# Wind turbine blades and bearing calculation help

Isimanica
Ok I have a design project that I am working on for the University this semester. Having a bit of problems trying to figure out and issue revolving around the wind force on our savonius blades. We are trying to make sure our blades will rotate with a wind speed of 10 mph. We have the blade total weight of approximately 75 lbs and are 6 feet tall by 4 feet wide. The bearings that the blades will be sitting and rotating on are tapered roller bearings, a set of two. I have read some where that my coefficient of friction for these bearings is around 0.0018 but to cover our selves I would like to use 0.05

Any formulas or directions on how to calculate this not just at 10 but other wind speeds would be wonderful.

You could just use Newton's method to calculate the lift generated by the profile of your blades. This method just integrates the pressure coefficient over the surface of the profile. That will give you the driving force and therefore the speed that they will rotate at. I would then compare this to CFD simulations. Come to think of it, this would make quite a nice Uni project. Working backwards from a required speed might be tricky though.

As a way of cheating, you could look up to see if a TSR (tip speed ratio) exists for your blade profile and that will give you the speed at which it will rotate at a particular wind speed. The other method would be impressive from a uni perspective though.

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Isimanica
savonius blades have a estimated TSR of one since it is a drag device and not a lift device. We can calculate the area of the blade that will have the wind pushing against. What I need to determine is if the blades will rotate on our bearings based upon the wind speed that is hitting it.

Homework Helper
I wouldn't worry too much about friction at the bearings. That will be negligible compared with the torque needed to accelerate the turbne and generator reasonably quickly from rest up to a practical RPM.

To give you an idea how low bearing friction is, you can easily turn the low speed rotor of a big jet engine (mass of rotor = about 2000lb) by hand with one finger. In fact for some test procedures you need to keep the rotor turning slowly (at 1 or 2 RPM) and one way to do that is to use an ordinary office fan to blow air onto the engine fan blades.