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Blade design is split into aerodynamic and structural design. The structural design is guided by external static and fatigue loads, taking into account the appropriate material choice. material combination and production technology. Light weight. flexible structures with high material damping will allow to achieve high life cycles, and a high resistance against static loading. The challenge is to design rotor blades for different applications (stall/active stall/pitch regulated wind turbines and/or constant/variable speed) which are optimized in weight and aerodynamic performance. The usage of advanced design tools, production technology and material choice are the dominating parameters. A weight/swept area ration in the order 1-I, 5 can be achieved with epoxy resin/glass fiber material up to a rotor diameter of 63m. 

4.1 Blade Design Aspects

The mayor applications of rotor blades are:

           Stall control with constant speed

           Stall control with variable speed

           Pitch control with constant speed

           Pitch control with variable speed

AERODYNAMIC DESIGN   

As the power curve is based on the cp-lambda characteristics of a certain rotor blade, the main parameters for obtaining an optimum power curve is the rotor diameter, rotational speed and pitch angle. E.g. a broad cp-lambda-curve for pitch controlled/variable speed turbine allows,  together with a high cp max in the order 0,49 aerodynamic value, result in an optimum energy capture, taking into account a lambda design value of 7.

PRODUCTION TECHNOLOGY

 

With composite material, production technology influences significantly the design. In the field of rotor blade production, the traditional hand lay-up procedure  using polyester and/or epoxy resin as technologies The application of pre impregnated material usually suffer from too high material costs, as it is more economic to use the raw materials itself in the production, thus using resin and fibers. An on-line impregnation technology during the production is to be used for an adequate production of unidirectional stiffness. The usage of raw materials such as filaments of glass fiber, is generally said cheaper than using fabrics with different lay-up combinations. Therefore, a compromise has to be found between the usage of the different raw materials and its consequences in design. Especially in the flexibility of the structure. E.g. + 45slayers help to improve the structural stability and improve the bending and torsion stiffness.

 

Design criteria to be taken into account for improvement and Standardization of rotor blade design and production:

 

MATERIAL PROPERTIES

 

Basically, there exist four material groups used for rotor blades:

• Epoxy resin/glass fiber

• Polyester resin/glass fiber

• Epoxy resin/wood

• Epoxy resin/carbon-glass fibers

Further improvements in the material choice, such as using carbon fibers in a hybrid system together with glass fibers has been currently only used for rotor blades larger than 25 m, as a sufficient bending stiffness is required (Schubert, M. 1997). Combined with an optimized structural design and thick profiles, it is also possible to use only glass fibers for rotor blade with a length of 30 m. The weight/strength ratio is the driving parameter for the determination of the optimum material and lay-up combination. Sandwich structures with foams are necessary for the structural stability.

On the other hand, material damping is one of the mayor issues concerning the dynamic behavior of the complete system rotor blades-wind turbines, especially for epoxy/glass fiber and polyester resin/glass fiber systems. The material wood, combined with epoxy resin, seems theoretically to have excellent performance, related to its dynamic behavior, but it still has to be approved in application. Research is carried out in order to use other raw materials, such as natural and aramid fibers, aiming at high material damping. On-going development aim the improvement of material damping, combined with a high flexibility of the structure.

 

WEIGHT

 

The blade mass is one of the most important parameters for dynamic loads of blade and wind turbine. The aim is to achieve an optimum between a low weight blade, related to low-cost production and a high performance. The blade weight can be reduced by thick profiles, thus increasing the moment of inertia of the blade cross section. This allows, taking into account the

material elastic properties, a high bending stiffness. Thus, the aim should be to achieve a blade weight/swept area ratio of 1 -1,5 kg/m2 considering rotor diameters from 43 to 62 m. this figure shows the corresponding increase of rotor weight with diameter.

 

T-BOLT ROOT CONNECTION

The T-bolt connection for rotor blade root in a reliable, simple and maintenance less joint. The

mayor criteria is to achieve a high level of pre-tensioning the bolt so that dynamic loading only

stress to a small level the bolts and the composite structure. In the past, no failures have occurred

with this type of root connection, which indicates its excellent characteristics in application.

NOISE

 

It has been found out that the mayor sources of noise emission of rotor blades are the

• turbulence inflow noise

• trailing edge

• tip

The aerodynamic lay-out, which aims an optimum aerodynamic performance, is influenced by the obligation to design low-noise profiles and to adapt the structural lay-out, especially the profile thickness (Scherer, R. 1997). Furthermore, dirt on the blade surface contributes to noise emission.

 

CONDITION MONITORING

 

The slope of the lift is no longer strongly positive, or even slightly negative, resulting in no- or negative aerodynamic damping of vibrations in the blades. Aiming at a condition monitoring of the blade during operation, the blades are nowadays provided with a vibration (acceleration) sensor, which is connected via a transmitter to the control system of the turbine to shut it shown with the tip brakes in case of high vibrations and related high structural loads

 

Another attempt for condition monitoring, especially for prototype testing, is the integration of optical fibers in the rotor blade structure (Giiemes et aI., 1998) . Signals are sent through the fibers and strains can be measured during operation. These "smart blades" are therefore provided with several optical sensor which allow, due to their easy mounting during manufacturing, the control of the blade at different locations simultaneously.

6.4 Composite rotors for gyros(p39)

Here we are presenting the results of testing  composite rotors for gyros:

Composite rotors

         are flight ready when you receive them

         require only to keep the surface clean and free from insect splatters

         do not have a limited life as normally associated with aluminum rotors

         weigh more than aluminum rotors and this allows for more stored energy for autorotations

         their high inertia accounts for a good deal of the safety, predictability and forgiveness

         have higher durability and damage resistance

         give the ability to optimize the efficiency of an element by orienting the fibers in directions that are most appropriate

         are "born-in" to their aerodynamic shape, and don't have the inherent internal stresses that metal blades have

         their tooling and fabrication techniques are relatively expensive and difficult to master, but the resulting rotor is superior in all areas

 

Special price 1200 EUR

Technical information :

Diameter set 7.60 m (25 ft.) or 8 m (26.6 ft.)

Airfoil NACA 8H12

Chord 203 mm

Weight for 25 ft. 30 kg (ca. 66 lb.); for 26.6 ft. 32 kg (ca. 70 lb.)

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