700 DFC tail issue

[Helos4me]

Guys, I had no hum with stock blades and kde metal links for 10 flights. Most of these were easy test flights and lazy get used to the new machine maneuvers. Part way through the last flight, what I now know is the hum sounded off and shortly thereafter the tail shaft snapped.

So this suggests to me that something else is occurring prior to the hum. After the tail rotor shaft snapped, postmortem inspection revealed cupped washers as one other person posted. This can't be good for two reasons, one the thrust bearings will be rotating on the loaded side, not touching completely all the way around.

Two, that has to throw off the balance of the disk especially if one washer deforms more than the other.

Three now that I think about it, going from left to right rudder has to snap the thrust bearing from one side to the other. Sharp impact on the TR spindle and drive shaft being as small as it is has got to fatigue it. Its possible also that the lighter align tail blades may get into a flutter aggravating all of this. The tail blade flutter would also produce the hum. Just as in a full size fixed wing aircraft a control surface might flutter if not physically and aerodynamically balance. That would seriously damage the structure.

Some of you may have noticed black grease being slung onto the tail blades. This is not a sign of normal bearing operation.

[HarveyR]

Some of you may have noticed black grease being slung onto the tail blades. This is not a sign of normal bearing operation.

Yes I have black grease, but I believe I've had this on most my newly built helies and not worried about it.

Anyway... there was no such issue with the V1 tail, so why not just use that one (my V1 has about 300 flights without any issue at all).

My V3 currently has about 30 flghts and I have noticed a slight difference in sound latelly... I wonder if this is the so called hum people are talking about... Is there anything I should check ?

[Steemo]

Originally Posted by Helos4me

...Some of you may have noticed black grease being slung onto the tail blades. This is not a sign of normal bearing operation.

I don't know of many helis which DONT get at least some TR thrust bearing grease thrown onto the blades from time to time.

At our club we have many MA, Hirobo, JR, Align and TT helis 450-700 size and I know all of the guys running them get some grease thrown from thrust bearings on their tail and main blades from time to time, worse when they have been freshly greased.

[msallua]

Originally Posted by Steemo

I don't know of many helis which DONT get at least some TR thrust bearing grease thrown onto the blades from time to time.

At our club we have many MA, Hirobo, JR, Align and TT helis 450-700 size and I know all of the guys running them get some grease thrown from thrust bearings on their tail and main blades from time to time, worse when they have been freshly greased.

+1, when you don't see grease in the blades it's time to re-grease the thrust bearings.

[HarveyR]

... just read through the full thread... and again, why not just use the V1 tail rotor hub and grips (the one with only one radial bearing)? Or even get the QuickUK tail hub, or use the plastic grips?

Because the rest of the design seems the same (the stepdown on the 6mm shaft has always been there), OK... it spins a little faster due to the new gearing, but could that really be it?

This whole thread is a nuisance.... I was fully happy with my 700DFC, getting worried now (I'll listen carefully for the humming issue).... I have a friend who's gone through 4 tail shafts in 3 months (and consequently a few other parts, but not too many luckily)

[RV_7A]

We all have had grease thrown from our thrust bearings onto the tail blades. I use a clear synthetic type which in the past has always been noticed on the blades on my previous makes of copters. What's different is I've noticed (before my thrust washer replacement) is that the slung grease is now black. That is the sign that something is experiencing some type of undetectable vibration within the grips.

[HarveyR]

Dino Spadaccini, I'm wondering if all this tail issue (humming and snapping) isn't getting psychotic.

As an official Align guy, do you have any idea of the probabilities of failure? How many kits have been sold and how many seem to have failed? I mean, there must be tens of thousands of 700DFC flying out there, and what.... 10 or 20 failures posted on HF and RR? Maybe even 100 failures? What's this, 0.1% chance of failure (or less, like 0.01%)?
As always, the forums give enormous weight to issues...

What's your honest opinion Dino Spadaccini, should we leave the 700DFC on the ground until Align release an update? (I'd prefer waiting a few weeks than breaking, I have other helies to fly on in the meantime).

[Helos4me]

First of all some of you are not paying attention. Grease on the blades by itself is not the issue. Of course the bearings sling grease. Black grease is a result of the metal bearings degrading. The metal oxidizes in the grease and turns to sludge. Then the metal particles continue to grind away at the balls, cage and races. Eventually the sludge inhibits bearing rotation and the ball start to skid rather than roll. Ever see other radial bearings spewing black grease? The ones I've removed were toast.

The tail rotor takes a fairly large chuck of power maybe 20% or more at extremes and a low 2-5% in forward flight. Spinning at 9000 rpm puts pretty heavy load on the two little bolts and shaft that support it all.

I think RV_7A is correct. The only way to get a hum is to vibrate something. What to check? I'm trying to read the signs and figure that out. I'm just about positive that the blades are fluttering, whether its the blades or the blades and grip I'm not sure. A couple engineers at work took one look and said fatigue failure. Just making sure everything is as tight as possible without binding. I'm going to put in a nitro shaft and get more vibe data and see if I can isolate it.

Harvey, you should be concerned if you are flying a DFC. This isn't a little machine that you pick up and fly again after it hits something. Failures can be catastrophic. And it is more than a nuisance, thank you very little.

[ddj74]

Originally Posted by HarveyR

Dino Spadaccini, I'm wondering if all this tail issue (humming and snapping) isn't getting psychotic.

As an official Align guy, do you have any idea of the probabilities of failure? How many kits have been sold and how many seem to have failed? I mean, there must be tens of thousands of 700DFC flying out there, and what.... 10 or 20 failures posted on HF and RR? Maybe even 100 failures? What's this, 0.1% chance of failure (or less, like 0.01%)?
As always, the forums give enormous weight to issues...

What's your honest opinion Dino Spadaccini, should we leave the 700DFC on the ground until Align release an update? (I'd prefer waiting a few weeks than breaking, I have other helies to fly on in the meantime).

Why you need an opinion? As already discussed use different tail blades like Edge or don't fly at all. Some people don't even use forums so many accounts go unreported and chalked up to pilot error/install error. Some don't use stock blades so problem never occurs. As far as how many posted about broken tail shafts here already...it's too much period especially happening all the same way and exact same description. Align is not going to admit the exact number of failures. By the time a fix is released, mass produced and in online stores and hobby shops we will be lucky to be flying Christmas if not later. So it's common sense just be patient and wait and don't fly it.

[K2Freak]

What's your honest opinion Dino Spadaccini, should we leave the 700DFC on the ground until Align release an update? (I'd prefer waiting a few weeks than breaking, I have other helies to fly on in the meantime).

Have you not read Dino's posts in all these threads?

I'll summarize: If you don't hear the "harmonic hummmmmm" then there is no problem and you can fly. If you DO hear it then switch to lighter tail rotor blades and see if it goes away.

[Spinning]

Damn i must say im quite disappointed to see all these problems after spending $2k on a hell im now scared to fly it together with some obvious geometry problems in the DFC head and linkages. I didn't have a hum at first but i've noticed with every flight a hum seems to be growing (i've now had 10-15 flights). I also noticed when I put the tail grips together there was a tiny bit of play which has now turned into a fair bit of play. Im wondering if the increasing amount of play is causing the hum to get worse.

I just pulled the rear apart and double checked everything and I cant see any problems other than the increased play in the blade grip bearings. It seems the 450 Main shaft shims fit perfect on the tail bearings of the 700 so i'm going to buy a set and shim the thing out along with some Edge 105 blades. Fingers crossed!

Dino it would be really helpful if you could tell us what parts are in testing to fix this problem so we can replicate it without having to ground the heli for months. If its just some new tail blades then we can all rest assured that changing the blades should make this thing ok.

Cheers.

Marcel

[Helos4me]

Spinning, check the washer on the screws that hold the grips on. Are they cupped? Deformed? This could be the issue. Also, I would pull the tail drive shaft and rotate it on something flat and see if the end is bent, where the hub mounts on. Don't fly it. I had the hum show on my last flight but didn't realize exactly what it was and the shaft snapped in flight.

[Spinning]

Originally Posted by Helos4me

Spinning, check the washer on the screws that hold the grips on. Are they cupped? Deformed? This could be the issue. Also, I would pull the tail drive shaft and rotate it on something flat and see if the end is bent, where the hub mounts on. Don't fly it. I had the hum show on my last flight but didn't realize exactly what it was and the shaft snapped in flight.

Sorry to hear about ur heli Helos thats gota suck. You were spot on, the washers were cupped. I found some thicker washers, added one shim and now there is no play. Also got some quality blades. Will see how she goes today

I made the desission to use retaining compound on the shaft grub screw so theres no way that's coming off for inspection!

[Steemo]

Some axial play is required for the thrust bearings to work correctly

You don't want radial play though.

I have also found cupped washers in my tail, Found some nice steel washers in my draws I think they were for the screws that secure the spindle on a 450

They are MUCH stronger than the stock tail washers

[Helos4me]

Excellent. Keep your ears open for the tell tale hum.

I put different spacers in my tail, about 0.003" thicker and have replaced the washers also. I need to get some with a little bigger outside diameter though. I have just a tiny bit of axial play and installed a nitro TR shaft. Same same on fit. That's gotta be tough on that shaft banging back and forth.

Keep us posted.

[Helos4me]

One other thing, I mentioned I think in another thread that black grease was getting slung out indicating the bearings were deteriorating. While that is true, it could also be the grease mixing with the carbon dust at the grip and blade bolt. IE the gel coat is wearing off the tail blade root and scuffing the carbon.

Sorry.

[Steemo]

My tail blade bolts had a few shiny bits on them

That would explain the black colour grease on my 500 which has 0 tail problems and nearly no play, would have to be the powder coated tail blade bolts

[satsuma]

So wheres the FIX ?

[Steemo]

Originally Posted by satsuma

So wheres the FIX ?

+1

However Dino did say he'd give us more info "before the end of the week", so I'm not grabbing my pitchfork and lighting the torches yet, at least not until midnight Sunday.

[Spinning]

Steemo, yes axial play is required as i found out haha. Tail went crazy, checked everything looked fine no binding, gyro doing the right thing. Took me ages to figure out that when there was load on the bearing they would bind up and make the tail do silly things. Even though when the tail wasn't spinning it was smooth as silk.

So i do not recommend the shims

Happy to report that once i removed the shims the new washers did the trick and the hum was totally gone.

Isn't there a nice bolt with a flat on the end to go in the tail grip holder? I remember seeing one but im not sure if it was for the main grips or tail.