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The accuracy for kV depends on how much drag your stick is making, how accurate you measure the applied voltage, and how bad the magnetic losses and bearing friction is on your particular motor. All these (except voltage measurement) depend to some extent on the no-load rpm you are running -----they get worse at higher rpm. So to minimize the effect, use the lowest cell count pack you have to run at the lowest rpm. Also using a disk instead of the stick might minimize the drag, but I am not sure how much that really matters if you keep the rpm's on the low side (e.g. 5-6000 rpm). I also note that the kV depends on how the ESC is setup--timing for example. So you see there are a lot of effects to worry about. Another way to try and take this into account is to also run some props (all this is at WOT) in the rpm range you plan to operate, and then plug in the measured rpm's, currents, and voltages into a motor program, which then tries to extrapolate your measurements to a real no-load kV. Probably this is the most accurate way to go. To be honest, super accurate kV knowledge doesn't matter that much to real world flying. It is basically fine to know at the 10% level --I think. |
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Outrunner....I wrap a file folder label around the bell, make two wide black stripes diametrically opposite....point a decent flashlight (I use a small Maglite) at the bell from the side... align your tach "along the beam"... read off the rpm using the reflected light, as if the tach is reading from a 2-blade prop. (in this photo the prop is on....too lazy to remove the tape for prop testing... obviously for no-load.. the prop was NOT on there!). |
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For inrunners just marking the prop adapter black on one side is often enough. (Multiply the RPM reading by two of course)
Pat MacKenzie |
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Got reflective silver striping trim
Pete |
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