700N Down

[SRH990]

Hi Lee,
I managed to get a shaft, its sold as the 'Torque tube rear drive gear set' at £8.75. It's all rebuilt and setup now, ready to go. The tail hub seems a tighter fit on this shaft, there's no play at all at the moment so it looks like there's no wear on the Quick UK components and the existing shaft end didn't appear worn either - perhaps the turned section was slightly undersize? I'll keep an eye on it over the first few flights. I did consider packing behind the hub with shims so it couldn't rock but as it's firm now I'll leave it.

The order from Fast Lad was £50.64, ordered Sunday, delivered via Special Delivery this morning. I couldn't resist adding a set of Maniac blades though so spent another £63 - you can't take it with you

Rob,
I was piroing at full stick which is a reasonable lick on the Spartan but if the shaft failed as I hit full rudder then the piro would have been a constant speed as it would have been full torque reaction - how fast that is I don't know (or maybe I do ) I did hit throttle hold fairly quickly after realising I hadn't got control, if nothing else I've learned to crash with dignity over the last few years

[robgt]

Hi Steve,
Glad to hear you're quick on the Hold switch - it has saved me a lot of crash cost in the past!

The Spartan gyro will control the piro rate at up to 2 revolutions per second (720°), but the heli will rotate much faster than that if the tail rotor no longer works.

It could well be that the timing of things happening here was crucial - you stop piroing and notice the heli doesn't so instantly hit hold which would not give the heli enough time to pick up piro speed, I think.

Did the heli continue to piro to the deck or did it straighten itself out before you controlled it in?

If it did stop piroing, that would suggest the tail was working - if it didn't... then the opposite

I'm intrigued by this as it's the first time I've heard of the tail output shaft shearaing, and I'm quite sure you'd like to know for sure what the cause of the crash was (I hate not knowing as there is that uncertainty that it can happen again!)

Cheers,
Rob

[SRH990]

To be honest Rob it was all a blur, piro started & didn't stop, then it started to roll and I knew there was no way I was going to get it level so hit throttle hold. As you say, this would have stopped or a least slowed down the piro but at this point it was a long way away from me.

I read a few forums and I've never heard of this happening before, certainly never heard of a tail shaft shearing so I'm working on the basis that this was an extremely rare failure but I think I will be restricting the first flight or so after the rebuild to upright flight & piros only!

[robgt]

I understand the blur issue
No worries, just trying to figure it out...
Good luck with the rebuild, and give it a thorough check over!

Cheers,
Rob

[SRH990]

While I've been rebuilding I've finished fitting the Quick UK link kit after buying the long blade grip balls and the ones for the flybar cage so I've had a good part of it in pieces. I noticed that the top mainshaft bearing is a little notchy so there's a new set of two on the way from Modelfixings. After looking at my earlier post I've realised the heli actually has around 10 gallons through it, around 80 or so flights and the only noticable wear is a little play in the bearings in the mixer & elevator arms, now also en-route from Modelfixings. Everything else is still tight and it doesn't take much cleaning for it to look and feel like it's still running in.

Despite the crash I'm still really stoked with the 700N, I'm enjoying it so much I haven't flown my 600N for two months now, just the Beam and the Protos for light relief, the only down side is the cost of 10 gallons of 30%

[colin n]

Bloody hell, thought mine was going then.

just been cleaning mine down and remembering ur prob i had a good look around the back end. dunno if u can see it from the pic but i thought the shaft had been scorred by the slider.

wasn't untill i looked at bobj's site that i saw the 700 has a ridge on the end of the shaft. phew...

anyhow maybe that turning process weakend yours some how. by the way i was flying my mates 600 today and the tail hub broke (maintainance). they dont spin slow .

[SRH990]

Here are some clearer pictures of the fracture area.

[colin n]

yes, thats where its gone.

where the diameter changes.

its not too clear from the second pic but it kinda looks like there is a mark from the grub screw but no recess there ?? tell me u tightened the grub screw on the correct side with the recess in. cos that would cause a vibe if it got loose.

[raptorheli2]

QUK tail has 2 grub screws FYI so that marking is correct.

cheers

[SRH990]

Yeas, Quick UK tail hub has two grubscrews. There is noticable wear on the section of the shaft the hub fits onto. The Quick UK hub 'teetered' slightly as if the hole in the hub was oversize, this may have contributed to the failure. Fitting the hub to the new tailshaft there isn't any play at all now so no wear to the hole in the hub, this leads to the possibility that the original shaft may have been turned down slightly undersize.

If the weather actually improves to let me get some flying time in I can see if the hub remains tight on the new shaft.

[SRH990]

Well, I finally got to fly the 700 again this weekend after the rebuild and it's working perfectly so I'm satisfied the shearing tail shaft was the cause of the failure, not a result of the crash. The tail hub is much tighter on the shaft and there isn't any teetering. What I have observed is that the Align tail has a 4mm grub screw which fits tightly into the depression machined in the tailshaft, the Quick UK tail uses two 3mm grub screws which can lead to some play. I'd advise anyone with the Quick UK tail to check that it doesn't teeter on the shaft, it looks likely that this has fatigued the shaft on my 700.

Another interesting thing is that I've been able to increase the gyro gain on both my 600N & 700N sfter fitting the QUick UK tail, blade bolt centre to centre the Quick UK tail is around 5mm shorter, this equates to about 13% less swept area which is pretty close the the increase in gain.