Dune and Demon.

[trillian]

Originally Posted by TheKingPhisher

When you spun up you tails without main blades there is no load on the tail? Did you hold the heli in place and apply rudder to see if there is vibes under load?

Yeah I did but only to a point because it's a bit awkward and scary. I didn't see any vibes at all :-o

[jiminator]

Put 3 flights on my Dune today. The more I fly it the smoother it seems to get. First few flights after it was built there was a very slight tail fin buzz. I kind of wondered if it just took some time for all the parts to bed-in together and that seems to be the case for mine. To a previous poster, the crown gear is adjustable like the Faifa and Srimok. The crown gear in mine was just a tiny bit tight in one spot when rotated, maybe some of these small imperfections lead to a bit of buzz until they're ironed out? Jim

[trillian]

Last night I checked a pair of Edge 95s I had used on the Dune before and they were very close but one just needed a tiny bit of tape and then the balance was perfect.

So today I tried those instead of the CYs but I still get the fin vibrations exactly the same. It only happens when spooling up, as it goes into a certain speed the fin vibrates a bit and then as the blades spin faster it smooths back out again. So I kinda think it's no big deal. My only working theory is that it is getting vibes from the motor and the reason it doesn't do it without any main blades is because only when there's a load is the motor making such solid contact with the main gear and at some resonant frequency the fin buzzes.

I can test this theory by swapping out the motor temporarily but at this point it's not a big worry (I hope). :-)

[Smoggie]

yeah, still getting a vibration on the tail fin here. i've got about 20 flights in and it hasn't really changed. mesh of both ends of the TT is perfect, tail blades are perfectly balanced as far as i can tell. Main blades are well balanced and tracking is spot op, and anyway the frequency is way too high to be main blades.

It doesn't stop the heli flying great, i love it! But it would be nice to be able to stop the vibe, it could conceivably be the motor (Pyro) but i'd expect a quality motor to be pretty well balanced.

[Boris the Blade]

I found pretty dramatic play of the shaft on the ball bearings. Ordered all new parts for the tail assembly. Just for a peace of mind. Hopefully will arrive today. (Miami-NYC-Melbourne-Adelaide). This tail have seen the world...

[Smoggie]

After my last post i got to thinking, so i checked the tail assembly once again on my Du-Bro prop balancer. Sure enough it was very slightly out of balance, I must have missed it first time because I only balanced the blades, not the complete assembly. It took a 20mm piece of tape to get the tail blade assembly absolutely perfectly balanced.

I'll report back on it that was the issue all along.

[trillian]

Originally Posted by Grumpy_Old_Man

After my last post i got to thinking, so i checked the tail assembly once again on my Du-Bro prop balancer. Sure enough it was very slightly out of balance, I must have missed it first time because I only balanced the blades, not the complete assembly. It took a 20mm piece of tape to get the tail blade assembly absolutely perfectly balanced.

I'll report back on it that was the issue all along.

Yeah I suppose mine could still be off a bit if it's due to the grips not being perfect. I could try the random piece of tape experiment next time out, put a little tape on one blade and see if it's better, if it's worse move the tape to the other blade.

[Smoggie]

I've got several flights on mine since precisely balancing the complete tail blade and holder assembly.. The vibration is still there, it appears unchanged. it's really not bad, the fin vibrates a few mm side to side, otherwise she flies very smoothly.

I think I'll just have to put up with it, though i may try swapping to alternative tail blades to see it that makes any difference. The other thing that might make a difference is adding a small weight at the bottom of the fin to shift the natural frequency, but it's really not a problem as it is.

[rchelijc]

"stress" is not good for any heli tail. That said, only some wound announce that stress to you, through vibrations. Vibrations are also a bad thing, and make the original stress worse.

An analogy is, imagine 2 cancers that would kill you unless treated early. One is excruciatingly painful from beginning to end. The other is painless, and only starts to get uncomfortable towards the end. Which would you rather have?

The Kasama tail is like that. Solve the vibes and your tail will last for years back to back flights, same bearings. Other heli tails will look vibe free, but bearings would wear out quicker since even though no visible vibes, the underlying stress of imbalance might actually be present, but there's no way for you to find out. Until you realise you're changing bearings every other week.

[Smoggie]

I'd like to solve the tail vibes but it's hard to know what to do without resorting to randomly changing stuff (which can get expensive very quickly). I've fine adjusted the front and rear torque tube gear mesh, I've painstakingly balanced the complete tail rotor assembly using a top quality balancer. None of this stuff made any appreciable difference.
Vibes appear too high frequency to be related to main rotor. I guess the vibes could be due to motor imbalance but it's a Kontronik so I'd expect it to be well balanced out of the box, I don't have a spare motor to try.

The vibes are quite minor, just a slight 'blurring' of the bottom of the fin when viewed from the rear. If there is a trick I'm missing I'm open to all sensible suggestions.

I'm stabbing in the dark but has anyone had luck with putting extra bearings to support the torque tube, possibly the tube itself is hitting some resonance?

[trillian]

The odd thing about the vibes on mine, and I also think it's a very minor thing, is that with main blades off that vertical fine was totally still, no vibes at all. So if it was the TT I'd expect to have seen the same vibes.

But it's too high a frequency to be the main blades (Radix V2 625). It goes away once a certain rpm is exceeded so it's gone by the time I take off. (It could still be the motor though and this is a 600MX, kinda known to not always be the best balanced cans).

The Faifa has the exact same tail design and I don't see any vibes on the Faifa tail fin. :-o The differences are that I'm running it at lower headspeeds and the tail blades are 105s. (and the Faifa boom is longer and slightly different construction).

[Smoggie]

I do have some experience of vibration and it's causes and how to fix it from my day job.

Personally now I've sat and thought about it i think, most likely, it's simply that the fin has a natural frequency that's close to the running speed of the tail rotor or main or a harmonic. Basically the tail fin is responding like a tuning fork to what could be a very slight excitation. Yes it vibrates much faster that the RPM of the main rotor but that doesnt mean it cant be the main rotor thats causing the vibration, the tail fin could be resonating at a harmonic of the main rotor speed.

I could prove this one way of another by adding a small weight on the bottom of the fin to shift it's natural frequency slightly... i'll do that next time out.

[Sage]

Hi,

To get rid of the tail fin blurring tail rotor assembly needs to be dynamically ballanced.
You've done static ballancing only.

1.Remove main blades & tail rotor push rod and spool up tail rotor.
2.Resize and reposition tape on the blade up and down until fin is smooth to touch.


Just give it a try. It will help.

[Smoggie]

Thanks for the suggestion and I'll bear that in mind, but I'd be suprised if dynamic balancing by trial and error would be any more accurate than whet i got static with the prop balancer. I already got it balanced to an accuracy equal to a couple of mm of tape... and bear in mind that it didnt actually make any difference anyway.

I do have a understanding of the differences between static and dynamic balancing from the day job. Dynamic balancing is really only necassary on rotating equipment that runs on a long shaft where balancing is needed at different point on the length of the shaft.
Providing the tail shaft is straight (which it is) then it's hard to see what attempting dynamic balancing could achieve on a tail rotor. To do true 'multi-plane'/dynamic balancing would require the ability to add weight at the drive end of the shaft, which would on a tail rotor be both pointless and impossible.

Further reading: http://www.dynamicbalancing.co.uk/whatis.html

But theory asside, I'll not rule it out if my other ideas prove fruitless.

[trillian]

Originally Posted by Grumpy_Old_Man

Personally now I've sat and thought about it i think, most likely, it's simply that the fin has a natural frequency that's close to the running speed of the tail rotor or main or a harmonic. Basically the tail fin is responding like a tuning fork to what could be a very slight excitation.

Yeah, it could be just the fact that the tailrotor is blowing air through the fin and will be in pulses as each blade passes. This could also account for mine not showing vibes without the main blades (no tailrotor pitch needed).

My other thought was motor vibes transmitted through the drivetrain but that's probably less likely.

The thing also with mine is that I never noticed any vibes with the stock carbon fin, it was only after fitting a bright Fusuno GRP fin, so perhaps the more rigid stock fin didn't vibrate (either that or it did but I just didn't see it as easily).