HUB BARS ON GYRO'S FOOD FOR THOUGHT ONLY

[NiqueNaque]

Brian,All metals work harden if they are deformed, some including aluminium work harden just with flexing alone.If you want to design a hub bar that won't ever break, you can't theoretically, because with even the most subtle of deformations, even elastic ones, it will work harden and fail. All you can design for is one that will last for as long as the machine.If you think about it, a hub bar does a lot of flexing, even the stiff ones, because there is a variation in the lift on the blades as they go around in their cycles, at least one per rev per blade, never mind rough air. at 330 RPM, that means:330rpm x 60minutes x 2000hours or cycles at 2000 hrs life or 39,600,000.00 times it gets flexed up and down, plus you have to add shock factors and the like when you are scrub cutting. 39 million times!I think the problem is that everyone thinks that aluminium is steel, which can be engineered for stresses below 50 mPa to last forever, aluminium cannot, repeat, cannot, it will always fatigue at some point in time. It is not an issue to be complacent about, we all have a time bomb ticking over our heads. I don't know enough about the fatigue properties of aluminium to commment on the life of a hub bar nor do I know what the stresses may be, but I would suggest that a thorough check every 10M cycles ( 500 hrs ) would be a damn good idea.As to the wedges, I checked and found that putting wedges in actually magnifies the extreme fibre stress at the base of the hub bar under the teeter block. A hub bar that has been deformed deliberately becomes stronger (work hardening) and the bending moment is reduced, it is therefore less likely to fail. You could find the hub bare, blade roots or the root mounting points failing in time with Rick's rotors or any other for that fact.Just get used to it, you will have to keep replacing high cyclic items such as masts, rotors and engine mount points for as long as you have a gyro and fly it, that is unavoidable.Hope this helps,Nick.I recently had Jack Allen make a new hub bar for me at $550, the old one to my knowledge only did 300 hrs tops, I'll keep it for a reserve, but at $550.00, my life is worth the expense.Cheers,Nick.

[Brian]

Thanks Nick for the info, much appreciated. I'm not unhappy ASRA has came out and put a time on hub bars, just inquisitive why some things break and some dont or last longer [seemingly]I'm also making the point that even a good looking well, designed gyro like the Magni might well break in say 1000 hours at some point because no one at this point has clocked up a couple of thousand hours in one. Also, we have made out gyros a lot heavier and have unwittingly, compounded a problem with the sleeping dog !!Just to relieve your concerns, my rotors are on their way up to Rob to have a couple of hub bars made so I'm not taking any chances even though my hub bar is a 27mm design which at this time has not been subject to cracking. Interesting too is that Rob is looking at making hub bars out of steel.Brian

[asra]

>"All metals work harden if they are deformed, some including aluminium work harden just with flexing alone.">"I think the problem is that everyone thinks that aluminium is steel, which can be engineered for stresses below 50 mPa to last forever, aluminium cannot, repeat, cannot, it will always fatigue at some point in time. "Good stuff!I'd better fire up the old forge and head to the tip for some heavy duty leaf springs!

[birdy]

If you think about it, a hub bar does a lot of flexing, even the stiff ones, because there is a variation in the lift on the blades as they go around in their cycles, at least one per rev per blade,Thats only if they aren't tuned Nik.A properly tuned and matched set of blades WON'T flex from changes in lift, coz there isn't any.[from the blades perspective] Granted, its not easy, but if your head/stick/machine has absolutly no feed back from the rotors, then the only flexing that'll be go'n on is up, when you take off and down when you land, thats two flexes per FLIGHT.Its the changes in the airflow direction; when the blades are fore n aft, then left n right that can't be avoided.[for n aft, the blades BOTH have less lift than when they are left n right.] And if thats the only stress on your rotor systm, then you'll die of old age before the bar fails. []Ignorance is bliss............but only till you realise you were.You can always get the answer you want, if you ask enough experts.

[Brian]

Well, if youre intersted Birdy, there's a job for you, CHIEF ROTOR TUNER . I'm sure there isnt too many perfectly tuned blades flying !!! My Patroneys are real smooth but they could do with a tune up !!Brian

[birdy]

Not me mate, I only fly um.Ignorance is bliss............but only till you realise you were.You can always get the answer you want, if you ask enough experts.

[Michael]

Brian - Re: The Magni hub bar and rotor design. The two components have been designed to compliment each other. Magni won't supply their rotors or hub bars to anyone other than a Magni owner.The most used M16 Trainer in South Africa has done 3,000 hours. The factory has NO time limit on the hub bar & blades provided no damage has occurred, ground strikes etc. The factory is very conservative and super safety conscious.In South Africa Magni Hub bars are given a provisional life of 3,000 hours, aluminium hub bars 500 hours. Doesn't that say something in favour of crome moly? The Magni hub bar system is 4130 Chrome moly, 1/4" thick vertical plates.All Magni hub bars are the same. Only the rotor blade length varies between models.

[Brian]

Thanks Michael,There's no question that chrome moly is lots stronger than alloy and 3000 hours provisional life is excellent going.I'm pleased that their design is hour proven re the South African trainer[3,000 hours] There are a lot of things that arent time proven and their pilots are basically test pilots, not good !!I would like to see some debate about the possibility of using steel instead of alloy for the hub bar, eg, weight, strength, is it practical? what are the drawbacks ? etc etc.The sportscopter heavy duty hub bar looks good to me.Brian

[Michael]

Correction - it is the Magni factory trainer that has done 3,000 hours, same rotors, same hub bar.Can't comment on your other questions as I don't have the knowledge. It does seem many members favour the old aluminium technology because its what they are familiar with. Now that we have had a big fright with 1 fatality and a near miss perhaps its time to take a good hard look at chrome moly. Especially when Magni have proved its worth with over 400 machines flying worldwide.

[NiqueNaque]

Michael,That is a point worthy of consideration.NO doubt Magni are very careful and very professional about the factory trainer, what would be interesting to see is the aggregate time they have in the air and status of all Magni rotor systems manufactured and the utility of the gyro attached to each rotor system.Though it is not a good thing, if you had a look at how many flight hours our gyros have vs failures of rotor systems from fatigue or the like, you will most likely find that in 'statistical terms' the rate may be reasonably low, say as compared to the bars that have been damaged from roll over accidents. If you look at it this way, we've been flying gyros for some time now and in the last few years we've started to have hub bar failures, and I will note, failures that could have been caught if appropriate inspections had taken place pre flight. This just confirms what has been said before, preflight inpsections are critical, no one can be complacent in any way. At least our new hub bar AD's address this issue.Cheers,Nick.

[asra]

preflight inspections are critical- yes Nick this is truth, but only 50% of success, additional 50% is understanding complicate stress behaviors,stress concentration and proper bar design specially for heavy extruded blades. Stan Rucinski

[Michael]

Preflight inspections of an aluminium hub bar? Ok if you have x-ray eyes. If you see a crack or fracture you would have to be amazingly lucky not to have crashed the last time out.Michael.

[Brian]

I have to agree there, very very dificult to see a crack in Aluminium but from what I have heard, the rotors develop an vibration as soon as the hub bar cracks. Now, its also fact that one pilot flew for another 30 hours from when a vibration was noticed indicating that it took some time for a failure to happen. This would indicate that a warning that all is not well is given well in advance of failure and should be picked up by any pilot with a bit of expereince.Also of interest is that there are rotors currently [or were] flying with over 5000 hours on them and at least one of these was a muster'er.Just a thought nick re the bending of hub bars is that the only rotors that havent cracked yet have been Ricks and there would be more Ricks blades flying around on mustering machines than any other type of blades and I'm fairly certain that the high hour rotors have been Ricks ! I have a sneeking suspicion that Ricks set up might well be a better set up. Just my humble opinion.Brian

[asra]

Brian ... sorry mate but I'm a little ignorant here. Just for my info can you tell me who Rick is please?Safe Flying ... ding Good judgment comes from experience and experience comes from bad judgment

[asra]

There has been some good discussion in this thread, particularly stuff from Brian and Nick. It has to be recognised that Australian conditions and useage of gyros puts us in a position far ahead of anywhere else, for example the Magni appears to be a very nice, well developed machine, and certainly 3000 hours on their trainer is commendable, but it's peanuts compared with what our musterers are doing in terms of not only hours, but consistent use of high energy manoevres. These manoevres appear to put enormous stress on the rotor system, but I must confess to misgivings when I see proposals to simply "beef things up". I think Nick referred to this as the problem of chasing a weakness around a system when you make things stronger, when really you should be designing in flexibility and resilience. Obviously things must be stronger enough, after all our rotor system is required to have a safety factor of 10! Where are the calculations to prove this in those in current use???Those with long memories will recall the discussions we had in the thread "Hub bars and starter motors" on this Forum. It's not clear what impact his has on our present problem, but as the cracks seem to be forming in the "lead/lag" plane it may well be a factor. The obvious solution of providing lead /lag hinges is apparently not appropriate in a teetering rotor system, apparently it is critical that the blades be kept strictly in pattern.There has been talk of using 4130 steel for the hub bar, the thought arises that 4140 might be even better, you know, it's those long flat bits of stuff holding up the back of your ute, ie - spring steel!So do you get a nice stiff thick lump of it and make it into a hub bar? Only if you want to move the stress elsewhere, such as into the blade/hub bar junction, or the blade itself. Funny thing, a common size seems to be 2 1/2 X 1/4, perhaps a couple of those would even have the tensile strength to give us the required safety factor of 10 despite the necessary holes being drilled in it! Food for thought!?!?!?!?John EvansThink logically and do things well, think laterally and do things better.