A wooden pole, with a white flag attached to it, used to touch the operating rotor blades, which have their tips covered with colored chalk. The marks left on the flag indicate the track of the main rotor. A wooden pole with a rubber wick coated with Prussian blue dye may be used in lieu. In this case, the blade riding the lowest in its track will have a blue mark on it.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved