# Calculating blade chord length and twist angle

**URL:** <https://forums.nlr.gov/t/calculating-blade-chord-length-and-twist-angle/1282>\
**Category:** Rotor Aerodynamics\
**Created:** [February 10, 2016, 10:36am UTC](https://forums.nlr.gov/t/calculating-blade-chord-length-and-twist-angle/1282 "2016-02-10T10:36:35Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![Morteza.Molaei](https://avatars.discourse-cdn.com/v4/letter/m/7ab992/32.png) [@Morteza.Molaei](https://forums.nlr.gov/u/Morteza.Molaei)\
**Post date:** [February 10, 2016, 10:36am UTC](https://forums.nlr.gov/t/calculating-blade-chord-length-and-twist-angle/1282/1 "2016-02-10T10:36:35Z")

</div>

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.

- [goo.gl/mh3w1l](http://goo.gl/mh3w1l)  
 ![20160210_112510 (2).jpg](https://europe1.discourse-cdn.com/nrel/original/1X/86a90806fab7befaef8bc0c91f01a7bad92c1a12.jpeg)

---

<div class="post-metadata">

**Author:** ![Jason.Jonkman](https://dub1.discourse-cdn.com/nrel/user_avatar/forums.nlr.gov/jason.jonkman/32/3086_2.png) [@Jason.Jonkman](https://forums.nlr.gov/u/Jason.Jonkman)\
**Post date:** [February 11, 2016, 4:19pm UTC](https://forums.nlr.gov/t/calculating-blade-chord-length-and-twist-angle/1282/2 "2016-02-11T16:19:32Z")

</div>

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](http://www.nrel.gov/docs/fy09osti/38060.pdf).

Best regards,