Showing posts with label Rotor blade. Show all posts
Showing posts with label Rotor blade. Show all posts
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Thursday, January 27, 2011
0 comments Labels: Main rotor hub, Mechanism, R44, Rotor blade, Swashplate
Types of main rotors
There are several different types of main rotor systems, and they are classified depending on how each rotor blade can move in respect to the main hub.
In fully articulated rotor systems, each rotor blade is attached to the main hub through a series of hinges, and each rotor blade can move independently of the others.The three possible movements are called flapping, leading/lagging, and feathering. These rotors usually have 3 or more blades.
The flapping hinge allows the rotor blade to move up and down, and is necessary to compensate for the asymmetry of lift.
The lead-lag hinge allows a rotor blade to move horizontally. The purpose of this hinge is to compensate for the acceleration and deceleration caused by the Coriolis-effect.
The last hinge is the feathering hinge, which allows a rotor blade to rotate along its length. Feathering is necessary to be able to change the lift generated by a rotor blade. Without a feathering hinge, it wouldn't be possible to control a helicopter.
Semi-rigid rotor systems are usually composed out of 2 rotor blades. The blades are connected to the main rotor shaft by a teetering hinge. The teetering hinge allows the two rotor blades to move up and down as a whole. When one blade goes up, the other goes down. Semi-rigid rotors don't have lead-lag hinges, so the lead-lag forces are absorbed through blade bending. These rotor systems do have feathering hinges though, because without them, the helicopter would be uncontrollable.
Rigid rotor systems only allow rotor blades to feather, all other forces are absorbed through blade bending.
Combination rotor systems are the most modern rotor systems, and may use the principles of all previous rotor systems. Some incorporate a flexible hub, which allows the blades to move without the need for bearings or hinges. They use flextures and elastomeric bearings to accomplish this. The advantages of this system include less maintenance, less vibrations, and a longer lifespan.
Source:
http://www.faa.gov/library/manuals/aircraft/media/faa-h-8083-21.pdf
In fully articulated rotor systems, each rotor blade is attached to the main hub through a series of hinges, and each rotor blade can move independently of the others.The three possible movements are called flapping, leading/lagging, and feathering. These rotors usually have 3 or more blades.
The flapping hinge allows the rotor blade to move up and down, and is necessary to compensate for the asymmetry of lift.
The lead-lag hinge allows a rotor blade to move horizontally. The purpose of this hinge is to compensate for the acceleration and deceleration caused by the Coriolis-effect.
The last hinge is the feathering hinge, which allows a rotor blade to rotate along its length. Feathering is necessary to be able to change the lift generated by a rotor blade. Without a feathering hinge, it wouldn't be possible to control a helicopter.
Semi-rigid rotor systems are usually composed out of 2 rotor blades. The blades are connected to the main rotor shaft by a teetering hinge. The teetering hinge allows the two rotor blades to move up and down as a whole. When one blade goes up, the other goes down. Semi-rigid rotors don't have lead-lag hinges, so the lead-lag forces are absorbed through blade bending. These rotor systems do have feathering hinges though, because without them, the helicopter would be uncontrollable.
Rigid rotor systems only allow rotor blades to feather, all other forces are absorbed through blade bending.
Combination rotor systems are the most modern rotor systems, and may use the principles of all previous rotor systems. Some incorporate a flexible hub, which allows the blades to move without the need for bearings or hinges. They use flextures and elastomeric bearings to accomplish this. The advantages of this system include less maintenance, less vibrations, and a longer lifespan.
Source:
http://www.faa.gov/library/manuals/aircraft/media/faa-h-8083-21.pdf
Rotor blade materials
The main rotor blades are a vital part of a helicopter, because they are responsible for supporting the entire weight of a helicopter. The forces on the rotor blades can increase even more when performing maneuvers, for example: pulling up at 1.5G means that the rotor needs to support 1.5 times the weight of the helicopter.
The first helicopter rotor blades were constructed out of laminated wood and fabric. One of the major drawbacks of using wood to construct the rotor blades is that wood absorbs moisture, which changes the mass of the rotor blade.
Wooden rotor blades were used up until the 1960s, until they were replaced by steel and aluminium. Advantages of steel and aluminium rotor blades is that they're cheaper and easier to produce, and that they do not suffer from moisture absorption. However, disadvantages include a low strength to density ratio and a poor resistance to fatigue.
Major improvements were made to the rotor blades by using composite materials. Composite materials are made by combining two different materials together. For example, glass fiber and plastic can be combined to form a composite material. The plastic binds the fibers together, and distributes the forces among them. The plastic also helps prevent the propagation of cracks. Composite materials are anisotropic, the material's properties depend on the direction of the fibers. Because of this, multiple layers are put on top of each other at 90° angles. Of course, glass fiber isn't the only material used in composite materials, carbon fibres, and many others are also used, depending on the specific requirements of the rotor blades.
Modern rotor blades start out with a core, made out of Nomex (a brand of aramid), or honeycomb aluminium, which is cut to size. Then, precisely cut pieces of composite materials are placed inside a mold, and are partially cured. The core is then placed within the mold, and is crushed into shape by a hydraulic press. The composite material is then cured using pressurized steam, and excessive material is trimmed off.
Rotor blades constructed out of composite materials can be up to 45% lighter than their metal equivalents, and they can be more easily manufactured in complex shapes.
Sources:
http://www.whystudymaterials.ac.uk/casestudies/helicopter.asp
http://www.madehow.com/Volume-1/Helicopter.html
http://www.advancedtechnologiesinc.com/rotor_blade_development.asp
The first helicopter rotor blades were constructed out of laminated wood and fabric. One of the major drawbacks of using wood to construct the rotor blades is that wood absorbs moisture, which changes the mass of the rotor blade.
Wooden rotor blades were used up until the 1960s, until they were replaced by steel and aluminium. Advantages of steel and aluminium rotor blades is that they're cheaper and easier to produce, and that they do not suffer from moisture absorption. However, disadvantages include a low strength to density ratio and a poor resistance to fatigue.
Major improvements were made to the rotor blades by using composite materials. Composite materials are made by combining two different materials together. For example, glass fiber and plastic can be combined to form a composite material. The plastic binds the fibers together, and distributes the forces among them. The plastic also helps prevent the propagation of cracks. Composite materials are anisotropic, the material's properties depend on the direction of the fibers. Because of this, multiple layers are put on top of each other at 90° angles. Of course, glass fiber isn't the only material used in composite materials, carbon fibres, and many others are also used, depending on the specific requirements of the rotor blades.
Modern rotor blades start out with a core, made out of Nomex (a brand of aramid), or honeycomb aluminium, which is cut to size. Then, precisely cut pieces of composite materials are placed inside a mold, and are partially cured. The core is then placed within the mold, and is crushed into shape by a hydraulic press. The composite material is then cured using pressurized steam, and excessive material is trimmed off.
Rotor blades constructed out of composite materials can be up to 45% lighter than their metal equivalents, and they can be more easily manufactured in complex shapes.
Sources:
http://www.whystudymaterials.ac.uk/casestudies/helicopter.asp
http://www.madehow.com/Volume-1/Helicopter.html
http://www.advancedtechnologiesinc.com/rotor_blade_development.asp
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