Hunting down tail vibration on on newly built legacy

[PlaneHazza]

FWIW, I've had my Rush700 legacy running at 2100 with Rail 696 and 116 tails since flight #1. Never had any problems, 50+ flights in.

[crashcopterman]

Originally Posted by Michaelinpr

That guy was one of a a small group that had issues. The Evo cnc tt gears IMHO are garbage. Out of round and yes they do strip sometimes. The regular ones were better to me. They are not 100%, but many and most had success with 115'sat 2100-2200 head speed.

Yup! Fighting that right now. I have 3 or 4 shafts and piles of gears and I have to mix and match to find a set that gives me good mesh. I had one assembly that was so tight I didn't even try to put it back into the frame. It's definitely a battle, but I've learned to just buy several sets of gears to find the magic set.

Steve

[Avitus]

So I'm at the field, rails blades are on, and it does get a little bit better. I can't tell by flight characteristics that it is vibrating (trying at a lower rpm of 1900), but I can still clearly see the vertical stabilizer vibrating laterally. Any pointers? One thing I notice is that there seems to be a bit of radial play on the tail "feathering shaft" (i can wiggle the tail grips as if the bearing diameter was too large), but perhaps this is normal? I could try uploading a video of it would help.

[Michaelinpr]

A little tail play can be fine.

Have you pulled the boom out yet? Check your boom supports too that the plastic ends are not loosened up. I have seen vibes and tail issues many times and it was just a boom support.

[Avitus]

I have not, I was hoping for the quick solution first I guess I'll start tearing it down tomorrow!

[Michaelinpr]

Here is what I would do. During this process try to feel the boom and frame to get a sense of where the vibes may be. Be careful.

1. Remove Main and Tail Blades
2. Spool it up at all head speeds and check for vibes
-If there are no vibes, this doesnt necessarily mean your tail blades are the problem. They can amplify vibes from somewhere else.
3. Remove tail hub, Try again
4. Remove tail case. Try again
Remove TT and bearings while leaving the boom and boom supports on, and try again.

Inspect each item thoroughly as you remove it. Bearings, rubber holders, shafts, TT gears, etc.

Let us know.

[Avitus]

I will try this. My sincerest gratitude for good, practical advice!

[Avitus]

So I ran the heli with no main or tail blades, and I had no vibrations visible to the eye, and could only feel a very low amplitude buzz when touching the helicopter. This buzz was uniformly distributed throughout the boom, tail case and fuselage, and I was really not concered about it. For the hell of it, I tried with tail blades on (no main blades), and still the same result, very low levels of vibrations.

When starting to take the tail apart however, I noticed that what I thought was just a seam in the plastic of the tail output shaft gear (On the "collar" of the gear I suppose), was a pretty bad crack (it is easy to see in the image, which is after I cleaned off the debris "masking" the crack with rubbing alcohol) (which in itself is weird, since I haven't once touched ground with running tail blades). The gear did not appear to be otherwise deformed, (but who knows, at 10k RPM, and with the added load from balancing the main rotor torque?)

I've changed the gear, and while I was at it, reamed the ball link or the tail pitch lever + at the servo side, which were really really tight.

Now just standing by for some better weather to test

[Michaelinpr]

Well I hope that is the issue. Good luck!

Nice catch. Just goes to show how much we need to look over our helis from time to time. Recently on a DT700E I was fighting a weird tail kick for almost a month. I was so confused that I mostly gave up and just flew accordingly to not push the tail. One day I noticed the boom support clamp was cracked in half. It looked solid and I could barely see the crack but the clamp was completely loose.

[Avitus]

Haha yes, coming from planks, it takes some getting used to not flying the heli if it so much as smells bad.

[Avitus]

Just for closure, and perhaps an addition to the knowledge base:

Finally both good weather and time to be on the field. Heli flies great at 2100 rpm now, good times!

[Atomic Skull]

Originally Posted by Avitus

So I'm at the field, rails blades are on, and it does get a little bit better. I can't tell by flight characteristics that it is vibrating (trying at a lower rpm of 1900), but I can still clearly see the vertical stabilizer vibrating laterally. Any pointers? One thing I notice is that there seems to be a bit of radial play on the tail "feathering shaft" (i can wiggle the tail grips as if the bearing diameter was too large), but perhaps this is normal? I could try uploading a video of it would help.

That's normal, almost all heli tails are like that to some degree because there are slight differences in the grips, bearings and hub from one unit to the next that must be accounted for so they have to design a little slop into the hub to ensure that the bearings won't be overloaded when the screw is tightened.

The only way around that problem is how SAB did it with their V2 tail i.e. use a spindle and dampers in the tail rotor so that they can design some preload into the tail.