# Edgewise blade root bending moment definition RootMxb1

Dear Jason,

I am curious why the RootFyb1 is a near harmonic signal with zero mean since the xb1 is rotating with the blade? This then causes the near harmonic RootMxb1. If this is caused by gravity, it may not be zero mean? I must miss some important information. It is appreciated if you have any comments. Thank you!

Xianping

Dear Xianping,

Best regards,

Dear Jason,

On one hand, I thought the harmonic single is nonzero mean but the result is almost zero mean. Now, I know that the gravity is pretty higher than the aerodynamic load so that should not be a problem again.

On the other hand, if the blade gravity is the main source of the RootFyb1, the RootFyb1 should be non-harmonic if the blade pitch angle is 90 degree since the blade coordinate is rotation with the blade pitch? I did try to verify this, this is not the case. In another word, We can assume the in-plane and out-of-plane aerodynamic forces of Blade 1 are Fb1_aero-in and Fb1_aero-out, so if or not the following equation is correct?

RootFyb1=Fb1_aero-incos(BldPitch1)+Fb1_aero-outsin(BldPitch1)+gravity*cos(BldPitch1)

Xianping

Dear Xianping,

The blade coordinate system in ElastoDyn is rotated from the coned coordinate system (where the out of plane and in plane loads are defined) by the blade-pitch angle.

For a rigid turbine spinning at a fixed speed, I agree with your equation assuming that aerodynamic and gravity forces are azimuthally dependent. However, blade deflection and inertia will impact the loads for a nonrigid turbine.

Best regards,

Dear Jason,

Understood. Yes, the flexibility also impacts the response. I did check the simulation setup again and verified the above equation roughly. Thank you!

Xianping Du