Chasing vibes after a repair

[Mateyhv]

Hi guys, my Trex crashed last week after a tail failure due to a long flying career, fortunately in a low level hover over a soft grassy ground and TH on. Damage didn't look severe, mainly in the tail section, although there is always more than apparent at first glance.

Anyway I rebult the tail with a new boom, Helioption blingy belt tail section and tail grips (pricy but very good quality indeed), Tarot metal slider and new tail shaft. Main body has no damage except a broken grip holder link. Main shaft is ok, bearings are smooth and so on.

The no grip trials were quite smooth at full speed with only minimal vibes. It was a big surprise when I added a new set of tail blades the vibes increased to an unflyable level. Added another set with the same result. Checked the blades for weight and CG, everything is perfect.

I have got an indictaion from the thin vertica stabilizer that vibes are way stronger in the horizontal plane (side to side) as far as I know that means a problem in the main shaft section, however I can't understand why there is a huge difference when the tail blades are off and on...

Any suggestions are welcome as I foresee no fun in spending long hours testing and replacing bits. Another approach might be rebuilding everything from scratch

[Abenn]

Is there something wrong in the tail geometry that's in effect giving you washout in the tail rotor?

[Mateyhv]

Yes, the boom has its front slot slightly offset so the tail shaft is tilted a few degrees downwards from horizontal, maybe <5º.

[Gladius]

Have you tried it with grips, but no blades? Do you have some other grips that you could throw on to see if they might be the problem?

[Abenn]

Originally Posted by Mateyhv

Yes, the boom has its front slot slightly offset so the tail shaft is tilted a few degrees downwards from horizontal, maybe <5º.

That wouldn't cause the problem by itself -- mine's been like that for a long while and is as smooth as butter. I did have vibration problems with the original plastic bits on, but upgraded all the tail to metal, with carbon blades, and the vibration disappeared.

Anyway, in your case I was wondering about the blades maybe not having exactly the same pitch, just like when the tracking is off on the main blades, due to some inaccuracy somewhere.

[Mateyhv]

Thanks guys for the quick response!

Originally Posted by Gladius

Have you tried it with grips, but no blades? Do you have some other grips that you could throw on to see if they might be the problem?

Yes I dids so, first checked Align plastic blades resulting in strong vibrations then no blades, OK with mild tipical vibes, KBDD new style tail blades, again strong vibes. Checked CoG and weight on both sets and they are perfect.

Originally Posted by Abenn

That wouldn't cause the problem by itself -- mine's been like that for a long while and is as smooth as butter. I did have vibration problems with the original plastic bits on, but upgraded all the tail to metal, with carbon blades, and the vibration disappeared.

Anyway, in your case I was wondering about the blades maybe not having exactly the same pitch, just like when the tracking is off on the main blades, due to some inaccuracy somewhere.

That is a very good point Abenn! I will surely check the tail pitch accuracy. I put a Tarot tail slider that might not be up the task, honestly the Helioption one is nice but hugely overpriced and since the plastic has worked great for the past 5 years I saw no reason for such an expense. By the way the slider gets a bit too hot, that means a lot of friction, does that happen in your case?

I will still check the main shaft section as I was dealing with the omnipresent wobling Align main gear and will share some conclusions shortly!

[Gladius]

If the slider is getting hot, then that's definitely friction. That really shouldn't happen, and you might want to take a real close look at that.

[Abenn]

Originally Posted by Mateyhv

. . . That is a very good point Abenn! I will surely check the tail pitch accuracy. I put a Tarot tail slider that might not be up the task, honestly the Helioption one is nice but hugely overpriced and since the plastic has worked great for the past 5 years I saw no reason for such an expense. By the way the slider gets a bit too hot, that means a lot of friction, does that happen in your case? . . .

Never even thought to check the temperature of mine.

My plastic Align setup worked fine for me for years too, but vibration became apparent when I installed an Align Hughes 500 fuselage (no other changes at that time) and its vertical stabiliser vibrated so much that the horizontal piece at the top flew off It was Align all-metal parts that solved that.

I've never experienced a main gear problem. If it's slightly eccentric it would cause a lateral vibration.

[Mateyhv]

Long story short, after many trials and changes I ended with a full plastic tail, replaced the thin glass fiber vertical stabilizer with the stock CF one and also both main an auto gears, just in case. The spindle shaft was very slightly bent so it was replaced too (that was not the cause of the strong vibes for sure).

Right now the vibrations has diminished to the point where the heli wont explode after a couple of seconds at full speed but are still pretty hard, the tail pushrod and support struts still get blurry I can't believe that as everything is smooth as butter, even the main rotor spins freely longer than in any previous setup. I have tested several speeds and selected a couple where things look smoother but not really happy at all.

I was so tired in the end that I just went to test it outdoors, no matter what was going to be the outcome. Well the spool up was nice and soft without any bouncing or so. But once airborne there was that wide slow side to side wag at about 1Hz.

So at this point there are two solution, pack everything into the rubbish container and forget about problems or replace the only part that has not been touched so far, the main shaft.

[acidla]

Matey,

You can check the gears of the plastic tail case housing by moving with your fingers both gears horizontal and vertical and see if they move.

Sometimes the bearing housings of the plastic case elongate thus creating movement of the tail gears.

[Relisys190]

not to be off topic, but which helioption tail box are you talking about? Sounds interesting.


Thank you.

[Dowquest]

From what I've read in your thread so far, you've rebuilt the tail section and replaced the spindle shaft in the head. And you've run it bladeless, ( mains and tail), and tail gripless. At which point the vibes were diminished. I would expect them to lessen, as you removed the blades if you had something going on anyway because the blades would amplify any problem you have. Continue removing components until it is absolute vibe free. I didn't catch if it was a belt or shaft drive, but do the obvious regarding the dif drives with the dissassembly, if you need to continue down to just the pod,, maybe you have a slight bend in your main shaft that is resonating in the tail boom. Regarding the side to side wag, check your tail control linkages. Sounds like something is too tight.

[dmacy]

I have never crashed that I didn't bend the main shaft or stripped a main gear. It is easy to check the main shaft just take the head off and spin it up real slow you will see it if it is bent.

[Mateyhv]

Just to close this old thread. The culprit was the only thing I was sure was not wrong: the main shaft. Once I change it the heli went smooth even though I couldn't tell by any means the shaft was bent. Thanks eveyone for the replies two years ago