# Calculating blade chord length and twist angle

Hello,
I am designing a 3-MW with a rotor diameter of 115.7 m and nominal speed of 12 m/s.
My question is about calculating c (chord length) when designing the blade.
Let me start with DU40-A17 profile used in 5-MW turbine designed by NREL as an example:
From the profile data table we get: alpha=6.5 and lift coefficient=0.9960 for maximal lift-to-drag ratio.
As you have calculated in the tables in the following link*: r=11.75 m, D=126 m.
But these values do not give me c=4.557 and AeroTwst=13.308 (as you have put in the table) when plugged in the formula.
I have attahced my calculations which lead to a wrong answer.
I am missing something for sure. Help for finding it would be appreciated.
Best regards and thanks in advance.

Dear Marteza,

I’m not too familiar with the equations you are using, but my understanding is that those equations only apply to an aerodynamic-only optimization under some assumed idealized conditions (like operating at a fixed wind speed). The NREL 5-MW blade was not designed in this way. Instead, the NREL 5-MW blade is based heavily on the DOWEC study and LM Glasfiber blades. See the NREL 5-MW specifications report and its associated references for details: nrel.gov/docs/fy09osti/38060.pdf.

Best regards,