Goblin tail nodding at higher rpm [Problem is back]

[sctholson]

I think I know what's causing it, but I'll ask as I haven't had a chance to change them out.
Since I replaced the head dampeners from the goblin stock to Lynx Ultra the heli nods at higher headspeeds. So at 1600 - 1650 its fine, at 1700 thru 1950 the tail nods. I've looked at the tail, no issues there, definitely had issues with the drive stack and 60t pulley and have since fixed those and that wasn't causing it. I think the main shaft bearings are okay, I had them out only 50 flights or so ago and they were fine at that point.

The stand out seems to be the dampeners. Has anyone had issues like this in the past at these headspeeds?
Thanks

[sctholson]

Update:
I replaced the dampeners with the SAB original pair that I had removed prior to the Lynx and the nodding was better, but not gone. I found a few bad bearings that I also replaced in the main blade grip.

I suspect that the SAB dampeners are old and need to be replaced with new ones, so that's next up to try.

Damn this heli flies well. 97 flights on the VX300TT and it sings.

[Doug Darby]

My experience has shown that when dampners are too hard you get the low head speed wooble that goes away at higher head speeds. Maybe the new dampners were too soft and you got a wooble at higher head speed?

If it was flying well for 97 flights where were you changing them? Or was there an issue?

[sctholson]

This new engine was installed into an airframe with over 100 flights. After the break in I started raising the headspeed and noticed at higher head speed the nodding. I decided to replace the dampeners as a precautionary measure. Then I scorched the piston and went back to all lower headspeed. Only over the last 40 flights since Al urged me to tune the engine at 1800-1850 rpm and not 1650-1700 rpm did I see the nodding come back. And then with the various drive stack issues appeared I thought that might be the cause of the nodding at higher rpm. Now that the drive stack is all fixed up and I still have the nodding am I going back to what I had changed before.

[Doug Darby]

I would install new dampners with the appropriate number of spacers to take all the axial play out of the grips but not enough to induce a notcfhy feel when you rotate the grips.

See if that does not help.

[sctholson]

Well, I received the new o-rings for the SAB dampeners. Left in place the delrin, lubed them up and reinstalled. No axial play and no notchiness.

Still the helicopter tail nods up and down.

I notice a few links that had worn and replaced those... didn't help either.

I'm all ears

[sctholson]

Just to try it out, I reset the Ridged tab in the BD3sx to Acro, in case it was a bad fbl setting that I had made, but that made no difference.

[jc23cl]

Try,

Using Elevator Filter setting, I had this very early on one of the Gassers.

I don't fly Demon any more (not because they weren't awesome, but I changed to Spirit for the Gov).

Good Luck ; )

[sctholson]

Thanks Joey,

I'll put that into the rotation of things to try.

In the meantime it's too dark to test anything else tonight, but I confirmed the main bearings are fine, I replaced the main gear as it was known to be wearing down and only thing I found out of the ordinary is the swash bearing isn't feeling silky smooth. If I had another swash or bearing I would have replaced it.

Tomorrow afternoon I'll have a go at it with the new main gear. And if that fails I'll give the elevator filter a try.

In the meantime, I'll scrounge around for another swash with a good bearing or just the bearing.

[sctholson]

Okay, today's testing.
New main gear - did not improve the situation
turned on Real Time tuning:
Elevator filter as high as 8 and low as 5 made still tail nods
Main gain as high as 68 and as low as 45 made still nods.

Bad set of main blades? I'll try that or any other suggestions tomorrow.

[Doug Darby]

Try experimenting with higher and lower head speeds to see if the nod changes. Might provide more insight to analyse.
Do you have a different FBL unit you could swap in and out to verify its good?

[sctholson]

Well the replacement main blades did the trick. After static and dynamic balancing to the new blades I decided to check the blades I had taken off. They were off both statically and dynamically. Sheesh. I don't drink, but it seems I was in a drunken-like stupor when I did that set.

The replacement blades didn't nod at higher speeds, I flew two tanks through that machine and then decided to finish the test and put on the original set of blades that I "newly" balanced. They too flew fine.

Glad it was easy. On the brighter side, I've done a whole bunch of maintenance and should be good for another 100 flights

[Doug Darby]

Im just curious which blades are you running, and how are you statically and more importantly "dynamically" balancing them?

I have not balanced a blade since I started using CF blades.

[sctholson]

I'm loath to buy blades that are $120 a set, so when I bought my DT700 I just used the DT blades that came with that. Last year they were $75 a set, this year they are up to $83 (ish).

Static: on a flat table - place an old main shaft - on top of that one blade. Roll blade left to right until the spot where the blade balances. Mark the balance point. Do the same to the second blade. Compare the two, by inserting a bolt through through the roots and seeing if the marks light up. If not place a new mark on the blade where it should balance. Place the blade to be balance on the main shaft and add (your choice here, but I use clear) scotch tape toward the outer tip of the blade until it balances at the new mark.

Dynamic balancing, I use a seesaw balancer. For the blade that is too light, I add the same scotch clear tape on the balance point of the light blade until both blades are level on the balancer. (I use a ruler to measure when both are the same height off the table)

I know it's coming in the way of a comment from someone, but to the point of just buying blades that don't need to be balanced. Last year when I first started with the HB gasser and I had four crashes (not due to the HB conversion) I just didn't see the need to throw money away on a set of higher priced blades that I wasn't sure were going to last more than a dozen flights. I do have a new set of SAB blades that came with the new conversion kit that I got from Carey two months ago and I expect I'll put those on as soon as I know the Comp kit won't meet an untimely accidental meeting with mother earth. Still I'll check those blades to see if they need balancing.

[Doug Darby]

Yep just like I did "in the day"....
perfectly good plan to keep the cost down...

[sctholson]

Well not so fast said the wise man!

Tail nodding is back, same conditions, higher head speeds. I made no changes (other than the blade balancing which I thought solved the problem. So, perhaps it's blade tightness, but I don't think so. They are tight enough so they don't fall when I turn the heli sideways, but if I shake the heli while sideways the blades move about 20 degrees.

So, my next test will be to lift the head (blades included) from my second gasser and put it on the one in question and vice versa.

If the problem follows the head it's the head assembly in some way, if the problem remains with the helicopter it's not the head and I'm back to square one.

Doug you suggested swapping out the fbl. I don't have another 3sx, Spirit is very happy on the new machine, no iKons hanging around, but I could throw on a Skookum 720be. And by so doing get a good vib reading too. This idea is growing on me.

[sctholson]

Slight delay in head swapping as the plug on this under one gallon (rebuilt) engine was looking dark grey. No matter what needle setting I tried I couldn't get the engine rich enough. Rebuild the carb and back to having full control with needles and to rich status once again.

Swapped heads over and that made no difference, helicopter still does this tail nod at 1500 rpm and not at 1300. Next step, tonight in fact will be to install the skookum 720be and see if that clears it up.

[sctholson]

Yesterday afternoon I was back at the flying field, while the head swap made no difference the rebuild carb and new spark plug did. I did two flights on the machine and problem is gone.

Does it seem odd that a non performing carb and old spark plug would cause this? I'll get more flights in this afternoon. Since the problem seems solved I did not do the fbl swap.

Just at the very last tank of my first gallon, so the long wait between flights is officially over.

[sctholson]

That darn tail nod... not gone only temporarily tamped down apparently.

I changed over to the Skookum720be fbl last night so I'll test that this morning. For those that are thinking it'll be too many vibs for self leveling, you are probably right and why I didn't bother to set that up. As a fbl, it'll be fine. Will report results in a few hours.

As I removed the wires/plugs from the BD to swap them to the skookum, the throttle servo plug signal wire broke away from the connector. I call that a good find!

[sctholson]

Well had a rough start getting the Skookum started... chose the wrong rx setting. All sorted and I did enough of a test fly today to learn two things:
at about 1600 rpms the skookum did not set off the vib warning, which surprised me and the second I still have the tail nod at 1600 rpm but not at 1400. I was not able to try 1800 but suspect it will nod to the point of I'll be afraid it'll break something.

I'm installing a new tail belt tonight. Will report back tomorrow.