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TH-67 (Bell 206B3) Limitation Question


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In the Dual Tachometer limits for Rotor Indicator it says "50-60% accelerate through this range." I am wondering the reason behind this. I have noticed that when sitting on the ground at engine idle the rotor RPM is around 55% which is right in the middle of that range.

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In Robinsons we have a similar lower yellow arc to avoid. It has to do with the vibration created at that speed which will damage the tail rotor driveshaft dampner. Perhaps yours is something similar?

 

By the way, what's a TH-67? :huh:

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The range gets typically into sympathetic frequencies (harmonics)with the cabin-tailboom mounting & driveline dynamics if my 40 year old micro brain is not mistaken - not the best range to "idle" or spend a great deal of time in. This is more of a situation that the airframe/TB will see cumulative (vs. immediate) effects & stresses, and one day when somebody else is flying, they get to deal with the consequences (sorta kinda like overspeed/overtorque). Similarly, it's also a good reason to keep the T/R tracked and for those who tend to do A LOT of orbits (LE), makes sense to keep trimmed orbits (I know, some helos need to be flow out of trim for TFO to have optimal views). Robinson had the safety bulletin issued not too long ago regarding this. I'd really like to have others thoughts/perspectives on this too, especially with the 206.

 

-WATCH FOR THE PATTERNS, WATCH FOR THE WIRES-

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Binder1.pdf post-4431-127533086711_thumb.jpgpost-4431-127533085728_thumb.jpg

In the Dual Tachometer limits for Rotor Indicator it says "50-60% accelerate through this range." I am wondering the reason behind this. I have noticed that when sitting on the ground at engine idle the rotor RPM is around 55% which is right in the middle of that range.

 

Arotrhd description is correct. The term that is used is "Sympathetic Resonance"

Edited by iChris
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