Dumb newb question

[RHGraham]

So from what I gather, the hub bars on most gyros are made from aluminum? Is this correct?

[DanielM]

That would be correct.

[RHGraham]

Thanks Daniel.

[Aussie_Paul]

Here in Oz because we do a lot more hours with the mustering guys there is now limits on the amount of hours that an alloy hub bar can do. After those hours either a new hub bar or have to old one NDT is required.

The limit for 2 seat and hard worked single seat mustering hub bars is 500 hours and for recreationale single seat gyros 800 hrs (I think 800 is correct)

Aussie Paul.

[RHGraham]

I'm a newb so there might be issues I don't understand about the situation, but the cracks I've read about sound like classic cyclic fatigue cracks to me, a particular issue with aluminum, and talking about a hub-bar, what a perfect place to put aluminum under repeated cyclic stress.
I personally think a properly heat-treated and formed alloy steel hub bar would not only completely shrug off such cyclic flexion issues, but likley be flexible enough to "pad" the blades as well.
Frankly, the use of aluminum in some of these gyro structures concerns me a little bit. Yup, aluminum is light.
And that's the one and only thing it's got going for it.
And, compare it pound-for-pound with 4130 or other alloy steels, I really don't see the advantage.

But like I said, maybe I'm missing something.

Randal

[autorotate]

yes given the size of the hub bar aluminium is kind of scary. and 500 hrs ide rather have a hevier head and longer tbo. i cant think of a beter place for titaniaum then the hub bar. the first benson gyro ive seen at a musem i couldnt belive how flimsey the head system looked! dosent mean it was insufficent but looked that way.

[RHGraham]

yeah, looks can certainly be decieving too, that's why I caveat by saying there might be more to it than I realize.
Plus, I have a long standing dislike of aluminum anyway, so I'm biased.

Titanium, that might be a good place for that perhaps.. But still, ya know, if it's flex you wanna take care of, and toughness you need, then all that fancy stuff, Ti, alum, composites, none of it will still stack up any better than a properly heat-treated chunk of low-alloy steel. Well, that's not true, kevlar composites actually come very very close to 4130 and similar alloys to cyclic fatigue resistance.

I honestly see a lot of things that strike me as being a deal where aluminum is being used because it's easy to work with. I get that, mostly, but still, I also wonder if there is maybe some erroneous thinking about what kind of properties some of these materials actually have. Aluminum is light and stiff... neither is good under cyclic loading.

non-gyro-educated opinion on my part.

[autorotate]

if im not wrong Titaniaum has a near infinate endurance limit. it rivials the stength of steel at half the weight. if it wasnt so expensive id say it would be foolish to use anything els.alumiunim seems to be seen as the best comprimise.

[Fl90]

Ti has it's issues as well. If you use titanium use caution when cleaning, some chemicals cause a corosion/embrittlement that will begin a stress crack. My experience is only associated with impact under tension, so I'm not sure what difference tension/torsion stress would be.

Phil.

[RHGraham]

Some Ti corrosion issues are worse with some alloys and not so significant with others, the 6Al-4V alloy with anodizing would be fine that way.
It has a somewhat similar cyclic fatigue limit as a treated 4130 ( for example), however it's "better" on the high-frequency side and tends not to test so well on the low-frequency end of the spectrum, where 4130 has an extreme limit throughout except for very high frequency where it drops off a little.
So if you stayed with the physical size of the standard aluminum bar but instead in Ti, it would be vastly tougher/stronger/more flexible.
And a material that has some "flexibility" will generally be more resistant to stress cracking and work hardening.

To use 4130 my personal opinion is that if it was shaped with distal taper from the mounting point out to the rotors, and specifically heat-treated for the application, you'd have an extremely tough bar that would shrug off any stresses the rotors/gyroplane would be able to inflict on it, essentially, indefinately. And with the shaping, the tapering of the bar, weight would be far lower than you'd first assume, way closer to the weight of the aluminum bar than most would expect I think.
If you take say a tube, one of aluminum, one of 4130, and say they both weigh the same amount per-foot, and we'll even give the aluminum a real break and allow it's tube diameter to be bigger, than the 4130 tube.
(make it 6061-T6)
Test them together, the steel tube will have a MUCH higher tensile strength still, the steel tube will be far more flexible, and be able to flex much much furthur without buckling or taking a "set" ( bend).
If you buckle the steel tube and you try to bend it back, it will, and it will take many multiple flexes to break it off finally. IF the aluminum tube is also bent back from it's buckled point, it will immediately snap off usually, very rarely it'll take one more flex to seperate the pieces.
The steel tube will test a higher impact resistace. The steel tube will have the better torsional strength.

But the aluminum bar WILL be "stiffer", and there are likely points where this would be a desireable trait. Aluminum is far easier to shape and tool, can be worked in it's heat-treated state without nessecarily requiring any furthur thermal or aging treatments, and it's far easier to cast complicated structures with as well. Steel can be cast just as well, but at a far far higher cost because of the much higher heat required.
Aluminum is the superior choice for beer cans since they tow behind the canoe easier, keeping the beer cooler, and crush compactly for carriage out of the fishing camp.



Hey, this is all just based on study and expirience on my end with stuff not all that gyro-related really, I actually havn't got the expirience with that kind of structure( the gyro) to *know* what the full envelope of structural requirements are for that craft. The stiffness and light weight on aluminum's behalf may sound trivial compared to the benifits of the 4130 tube I listed, BUT, if I've learned one thing over the last 20 years, it's that every sigle attribute of any material can have extremely important ramifications in any particular application. Light and stiff is a big deal sometimes. That's why carbon fibre is used.

Regardless, the whole point is simply I wonder how many of these things have been seriously studied for the gyroplane yet, it really does seem to me that perhaps at least the hub-bar is a place where I wouldn't think aluminum is the best choice overall.

My two canaidjun cents eh.... may not be worth that much...

( in reality, I just really hate aluminum.)

[scottessex]

Randal, have you ever worked in the aviation field?
Every material has its pro's and con's, Many aircraft are and have been for years made from Chromo tubing, but Wood, aluminum, composites all have been used and still are, and remember there are different grades of alum, just like steel,
You wouldn't use mild steel tubing for a gear leg, Just as you wouldn't use 5052 alum or any -0 for anything structural on an aircraft.

Besides whenever you bend anything on an aircraft, you never bend it back, it goes in the scrap heap, aluminum, or steel.

[groundhog]

distal taper ?

[RHGraham]

Originally Posted by scottessex

Randal, have you ever worked in the aviation field?
Every material has its pro's and con's, Many aircraft are and have been for years made from Chromo tubing, but Wood, aluminum, composites all have been used and still are, and remember there are different grades of alum, just like steel,
You wouldn't use mild steel tubing for a gear leg, Just as you wouldn't use 5052 alum or any -0 for anything structural on an aircraft.

Besides whenever you bend anything on an aircraft, you never bend it back, it goes in the scrap heap, aluminum, or steel.

No, not aviation. Performance Auto and marine, and standard fab stuff.

And yup, I realize and agree about the pros and cons, and with everything you said. Liek I said in my post, I don't *know*with regards to gyros, I do have more than a little bit of expirience with all the materials commonly used for them though. And heat-treating, stress/failure analysis, destructive and non-destructive testing. It leaves me feeling that aluminum is a poor choice for a hub bar, but it's just my opinion. I won't fly under an aluminum bar, once it comes time. Everybody can make thier own choices though.

[RHGraham]

Originally Posted by groundhog

distal taper ?

A taper in thickness from one end of an object to another.
It helps spread a "bend" throughout the length of a piece instead of it concentrating in one spot. You can also adjust how "flexible" a part will be by varying the taper and thickness. In a piece that has an even thickness throughout, it'll tend to bend in one localized area, focusing all the stress there.

[scottessex]

Randal, the aluminum hub bar along with most of the other rotorhead components are WAY over built for the stresses that a gyro will see over it's lifetime,

Next time you are on a commercial airline flight, watch the wingtips flex, they all use aluminum and an aluminum spar, these are good for many tens of thousands of flight hours and thousands of cycles, Many smaller aircraft use heat treated aluminum spring landing gear, again many thousands of cycles.

Steel rotor blades might be more durable themselves, but then you get into serious weight issues, again everything is a trade off. if you add some weight here and there on a race car or boat, to add strength, you might not hurt performance much, on an aircraft, overbuilding here and there CAN and does seriously affect performance. Ti is very cool stuff, hard to work with, and the cost is astronomical,