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10.2 Design evaluation<br />
ALTERNATIVE BLADE DESIGN<br />
From appendix J it is seen that profile A exhibits the best aerodynamic properties. These<br />
are repeated on figure 10.1, which shows the lift coefficient Cl, the drag coefficient Cd and<br />
the glide ratio GR of profile A. The error bars of figure 10.1 show the standard deviation<br />
of three independent tests. This is further elaborated in appendix J.<br />
Figure 10.2: Lift coefficient Cl, drag coefficient Cd and glide ratio GR of profile A<br />
The glide ratio is approximately 10% of the NACA 4412 glide ratio, which indicates a<br />
significantly lower aerodynamic performance.<br />
The airfoil data is employed in the rotor design tool by the method described in Appendix<br />
D. On basis of this, a three-bladed rotor is designed to have the same power output as the<br />
NACA-based rotor at a wind speed of 12 m/s. U<strong>sin</strong>g a tip-speed ratio � of 4 and a fixed<br />
blade pitch � of 6�, the resulting blade radius is 2.05 m and the rotor efficiency is 40% (Cp<br />
= 0.24). This should be compared to the 1.35 m blade radius of the current design that<br />
has an efficiency of 93% (Cp = 0.55). Full details of the rotor tool calculation are provided<br />
as att. 5.<br />
The more than 50% increase in blade length results in considerably higher loads on each<br />
blade. Whether it is possible for the blade to structurally withstand the loads, depends on<br />
the material used. It is considered unlikely that a section-cut plastic drain pipe of the<br />
given length is strong and stiff enough, but a steel plate or a reinforced pipe section might<br />
suffice. Further development of the airfoil and structural verification of the blades that<br />
are based on it is left to a future project, as described in chapter 12.<br />
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