Disc Lauching Velocity

[Orion.DeYoe]

Originally Posted by Ether

Not to annoy you, but I think this is worth discussing because there are some things to be learned here.

I was trying to help you understand the mystery behind your earlier comment:




If you plot the 3 points for which you actually have data, two things are clear:

1) the graph's 2nd derivative is negative (it's concave down), and

2) the X intercept is not trending toward zero.

These two observations comport well with what would be expected based on the system that's being modeled: it takes some non-zero voltage to move the belt, and the losses are greater at greater speeds.

So to get a good extrapolation you need to take the above two things into account.

Forcing a zero-intercept and fitting a quadratic gives a trendline which clearly violates those two observations, and gives a misleading answer.

If you fit a quadratic to your actual data (without forcing it to go through zero), the quadratic will be concave down (as expected) and will extrapolate nicely to 4400 rpm at 12 volts.

If you run 4400 through your calculations, you'll see it agrees much more closely with your observed frisbee speed.

Your extrapolation would line up perfectly with the properties of the motor. You have to remember that the motor won't move at 0.001 VDC. It takes a little (or a lot of, depending on the load) voltage to get it to start moving. This would account for the non-zero intercept. I would have to agree with the 4,400 RPM number (that's a little above the normal operating speed of a CIM). Everyone seems to be doing all of their calculations on disc exit velocity using the FREE LOAD SPEED of the motor. This is incorrect. The "free load speed" is the speed of the motor's shaft when you hold the motor in your hand and give it full power (12 VDC in this case). The normal operating speed usually ends up being about 80% of that number due to friction and inefficiency of the mechanism. You'll never get 5,300 RPM (free load speed of a CIM) out of a CIM attached to anything.