by Tommy H. Thomason

Tuesday, April 23, 2013

A Note on Rotor Blade Droop

Most helicopter rotor blades droop when the rotor is not turning, some more than others depending on the type and design of the rotor hub and the stiffness of the blade. However, as a general rule:

1. Most rotors have a "droop stop" at the hub. This keeps the blade from hitting the fuselage when the rotor first starts turning. That means the blade will extend straight out or at a small downward angle from the hub with little bend for the most inboard portion of the blade.

2. The outboard end of the blade doesn't curve as much as the middle. That is because there is less weight outboard to cause bending as you move outboard on the blade.

3. The most curve is very roughly somewhere between 20% and 50% of rotor diameter. The inboard end doesn't curve as much because of 1 and the outboard end because of 2.

This is an illustration from a discussion of the shear and deflection of cantilever beams (http://www.codecogs.com/reference/engineering/materials/beams/cantilever_beams.php) that illustrates the general rule:
 An example from the Emil Buehler Library at the National Museum of Naval Aviation:
Note that the root of the blade that extends aft angles upward because the rotor mast of the Sikorsky H-3 is tilted forward relative to the fuselage waterline.

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