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Startup Shakes

shake startup vibrations

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#1 overtorque

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Posted 31 July 2018 - 20:00

What causes that shake when the rotors begin to turn? After cranking up the engine, the blades will move from a stand still and slowly gain speed, during this moment, in the Robinson, you will feel a definite shake which subsides as the RRPM approaches the higher percents.

 

Here's a video of it very evident in the tail boom:

 

It's common in the Robinson but videos of other helicopters aren't quite so shaky. I would imagine that it's not an imbalance of the blades...



#2 Eric Hunt

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Posted 31 July 2018 - 20:34

Piston engines and rubber bands have this effect, though a B206 also wobbles on startup. Lots of different things happening, aero forces on blades, harmonics of the tail boom, torque effects, coriolis effect, ground effect, sky effect, clumsy hands effect, lack of coffee, flimsycopter tail effect, DILLIGAF effect.

 

Don't sweat it, just go with the wobbles.


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#3 Discap

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Posted 01 August 2018 - 12:10

One thing Eric left out is a phenomenon called "whip".  Any time you rotate a shaft that is supported only on one end, there is an rpm that will cause the shaft to rotate outside of concentricity.  For example stick a welding rod in a drill and start spinning.  It will shortly bend at 90 degrees once the rpm is hit.

 

In my Safari this occurs between 80% and 87% RPM.  You don't dawdle in that range.

 

Bill


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Still Learning


#4 mudkow60

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Posted 02 August 2018 - 08:33

I invite you to experience the wobble of a Huey on start-up (more on shut down).






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