Final Project advice

I'm working on my final project for my Expiremental Methids course. My project is using an op-amp in order to controll a propellor. The propellor is attached to a wooden frame and will go to different heights depending on the voltage going into the op-amp.

I spoke with my professor, and he suggested getting feedback of the propellers height. What is the most convient way to get this sort of feedback? I need something relatively basic (no use of audrino or anything digital)

If you could also tell me a bit on how I could implement a sensor like this, that would be great!