jeffrizza:
Ah ok yes, second spindle. Good idea. And then have an optical sensor measuring reflectance off the end of it or something like that? What do you think the best way to alter the reflectance is? Black dots around perimeter of white spindle or something? Maybe holes around the perimeter of the spindle?
All are good options. I would prefer holes & IR sensor to minimise influence from ambient light. But it very much depends on what you can build in to that drone. Also whether you want to measure complete rotations only (magnet + reed contact work great) or very precise parts of rotation.
The payload will be about 1 kg, so the motor would need to put out a decent amount of torque, but it doesn't need to rotate at a high speed. In fact, I'd rather it rotate fairly slowly.
Electric motors normally run at high RPM, so you're going to need a gear box anyway. Get one that produces the speed you need. Torque increases as RPM lowers.