle (the angle between the resultant relative velocity and the blade rotation direction) and Φ is the blade pitch angle. 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 when the blade is acting as a wing producing much more lift than drag. However, 'lift-and-drag' is only one way to express the aerodynamic force on the blades. To explain aircraft and engine performance the same force is expressed slightly differently in terms of thrust and torque[28] since the required output of the propeller is thrust. Thrust and torque are the basis of the definition for the efficiency of the propeller as shown below. The advance ratio of a propeller is similar to the angle of attack of a wing.
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. Therefore, most propellers use a variable pitch mechanism to alter the blades' pitch angle as engine speed and aircraft velocity are changed.
A sailor checks the propeller of a Landing Craft Air Cushion hovercraft
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 result in an excessive propeller diameter. 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. The performance of a propeller suffers when transonic flow first appears on the tips of the blades. As the relative air speed at any section of a propeller is a vector sum of the aircraft speed and the tangential speed due to rotation, the flow over the blade tip will reach transonic speed well before the aircraft does. When the airflow over the tip of the blade reaches its critical speed, drag and torque resistance increase rapidly 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 have been propeller aircraft which attained up to the Mach 0.8 range, but the low propeller efficiency at this speed makes such applications rare.
𝐁𝐥𝐚𝐝𝐞 𝐭𝐰𝐢𝐬𝐭
The tip of a propeller blade travels faster than the hub. Therefore, it is necessary for the blade to be twisted so as to decrease the angle of attack of the blade gradually from the hub to the tip.
𝐇𝐢𝐠𝐡 𝐬𝐩𝐞𝐞𝐝
There have been efforts to develop propellers and propfans for aircraft at high subsonic speeds.[30] The 'fix' is similar to that of transonic wing design. Thin blade sections are used and the blades are swept back in a scimitar shape (scimitar propeller) in a manner similar to wing sweepback, so as to delay the onset of shockwaves as the blade tips approach the speed of sound. The maximum relative velocity is kept as low as possible by careful control of pitch to allow the blades to have large helix angles. A large number of blades are used to reduce work per blade and so circulation strength. Contra-rotating propellers are 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 .
𝐏𝐡𝐲𝐬𝐢𝐜𝐬
Forces acting on the blades of an aircraft propeller include the following. Some of these forces can be arranged to counteract each other, reducing the overall mechanical stresses imposed.[31][1]
Thrust bending
Thrust loads on the blades, in reaction to the force pushing the air backwards, act to bend the blades forward. Blades are therefore often raked forwards, such that the outward centrifugal force of rotation acts to bend them backwards, thus balancing out the bending effects.
Centrifugal and aerodynamic twisting
A centrifugal twisting force is experienced by any asymmetrical spinning object. In the propeller it acts to twist the blades to a fine pitch. The aerodynamic centre of pressure is therefore usually arranged to be slightly forward of its mechanical centreline, creating a twisting moment towards coarse pitch and counteracting the centrifugal moment. However in a high-speed dive the aerodynamic force can change significantly and the moments can become unbalanced.
Centrifugal
The force felt by the blades acting to pull them away from the hub when turning. It can be arranged to help counteract the thrust bending force, as described above.
Torque bending
Air resistance acting against the blades, combined with inertial effects causes propeller blades to bend away from the direction of rotation.
Vibratory
Many types of disturbance set up vibratory forces in blades. These include aerodynamic excitation as the blades pass close to the wing and fuselage. Piston engines introduce torque impulses which may excite vibratory modes of the blades and cause fatigue failures.
Torque impulses are not present when driven by a gas turbine engine.
𝐕𝐚𝐫𝐢𝐚𝐛𝐥𝐞 𝐩𝐢𝐭𝐜𝐡
Main article: Variable-pitch propeller (aeronautics)
The purpose of varying pitch angle is to maintain an optimal angle of attack for the propeller blades, giving maximum efficiency throughout the flight regime. This reduces fuel usage. Only by maximising propeller efficiency at high speeds can the highest possible speed be achieved.
Effective angle of attack decreases as airspeed increases, so a coarser pitch is required at high airspeeds.
The requirement for pitch variation is shown by the propeller performance during the Schneider Trophy competition in 1931. The Fairey Aviation Company fixed-pitch propeller used was partially stalled on take-off and up to 160 mph (260 km/h) on its way up to a top speed of 407.5 mph (655.8 km/h).[34] The very wide speed range was achieved because some of the usual requirements for aircraft performance did not apply. There was no compromise on top-speed efficiency, the take-off distance was not restricted to available runway length and there was no climb requirement.
The variable pitch blades used on the Tupolev Tu-95 propel it at a speed exceeding the maximum once considered possible for a propeller-driven aircraft using an exceptionally coarse pitch.
𝐌𝐞𝐜𝐡𝐚𝐧𝐢𝐬𝐦
Cut-away view of a Hamilton Standard propeller. This type of constant-speed propeller was used on many American fighters, bombers and transport aircraft of World War II
Early pitch control settings were pilot operated, either with a small number of preset positions or continuously variable.[1]
The simplest mechanism is the ground-adjustable propeller, which may be adjusted on the ground, but is effectively a fixed-pitch prop once airborne. The spring-loaded "two-speed" VP prop is set to fine for takeoff, and then triggered to coarse once in cruise, the propeller remaining coarse for the remainder of the flight.
After World War I, automatic propellers were developed to maintain an optimum angle of attack. This was done by balancing the centripetal twisting moment on the blades and a set of counterweights against a spring and the aerodynamic forces on the blade. Automatic props had the advantage of being simple, lightweight, and requiring no external control, but a particular propeller's performance was difficult to match with that of the aircraft's power plant.
The most common variable pitch propeller is the constant-speed propeller. This is controlled by a hydraulic constant speed unit (CSU). It automatically adjusts the blade pitch in order to maintain a constant engine speed for any given power control setting.[1] Constant-speed propellers allow the pilot to set a rotational speed according to the need for maximum engine power or maximum efficiency, and a propeller governor acts as a closed-loop controller to vary propeller pitch angle as required to maintain the selected engine speed. In most aircraft this system is hydraulic, with engine oil serving as the hydraulic fluid. However, electrically controlled propellers were developed during World War II and saw extensive use on military aircraft, and have recently seen a revival in use on home-built aircraft.[citation needed]
Another design is the V-Prop, which is self-powering and self-governing.
𝐅𝐞𝐚𝐭𝐡𝐞𝐫𝐢𝐧𝐠
Feathered propeller on the outboard TP400 turboprop of an Airbus A400M
On most variable-pitch propellers, the blades can be rotated parallel to the airflow to stop rotation of the propeller and reduce drag when the engine fails or is deliberately shut down. This is called feathering, a term borrowed from rowing. On single-engined aircraft, whether a powered glider or turbine-powered aircraft, the effect is to increase the gliding distance. On a multi-engine aircraft, feathering the propeller on an inoperative engine reduces drag, and helps the aircraft maintain speed and altitude with the operative engines.
Most feathering systems for reciprocating engines sense a drop in oil pressure and move the blades toward the feather position, and require the pilot to pull the propeller control back to disengage the high-pitch stop pins before the engine reaches idle RPM. Turboprop control systems usually utilize a negative torque sensor in the reduction gearbox which moves the blades toward feather when the engine is no longer providing power to the propeller. Depending on design, the pilot may have to push a button to override the high-pitch stops and complete the feathering process, or the feathering process may be totally automatic.
𝐑𝐞𝐯𝐞𝐫𝐬𝐞 𝐩𝐢𝐭𝐜𝐡
Thrust reversal
The propellers on some aircraft can operate with a negative blade pitch angle, and thus reverse the thrust from the propeller. This is known as Beta Pitch. Reverse thrust is used to help slow the aircraft after landing and is particularly advantageous when landing on a wet runway as wheel braking suffers reduced effectiveness. In some cases reverse pitch allows the aircraft to taxi in reverse – this is particularly useful for getting floatplanes out of confined docks.
#𝐟𝐨𝐥𝐥𝐨𝐰-𝐚𝐞𝐫𝐨
#𝐚𝐞𝐫𝐨𝐧𝐚𝐮𝐭𝐢𝐜𝐬
#𝐚𝐯𝐢𝐚𝐭𝐢𝐨𝐧
