sin αst

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Alternative blade design
The option for an alternative blade design, which is based on a simple airfoil, is investi-
gated in this chapter.
The blade geometry of the design proposal, described in section 6.2, is based on a NACA
airfoil which has good aerodynamic properties and decent manufacturability. It is however
possible to utilise simpler airfoils that have reduced aerodynamic properties, but superior
producability when considering the available manufacturing options in developing coun-
tries. Among home builders of wind turbines it is a widespread concept to use simple
curved airfoils, which enable the turbine blades to be made from e.g. cut-out sections of
plastic drain pipes or from rolled steel plates.
The considered potentials of using an alternative blade design based on a simple airfoil
are:
� Blade that may be produced easily and by simple means
� Increased flexibility as the alternative design may be used when the NACA-based
design is inconvenient
It is beyond the scope of this project thesis to fully design a blade other than the one al-
ready described in section 6.2. It will however perform a preliminary investigation of the
possibility of using simple airfoils. This is done firstly by testing lift and drag properties of
a simple airfoil. Although widely used, virtually no data is available on the simple airfoils
and it is therefore found necessary to design the test airfoil. Two different candidates are
designed and the best is selected from the test results. Secondly the test data is employed
in the rotor design tool with the aim of designing a rotor with performance equal to that of
the current NACA-based rotor at a wind speed of 12 m/s. This provides a preliminary view
of the resulting blade dimensions and the rotor efficiency when using a simple airfoil, and
thus gives an indication of whether it is applicable in the design.
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