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Home✈️Aircraft🚁HelicopterGasser Model RC HelicoptersOther › Spectra Tail...again !!
11-13-2005 11:40 AM  14 years ago
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amphip

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Perth, WA, Australia

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Spectra Tail...again !!
Anybody had problems with the Spectra throwing tail blades ??

Today had occurrence no.3 !!

By my eye, the threads in the hub/retainer Nyloc are stripped. Were put on with Locktite. The tail was balanced and up to the point of letting go, there was no visible vibration in the tail area. I am using the replacement closed tail gear box, 105 mm blades and Gy 611 gyro at 35 gain
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11-13-2005 01:05 PM  14 years ago
aileron bail

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What kind of blades?
Was it the same type of blades all 3 times?
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11-13-2005 02:49 PM  14 years ago
amphip

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Perth, WA, Australia

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Tail Blades - Spectra
1st time 105mm Zigsaws
2nd time 105mm SAB
3rd time 105mm Radix

I always carefully balance whole hub/blade assembly.

The heli is very new - less than 2 gals thru motor, so I was only hovering - no sign of vibs or oscillation prior to "let-go"

Just hovering nicely, very stable and easy...then "bang"..down she comes !!!
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11-13-2005 03:17 PM  14 years ago
FCM

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Surrey, England

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By my eye, the threads in the hub/retainer Nyloc are stripped. Were put on with Locktite.
Can't be sure if this is relevent to your problem, but you should not use threadlock togther with nylock nuts as most threadlock fluids destroy nylon. Tamiya threadlock is an exception and will not damage plastics /nylon.

Paul.
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11-13-2005 05:56 PM  14 years ago
thenewguy

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Corvallis, Oregon Where there is liquid sunshine!

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My Spectra threw the tail blade after a summer of use.
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11-13-2005 11:39 PM  14 years ago
TheRickster

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Beaumont Texas

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Finally got almost a gallon of fuel through the new 231 in mine today.. Got it running exceptionally smooth with the through bolted front mount on the motor and the Bracket from the head down to the main plate.. The fuel pretty much just at there with very little agitation.. All was well till it slung the tail blade/Grip assembly.. Broke the yoke right at the base of the threads... Not a pretty sight... Kept the damage down.. Looks like canopy(Custom painted one is now toast!!), boom, tail drive output shaft, split main gear for drive , main blades and the related shafts.....

All in all it could have been worse... Pisser here is what to do to keep it from happening again??? HAs anyone looked into a better yoke ??Is there one? I am not willing to spend a ton of money getting this thing going only to have it do it again... I can live with me driving it into the ground but repitious parts failures by numerous individuals with no known cause is a little hard to swallow.. I do not have the financial where with all to do the R&D for any company...


Rick
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11-14-2005 04:12 AM  14 years ago
TheRickster

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Beaumont Texas

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Just did a quick tally on the obvious stuff damaged.... Looks like around $450.00 so far....

It will be down for a while....

Rick
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11-14-2005 06:20 AM  14 years ago
amphip

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Perth, WA, Australia

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Spectra Tail
Rickster

Fully agree with your sentiments

My 3 episodes all up are about $1000 Aus.

BTW, I am not so sure that the issue is about the yoke failing, I think the hub thread and nyloc compatibility is somewhere in the equation, when this nut lets go, the blade and blade holder comes off and then this would certainly rip the yoke to pieces........it would be impossible for the yoke not to fail at this point in the "disintegration" process.

When the nyloc is fully tightened, the top of the hub threads are JUST at the end of the nut, if the nyloc is 50% metal threads and 50% nylon threads, it means that the blade holder is basically secured by about 3 metal threads ???

I have written to MA with all the detail, hopefully they can shed some light on the issue.
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11-14-2005 06:56 AM  14 years ago
FCM

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Surrey, England

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Is this an M3 size nut?

Paul.
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11-14-2005 11:27 AM  14 years ago
amphip

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Perth, WA, Australia

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Spectra Tail
The nut in question is an M4 Nyloc...
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11-14-2005 11:55 AM  14 years ago
FCM

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Surrey, England

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M4 should be more than big enough to handle the loads but I think you have a good point about the amount of thread contact you are getting with a nylock nut i.e. not much!

I am just thinking how Vario do this and although they use a smaller M3.5 thread, it is a cap headed bolt that screws into the tail rotor hub so you have, without measuring it, probably a good 6mm of thread contact.

With what you have told me, I think the nylock should be replaced with a deeper plain M4 nut installed with threadlock or maybe 2 shallow plain nuts if that's all you have.

Hope this helps,

Paul.
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11-14-2005 12:11 PM  14 years ago
TheRickster

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Beaumont Texas

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On mine the threads broke. The nut was still in the blade grip with the threaded portion still inside the nut. It broke at the base of the threads, using the deep thread as a stress riser. The break was clean, no sign that it was fractured for a period of time then let go. In fact, I had been flying most of the day with MAH 110mm tail blades and because they had uneven lead/lag I switched to a pair of 105mm NHP which are almost half the weight of the MAH blades. If it was a small fracture you would think it would have slung the heavier MAH blade....

This is all very puzzling and troubling... Purhaps it might be wise to look into retrofitting a different yoke, Maybe a Predator or look into Bergen.. I have not heard of any hub issues with either of those Manufacturers and some people swing 115mm on the Predator so the cetrifugal weight pulling on the hub should be greater..

This problem to me is purely a pull apart problem.. If it was a Blade Flap issue it would be breaking at the base of the shoulder at the Hub, instead this as at the retention point...

Rick
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11-14-2005 12:53 PM  14 years ago
FCM

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Surrey, England

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That sounds nasty Rick!

My best guess is that MA are using a grade of steel for the hub that is too hard for the job. The very idea of using a 4mm thread in tension is flawed anyway as this thread form is not suited for high tension loads but you should get away with it due to its size (after all we do with the main blade pitch arm retention) and in your case anyway, it seems like the steel itself is at fault, not the M4 thread.

Does anyone make an upgrade for this?

Paul.
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11-14-2005 01:24 PM  14 years ago
amphip

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Perth, WA, Australia

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Spectra tail
Rick

I was considering using a high tensile plain nut on my next repair job, to get a bit more holding power in there. But it seems that the hub end on your tail actually broke off ??

This would seem to suggest that the use of high tensile nuts would achieve very little if the hub threads / shaft cant handle the load ??

I am also wondering if the sequence of failure could be the plastic yoke failing first, thus leaving that blade to wildly oscillate in the hub, having no control mechanism left...from this point it is not difficult to envisage that the hub failure could be attributable to this.

It is damm near impossible for mere mortals to define these happenings without serious technical knowlege/measurements.

I am still confused about how to go about building a new tail that wont disintegrate within short time !!!
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11-14-2005 07:34 PM  14 years ago
AceBird

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Utica, NY USA

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QUOTE>>>

It is damm near impossible for mere mortals to define these happenings without serious technical knowlege/measurements.

END QUOTE>>>

Amphip,

It is also damn near impossible for one who does have the knowledge, to be able to analyze what someone else has done wrong. All we can do is just guess.

Did you over-torque the nuts for instance?

The nylon in the nut is only there to prevent the nut from unthreading. Only the steel part of the nut is supporting the load from the blades. When you over-torque the nut you can stretch the threads or even strip them. All you need to realize is that the forces are additive, meaning the torque of the nut can preload the feathering shaft a hundred pounds or more, then the force created by the blades spinning adds enough to cause the failure.

Our hobby is inundated with designs that were developed mostly by trial and error methods. In many cases the manufacturer does not know what “factor of safety” the design really has. If you were building a real helicopter nothing would be left to chance. All torque specifications and procedures would be called out and you would have to be trained before you would even be allowed to do the work. NO DEVIATION WOULD BE ALLOWED TO ANY SPECIFICATION. Think about it, no one gives a second thought about changing engines, gears, blades, frame parts, servos etc. that may affect the integrity of these R/C machines. It is considered a recreational hobby and whether you know it or not you as the operator accept all liabilities regardless of your training and knowledge.

What am I trying to say hear? If you are a mere mortal and do not understand what has gone wrong, then do not deviate from what has been supplied in your kit. Just because someone has successfully changed to larger blades or faster gear ratios will not guarantee you will have the same success. There is no way of knowing what you have learned wrong or don’t understand about the mechanics.


Ace
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11-14-2005 09:41 PM  14 years ago
amphip

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Perth, WA, Australia

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Spectra Tail
Ace

Your point about tourquing is noted and accepted. In the Spectra tail, the nyloc is tightened only to the point of light pressure on the shim, so allowing the free movement/rotation of the blade holder without radial slop. To get this nut into an over tightened (stressed) condition would require substantial tightening force...at this level of tourque the blade holders would be locked solid !!!

I do not know of course exactly what tourque was applied here, but it was minimal....this is a 4 mm nut after all and by any objective view should be capable of hanging on. If tourque is critical to the failure potential, I am sure MA would note that in the build manual...instructions call for only enough nut pressure to snug- up the blade holder without binding..a procedure I rigidly followed...on THREE ocasions.

The purpose of these forums is to honestly describe our own interpretation of what happenned and what circumstances lead up to any issues so that others can
a) spot any faulty techniques by their own experience and offer feedback/help
b) Identify common failures that may be emerging evidence of faulty parts/handling.
In this case I have not deviated from MA procedures. I dont want to simply repeat the failure...too much cost, too much downtime, too much angst.
Amphip
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11-14-2005 10:42 PM  14 years ago
TheRickster

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Beaumont Texas

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Amphib,

You echo my sentiments exactly.. I am not denying that this is a recreational hobby and that some times sh^t just happens... I for one am not a metalurgist nor do I have access to X-Ray , Magnifluxing and Rockwell hardness testing equipment . I look at everything from a Mechanical standpoint and use some common sense.

I to thought about the pitch control yoke breaking on mine first thus allowing the blade to flutter and come off.. BUT, I would think that if that is the case the shoulder on the hub(the lateral load bearing surface) would be bent or at least that junction be where it seperated at. Clearly to my simple mind it appears that the stud/nut portion is a retaining fixture to hold the blade grip assembly on from the centrifugal force imposed from spinning, all lateral and flapping loads are supported by the shouldered portion of the hub. It appears that the threaded area near the base to be a stress riser.

To over tighten and fatigue the threaded stud of the hub would require substantial force and as you stated would bind the ballraced bearing and the thrust bearing to the point of no movement..

It is a mystery to me and I spoke with miniature today and they want me to send them the tail group for inspection and testing to try and figure what happened..

we will see....

Rick
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11-14-2005 11:29 PM  14 years ago
amphip

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Perth, WA, Australia

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Rick

Thanks for that !! So far the evidence points to the nut ?? (unstructured opinion - not proven fact )

I have today sent my hub and associated bits to MA for examination also.

When I put the nut on the hub without the blade holder, I can pull it off easily with fingers....something has definitely stripped here !!

David (aka Amphip)
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11-14-2005 11:48 PM  14 years ago
amphip

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Perth, WA, Australia

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Spectra Tail
Ace

you've got me a bit stirred up !! Whilst I agree absolutely that as a model heli hobbyist I accept entirely the consequences of my lack of knowlege, technique, understanding and assembly quality without question, what I will not accept is the machine failures attributable to manufacturing/design etc (not neccessarily proven in the case of the Specra tail in question )

Any manufacturer of any product has a duty of care to put product to the market "that can be assembled and operated by a reasonable person with reasonable knowlege" That is the Law here in any case.

40 years of aeromodelling, including 25 years of helis...beginning with Micro Mold "Lark" should qualify me as having at least the basic competance to put together and fly a modern heli...

3 exactly the same failures in exactly the same place leads me definitely to think something is happening that is suspect...again, not proven, but suspect.

MA will, I know, treat this issue with seriousness and give me an honest opinion of what is going on in their view. If there is any doubts in their mind about product quality they will replace parts and even go to redesign and free upgrades (remember the open gearbox ??)

Amphip
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11-15-2005 01:33 AM  14 years ago
FCM

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Surrey, England

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IMHO that's a strange assembly that MA have there, having to lightly torque the nut to prevent binding. I thought all heli manufacturers had got around this now by using spacers and shims together with precision machined hubs to allow the whole assembly to be properly tightened up trapping the ball bearing's inner race thereby preventing any unwanted movement and also removing the assemblers interpretation of what is the correct torque/clearance required.

I can't help but think this is not the way a tail rotor should be assembled.

Paul.
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Home✈️Aircraft🚁HelicopterGasser Model RC HelicoptersOther › Spectra Tail...again !!
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