# How does WT_perf calculate the flapwise bending moment

Hi all

can you please explain to me how WT_perf calculates the flapwise bending moment in the .oup file?
I wrote an own code and compared two blades to validate my own program with WT_perf.
All outcomes of the .bed file are the same for both blades.
The first blade I implemented in my own code had almost the same outcomes in the .oup file up to a difference of 1-2%.

For the second wing all outcomes (.oup file) were in the 1-2% range too. But the flapwise bending moment had a difference of 38%
I tried to find a bug in my program but could not find one.
Than I tried to take the thrust force / length from the .bed file and calculate the flapwise bending moment with my numerical algorithm but I got bad values with a difference of about 38% again.

Is there something I am missing. My bending force is always ~38% higher than the WT_perf one. The second wing is rotating a lot of faster than the first one but friction should be of no importance for the flapwise bending moment. Than the Torque should have an even bigger difference.

I assumed a linear function between the forces calculated at each node. What kind of numerical integration is used in WT_perf?

Thank you for the help

Andreas

The short answer is that it does rectangular integration. It multiplies each local thrust by the distance to the hub radius. If you want more details, please look at the code. Look for FlapMom in WTP_Subs.f90\RotAnal().

Hi again,

I now had a closer look at the computation of the flapwise bending moment FlapMom in WTP_Subs.f90\RotAnal().
If I understood everything all right you calculate the moment that act at the point where the Blade is attached to the hub? Could you please tell me if thats right? If yes than all my questions would be solved, and it would make totally sense!

Thats an excerpt from the WT_perf file WTP_Subs.f90:

`````` ! Local blade element data.
ThrLoc = QACC_NS*( ClLoc*CosAF + CdLoc*SinAF )
FlpLoc = ThrLoc*( RLoc(ISeg) - HubRad )

! Increment rotor dimensional data for single blade.
Thrust  = Thrust  + ThrLoc
FlapMom = FlapMom + FlpLoc
``````

Thank you. all problems solved 