DWM50172 Propeller Fundamentals PDF

DWM50172 PROPELLER Chapter 1 : Fundamentals  Topics Definition Function History of propeller Type of Aircraft Propeller Rotational speed...

DWM50172 PROPELLER Chapter 1 : Fundamentals  Topics Definition Function History of propeller Type of Aircraft Propeller Rotational speed Propeller Slip Forces Acting on the propeller Torque Reaction Vibration and Resonance Aircraft maintenance Department 2  Definition Aircraft maintenance Department 3 Definition Propeller : a mechanical device for propelling a boat or aircraft, consisting of a revolving shaft with two or more broad, angled blades attached to it. A propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid (such as air or water) is accelerated behind the blade. Propeller dynamics, like those of aircraft wings, can be modelled by either or both Bernoulli's principle and Newton's third law. Kota Kinabalu(BKI),Malysia 27,Apr.'90 Aircraft maintenance Department 4  An aircraft propeller or airscrew converts rotary motion from an engine or other mechanical power source, to provide propulsive force. 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. Aircraft maintenance Department 5  a. A propeller converts engine torque into propulsive force. A rotating propeller gives rearward motion to a mass of air and the reaction to this is a forward force on the propeller blades. A propeller moves a large mass of air rearward, at a relatively slow speed, as opposed to a gas turbine engine, which moves a small mass of air rearward at a high speed. b. Each propeller blade is of an aerofoil cross-section. c. The lift and drag forces of an aerofoil are equivalent to the thrust and torque of a propeller blade. d. The magnitude of thrust and torque produced will depend on the size, shape and number of blades, the blade angle, rotational speed, air density and forward speed. Aircraft maintenance Department 6  History of Propeller Aircraft maintenance Department 7  History of Propeller The twisted aerofoil shape of modern aircraft propellers 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. They also introduced a twist along the length of the blades. This was necessary to ensure the angle of attack of the blades was kept relatively constant along their length. Aircraft maintenance Department 8 9  Type of Propeller Aircraft maintenance Department 10  Type of aircraft Propeller In designing propellers, the maximum performance of the airplane for all condition of operation from takeoff, climb, cruising, and high speed. The propellers may be classified under eight general types as follows: 1. Fixed pitch: The propeller is made in one piece. Only one pitch setting is possible and is usually two blades propeller and is often made of wood or metal. Constant pitch propeller whose blade angle is fixed with respect to hub is suitable for low speed airplane. Wooden Propellers : Wooden propellers were used almost exclusively on personal and business aircraft prior to World War II.A wood propeller is not cut from a solid block but is built up of a number of seperate layers of carefully selected. Any types of wood have been used in making propellers, but the most satisfactory are yellow birch, sugar maple, black cherry, and black walnut. The use of lamination of wood will reduce the tendency for propeller to warp. For standard one-piece wood propellers, from five to nine seperate wood laminations about 3/4 in. thick are used. Metal Propellers : During 1940 , solid steel propellers were made for military use. Modern propellers are fabricated from high-strength , heat-treated aluminum alloy by forging a single bar of aluminum alloy to the required shape. Metal propellers is now extensively used in the construction of propellers for all type of aircraft. The general appearance of the metal propeller is similar to the wood propeller, except that the sections are generally thinner. Aircraft maintenance Department 11  Type of aircraft Propeller 2. Ground adjustable pitch: The pitch setting can be adjusted only with tools on the ground before the engine is running. This type of propellers usually has a split hub. The blade angle is specified by the aircraft specifications. The adjustable - pitch feature permits compensation for the location of the flying field at various altitudes and also for variations in the characteristics of airplanes using the same engine. Setting the blade angles by loosened the clamps and the blade is rotated to the desired angle and then tighten the clamps. 3. Two-position : A propeller which can have its pitch changed from one position to one other angle by the pilot while in flight. Aircraft maintenance Department 12  Type of aircraft Propeller 4. Controllable pitch: The pilot can change the pitch of the propeller in flight or while operating the engine by mean of a pitch changing mechanism that may be operated by hydraulically. Aircraft maintenance Department 13  Type of aircraft Propeller 5. Constant speed : The constant speed propeller utilizes a hydraulically or electrically operated pitch changing mechanism which is controlled by governor. The setting of the governor is adjusted by the pilot with the rpm lever in the cockpit. During operation, the constant speed propeller will automatically change its blade angle to maintain a constant engine speed. If engine power is increase, the blade angle is increased to make the propeller absorb the additional power while the rpm remain constant. At the other position, if the engine power is decreased, the blade angle will decrease to make the blades take less bite of air to keep engine rpm remain constant. The pilot select the engine speed required for any particular type of operation. Aircraft maintenance Department 14  Type of aircraft Propeller 6. Full Feathering : A constant speed propeller which has the ability to turn edge to the wind and thereby eliminate drag and windmilling in the event of engine failure. The term Feathering refers to the operation of rotating the blades of the propeller to the wind position for the purpose of stopping the rotation of the propeller to reduce drag. Therefore , a Feathered blade is in an approximate in-line-of-flight position , streamlined with the line of flight (turned the blades to a very high pitch). Feathering is necessary when the engine fails or when it is desirable to shutoff an engine in flight. Aircraft maintenance Department 15  Terms of Propeller Aircraft maintenance Department 16  Terms Leading Edge of the airfoil is the cutting edge that slices into the air. As the leading edge cuts the air, air flows over the blade face and the camber side. Blade station is a reference position on a blade that is a specified distance from the center of the hub. Aircraft maintenance Department 17  Term A well-designed propeller typically has an efficiency of around 80% when operating in the best regime. The efficiency of the propeller is influenced by the angle of attack (α). This is defined as α = Φ - θ, 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. Angle of attack is similar to advance ratio, for propel Pitch distribution lers. α = Angle of Φ Attack Blade angle is Θ = Helix angle or Pitch Distribution the angle the gradual twist in the between the propeller blade from shank to propeller’s tip. plane of rotation, and the chord line of 18 Aircraft maintenance Department the blade  Term  Propeller Lift and Angle Of Attack a. The angle of attack is the angle between the aerofoil chord line and the relative wind b. The relative wind is the resultant of the combined velocities of propeller rpm and airspeed. c. The relative wind changes due to different flight conditions. d. As the relative wind changes, so do the angle of attack. e. The most effective angle of attack should be between 2° - 4°. Any angle above 15° is ineffective because of the possibility of a stall. Aircraft maintenance Department 19  Term Summary : Aircraft maintenance Department 20  Propeller Slip Aircraft maintenance Department 21  Propeller Slip Aircraft maintenance Department 22  Propeller Slip Aircraft maintenance Department 23  If a propeller has a pitch of 50 inches, in theory it should move forward 50 inches in one revolution. But if the aircraft actually moves forward only 35 inches in one revolution the effective pitch is 35 inches and the propeller efficiency is 70% Aircraft maintenance Department 24  Forces Acting on the propeller Aircraft maintenance Department 25  Forces Acting on the propeller Aircraft maintenance Department 26  Forces Acting on the propeller Aircraft maintenance Department 27  Forces Acting on the propeller Aircraft maintenance Department 28  Forces Acting on the propeller Aircraft maintenance Department 29  Forces Acting on the propeller Aircraft maintenance Department 30 Aircraft maintenance Department 31  Torque Reaction Aircraft maintenance Department 32  Torque reaction Torque reaction involves Newton’s Third Law of Physics—for every action, there is an equal and opposite reaction. As applied to the airplane, this means that as the internal engine parts and propeller are revolving in one direction, an equal force is trying to rotate the airplane in the opposite direction. Aircraft maintenance Department 33  Torque reaction  When the airplane is airborne, this force is acting around the longitudinal axis, tending to make the airplane roll. To compensate for this, some of the older airplanes are rigged in a manner to create more lift on the wing that is being forced downward. The more modern airplanes are designed with the engine offset to counteract this effect of torque.  Generally, the compensating factors are permanently set so that they compensate for this force at cruising speed, since most of the airplane’s operating lift is at that speed. However, aileron trim tabs permit further adjustment for other speeds.  When the airplane’s wheels are on the ground during the takeoff roll, an additional turning moment around the vertical axis is induced by torque reaction.  As the left side of the airplane is being forced down by torque reaction, more weight is being placed on the left main landing gear. This results in more ground friction, or drag, on the left tire than on the right, causing a further turning moment to the left. The magnitude of this moment is dependent on many variables. Some of these variables are: 1) size and horsepower of engine, 2) size of propeller and the r.p.m., 3) size of the airplane, and 4) condition of the ground surface.  This yawing moment on the takeoff roll is corrected by the pilot’s proper use of the rudder or rudder trim. Aircraft maintenance Department 34  Vibration and Resonance Aircraft maintenance Department 35  Propeller Vibration 1. In most cases the cause of the vibration can be determined by observing the propeller hub, dome or spinner while the engine is running within a 1,200 to 1,500 r.p.m. range, and determining whether or not the propeller hub rotates on an absolutely horizontal plane. 2. If the propeller hub appears to swing in a slight orbit, the vibration will normally be caused by the propeller. If the propeller hub does not appear to rotate in an orbit, the difficulty will probably be caused by engine vibration. 3. When propeller vibration is the reason for excessive power plant vibration, the difficulty will usually be caused by propeller blade unbalance, propeller blades not tracking, or differences in propeller blade angle settings Aircraft maintenance Department 36  ANY QUESTION ? END Aircraft maintenance Department 37