ZU-DBV, ZU-RHO & RAF 2000 hub bar bolt torque Threads Merged

[Hub-Bar]

MODERATED

The more I thought about the ZU-RHO accident report I realised that the AR failed to identify the root cause of the accident. Refer to my attached formal objection to the SACAA. Whether the SACAA is going to act on it or not, is not important to me. I notified them and my conscience is clear.

The PROBLEM, however, remains unresolved. By problem I mean the bolted joint in question. First of all, the RAF NAS 501-012 AFC bolt used in the assembly is a custom made bolt produced for RAF exclusively. (I could gather no information to indicate the contrary.) The properties and characteristics of any other AN or NAS bolt used in the aerospace industry is readily available in the public domain and production thereof is subject to rigorous quality and conformance control, but the RAF NAS 501-012 AFC bolt on which I must trust my life, is cloaked in darkness. Rafsa blankly refuses to supply the information so it is also possible that it officially does not exist. There is also the problem with the chamfered washer used under the bolt head in addition to the complete lack of a procedure describing the modification process thereof. PN-39 also does not describe the washer used under the nut. Crucial to the proper operation of this tension joint is the procedure of assembly and quality control thereof which, sadly, is also cloaked in darkness. Do not think that the SACAA is going to cause the problem to disappear, it is not their beef, it is the beef of the owner/operator of the ZU craft.


I rest my case for now.

Rgds
Francois Marais


My Objection :

DATE : 10 APRIL 2017

TO : The Director SACAA and
Mr J Visser : SENIOR AIRCRAFT ACCIDENT AND
INCIDENT INVESTIGATOR SACAA

FROM : HF MARAIS 0272201179 ZU-DTS
ID 5810065081082
PO Box 585
GANSBAAI, 7220
0827712073


FORMAL OBJECTION TO ACCIDENT REPORT CA18/2/3/9462 RE ZU-RHO

Sir,

Herewith my formal objection re the content and findings of the AR above. The withheld and/or misinformation presented to the SACAA skewed the investigation and therefore the SACAA was prevented from reaching a definitive conclusion. Refer Appendix D Para [12]. I have no objection to the CrashLab report. The bolt in question failed simply because a non conforming washer was installed under the bolt head, implying that the proper procedure was not followed and the subsequent quality control failed or was non existent. Negligence was the cause of this accident which very easily could have cost the lives of a father and a son. I will endeavour to explain.

The information as per AR CA18/2/3/9462 Para 1.16.7 : RAFSA Procedure No7 issued April 2007, revised 28 February 2011 (rev 001) which was supplied by RAFSA to the SACAA is incorrect and incomplete. This assembly procedure, including pictures, relates to the use of the AN12-34A bolt, NOT the RAF NAS 501-012 AFC bolt, washers and castellated nut assembly which failed. Please compare the photographs of this paragraph to that of the CrashLab report. (Para 1.16.8.1 is another point in case - apart from the grossly erroneous stress level figures quoted for the bolt.)

RAF Product Notice – 39, dated 15 December 2004, announced the introduction of the RAF NAS 501-012 AFC bolt, washers, new form winglet and castellated nut. I see that an incomplete copy of this PN-39 is attached to the AR as appendix F. I do not know if this was presented to SACAA by RAFSA as published, but I would like to point out that it is also incomplete as it does not show the pictures of the mentioned hardware. For clarity I will refer to the RAF NAS 501-012 AFC assembly currently in use as the ‘new assembly’ and to the superseded AN12-34A assembly as the ‘old assembly’. The ‘new assembly’ incorporates a chamfered and modified washer under the bolt head as well as a washer and castellated locknut with split pin on the other end of the assembly. The modified washer is a specially chamfered hardened, round, flat washer of which two opposite (round) ends are trimmed (cold ground or cold sawed) straight according to the bolt head dimension in order for it to not contact the rounded surface of the hub bar when the bolt is fixed. CrashLab photo no 12 shows these washers and evidently they were totally round and not modified. The ‘old assembly’ did not have a washer under the bolt head and utilised an uncastellated locknut and washer. Due to these fundamental differences, the assembly procedures for these two vastly different configurations can NOT be the same.

As background I present the following information. The kit for my RAF was ordered somewhere in 2004 and completed early 2005. It came with the old style winglets and the ‘old assembly’ hub bar. In the beginning of 2006 another pilot had a roll-over with this gyro and amongst the replacement spares, new rotor blades and hub bar was ordered. What arrived from RAF Canada was the new generation winglet with the ‘new assembly’ hub bar. So this was beginning 2006. In the beginning of 2008 I noticed a very fine crack developing on the side of the winglet closest to the cylindrical steel insert used as bearing surface for the pitch adjustment screw. I ordered a complete new hub bar from Rafsa as Rafsa informed me that RAF Canada refused my claim. What I received from Rafsa was again the new generation assembly. That was beginning 2008. Both hub bars thus was complete with the ‘new assembly’ incorporating the correctly modified washers and this leads me to believe that a procedure for the ‘new assembly’ as well as a specification for the chamfered washer and it’s modification had to exist from early on. Especially since RAF Canada was subject to the American law system.

Now the question is, why would the assembly procedure, RAFSA “Procedure No7 issued April 2007”, describing the ‘old assembly’, be issued in April 2007 if the hardware production and assembly had been announced to change December 2004 and there is evidence that production had indeed changed by beginning 2006? Even more astounding is the fact that this procedure was revised, still wrt the outdated ‘old assembly’, in February 2011! : RAFSA “Procedure No7 issued April 2007, revised 28 February 2011 (rev 001)” as per this AR. Very strange indeed.

Furthermore, according to Appendix C : SACAA and RAFSA Meeting on 21 September 2015 point 7, it is stated “The organisation was unable to provide the design data, not even in the form of a drawing of the washer used with the effected bolt during the visit. They stated that the washer was deemed a standard item.” I find this ignorance very concerning. I can only assume that Rafsa at this point did not inform SACAA that the washer needs to be modified for use in the assembly. This washer on it’s own is also a critical part of the joint and must be of hardened steel with adequate properties in order not to compromise the joint. Inspection of this modified washer showed me that the modification is a procedure performed after it was produced and plated in the factory. This modification process needs to be specified and quality controlled so that the washer does not heat up sufficiently to lose it’s temper during this process. Clearly, absence of an approved procedure is a grave shortcoming. I (and others) have previously indicated to Rafsa that I am not happy with the grossly excessive chamfer to this washer. This arrangement unduly restricts the available area for load transfer between the bolt head and the washer and leads to increased stress and bending moment levels to the head of the bolt. In due course I will endeavour to quantify my argument. This washer is not just ‘a standard item’.

Rafsa publicly (it is even quoted in AR) claims safety factors of 4 to 5 for the bolt in question. This is not true. My initial calculations assuming reasonable bolt parameters and considering ONLY the main forces at play, indicates a SF of 1.8 to 1.9, but this could change either way depending on how the joint is assembled. A split pin can only be inserted at 60 degree turning intervals on the castellated nut which means that you can only achieve specific torque intervals when fixing the joint. These ranges are also going to differ from joint to joint if there is significant dimensional variation in the components of the joint. Temperature is also going to have it’s hand in the pie. Obviously in this arrangement the position of the torque ranges can be altered by the use of shims, but the 8 joints which I had a look at, contained no shims. All these things make this critical joint a complicated one. Experiments that I have done since the AR was released, indicated that on this joint there are only three slots available to turn the nut to after the ‘snug’ position. Either by design, or by coincidence, the hole in the bolt lined up more or less with a slot in the nut at the snug position. Torque on the joint at the first slot will be fatally low, at the second slot is seems to be in the right region and at the third slot it is too high as deformation of the aluminium under the washer starts taking place. I have to mention here that it is a recognised practice in industry to torque a joint by tightening the nut a certain number of degrees from the snug position and this can, under controlled conditions, be more accurate than using a torque wrench which anyway, even if calibrated correctly, yields uncertainty of +/- 25%. I am compelled to come to the conclusion that this is the way that the joint under discussion is being assembled.

I can now only wonder what the person who assembled RHO’s hub bar did. By turning the unmodified washer so that it’s unmodified sides touches the rounded sides of the hub bar, as would be the case as seen in RHO’s hub bar, a gap of about 1,5mm exists between the washer and the hub bar where it must seat. That equates to force turning the nut (and in the process deforming the washer) through at least 6 consecutive slots before the washer seats against the hub bar. It will now only be in the snug position offering no stiffness to the joint. The nut will still need to be turned through two more slots for the joint to be able to work. So what the operator did now is anyone’s guess, but whatever he did from here, one slot or two slots further, the failure mechanisms had already been set up and sooner or later the end result would have been the same – the bolt did not have a chance. From this we learn a number of things about the operator who assembled RHO’s hub-bar. The most important being that he(she) had no technical background, had no training with respect to the procedure he was about to perform and had no previous experience assembling the RAF hub bar. Had he been simply told to snug the washer against the hub bar and then turn two more slots? This also tells a story about the facility. Could this have happened in Canada or did this happen at Uppington? Whatever the case, gross negligence was perpetrated and accountability in this case is clear.

From practice we know the joint works but we do not know how well it works. I must add that it was a great relief for me to realize that RHO’s bolt failed due to negligence and not anything else. For me, or anyone for that matter, to analyse what we are dealing with here, it is necessary to know the specifications for the RAF NAS 501-012 AFC bolt, washers, nut and the assembly procedure. I have on a previous occasion asked Rafsa kindly to provide me with these details for purpose of analysis, but was stonewalled completely, them claiming proprietary confidentiality. This is utter nonsense. Information on any approved fastener product used in the aviation industry is available in the public domain for reason of accountability (amongst others) and Rafsa can be no exception to this rule. Even the more so in this critical application where custom made fastener equipment is used. In the interest of safety and accountability, this information must be included in the AR.

In view of the above, to be fair to the survivors of ZU-RHO, to serve the general aviation community as a whole and in the spirit of truth, transparency and safety, I kindly request you to re-open the investigation and revise AR CA18/2/3/9462.


Thanking You &
Kind Regards



_____________
Francois Marais

[Hub-Bar]

Hi all my fellow RAF owners/pilots.

The following is for clarity on the bolt question. I am at this stage not claiming that the RAF NAS501-12 AFC bolt used in our hub bar assemblies is not good enough, even if it is not a certified item. It may, for all we know, be better than the AN12-34 used previously. That we can only evaluate if and when Rafsa decides to make the bolt specs and joint assembly procedure publicly available. The overly chamfered washer is clearly a bad idea, but how bad it is, I yet have to determine. I have now started the process of analysing the joint and as soon as I have some answers, I will post my findings. Due to lack of said info, I have to base my analysis on the characteristics of the AN12-34 bolt adapted to the physical properties of the RAF NAS bolt which I can physically measure. Frustrating!

Did I stop flying my RAF? NO. Is there still something bothering me at the back of my mind when I fly, even now that I know that RHO’s bolt failure was due to gross negligence? YES – for my own peace of mind, I need to know the margins involved both in the design and the assembly. Without that knowledge you have two choices, you either trust Rafsa unconditionally or you stay away. Your choice.

Hopefully I will in due course have a clearer picture.

Kind Regards
F

[Kolibri]

Howdy all,

MODERATED

I've reposted below two public safety photos, so that RAF owners may easily confirm for themselves if their hub bar contains the correct washer (with flat grinds) vs. those installed by RAFSA on ZU-RHO. (One wonders why such photos are not on the manufacturer's own website.)

I applaud Francois's posts and continuing efforts in the matter of the ZU-RHO crash.

The strength, however, of the NAS RAF bolt vs. the older AN12-34 bolt is almost moot.
The REAL ISSUE is the jointed hub bar/winglet, which is vulnerable to constant lead/lag bending forces.
I am unaware of any other gyro which contains two-part hub bars with huge cutouts and tensile bolts.

Any disassembled RAF "joint" will show evidence of fretting, and the bolt holes will become ovoid in time (as did mine after only 234 hours). Don't believe me? Take apart yours and see for yourself.

Whether the bolt was AN or NAS is almost immaterial.
Bolts are made to resist only two primary forces: tensile or shear.
They are not made to resist vigorous forces of cyclical bending, especially in all directions (horizontal, and coning).

Is there still something bothering me at the back of my mind when I fly, even now that I know that RHO’s bolt failure was due to gross negligence? YES – for my own peace of mind, I need to know the margins involved both in the design and the assembly. Without that knowledge you have two choices, you either trust Rafsa unconditionally or you stay away. Your choice.

I've "stayed away" from RAF's rotor system. So has Peter Goldsmith.
What the rest of you RAF owners fly with is your choice, but you're fortunate to have a choice other than the OEM rotor system.

I've no plans to post further about ZU-RHO. Key people already have my pdfs on the matter.

Regards,
Kolibri

[Kolibri]

Even if we forget about, for the moment, the RAF hinged hub bar and its beleagered tensile bolt, there remains the crucial matter of material fatigue. Aluminum has no clear endurance limit. The one-time tensile yield strength of 6061-T6 is ~40,000 psi. However, aluminum quickly loses strength when subjected to vibrational cycles, and a gyro hub bar sees a lot of action. With 2 cycles/rev, at 350rrpm, that's 700 cycles every minute. Nearly 12 every second.

From Kent's Mechanical Engineer's Handbook, and using 350rrpm:

100K cycles: 31,000 psi 77.5% of original 143 minutes
1M cycles: 22,500 psi 56.3% of original 24 hours
10M cycles: 17,500 psi 43.8% of original 238 hours
100M cycles: 15,000 psi 37.5% of original 2380 hours
500M cycles: 13,500 psi 33.8% of original 11,900 hours

That's right, in your very first 2.4 hours of gyro flight, a hub bar loses nearly a quarter of its original material yield strength. By the time I upgraded my RAF rotor system at 234 hours, I didn't know that I'd been flying OEM with less than half of its original strength.

And, remember, those figures are per one type of vibrational cycle. Combine ALL types of simultaneous stresses (2/rev, droop/coning loads, centrifugal loads, in-plane bending) and yield strength is even more quickly lost.

ZU-RHO's faulty washered bolt endured nearly 2,000,000 lead/lag cycles before popping off the head <50 hours. That NAS bolt certainly tried its mightiest.

As gyro expert Doug Riley explained on RWF:

Bottom line: when designing aluminum parts that see a lot of vibration, you have to use a very conservative estimate of the strength of the material, AND do everything you can in the design to avoid stress raisers. This means big, smooth radii, careful choice of hole locations, and gradual tapers from rigid areas to less rigid ones.

Those of you flying an RAF have two choices in hub bars:

1) You can settle for your 500 hour controversial OEM hinged hub bar (meanwhile wondering how much to trust even that) that your own SACAA itself has recently criticized:

4. SAFETY RECOMMENDATIONS
4.2 It is recommended that the SACAA Certification Department assist RAFSA to correct the designs and manufacturing issues relating to the rotor hub bar bolts that caused it to fail. This intervention is required urgently, as all indications show that over last few years RAFSA has been unable to get it right themselves.
http://www.caa.co.za/Accidents%20and%20 ... s/9462.pdf

2) You can fly a never-once-ever-failed 3500 hour hub bar (massively constructed in one piece, huge radii, no cut-outs, and an integral lead/lag pivot absorbing that stress). 3500 hours? What RAF has had that? Buy your last hub bar.

From practice we know the joint works but we do not know how well it works. I must add that it was a great relief for me to realize that RHO’s bolt failed due to negligence and not anything else.

Francois is attempting to find "peace of mind" in his OEM hub bar.
I can empathize, as I went through the same process.
I imagined the lead/lag bending loads happening 12 times every second to my hinged hub bar.
Basically, I got to it, before it got to me.
I decided to no longer trust a hub bar which the manufacturer itself didn't trust past 500 hours.

There are only two ways to have "peace of mind" about any matter:
Remain ignorant, or become informed and act from there.

I was, like most of you, the first. I'm glad to have nevertheless survived 234 hours in the air.
Today, I am in the second category and I never worry about my rotorhead, hub bar, or rotors.
That is true peace of mind, and I wish such for you all.

Have my posts made some of you RAF owners "nervous"?
Good, because that means you're moving out of ignorance into knowledge.
After knowledge comes wisdom, and from wisdom, action.
Wise action will save your life.
You're on your own. Never forget that.

Good luck,
Kolibri

[Rotor-Blade]

With 2 cycles/rev, at 350rrpm, that's 42,000 cycles every minute. 700 every second.

Please can you explain the high values above, I would have thought @350RPM x 2 cycles/rev = 700 cycles per minute or 11.666 cycles per second.

[spinkaan]

All RAF owners that has to replace their rotors at 500 hours as per recommendations, or for safety and peace of mind can now buy the Sportcopter rotor system from me as the fatory appointed sole agent in South Africa.
Interested PM me.
spinkaan

[Hub-Bar]

Kolibri, Kolibri, Kolibri… You are at it again! Thank you for reposting the washer pics. Thank you for sending me the PDF backups of the deleted threads as well – it is useful for the record. (Very efficient of you, I might say.) If anybody should be interested in it, it is available. Incomplete public records makes for ‘darkness’. MODERATED

Anyway, Kolibri, you are not making me nervous with you last post. The info which you have posted may very well be out of context for all I know. If you were to present to me a detailed analysis of the RAF hub bar, I will study it and only then become nervous or not. That would be a mouthful. Somebody is not telling you the truth… yield for 6061T6 is known to be 37 ksi in t&c and 19ksi for shear.

Just on the bolted tension joint. A shear joint is much less complex than a tension joint but, conceptionally there is absolutely nothing wrong with a WELL DESIGNED and more importantly, a WELL ASSEMBLED tension joint, even in the application of the RAF hub bar. You will be surprised by the number of bolted tension joints we trust our lives to on a daily basis by just doing normal things. ZU-RHO has highlighted the potential threat of this joint and therefore my quest, especially in the light of information denied by Rafsa. (That would hopefully change in due course.)

On the bright side, ZU-RHO has brought a new awareness re bolted joints to me. It is a massive field on it’s own and I would like to encourage anybody who puts a spanner to his aircraft to spend some time reading up the fundamentals involved. By Googling there is more than ample information available that can be understood by anyone. A joint isn’t just a joint, unless you smoke it of course…

Kind Rgds
F

[Kolibri]

All RAF owners that has to replace their rotors at 500 hours as per recommendations, or for safety and peace of mind can now buy the Sportcopter rotor system from me as the fatory appointed sole agent in South Africa.
Interested PM me.
spinkaan

Congratulations, spinkann!
I wish you and your clients many joyous hours under that well-proven and reliable rotor system.

__________
You're welcome, Francois. Glad to help.

If you were to present to me a detailed analysis of the RAF hub bar, I will study it and only then become nervous or not.

Well, nobody to my knowledge has ever seen a detailed analysis of the RAF hub bar,
including, apparently, RAF Canada or RAFSA. The implication is that no such analysis was ever conducted.

Somebody is not telling you the truth… yield for 6061T6 is known to be 37 ksi in t&c and 19ksi for shear.

Those were figures posted by Doug Riley on RWF years ago, so I'd be surprised if they were inaccurate. You're welcome to post competing figures of material strength decline of 6061-T6. I'm sure the relationship is about the same.

You will be surprised by the number of bolted tension joints we trust our lives to on a daily basis by just doing normal things.

Such joints are not standard in the rotorcraft world, though.
Even to a non-engineer, the RAF design is completely counter-intuitive.
The whole reason for it was to allow rotor pitch adjustments, but there are safer designs for that.

On the bright side, ZU-RHO has brought a new awareness re bolted joints to me. It is a massive field on it’s own and I would like to encourage anybody who puts a spanner to his aircraft to spend some time reading up the fundamentals involved.

If the joint were rock-solid, that'd be one thing, but it's not solid.
It's a hinge. The internal faces of the hub bar and winglets move against each other.
Pictures of both ZU-RHO and my own RAF have proven that with the blackened fretting evidence.
Furthermore, the parts move around the big bolt, ovalizing the hole. Great.

Finally, if none of that is enough of an eye-opener, with the castellated nut, the owner cannot verify proper torque.
Pre- and post-flight inspections will confirm only that the big bolts are still there, and nothing more.
How loose the assembly becomes over time cannot be reliably discerned by the owner.

Think of it this way: if RAFSA offered, at no extra cost, your choice of rotor system, which one would you more trust?

Regards,
Kolibri

[Hub-Bar]

Kolibri, come now, you have to admit that the RAF hub bar & winglet combination is a sexy looking piece of machined hardware. Ok, ok, ok – yes, before you can tell me so, the years have also taught me that the bit not revealed by the sexy look, is what is the really important part. (I first wrote ‘juicy part’, but then remembered my age.)

Kolibri, I hear what you say. I also see some of the stuff that you see and an explanation for that must be found. I am however not ready to give up on the RAF hub bar just yet. Key to what I eventually may find, is for Rafsa to make the boltspec AND the assembly procedure that they implement, available to me or rather, publicly. If they do not do so, there is no way on this earth that I, with a clear conscience, can defend the hub bar that I fly. I have already stated that a trustworthy tension joint consist of both a good design AND a good assembly procedure. If either of the two is lacking, a tension joint will eventually fail. So, the ball is now totally in Rafsa’s court.

To my fellow RAF owners I would like to say : Do NOT touch this joint unless you are a tension joint expert, you have fully analysed the joint and you are very, very sure that you know exactly what you are doing. Do not even try to check the torque because you will NOT get a meaningful indication. If none of the components of the joint have ever gone beyond it’s yield point, as should be the case for a well engineered design/assembly, there is no reason for the set clamping force to change. I would hope that with the implementation of the castellated nut, the original designer aimed to achieve a fail-safe assembly procedure. That would have been a very smart move, but that remains to be verified.

Kolibri, thanks again for your contribution in this matter. I really appreciate it - it keeps the issues focussed. I have now said everything that I needed to say.

Fly safe &
Kind Regards
F

[Kolibri]

Rotor-Blade, thanks for pointing out my copy/paste error, which I've corrected in my post:

With 2 cycles/rev, at 350rrpm, that's 700 cycles every minute. Nearly 12 every second.

___________

Kolibri, come now, you have to admit that the RAF hub bar & winglet combination is a sexy looking piece of machined hardware.

LOL, well Grasshopper, actually, the RAF was the first gyro hub bar I'd ever looked at, much less owned.
Now that I've owned and flown something else, there's nothing "sexy" about the RAF rotor system to me.
In fact, the more I ponder it, the more amazed I am at how wrong it is, in every important respect.

Key to what I eventually may find, is for Rafsa to make the boltspec AND the assembly procedure that they implement, available to me or rather, publicly. If they do not do so, there is no way on this earth that I, with a clear conscience, can defend the hub bar that I fly.

The assembly spec is probably nothing more elaborate than torquing the big bolts to 180-200 ft/lbs.
(I've read both figures, 180 and 200, as secondhand reports from RAF Canada. Your test confirms those general values.)

The NAS bolt spec is public knowledge.
What is not public knowledge are measurements of all the forces acting on those two bolts in flight.
Do you really think that RAF Canada or RAFSA has ever conducted any engineering study?
If they had, they'd have boasted about it by now.
When the SACAA asked RAFSA for test data, all Eben could provide was a tensile test.
Well, what about lead/lag forces? Coning and drooping loads?

However, it seems to me that you're still missing the larger issue: its flawed basic design.

This was commented on this forum by an engineer, coline on Wed Oct 19, 2016 4:07 pm, before even the SACAA report was published:

Guys, I don't normally comment on threads like this, but I saw the picture of the failed part and feel I have to.

I am an engineer designing safety critical heat treated fatigue parts that at least 80% of you all use every day.

The pictures on Greg Vos's post scared me on the first look because the design is simply wrong! A bolt is designed to be used in tensile or shear, not in bending. It will fatigue and break when subject to repetitive bending.

Furthermore, the calculations presented are wrong! The initial bolt torque introduces a tensile force into the bolt which has not been taken into account. Secondly the bolt head fixings as used will in effect create a catenary tensile force when the bolt is in bending. No allowance is made for this.

The bolt failed due to fatigue in a bending direction.

My advice is to stay well away from this setup! Please.

Remember, the current (i.e., 2005-on) hub bar/winglet/bolt system is merely a fatter version of the failed 100 hour and 200 hour previous versions. They've simply added weight to make it into a "500 hour" part. One cannot fool the fundamentals. The Canadians who designed the RAF hub bar clearly did not know what they were doing.

Show your RAF hub bar to any helicopter A&P, or mechanical engineer. Show them photos of the surface fretting.
Report back with their opinion.

I have already stated that a trustworthy tension joint consist of both a good design AND a good assembly procedure. If either of the two is lacking, a tension joint will eventually fail. So, the ball is now totally in Rafsa’s court.

As you yourself pointed out, "From practice we know the joint works but we do not know how well it works." We also know that the joint doesn't work. Don't forget, you've already had a current version that developed cracks long before 500 hours, in just 2 years of operation. Perhaps you've already had your "warning"?

If none of the components of the joint have ever gone beyond it’s yield point, as should be the case for a well engineered design/assembly, there is no reason for the set clamping force to change.

Ah, but the "set clamping force" cannot help but decrease. Here's why (and I've posted photos as evidence):
The OEM parts are wearing themselves, thus increasing play and decreasing bolt tension.
The washers (bolt head and nut) are compressing the aluminum faces, which also decreases bolt tension.
Once that bolt tension has been sufficiently reduced, the lead/lag bending forces have much more effect on the bolt head.
In the two cases that we know of, the bolt head was "bottle-opener" popped off, vs. the threads failing.
Meaning, the bolt did not live up to its tensile specs (which are based on the weakest part, the threads).
The bolt failed because it's being used inappropriately (in bending loads), and the tensile specs don't account for that.

So, RAF owners: how much have your hub bar parts been worn and loosened in flight?
What is the current actual torque of those big bolts?
You don't know, because you cannot know from mere post-flight inspection.
You can only visually check the cotter pin.

Will your RAF hub bar system last to 500 hours?
Such is a statistical probability, I will grant, but is that really enough assurance?
Only you all can answer that for yourselves.

For myself, I see no logical choice but to fly a rotor system that has not even once failed.
Something that is designed right, overbuilt, and will outlast the machine itself.
In a gyro, the rotor system should be the most reliable and strongest of systems. Nothing else is as important.

Even your own SACAA does not believe in the RAF hub bar system, and wrote that a redesign is "urgently" needed.
Can't ZU-RHO be the last RAF to throw a rotor blade?

Regards, Kolibri

[John Boucher]

I see there is some condescending "mud" being thrown at the MODS...

Firstly:
This forum has always been a wealth of information and we intend keeping it that way.
However, when posters openly challenge or attempt to make their personal gripes that of the general user of Microlighters, then the MODS do what they are there for.
Most of the time, the forum self regulates and we very rarely have to intervene

Secondly:
The deletion was done by me after Microlighters.co.za & the MODS were courteously requested legally to address the nature of some of the posts.

Finally:
If posters persist with being personal or revert to slander, we'll can this one as well and take action against the individual users. Keep it civil please so that ALL can benefit.

Post Note: SAGPA is most welcome to nominate additional MODS for this subsection.

[Kolibri]

Comment removed by MODERATOR



My below post has no such verbiage, nor none of RAFSA.
I consider the information vital for RAF owners, and request that this thread be allowed to remain, thank you.

______
Francois provided some valuable data when he experimented with different tightenings after hand snug:

1st castlellation 100 Nm 74 ft/lbs
2nd castlellation 240 Nm 177 ft/lbs (RAFSA setting is180 ft/lbs, maybe 200.)
3rd castellation 310 Nm 229 ft/lbs

1Nm = 0.73756215 ft/lbs.
1 ft/lb. = 1.35581795 Nm

(As Francois reported, "This [3rd castlellation] took some real effort and also caused permanent local deformation to the throat of the hole in the alu winglet under the washer to the nut. I think it is fair to say that we can disregard the possibility of anyone torqueing the nut to the 3rd slot.")

(192-208 ft/lbs is max with AN310 full-height nuts)

The NAS501-012 bolt is ¾ UNF with 16 tpi (rolled threads vs. cut). Each castlellation of tightening must stretch the bolt (as well as compress the aluminum hub bar and winglet) a total of .01042" (1" / (6 x 16)).

Ten thousandths. That's about the thickness of 2½ sheets of copy paper.

Below are photos of my RAF hub bar after 234 hours with deeply compressed nut washer and winglet faces.
(One of the steel nut washers had been greatly compressed, forming a 4+mil "race-track" underneath the nut.)
Total compression was at least 10 thousandths.

Ten thousandths. That's about the thickness of 2½ sheets of copy paper.

Assuming my RAF hub bar had been properly torqued by RAF Canada to the second castlellation (and thus 180-200 ft/lbs), after 234 hours of flight the aluminum parts and steel washers had been compressed by a total of at least 10mil.

Effectively, through parts compression, the bolt torque was reduced one castlellation in tightening, i.e., from the factory 2nd castlellation to the 1st. The difference between those two, according to Francois, is 140 Nm or 103 ft/lbs. Instead of flying with about 177 ft/lbs, I was (before replacing it for Sport Copter's system) flying with no more than about 74 ft/lbs, which was only 42% of the factory setting.

My RAF parts had been so compressed, that I was effectively flying on the 1st castlellation of tightening past snug, not the 2nd. And how was I to know that I'd needed to tighten by as much as a castlellation to make up for the parts compression?

Are you all now getting the picture? Are you finally wondering what actual torque is today keeping your RAF hub bar/winglets together? One thing is for certain: it's no longer the factory setting of 180 ft/lbs (~240 Nm), because your parts have been compressed!

And that's the problem with the castlellated nut; it doesn't lend itself to maintaining a consistent 180 ft/lbs with squished parts under such vigorous gyro flight dynamics. A regular Nyloc nut, however, could at least be periodically checked for proper torque.

The bolt head in ZU-RHO popped off in <50 hours, because of being greatly undertorqued due to that oversized washer being squeezed inside the hub bar cut-out. Its actual compression on the entire joint was no doubt far less than my 80 ft/lbs after 234 hours.

Here's something that I strongly urge you RAF hub bar owners to consider: remove the cotter pin from the nut so you can confirm its present torque. Start with a mere 20 ft/lbs, and ratchet it up in 10 ft/lb stages. If you've suffered no parts compression (heh, fat chance!), then the nut won't turn once you reach its presumed factory setting of 180 ft/lbs, right?

HOWEVER, if the nut begins to turn well before 180 ft/lbs, then you've proven that your parts have compressed. This will greatly increase the bolt head to bending/prying forces of the lead/lag winglet.

OK, now what do you do? You've five alternatives:

1) Back off the nut to realign the castlellation with the hole and install a new cotter pin, knowing that the assembly has only ____ ft/lbs of torque. (Do you really want to take the chance that by 500 hours there will still be enough torque to keep the lead/lag bending forces from popping off the bolt head?)

2) Tighten the nut. If you're "lucky" the next castlellation will line up once you reach 180 ft/lbs. (If not, then you'll have to install hardened shim washer(s). And where will you procure those? If your winglet is compressed, you should replace it, as well as any compressed washers. Perhaps with new parts, torque will again be 180 ft/lbs on the second castlellation.)

3) Dispense with the castlellated nut and just use a Nyloc nut. (You could paint it to confirm that it hasn't loosened.)

4) Buy a new hub bar/winglets assembly from RAFSA. (And if you replace it all at 500 hours, you'll have to go through all the ennui again during the next round. )

5) Dump the RAF rotor system for a superior engineered replacement, and thus never again have to worry about this crap again.

Go confirm the torque of those nuts. Like, right now.
Do you want to hope they're still at 180 ft/lbs, or do you want to know?

Regards,
Kolibri

[Learjet]

Re: ZU-DBV, ZU-RHO Threads?
Unread postby John Boucher » Thu May 11, 2017 4:13 pm

I see there is some condescending "mud" being thrown at the MODS...

Firstly:
This forum has always been a wealth of information and we intend keeping it that way.
However, when posters openly challenge or attempt to make their personal gripes that of the general user of Microlighters, then the MODS do what they are there for.
Most of the time, the forum self regulates and we very rarely have to intervene

Secondly:
The deletion was done by me after Microlighters.co.za & the MODS were courteously requested legally to address the nature of some of the posts.

Finally:
If posters persist with being personal or revert to slander, we'll can this one as well and take action against the individual users. Keep it civil please so that ALL can benefit.

Post Note: SAGPA is most welcome to nominate additional MODS for this subsection.John Boucher
MISASA Chairman 2016/17 (... back in the seat!)
jb.brokers@gmail.com
chairman@misasa.org
Cheetah No 1 CMT - the project should get some attention now that I have some free time on my hands!User avatar
John Boucher
The Big Four K
The Big Four K

Posts: 4245
Joined: Mon Jan 29, 2007 5:23 pm
Location: Dana Bay, Western Cape South Africa
Private messageE-mail John BoucherWebsiteMSNM/WLM

Given the overriding safety critical nature of this issue I am disappointed that mods saw fit to close down the discussion for the reasons provided. That vested commercial interests ( a la the "legal request") should take precedence over potential pilot safety is deplorable. John I have always respected your "straight shooting" approach and I am genuinely puzzled by the muzzling going on?

MODERATOR NOTE : Dave, I think you and I have resolved this matter offline

[Horned Adder]

Re: ZU-DBV, ZU-RHO Threads?
Unread postby John Boucher » Thu May 11, 2017 4:13 pm

I see there is some condescending "mud" being thrown at the MODS...

Firstly:
This forum has always been a wealth of information and we intend keeping it that way.
However, when posters openly challenge or attempt to make their personal gripes that of the general user of Microlighters, then the MODS do what they are there for.
Most of the time, the forum self regulates and we very rarely have to intervene

Secondly:
The deletion was done by me after Microlighters.co.za & the MODS were courteously requested legally to address the nature of some of the posts.

Finally:
If posters persist with being personal or revert to slander, we'll can this one as well and take action against the individual users. Keep it civil please so that ALL can benefit.

Post Note: SAGPA is most welcome to nominate additional MODS for this subsection.John Boucher
MISASA Chairman 2016/17 (... back in the seat!)
jb.brokers@gmail.com
chairman@misasa.org
Cheetah No 1 CMT - the project should get some attention now that I have some free time on my hands!User avatar
John Boucher
The Big Four K
The Big Four K

Posts: 4245
Joined: Mon Jan 29, 2007 5:23 pm
Location: Dana Bay, Western Cape South Africa
Private messageE-mail John BoucherWebsiteMSNM/WLM

Given the overriding safety critical nature of this issue I am disappointed that mods saw fit to close down the discussion for the reasons provided. That vested commercial interests ( a la the "legal request") should take precedence over potential pilot safety is deplorable. John I have always respected your "straight shooting" approach and I am genuinely puzzled by the muzzling going on?

Try Avcom - at one stage there where some real cry baby mods that blocked you if you questioned their personal interference and reasoning. I also agree that this kind of info needs to get out there and as long as it is factual it should stay open. The members slinging mud at each other should just be blocked from that thread or their comments deleted...

[gyropilot]

I for one found it very interesting & informative and since it's ALL ABOUT SAFETY & SAVING LIVES the issue needs to be resolved, this is why we have forums! and I am disappointed that the company in question - the same one myself & a mate of mine were going to buy a kit from a few years ago - did not come to the party and defend themselves or offer any solutions or any transparency on the issue - that says a lot!! they shot themselves in the foot!