High energy manuvers.

[birdy]

Been thinkn bout wots been posted about hub bar failures and high energy manuvers, and i think a point may have been missed.Wot most people think of when they hear the term 'high energy manuver' mite have some thinkn they're safe coz they don't do them.And this could be a problem i think.Anyone can stress rotors, experianced or not, and its got less to do with the style of flyn and more to do with an understanding of wot stresses a blade in a way that its not desigened to handle.Ozy Woz,[ for those who saw him at Cooma this year], was performing wot i recon most people would call 'high energy manuvers'. In a sence they'd be right, but not in the sence of applying damageing stress on a rotor/ hub bar.The stress he was dishn out is the sorta stress a rotor CAN handle.[ solong as you apply this stress properly.][ sorry if i'm not explaining meself very well.]The main things that stresses a hub bar in a detramental way may be,rotor strikesrotor flappingignoring pitch feed backrough ground handelingpoorly tuned baldesPoor rotor RPM management.[ yes, it don't just apply to choppers]and prolonged high speed.[ please correct me if im wrong, or missed something.]The abuse above can be served to anyone's rotors, but more likely the rotors over a novise or sumone who's ignorant of where the dangers lie.More experianced fliers, especialy ones who apply their machines to situations that require maximum performance/ effect are less likly to stress their blades detramentaly coz they learn that if the blades are managed properly, they'll get maximum effect.Once you get the hang of 'flyn the rotors' you realise wot potential there is in um.I'm not layn blame on anyone or defending anyone, i just don't want the more likely people to cause blade/bar stress to mistakenly think that its not go'n to happen to them, coz they're just weekend putters. We'll probably never know the true history of the offending blades/bars, but i'll bet a blade that's looked after, respected, and flowen appropriatly will accumulate many more hours than the ones that are abused unintentionaly.Probably a more appropriate term mite be sumthn like 'high energy incidents', where its clearer that any energy absorbing impact, manuver or incident will stress the systm.Any feed back through the stick such as blade flap, pitch feed back, blade strikes, cone shake or any general stick shake is stressing the blade/bar and should be avoided.I think the 800 hour limit is a good starting point, no matter wots happened in those 800 hours. They're only blades, its not like they're made of gold or sumthn.If no blades fail under the 800 hours then we know where to start, and maybe stretch it out a few hunderd hours in a few years.Waiting for a blade to fail to find out when its going to fail ain't a real smart way of go'n bout it.[and wouldn't it be good if EVERYONE was 100% honest bout the history of their blades][}]There are alota ways to fly gyros, but only one way that's going to ensure you'll get another go at it.[]Ignorance is bliss............but only till you realise you were.You can always get the answer you want, if you ask enough experts.

[Greg Mitchell]

Hey Birdy,Good post Dude. Couple of questions.What is pitch feed back?What is cone shake as opposed to "any general stick shake"?And will you take me for a blap in your Raf if I get up to Biggenden?Cheers Mate.Mitch.www.thebutterflyllc.com

[asra]

All sounds about right Birdy800hrs is fine if youd bet your life on the accurasy of those hours which is no problem if youv owned em from new but if the bloke you buy your secondhand blades from tells you 150hrs no rotor striks no flaping & truth is 450 of the hardest hours ever then youv got a problem

[Tim]

Another thing that causes undue stress is getting up the machine with low rotor speed. The over excess coning is clearly visible from the side view and I would suggest that there have been many cases of hub bars bending up as the rotors try to catch up and then bending down again as the centrifugal loads take over.Low hour pilots are the biggest culprits here because they haven't yet learn't that they would actually get off shorter (maybe not quicker)if they took it a little easier, with the side benifit of less stress on the blades..Tim McClure

[asra]

Just a couple of points.We haven't had any blade failures to my knowledge, just hub bars.The failures have all been on machines flown by high hour pilots.All have been subjected to high impact starter motor pre-rotators.There appears to be a lack of proper engineering analysis in the design of some hub bars.Safety factors have been eroded as machines and blades have gotten heavier.The cracks have occurred in the lead/lag plane rather than the bending plane as would be expected from comments in the previous posts.Historically there has been a tendency to under-estimate the lead/lag stresses - evidenced by Courtney's loss of one blade, fortunately followed by another, allowing him to survive the crash of an early Cierva. Cierva very reluctantly fitted lead/lag hinges to cope with this.I am bracing myself for expected attacks[]John EvansThink logically and do things well, think laterally and do things better.

[birdy]

Mitch,What is pitch feed back?Usualy occers at low AS, low RRPM, high power setting and high rudder input. Basicaly, the disc has a large pitch change command and dosn't have the rpm or time to keep up with the machine. As the disc tryes to keep up it needs huge 2 per rev cyclic pitch changes, and the harder the rudder is stood on, the more the rotor will scream "slow down"[ by shakeing the stick.]What is cone shake as opposed to "any general stick shake"?'Out of cone' shake is a tune thing, if the rotors arn't cone tuned to suit the weight of the machine they'll shake.But, when a set of perfectly tuned blades are pushed hard, there is a momentary shake as they move above or below the setting due to a rapidly changing rrpm/G load situation.And will you take me for a blap Stupid question Mitch[}], corse I will, id be offended if you didn't[].[ you scratched mine, I'll scratch yours[].]I know wot your sayn Butch, and thats why I said "wouldn't it be good...."If you don't know or trust the other bloke, get um checked out, afterall, its your neck init??Another thing that causes undue stress Thats wot I was refer'n to when I mentioned " managing RRPM," Tim,on the ground or in the air. No dout low hour pilots will do it till they get the "feel", but so do high hours ones, take'n off from short pads AND too haisty on the stick when in the air and not giv'n the rotors time to rev up.just hub bars.Point taken Doc, but if the rotors are respected/ looked after, the bar will be too. A bar won't stress out if the blades arn't abused, and its the abuse of the blades thats caus'n greef to the bar. Init?? The failures have all been on machines flown by high hour pilots.This don't mean much, high hour pilots can be just as abusive, or even complaicent.[ i know, i'm as guilty as the next bloke[V]] I think, coz SUM high hour pilots/ blades have failed, and other high hour pilots/ bars didn't , points to sumthn other than just 'hours'. If a newby flogged his blades the same as a high hour bloke, they will still be stress'n the bars, then you'll have a low hour pilot/ bar fail.Like I said, I'm not say'n we [ high hour/muster'n typs] don't stress our blades, i'm just warning EVERYBODY to wot 'high energy manuvers' means, and that EVERYBODY is capable of do'n it.Respect your blades, and they'll do the same for you.Ignorance is bliss............but only till you realise you were.You can always get the answer you want, if you ask enough experts.

[Brian]

John, there are plenty of high hour rotors around [3000 hours plus]that have electric pre rotators. I take your point seriously but I'm yet to be convinced of your argument.The lead lag thing could well answer a lot of questions. Also we havent any detail of sizes ,hours thickness any really any information of the hub bars that have failed or cracked as yet. Also needed is machine types, weights [machines] and type of flying done.We need a comphensive look into the hub bar failure profile.Brian

[asra]

Actually Brian I am not putting up an argument for anything other than a rational approach. I hope I'm wrong, but I don't believe any [perhaps with one exception] of our 'manufacturers' can provide proper engineering specs for their rotors.Perhaps it's time for the 'consumer' to start demanding evidence of the safety of their product. Now that we have five failed hub bars we need to reject the farce of 'history of safe operation' as the delusion it has always been. We have only now become painfully aware that we have a problem as we have eroded the factor of safety down to nothing.Did you know that our accepted design standard, BCAR section T requires a safety factor of 10? I understand these specs are available on this site.John EvansThink logically and do things well, think laterally and do things better.

[Tim]

We did actually have a rotor failure about 10 yeasr ago. A CASA expert determined it to be fatigue (after a visual inspection)I would have preferred to blame "manner operated" because the last hole in the strap where it broke was severely burred, elongated and raised which could have only come from excessive up and down flexing. The pilot was known to taxi long distances with the rotors either tied or turning at hand speed, over rough ground. The stress witnessed on the strap mounting holes became less as they got closer to the hub bar.Back then, the hub bar was stronger than the blades.I will attach a diagram.Download Attachment: [img]icon_paperclip.gif[/img] Broken Rotor.doc20.44 KB Tim McClure

[Greg Mitchell]

Birdy,Thanks for the explanations, very helpfull.Looking forward to catching up.[]Mitch.www.thebutterflyllc.com

[asra]

Obviously the forms of abuse Birdy describes are rough on the rotor, BUT, isn't that exactly what the rotor on a gyroglider puts up with? run after run, student after student, yet failures are unheard of.Let's keep this discussion rational and not go off on a tangent blaming some possible and un-identifiable event in the past for something that can be calculated and predicted.John EvansThink logically and do things well, think laterally and do things better.

[asra]

The difference I think John, is the gyroglider only accumulates minutes of air time. Each run probably only seconds, BUT I appreciate that a gyroglider is generally operated in a rough environment on the ground.Aussie Paul. []www.firebirdgyros.com

[Tim]

quote:Originally posted by JohnI am bracing myself for expected attacks Not an attack John, just a difference of opinion, with due respect."blaming some possible and un-identifiable event in the past"This event wasn't just possible, it did happen, and it was more identifiable than most of our current breakages. I still have all the details including a bit of the broken rotor if you wish to see them. "Let's.. not go off on a tangent blaming some possible and un-identifiable event in the past". I do not believe it is going off on a tangent to bring up a rotor failure, when the subject is the causes of failures, even if it was in the past. We must always rely on the past to give us today's information."for something that can be calculated and predicted" I believe that this is our main problem - It cannot be calculated and predicted. If it could, we wouldn't have this problem. There are way too bany variables involved here, such that even a top physicist could not predict failure with any accuracy. And finally "we need to reject the farce of 'history of safe operation' as the delusion it has always been"A "history of safe operation" can be a farce if used incorrectly. As used by CASA, it will give you a far more accurate figure than experts second guessing at some un-calculable figure. Many of the fatigue life figures used by CASA are derived from "use in the field" (history of safe operation) The blade life of a Robinson is an example, that recently had to be reduced due field results. I must admit that CASA approved metalurgists findings can be misleading as well. The recent failure in WA is a good example. Though fatigue may have played a part in the final separation of the metal, it was not the primary cause. The hub bar was bent up some 20 degrees before it failed. I can only guess that the metalurgist assumed that the rotor would bend up when the spar failed, as the wing on a plane would - A total lack of the understanding of the forces involved.I believe we must look at all failures and do our homework from there. We have to gather all the information on all the mitigating circumstances that we possibly can (regardless of how small or insignificant), and put it all together in one probability document or chart. I intend to propose that we send out a comprehensive questionaire to all those that have had failures and see how many common threads we can find to come up with a scaled probability graph.Tim McClure

[Greg Mitchell]

Tim, John,Could one of you guys highlight in BRIEF the 5 cases of hub bar failure for us here.Thanks.Mitch.www.thebutterflyllc.com

[asra]

This will be messy as Llewella is away.Taking your forst 2 points in blue Tim, these were not in any way aimed at your mentioning the Wilcania failure, I gather the preceding abuse was well known, together with a failure to investigate a rotor shake that constituted a warning. I was actually referring to the points Birdy was making, those factors can be hard to identify unless documented in the gyro's log book. I was wanting to draw attention to the need to use good design and hopefully see more of the information in the FAA's "Acceptable Methods

"incorporated into the design process. I'm no Engineer, but I do know that great care has to be taken in the design of stressed critical components to ensure the fabrication process does not impair their integrity. While appropriate calculations may not be infallable they are capable of distinguishing good from bad design.for something that can be calculated and predictedWe will have to agree to differ on this, much progress has been made since the beginning of the Industrial Revolution, we are now in the 21st century, we have tools at our disposal to make many of these predictions resonably accurate. The strength of the materials used are known, their fatigue characteristics are known, and many [ unfortunately not all] of the stresses encountered are known.Those unknowns are why we have to have appropriate Factors of Safety, our standard for the rotor is supposed to a FoS of 10. Can someone tell me what it actually was for the failed hub bars?History of Safe Operation, used properly is obviously valid, and yes, a rotor with 3000 hours on it would appear to have one, but there may be a signigant scatter in the time to failure of a batch of identical rotors, it is conceivable that the worst one in the batch will fail before 1000 hours. How can we tell? Hopefully good log book records and the required crack testing will give us some answers. John EvansThink logically and do things well, think laterally and do things better.