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T-REX 450 › Very strange and persistent tail vibration - Solved
11-27-2007 08:52 PM  9 years agoPost 1
ChristianM

rrVeteran

Oslo, Norway

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Let me start by saying that I have done a search and have not found any posts that fits my problem and that I am not a newbie to vibration problems.

It started with a crash where the heli went in on its side in idle up. The usual bits plus the CF tail boom and the bottom frame plate broke. I have the following setup:
  • Trex 450SE V1
  • V2 tail rotor hub/grip/pitch lever
  • V2 main gears
  • Slow down TR gears
  • Head speed: 3000+
Here are the bits that I have changed:
  • All the shafts
  • Main shaft bearings
  • Tail rotor hub/grip/pitch lever (back to V1)
  • TR case bearings
  • TR belt
  • Metal Tail Drive Gear Assembly (HS1216)
  • Pinion
  • Motor
  • Main gear
  • TR blades
The problem is that I still get high frequency vibrations on the tail and this is even when I have removed the entire head AND the tail rotor blades.
The vibration increases with increasing RPM. It is not a resonance issue since the vibration does not change when I hold the TR boom or not. I have tried two different TR shafts with no improvement. I am also doing this test with the TR servo disconnected so the gyro / servo is not influencing this issue. I have also tried both the carbon fiber and the aluminum boom with no difference.

I have noticed that there is some slight radial play on the TR shaft from the bearings in the TR case. The only thing I can think of is to change the TR case but I am reluctant to buy even more parts for this heli that I probably will not need. Anybody have had any similar problems? I am really frustrated with this heli right now.

Christian

Burn fuel, be happy

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11-27-2007 09:16 PM  9 years agoPost 2
TomRex

rrElite Veteran

West Palm Beach Join Date: 12-28-2005

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Have you removed the tail hub/grips and check for vibs? Check the grip screws also, be sure their tight and seated. Side play can be worked out by sanding the tail box spreaders a bit.

Ignorance is the absence of facts.Stupid is lacking the intellectual capacity to comprehend the fact

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11-27-2007 09:35 PM  9 years agoPost 3
copterklutz

rrApprentice

Western Massachusetts

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Check main shaft bearings for notchiness. Pull the head and spin the main shaft by hand while putting a sideload on it. Notchy main shaft bearings will transmit vibes through the whole heli as will out of balance main blades and out of tracking main blades....






" Well, Um.... it's fixable......"

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11-27-2007 09:37 PM  9 years agoPost 4
ChristianM

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Oslo, Norway

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Thanks for the reply. With the TR hub and grips removed then there is no visible vibration on the fins but you can feel some slight vibration. I suspect this is from the gears but I can't be sure. Without the hub there is very little spinning mass to amplify the vibration.

I have replaced the screws that hold the grips to the hub and they were bent. However I have also changed to the old V1 hub/grip/pitch lever which did not have any vibrations issues before I changed them out for the V2 so I doubt that the issue is in these parts.

Christian

Burn fuel, be happy

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11-27-2007 09:38 PM  9 years agoPost 5
gian

rrVeteran

AZ

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Well, if your bearings in the tail case have play, go ahead and loctite them in there. I'd guess that, considering what you've changed out, this may be your issue.

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11-27-2007 09:40 PM  9 years agoPost 6
ChristianM

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Oslo, Norway

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Check main shaft bearings for notchiness.
I forgot to include that in the first post but I did change the main bearings as they were notchy after the crash. I will edit it to reflect this. Good suggestion though.

Christian

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11-27-2007 09:48 PM  9 years agoPost 7
ChristianM

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Oslo, Norway

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Well, if your bearings in the tail case have play, go ahead and loctite them in there. I'd guess that, considering what you've changed out, this may be your issue.
I have added loctite to the outside of the bearing to secure them and I also heated the tail case in order to replace them so they are not moving. I have tried two different bearings on the tail rotor side of the casing without any noticeable difference. I believe the radial play is between the bearing balls and the inner and outer race and perhaps between the shaft and inner diameter of the ball bearing. I will try to add loctite here and see if it makes any difference.

Christian

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11-27-2007 10:04 PM  9 years agoPost 8
ChristianM

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Oslo, Norway

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I just added loctite between the shaft and the bearing. I will let if cure until tomorrow and then try again. I did recheck the radial play and it appears that most of the play comes from the gap between the shaft and the ID of the bearing. It is very minor but it is there. I can move the tip of the shaft maybe 0.1 mm. How much radial play do you guys have here?

I am going to bed now and I will probably have nightmares about this thing now

Christian

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11-27-2007 11:30 PM  9 years agoPost 9
yamahajoe

rrApprentice

ROGERS , AR 72758

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if you have another main shaft i would try it. i have had new shafts that were bent right out of the package. i had a new v2 with this same problem and it had never been flown.

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11-28-2007 08:05 AM  9 years agoPost 10
ChristianM

rrVeteran

Oslo, Norway

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if you have another main shaft i would try it. i have had new shafts that were bent right out of the package
I have tried two different main shafts and they certainly will cause a vibration problem when bent. However I am diagnosing this problem without the rotor head mounted so there is no mass apart from the shaft it self to generate the vibration energy if it was bent. Even if it was slightly bent it would not generate the intense vibration that I am seeing right now.

Right now my theory is that the tail case somehow has been damaged during the crash such that it no longer is stiff enough to support the TR shaft properly. Once the vibration starts it will get stronger with increasing RPM even when I hold the tail boom. With the tail blades mounted it will vibrate to the point where my fingers will get numb when I hold the boom. I tired hovering like this once and it vibrated so bad that it could hardly sustain a hover. This is very frustrating.

Christian

Burn fuel, be happy

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11-28-2007 12:29 PM  9 years agoPost 11
TomRex

rrElite Veteran

West Palm Beach Join Date: 12-28-2005

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Try an RPM higher than 2200.

Ignorance is the absence of facts.Stupid is lacking the intellectual capacity to comprehend the fact

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11-28-2007 01:06 PM  9 years agoPost 12
ChristianM

rrVeteran

Oslo, Norway

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Try an RPM higher than 2200.
I normally have it setup for a head speed over 3000 RPM in Idle-up but now during diagnosing I have installed a larger pinion that allows me to drive the main shaft at over 4000 RPM. The vibration keeps getting worse with increasing RPM so it is not a resonance issue. Keep the suggestions coming though.

Christian

Burn fuel, be happy

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11-28-2007 01:29 PM  9 years agoPost 13
Boidman (RIP)

rrKey Veteran

The Home Stretch

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Balance the tail at operating RPM? Main blades removed, frame held in hand, clear Scotch tape added or removed on tail blades until vibration disappears? Spare and true tail shaft with the hub, grips and blades in the center, balanced on a pair of glass rims? Both methods have proven to work on problem children.

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12-04-2007 02:37 PM  9 years agoPost 14
ChristianM

rrVeteran

Oslo, Norway

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Problem Solved

Well I ordered the SE TR case and it did fix the problem. Somehow the crash has damaged the TR case sch that it no longer can support the TR shaft properly. The shaft will deflect off its axis and cause vibrations. There was no help in trying to balance the blades with tape or anything. It is very strange as I can not observe any damage to the TR case but it just does not work with it. Now I finally can get the heli back in the air.

Christian

Burn fuel, be happy

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T-REX 450 › Very strange and persistent tail vibration - Solved
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