Hub-bars

[Tim]

The latest hub-bar failure (http://www.asra.org.au/smf/index.php?topic=2221.0) will no doubt open up plenty of discussion, and so it should, now proving to be the weakest link in gyroplane technical engineering. This is without doubt the biggest crisis threatening the safety of gyroplanes today. I also believe that this need not be the case.There are many rotors that have done thousands of hours before, and on heavy machines, without a problem so why is this problem so prominent now, manifesting itself so blatantly in the last four years?The tensile strength of the bars is not in question, with the ability to withstand something like ~90,000 lbs, and only subject to ~12,000 lbs of centripetal force. The hub-bars have failed purely through fatigue, which is a “weakening of metal after repeated variations of stress”. One of the most important factors is the number of cycles that the bars are subject to. A rotor turning at 350rpm is subject to 700 cycles per minute, or 42,000 cycles for every hour that it is flown. This is not a problem with 6061 T6 or better grades of aluminium on the condition that the flexing is within the “elastic” limits of the Bar.The second biggest factor is the “variations of stress”, which is the amplitude of the flexing, or the maximum extent of flexing from the point of equilibrium. If this is outside the elastic limit of the bar then fatigue will multiply at an exponential rate. The bars must be exposed to flexing outside their elastic limit for failure to occur prematurely. There are two distinctly different types of failure that have occurred, but both are fatigue related. (1) In the earlier failures, the cracking occurred approx 130 mm from the end of the hub-bar. These cracks started at either the leading edge or the trailing edge of the bar, meaning that the stress was “in plane” or lead-lag of the rotor. As stated before, this happens at 42,000 cycles every hour. This type of failure can also occur if the rotors have incorrect chord-wise balance, and they flex back or forward to find their point of equilibrium to align the blades C of M with the centrifugal force.

[GraemeMonro]

A
..........

[Brian]

It is very interesting that basically, all seemed well on the western front, since the AD"s have come in and probably more pilots are complying to the AD"s than in the earlier days AND hub bar cracking seemed like it was now under controll BUT THIS ONE IS VERY LOWS HOURS so whats happening? Not wanting to put foot in mouth too early, it would seem that an obvious cause could be that the gyro [rotors] were subjected to very high forces of pressure and or perhaps dinged [lightly] Anyway, its a worry that shouldnt be there. My Ricks 27"s had at least 2,500 hours [before the AD] and everything is fine and they were all working hours including a bit of scrub cuttting. There are other blades with way over that figure. My gyro isnt light and allegedly, I am classed into the 500 hour hub bar repacement so I should be a leading contender for faiure .After going to two Nationals and watching others fly I"m wondering if some are sticking too much speed into their rotors too quick. Anyone care to comment??

[Russell]

Sorry mate............don"t know the answer to your query...........but here"s another query....Too often do i see guys spin up their blades, then off they goes bouncin over the ground getting em honking good.The prob is...........their blades are going too slow initially, they think because they is spinning, they can ground roll fairly quickly....................too quick, too soon. I see them blades flexing sumin shocking, as they bounce around etc etcMakes me whince at times.

[birdy]

With all the talk bout rotor management over the last few years, iv not seen the slitest improvement wen i go to flyins. >And i mean rotor management on AND off the ground.I also think theres a few "older" blokes who"v switched from the older, lighter blades to the later heavier ones and havent changed their style to suit the new blades.I know i used to be able to put in sum seriously quick stick inputs with the old Goodwin blades and they would respond as needed with rrpm very quickly, but iv never even tryed [ deliberatly] to command the same rate of cyclic with the AK"s. Its not rocket science, but i also dont think its common knowlage either. Im no brain surgen Tim, but i recon these 3 points have alot to do with it.Rotor management, its everythn.

[Ken Watson]

I pulled the Block off my AK"s to check. In one hub bar whole i noticed 2 scratches. When I checked the bolts they both had a small flat spot extending half an inch from the head about 2mm wide and 8mm long. This had sharp edges on each side and had scratched both holes; one on the smallest cross section. The other thing I noticed is that one bolt is tight to press through the bar and the other slips freely, so that if you let it go it falls out. When you put the block on it binds it all tight.I"m curious to know if any other bolts are the same, especially the flat spot.My blades are 29" with 2" bar and have done 120hrs with no pre-rotator and perfect balance and no mistreatment.My blades are always supported when hangared. Ken

[Mark Horan]

Iv"e said it before,I always like the posts that Tim puts on.Stiff masts and stiff blades mean somewhere somehow something has to give, particularly as increased weights increase forces etc. Good on you Tim.....And check it out, looks like we are rightback onto rotor management....Not a bad thing....And another happy coincidence, there appears to be a post by my mate Ken while Ive been two fingering the keyboard.I guess that the appearance of the bolts from Kens hub bar assy just goes to show the enormous forces at play ay the wiggly bit at the top of the mast. No doubt it increases as the weight of machines increaase and the centrifical forces of longer blades add to the formula.The thing is, not being an engineer, I just dont know. I can only guess!We all know that the rotors will only spin fast enough to keep us in the air, so does this rule counter act some of the forces that we imagine COULD be going on.What I doo know is that I have been in a few machines that have a very pronounced stick shake....I dont like them. In every one of my own machines, I have been able to remove the parkinsons twitch and that is with 3 different blade manufacturers and 5 different sets of blades. My thought is, If the steering wheel in my car is shaking, something is wearing out. In the car, it is usually the tyres...In the Gyro, its usually my life expectency.Anyway, Im sure that Tims post is going to raise some good disscussionMark.

[Brian]

Rotor management [or the lack of] and hub bar discussions are excellent topics and another thing is that a flight is not over till the rotors are stopped. All common sense stuff but seemingly forgotten or perhaps never instilled into the student.Russ bought up the taxying while rotors are not up to speed, its basic and we know from past experience that this very thing has resulted in a fatality.I got to say that I have certainly been guilty of pushing a gyro off the ground early, mind you, was only done under extreme duress but it can be done easily with lots of horses and big blades and it would be interesting to see a video of the blades while this was happening.

[NiqueNaque]

To quote Mr Horan:"My thought is, If the steering wheel in my car is shaking, something is wearing out. In the car, it is usually the tyres....In the Gyro, its usually my life expectency"Not a truer word said, you might consider every cycle on a joystick another minute off your life, potentially..

[Tim]

Couldn"t agree more Nick. If a steering joint in your car is just sitting there not moving while the car is travelling, it will never fatigue. If wheel wobble has it constantly shaking or vibrating, then it is wearing and fatiguing.

[Ken Watson]

I have checked both my hub bars and neither are flat on top. The teeter block is sitting on a slight arc. I plan to remedy this by milling and blending it flat. I think it would be wise for others to check this as I believe this may be the norm. when you check see if it is possible to fit a feeler guage under the block on the diagonals not bolted. Mine would fit a .05mm in for 20mm at rest.In my opinion flush must be best.Ken

[Ken Watson]

I think the curve in the top of the hub bar may be a blessing in disguise. I now plan to lap the block to fit the curvature of the bar. This will make it all stronger and make it near impossible for the block to move on the bar. Surface to surface contact will be improved and also for the block to move it will not only have to put shear on the bolts but also tensile pull. Ken

[NiqueNaque]

Ken,"I now plan to lap the block to fit the curvature of the bar"Out of curiosity Ken, how are you doing this? The old wet and dry on the hub bar and rub the block until it fits?One thing to consider in this is that the hub bar as you see it is unloaded, so will this have an effect on the curvature of the bar? Regardless of which, any attempt at lapping the teeter block to the curvature is a worthwhile thing to do, it is better than having the bar bending over the square edges of the teeter block and suffering undue stressed due to same.Another consideration is to what happens to the bar when it is flexing downwards if you are traversing rough ground - can / does this make the bolts bite into the hub bar any more than it would prior?If you are curving the block and your bolt holes do not change then does this mean the bolts will bite in on the arc of contact closest to the teeter bolt - washers will help to diffuse the load but it still means the outer arc of contact for the bolt head may in fact not be in contact at all - check it with feeler guages.Good to see you"re making an effort with this idea Ken - I"ll look forward to your results.Cheers,Nic.

[Ken Watson]

The arc is across the hub bar not along it. The extrusion is not flat but rolls of towards the side of the bar. The point or bed of contact misses the sides completely and only goes halfway through each hole. My block is 63mm wide and 49.5 long. The contact patch when measured was only 43mm wide but still 49.5 long. See the problem. This fitting relies on a contact patch as well as sheer strength of bolts. The smaller the contact patch the smaller the resistance and so more emphasis placed on the sheer of the bolt.Furthermore a block fitted to that arc in my belief will have a much greater resistance to any movement. The fact the block is suspended over the centre also tends for you to over tighten to try and close up the gap. Thanks for the comment Nick. What are your thoughts now.

[NiqueNaque]

Ken,You sure do know how to throw a curly one... that makes it intensely interesting because it raises again the spectre of the "extreme fibre stress". With the shape of the bar being like a bent rectangle across the width of the bar that means that the bottom corners are even more stressed than I would have thought, and if the teeter block /