hi, i was trying to animate and render a rotating helicopter blade, but when i applied a image based motion blur on the blades, for my surprise they looked like a linear animation, like each blade is moving to the side, in a square movement, like that attatched file

obviously those are not my blades, im only showing the effect
the only way i managed to get they to be blurred correctly was rotating passes, but to make it actually blur, in these blades’ speed, i need at least 80 samples, and no way in hell i can afford spending 80 times the render time!

is this motion blur supposed to work like that or im doing something wrong?
thanks

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is this motion blur supposed to work like that or im doing something wrong?

Yes, image motion blur does work like this. Image motion blur is really just a cheap fast cheat for motion blur. It is fast, but at the expense of quality and accuracy.
Each pixel has a motion vector, it knows how fast it’s moving, and in what direction. The direction vector is a straight line. IMB simply blurs the image after rendering one pass according to that speed and direction.
To get the proper effect, you will have to go with the slow but accurate multi pass method.
The differences and limitations of the MB methods are documented.