Out of plane force blade root sign

Wind & Water Rotor Aerodynamics
1

I would like to ask why the out-of-plane shear forces RootFxc1-3 at the blade root are positive? Shouldn’t be negative for NREL 5-MW upwind turbine?

2

Dear Koen,

The blade-root out-of-plane shear force outputs from the ElastoDyn module–e.g. RootFxc1, RootFxc2, and RootFxc3 for blades 1-3–are the reaction loads applied to the hub from the blade at the blade root. Thus, for positive rotor thrust, I would expect positive-valued RootFxc1, RootFxc2, and RootFxc3.

Best regards,

3

Dear Jason,

thanks for the answer. I agree that the thrust force is positive as reaction to the incoming wind, but if we exclude all types of loads apart from aerodynamics, then the normal forces acting on the airfoils cross-sections are negative. The sum of it being RootFxc1 should thus be negative, no?

Best regards,
Koen

4

Dear Koen,

I’m not sure why you say that, " the normal forces acting on the airfoils cross-sections are negative." I would say that the aerodynamic forces normal to the plane (or chord) are typically positive.

Best regards,

5

Dear Jason,

Performing a force decomposition on an airfoil (let’s say only aerodynamic loads) the normal force component points towards the downstream direction. This is the load that the blades feel, which should be negative according to the coordinate system that I see in pg.9 of http://wind.nrel.gov/public/bjonkman/TestPage/FAST.pdf. So, I don’t know why the RootFxc are positive.

Best wishes,
Koen

6

Dear Koen,

Figure 8 on page 9 of the old FAST User’s Guide you linked to is showing a downwind wind turbine, where the wind is moving toward the right. Thus, a positive thrust is along positive x_c.

Best regards,