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Question on flywheel design

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Chuck in E. TN:
I have been thinking about size and weight of flywheels for our little engines. I don't have a large selection of material on hand and want to make the best use of what I have.
I understand the basic flywheel effect, but was wondering about the relationship between size, both dia and width, mass and material.
Is a 1/2" thick 3" flywheel, made from aluminum as effective as a 2" dia x 3/8" thick flywheel made from brass? Why or why not?

Chuck in E. TN

kwackers:
you need to do the maths to answer that.

If you understand the flywheel effect then you'll know that it's all to do with the velocity and the mass - 1g at the edge is travelling faster than 1g in the middle and therefore has more inertia.
Your 3" disc has a larger circumference and therefore more inertia for a given mass than your 2". Different thickness and materials mean differing mass and therefore differing inertia.

In simple terms if your two discs weighed the same then the 3" disc would have more of it's mass travelling faster and thus would have more inertia.
Beyond that you need to do some maths to find the average energy in the disc for a given RPM and compare the two.

The relevant equations can be found here:
http://en.wikipedia.org/wiki/Flywheel

kwackers:
Further (whilst waiting for a build):

Volume of a cylinder = PI * R^2 * H
So your Ali = 3.142 * 3 * 3 * 0.5 = 14.14 cubic inches (0.000231 m³)
your brass = 3.142 * 2 * 2 * 3/8 = 4.712 (7.721584e-05 m³)
Mass of your ali disc is about 2700kg/M^3 = 0.624kg
Brass is about 8550kg/M^3 = 0.66kg

Moment of inertia = 1/2 * M*R^2
So for your Ali = 1/2 * 0.624 * 0.0762^2 (converted R to meters) = 0.00181
for the brass = 1/2 * 0.66 * 0.0508^2 = 0.00085

So your ali disc has about double the 'flywheel' effect of your brass disc.

(Assuming my calcs and assumptions are correct - haven't checked...)

jim:
given my understanding of maths, i'm going to stick with my theory that if it looks ok, it'll be alright!! :D

mklotz:
Kwackers,

Although it doesn't affect your conclusion, I believe you've used as your radii the diameters the OP gave.

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