BeamDyn versus ElastoDyn


I am trying to compare blade 1 flapwise bending moment computed by BeamDyn and ElastoDyn at a mid-span location (assuming all other inputs are exactly the same).

In ElastoDyn I define the sectional flapwise moment as (i is some integer):
SpniMLyb1 Blade 1 local flapwise moment at span station i

In BeamDyn I define the sectional flapwise moment as (j is some other integer):
NjMyl Sectional moment resultants at Node j expressed in l

My question is, will SpniMLyb1 and NjMyl be comparable assuming I manage to define them in the same location along the span of the blade?
Or will NjMyl need to be transformed to be directly comparable to SpniMLyb1?

Thanks very much

Dear Imad,

Yes, those outputs should be equivalent between ElastoDyn and BeamDyn.

Best regards,

Thanks very much for the prompt reply.

Dear Jason,

is there a documented comparison between BeamDyn and ElastoDyn outputs? Espcially for outer sections along the span of the blade.

I am only aware of this:

Best Regards,


Dear Imad,

We’ve made a lot of comparisons, but little is published. The paper you referenced by Guntar et al described an effort that was very thorough (although only a small subset of the results are presented).

To get more comfortable with BeamDyn, you can compare the results of Test18 and Test26 in the FAST v8 CertTest. These are identical simulations of the NREL 5-MW baseline turbine, but Test26 models the blades with BeamDyn and Test18 uses ElastoDyn.

Best regards,

Good stuff, thanks a lot.


Dear Jason,

I would like to follow up on this.

So, I run and compare the output of Test18 and Test26 in CertTest. I compare the flapwise bending moments along the span of the blade, for a steady mean wind speed of 8m/s, no shear, simulation time = 750s (with the first 150s omitted).

The ElastoDyn sensors I use are (for blade 1): Spn1MLyb1, Spn2MLyb1, Spn3MLyb1,…, Spn9MLyb1
The BeamDyn sensors I use are: N1Myl, N2Myl, N3Myl,…, N7Myl

In BeamDyn I use order_elem=6 with a time step of 0.0005. I am using FAST_v8.15.00a-bjj.

FAST with BeamDyn and ElastoDyn give very similar operational output of the wind turbine: RotSpeed, RotTorq, GenSpeed, GenTq , GenPwr

The ElastoDyn flapwise bending moment distribution along the span of the blade looks good. However, apart from the blade root node, the BeamDyn N2Myl… N7Myl are very close to 0 kNm.

Is there some bug somewhere? Not sure what I am doing wrong. I would appreciate it if you could point me to how I can get Test18 and Test26 to give similar/comparable values of the flapwise bending moments along the span of the blade.

Best regards,


Dear Jason and Imad,

I am facing the same problem as was posted by Imad in previous post. I wanted to compare the flapwise and edgewise bending moment on blades using Elastodyn and BeamDyn (Test18 vs Test26 - without changing anything). However, the result showed apart from bending moment on the blade root node, all bending moment on other nodes (Node 2 to Node 6) are close to zero.

Before I came across comparing Test18 and Test26, I was using my 3MW Windturbine and I ran FAST to compare bending moments on blades by using ElastoDyn and BeamDyn. I got the same result where bending moment on node 1 is similar and all other bending moment on node 2 to node 6 are close to zero. I cross checked my Mass and Stiffness Matrix for BeamDyn and confirmed they are correct. I am also not sure what I am doing wrong or is there bug somewhere. Hence, I tried to look for some hints in this forum and came across this Test18 and Test26. I tried and also ended up with the same result as Imad.

Appreciate help from both of you you and also kindly let me know if these were resolved and discussed somewhere else in this forum. Thank you in advance.

Best Regards,

Dear Intan and Imad,

Sorry, I thought I had responded to Imad earlier.

We are aware of a bug in how BeamDyn calculates the local sectional loads (forces/moments) that are output. While the internal blade dynamics and deflections are calculated properly by BeamDyn, the local sectional load outputs are not calculated correctly, except at the blade root. We are working on a fix, which will be included in the next release of BeamDyn.

Best regards,

Hello FAST Forum Friends,

When I place a (redundant) gage at the blade root and compare "Spn1MLxb1"to the “RootMxb1” the values are not the same. (here using Test18)

“RootMxb1” is ‘about the yb1-axis’ and “Spn1MLxb1” is calculated ‘About the local xb1-axis’. So I think they should be the same.?

Mark C

Dear Mark,

A similar question has been asked and answered in the following forum topic:

Best regards,