Need help diagnosing a tail hum

[Chris Wall]

One of my E7se birds has developed a tail hum at 2050 & 2150 RPM with the standard tail ratio.  I assume it has to be a bearing, but anyone run across this so I just don't have to pull all bearings?  I was thinking torque tube first, but am open to ideas...

[Ross Lawton]

Hi cwall64,

I've not come across this personally with the E7SE but I can recommend a few things to check.

Has this hum developed steadily over time or has it developed suddenly?

First thing I would check would be the torque tube guide bearings just in case one of them has come loose and shifted, If there has been any impact or Knicks to the tail rotor assembly then I would check the bearings and thrust races, including the tail output shaft, tail rotor hub.

Just a couple of things to check but I would check the torque tube bearings initially for and loose guide bearings.

Let us know how you go and if you find anything.
Hope this helps and if we can help any further just give us a shout.
Cheers,
Ross

[Chris Sexton]

If the tail has ever touched the dirt I can see one of the tail shaft bearings being damaged. If it's never crashed I would just start at the back and work backwards until you find the notchy bearing.

Good luck

sent from my Verizon Samsung Galaxy Note 2

[Chris Wall]

It has not touched the ground and it has gotten progressively worse over time.  Luckily I have one of Ralph's pre-assembled tail boom/torque tube units ready to go!  Then I'll take the tail apart and check bearings...

[curmudgeon]

I would make sure that the torque tube is 10mm to 11mm shorter than the boom.  If the TT is less than 8mm to 9mm shorter than the boom, the TT may be too tight against the gears.  That may cause an unpleasant humming noise.

[Jean-Luc Bolduc]

First on my list would be tail shaft. Is it 100% straight. could be 10 thou bent, enough to hum.

[Chris Wall]

First on my list would be tail shaft. Is it 100% straight. could be 10 thou bent, enough to hum.

I haven't dug into it yet, but I do not see how the tail shaft could have been bent without some contact.  This just came on in the last few flights, probably after 100-150...  That is why I am thinking just normal maintenance.

[Jon Mills]

I have chased a similar problem down to the tail shaft as well. As far as the hum itself it was the bearing absolutely, but my very slightly bent tail shaft caused those bearing to go bad.

[Jon Mills]

Ralph's plug and play parts are awesome! Thanks Ralph!

[Chris Wall]

I have chased a similar problem down to the tail shaft as well. As far as the hum itself it was the bearing absolutely, but my very slightly bent tail shaft caused those bearing to go bad.

Relatively inexpensive part, so I'll make sure and change it out also.  Thanks for the heads up.

[Chris Wall]

So, for anyone following this thread (or having same issue) Chappa must have been dead on!  The torque tube spun freely and true, so I didn't replace it.  I had a rebuilt tail assembly sitting in a drawer, due to breaking off hex driver in the set screw previously, so I put it on and hum gone!  So, either bent shaft or worn bearings on tail shaft...

Edit - could just be I committed a mortal sin by running Curtis Youngblood blades on a Synergy!!!

[Chris Sexton]

Glad you narrowed it down at least!

Shame on you with your blade choice

sent from my Verizon Samsung Galaxy Note 2

[Kevin Feil]

FYI , I have seen when The TT ends fit tightly in the Mushroom Gear on the inner transmission as well as the Tail Gear Box, what has happened in the past is ( if they are really tight, when you Land the Tail boom assembly into the frame and Tail Gear box. You can actually slightly bend the TT ever so slightly which will cause the Slight /almost un noticeable Vibration. When the bearings start to wear due to this, they will only get worse and really start to hum.

 So at the end of the Day this could happen, check fit before you assemble. I have had to change Many (Carrier Bearings) on "Real Helicopters" due to Bearings Fatigue.. I hook up a (Micro-Vibe) to it and it while pick it up.. *** Also had a TT bent on a MD500E, I couldn't roll the throttle to 60-70 % with out getting a wicked BUZZ in the rudder peddles.. Damn near put your feet asleep...

Congrats on the fix..

Kevin

[Ross Lawton]

Glad you narrowed it down and found the cause. 

Cheers,
Ross