Aircraft Propellers

Aircraft Propellers General Information Thrust is the force that move the aircraft through the air.Thrust is generated by the propulsion system of the aircraft. There are different types of propulsion systems develop thrust in different ways, although it usually generated through some application of Newton's Third Law. Propeller is one of the propulsion system. The purpose of the propeller is to move the aircraft through the air. The propeller consist of two or more blades connected together by a hub. The hub serves to attach the blades to the engine shaft. . The blades are made in the shape of an airfoil like wing of an aircraft. When the engine rotates the propeller blades, the blades produce lift. This lift is called thrust and moves the aircraft forward. most aircraft have propellers that pull the aircraft through the air. These are called tractor propellers. Some aircraft have propellers that push the aircraft. These are called pusher propellers. Description 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 cambe side. Blade Face is the surface of the propeller blade that corresponds to the lower surface of an airfoil or flat side, we called Blade Face. Blade Back / Thrust Face is the curved surface of the airfoil. Blade Shank (Root) is the section of the blade nearest the hub. Blade Tip is the outer end of the blade fartest from the hub. Plane of Rotation is an imaginary plane perpendicular to the shaft. It is the plane that contains the circle in which the blades rotate. Blade Angle is formed between the face of an element and the plane of rotation. The blade angle throughout the length of the blade is not the same. The reason for placing the blade element sections at different angles is because the various sections of the blade travel at different speed. Each element must be designed as part of the blade to operate at its own best angle of attack to create thrust when revolving at its best design speed Blade Element are the airfoil sections joined side by side to form the blade airfoil. These elements are placed at different angles in rotation of the plane of rotation. The reason for placing the blade element sections at different angles is because the various sections of the blade travel at different speeds. The inner part of the blade section travels slower than the outer part near the tip of the blade. If all the elements along a blade is at the same blade angle, the relative wind will not strike the elements at the same angle of attack. This is because of the different in velocity of the blade element due to distance from the center of rotation. The blade has a small twist (due to different angle in each section) in it for a very important reason. When the propeller is spinning round, each section of the blade travel at different speed, The twist in the peopeller blade means that each section advance forward at the same rate so stopping the propeller from bending. Thrust is produced by the propeller attached to the engine driveshaft. While the propeller is rotating in flight, each section of the blade has a motion that combines the forward motion of the aircraft with circular movement of the propeller. The slower the speed, the steeper the angle of attack must be to generate lift. Therefore, the shape of the propeller's airfoil (cross section) must chang from the center to the tips. The changing shape of the airfoil (cross section) across the blade results in the twisting shape of the propeller. Relative Wind is the air that strikes and pass over the airfoil as the airfoil is driven through the air. Angle of Attack is the angle between the chord of the element and the relative wind. The best efficiency of the propeller is obtained at an angle of attack around 2 to 4 degrees. Blade Path is the path of the direction of the blade element moves. Pitch refers to the distance a spiral threaded object moves forward in one revolution. As a wood screw moves forward when turned in wood, same with the propeller move forward when turn in the air. Geometric Pitch is the theoritical distance a propeller would advance in one revolution. Effective Pitch is the actual distance a propeller advances in one revolution in the air. The effective pitch is always shorter than geometric pitch due to the air is a fluid and always slip. Forces and stresses acting on a propeller in flight The forces acting on a propeller in flight are : 1. Thrust is the air force on the propeller which is parallel to the directionof advance and induce bending stress in the propeller. 2. Centrifugal force is caused by rotation of the propeller and tends to throw the blade out from the center. 3. Torsion or Twisting forces in the blade itself, caused by the resultant of air forces which tend to twist the blades toward a lower blade angle. The stress acting on a propeller in flight are : 1. Bending stresses are induced by the trust forces. These stresses tend to bend the blade forward as the airplane is moved through the air by the propeller. 2. Tensile stresses are caused by centrifugal force. 3. Torsion stresses are produced in rotating propeller blades by two twisting moments. one of these stresses is caused by the air reaction on the blades and is called the aerodynamic twisting moment. The another stress is caused by centrifugal force and is called the centrifugal twisting moment.