螺旋桨基本原理1

• A blade station is simply put, a reference position on a blade that is a specified distance from the center of the hub.
Blade Station Numbers
• When a propeller rotates through the air, a low pressure area is created in front of the blade (forward of the blade back; or curved side of the blade), much like the wing’s curvature creates a low pressure area above the wing. • The low pressure area just mentioned, along with the constant, or high pressure area behind the blade, allow a propeller to produce thrust. • The amount of thrust produced depends on several factors including; the angle of attack of the propeller blades, the speed the blades move through the air and the shape of the airfoil.
Types of Propellers: Fixed pitch Ground Adjustable Controllable Pitch Two-Position Pitch Constant Pitch Automatic Pitch These types of propellers will be discussed in detail in the future.
• The blade shank on this type of blade is typically round and extends out to at least the end of the hub assembly; However in some cases, the shank may extend beyond the hub assembly, into the airstream. When this is the case, blade cuffs may be installed to improve air flow around the blade shank.
• When a propeller rotates on a stationary aircraft, the direction of the relative wind is exactly opposite to the rotational movement of the propeller. Therefore, the propeller’s blade angle of attack is the same as the propeller blade angle.
• A blade cuff is an airfoil-shaped attachment made of thin sheets of metal, plastic or composite material. Blade cuffs mount on to the blade shanks and are primarily used to increase the flow of cooling air to the engine nacelle (cowling). Mechanical devices and bonding agents such as rubberbased adhesive or epoxy-based adhesive are used to attach the cuffs to the blades.
• The angle of attack of a propeller blade is the angle formed by the chord line of the blade and the relative wind.
• The direction of the relative wind is determined by the speed an aircraft moves through the air and the rotational motion of the propeller.
• We will end the study of the theory of operation here but will revisit this and continue further in next week’s lesson.
• Materials: Wood: - In the WWI era, the early propellers were made generally made of wood.
- This propeller, made of hardwood, shows the reinforcement of the attachment to the engine crankshaft with a metal hub assembly inserted into the hub bore to accommodate the mounting bolts and face plate.
An example of Blade Cuffs.
• To aid in identifying specific points along the length of a propeller blade, most blades have several defined blade stations.
Forward travel of aircraft
• Propellers which allow changes in blade angle have removable blades that are secured to a hub assembly by a set of clamping rings. Each blade root has a flanged butt, or shoulder, which mates with grooves in the hub assembly.
Materials: Steel: - The next progression of propeller construction, used steel as the material.
Materials: Aluminum (Alloy): - Aluminum Alloy is the current production standard, with composite blades the most recent variation.
• Since propellers and wings both produce lift, they share most of the same nomenclature. • All propeller blades have:
- A leading edge, trailing edge and a chord line.
Pusher Type: - Pusher type propellers are mounted on the aft end of an aircraft and push the aircraft through the air.
Tractor Type:
- Tractor propellers are mounted on the front of an engine. Tractor propellers pull an aircraft through the air.
Cessna Skycatcher, compliments of Cessna Aircraft Co.
Propeller Principles
• Propeller-Purpose:
- The purpose of a propeller is to convert the engine’s power into thrust (to driห้องสมุดไป่ตู้e the aircraft forward).
- Propellers can be used with piston engines or turbine engines (turbo-props). - Early propellers consisted of fabric covered wooden panels, wirebraced designs or simple flat boards that pushed air backwards. - As aeronautics developed, it was realized that a propeller with an airfoil shape was the most efficient design. - The blades of a propeller are airfoils, similar in profile to the airfoils of the wings of an aircraft.
- The curved, or cambered side of the propeller blade is the blade back and the flat side is the blade face.
- The blade angle (or pitch of the blade) is the acute angle formed by the propeller’s plane of rotation and the blade’s chord line.
• Nomenclature:
• Nomenclature (continued):
- Each blade acts as an airfoil, to produce lift and pull the aircraft through the air, much the way the wing of the aircraft produces lift to allow and aircraft to climb through the air.
• Nomenclature: - Basic terms and component names: - All modern propellers consist of at least two blades that are connected to a central hub. - The portion of the blade that is closest to the hub is the blade shank. - The portion of the blade which is the furthest from the hub is the blade tip. - The propeller hub, or hub assembly, is bored out to create a hub bore which permits a propeller to be mounted to the engine crankshaft or to a reduction gear assembly