Poor man's dial indicator

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This is a technique for testing the straightness of main masts and tail shafts, particularly applicable to electric helicopters.

Remove all the rotor mechanics (for safety, both main and tail rotors), leaving the bare shafts. Ensure nothing can become wrapped around either shaft while working---tie back hair, remove necklaces, and so on. Take a permanent marker and rest your hand on the frame or on the table such that you can precisely control the position on the pen relative to the shaft.

Use the transmitter to start the shaft turning at a moderate speed, and slowly bring the marker towards the shaft until contacts (you'll see the marker draw on the shaft); almost immediately pull the marker away from the shaft. Stop the shaft spinning with the transmitter and examine the line drawn on the shaft. Any eccentricity will be indicated by breaks in or changes in thickness of the circle drawn around the shaft.

Internal combustion engines in general create too much vibration for this method to work well, and should not be run without any load anyway.

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