TREX700N DFC - Tail shear possible issue

[Gareth-71]

Hi All

Not posted in a very long time

Just thought I would let any of you with the new trex700 dfc than there may be a problem with the tail shaft or to be careful about over tightening the grub screws, mine sheared today and the blades, hub etc flew off never to be found.

Apparently there are other people that have had the same issue, so it might be a good idea to check yours (exactly how one goes about 'checking' I don't know)



ATB
Gareth

[waveydavey]

Hey Gareth hope you're well.

[Chris Boughton]

Thanks for tht garath, im hopeing this wont happen to mine!!!! Wont be very happy if it does, as this is ment to be the best 700n version and to hav a problem like tht is bad. Hopefully align will sort out this problem soon and design a better tail shaft

Cheers Chris

[Ross Northcott]

Align cheese strikes again, when will they learn?

[tonystevens]

this has always been a problem with the 700 not just the DFC

[cupracing]

this isnt good, Align will obviously blame the user :-(

[heathy]

TREX700N DFC - Tail shear possible issue

Maybe if they spent less time and effort printing align on everything they could sort these little problems out.

[anjki]

Hi Gareth.

This can happen if the tailshaft has been stressed from a high speed vibe that seems to be affecting a few models.700 owners should Keep an eye out for a hum from the tail and investigate further if heard. At the moment there are a few things that have fixed this vibe for other users ,the first thing to try seems to be lighter tail blades.There is lots of info on the amercan forums which will be worth a look for anyone who may of heard this hum.

cheers
andy k

[anjki]

just in time....looks like the fix is here!

ALIGN New Torque Tube Drive Gear Set Crash test! - YouTube

cheers
andy k

[tonystevens]

Good find Andy

[coolice]

Hey Gareth.

Sorry to see this mate, there have been a few isolated incidents and it seems linked to what AndyK has mentioned regarding a buzz/hum from the tail rotor. This them fatigues the tail shaft prematurely. This tail shaft is not a new part and so it is a problem brought about by the increased tail speed.

To add to Andy's post, what appears to be happening is the tail blades may be going out of track due to the retaining washers within the tail rotor grips not being strong enough and ending up conical shaped where the centrifugal pull on the tail grips is bending the washers.
The fix, although no good to you now , is to fit some thicker steel washers to the tail grip retaining bolts, I used some 600 frame finishing screw caps on mine.

Did you retrieve the tail hub and grips ok? If so would you mind removing the tail grips and letting me know if the washers have cone shaped.

Also what boom where you using? The all aluminium boom appears in some cases to help the buzz/hum, as does a little vertical play in the mainshaft.

How bad is the model damage, or is it luckily only limited to the tail shaft?

Known fixes to date;

1. All aluminium boom, less flex.
2. some play in the main shaft
3. Lighter tail blades, I read Rails were good and this was them confirmed by a club mate who tried a set on his humming DFC.
4. Changing the tail grip washers for thicker ones, the originals are too thin.
5. Run your tail blades looser in the grips. I always get ribbed for how floppy I run mine to allow them to lead/lag easier in flight and deflect should I touch them on the ground. The Osbourns had a conversation with Colin Mill and he suggested to them the same, as this allows the blades to lead/lag in flight. They were also advised to try a special grease on the blade roots/bolts to help them pivot better.

On my machine it was noted it had a slight hum when pushed hard, but no where near as bad as others. A combination of the all ali boom, thicker tail grip washers and my floppy blades and my machine is free from noise. All along the Vbar has not shown there to be any vibrations, but others have had vibes logged and so if you can retrieve vibe logs this may tell us more.
.
.

[ChrisB]

Agreed this issue is very much isolated,my model has carbon boom,stock tail blades,original grip washers etc and it does not or never has had a tail hum.

I will however,due to the heads up,keep an eye on the tail assy condition in more detail as part of my routine checks.

[coolice]

just in time....looks like the fix is here!

ALIGN New Torque Tube Drive Gear Set Crash test! - YouTube

cheers
andy k

Hey Andy.

Ah good one, I hadn't seen the new tail shaft without the machined step yet only the new gears cutting wood

Hopefully the washer problem will fix the cause of the buzz and so remove the cause, the stronger shaft will just be more tolerant to breaking.
.

[anjki]

Looks like a decent fix to me Ian.Looks like we should be able to put this behind us now and concentrate on flying.
Fair play to align for addresing this problem so promptly,They definatly seem to be on the ball now.

cheers
andy k

[miniman]

AS someone who has only recetly purchased their 700, I hope that Align will offer a free upgrade to these parts. Something in my mind says this will not be the case.........

Peter