Eli5: How is the speed of a rotating object defined (e.g. a windmill) if you don‘t use rpm. Where do you measure the speed in mph?

Question

I had a look at a huge windmill and was wondering how fast the blades are spinning. Obviously, the distance the blades make is different on the outside than on the inside. Is the speed of a rotating object always measured in rpm and not in mph?

In: 3

Answer 1

As you pointed out the outside is traveling faster than the inside. So there is no one velocity (speed) answer like mph, here let’s use Meters per Second (m/s).

Looking at a large windmill (turbine) is an interesting case because the velocity of the blade, and specifically through or relative to the air is important, primarily because the force on the blade, being used to convert the wind to energy, will vary along the length of the blade.

So if you look at one of these enormous blades its shape changes along its length.

So the engineers that design the blades need to do complex modeling (in CAD) on the blade design.

Another issue is the tips, which are critical to managing the drag of the entire system, are traveling the fastest from a velocity perspective, in fact quite fast. They suffer from erosion and pitting, which impacts the required maintenance and inspections needed.