Spring Steel Hub Bar

[asra]

Even though I know of no hub bar failures in Jack Allen rotors, I do have to taxi over a not very smooth paddock to get to the strip, and I have to go through 2 gates, making the recommended practice of always having the rotor turning impractical.Col Smith and I hatched up a spring steel hub bar, Jack didn't enjoy drilling it much!I spent a bit over an hour today abusing it, trying to flap it, accelerating hard for short takeoffs, thumping it down out of high roundouts etc. It was incredibly docile!Pictures attached.Image Insert: 66.38

[birdy]

Funny you should mention spring steel, coz i woz wunder'n the same thing, use'n a spring leaf or a bar made outa the same material those aircraft bolts are made of, coz i know that stuff can handle some abuse. Where did you get that spring material from Doc??Ignorance is bliss............but only till you realise you were.You can always get the answer you want, if you ask enough experts.

[asra]

Dave it's 4140, I got this from Dumbrell's a spring maker in Newcastle. The main downside is that it is heavier than the alloy bar by 4 KG - it's a 4 ft hub bar.Actually there are different aircraft bolts, the ANs are said to be equivalent to grade 5, they are meant to tolerate deformation without letting go, as you know 4140 tolerates amazing punishment and we don't seem to see many broken springs!John EvansThink logically and do things well, think laterally and do things better.

[NiqueNaque]

John,Can you advise if the bolts on the blade root end of your hub bar that are in the vertical axis are in clearance holes, even if just slight ones?My reason for asking is that when your hub bar flexes up and down it will be trying to both bend and shear the bolts by a small amount each time it flexes. In addition the teeter bolts will be subject to the same issues if there is a disymetry of lift and the hub bar tries to shape itself as a parallelogram as the bottom bar slides relative to the top. The bar is obviously experimental as I note the teeter bolts have been put on the diagonal..Jack needs some tungsten carbide drill bits.Cheers,Nick.

[asra]

Nick, nope, no clearance, the pitch block bolts are locktighted in.Actually there is minimal shear on the bolts as the pitch block forms the base of a triangle with the apex under the teeter block, the long sides of this isosceles ?? triangle flexing to provide coning and other movements.So you see there is no real parallelogram movement.The bolts through the teeter block are on the neutral axis i.e. not on the diagonal as you suggest.Experimental, well yes, but it is engineered to give the X 10 Factor of Safety required by Section T, the fatigue characteristics of 4140 are superior to aluminium, and have been further enhanced by shot blasting.John EvansThink logically and do things well, think laterally and do things better.

[asra]

Nicholas,Your quite right in the fact that the bolts are under shear, first in one way and then the other, that is why car/truck/trailer springs have all the leaves tied together in the middle (axel) and other than the main leaf the rest are held in a slider shackle, sometimes the second leaf is rounded/curved at the end to allow it to slide (move back and forth on the main leaf), however as I do not make specific statements unless I have actually built/tried/tested (so I can say what actually happens through observation), I went to the trouble of duplicating the four foot leafed Hub-bar, and the results are as follows:- (looking at any end) lifting or lowerering only two inches from the level datum line produces a 0.55mm shear in one direction and the same 0.55mm shear in the other direstion, or overall 1.10mm shear, placed on the bolts, the bolts stressed one way is tryin to "Z" shape them, and trying to reverse "Z" shape them the other way, however thats not the end, there is also the little problem of squared centrifugal force going on in flight.One positive that comes out Johns experiment is, depending on load etc is that the blades will find there natural coning angle with the bar, that is, light blades are not for example trying to bend a preset bar up that has not enough pre-set coning angle, or heavy blades trying bend the bar down (Gull winging) if there is too much preset coning angle.Pete Barsden

[NiqueNaque]

Pete,Acknowledged, thank you. Your figures are more than I imagined and I have no doubt as to their accuracy, it is a little disconcerting.John,I suggest an application of the propane torch to remove the locktite and an examination of the bolts to check them for any shear deformation or alternately elongation of the holes in the steel. I don´t think the idea of having oversized holes will help because it will mean the bolt will suffer alternating bites from the top and then bottom of the steel at every oscillation, this could be quite an accelerator of fatigue in the bolts or the bar. If the pressure at the bite point is below 50 MPa then in all probability neither the bar nor the bolt will deform, but I think with about 1500 kg of centrifugal force on the bolts they will deform.I trying to think of a solution which may involve spherical washers, or, if Mr Dumbrell is a wizard at making springs, to form the hub bar up with rolled ends or blade sockets of some kind. You may have to make the bar with the laminates together to the blade root end with a forged connection to the blade roots to take the bolts like a pair of back to back L´s, so the action of the blades is not abnormal to the root axial bolts as opposed to the root shear bolts, however, this means the blade roots would then take a hammering. It may be a better idea to just have a single leaf as opposed to a double, my guess that you didn´t do this is because this interfered with Jack´s alignment pin at the centre of the blade root. You could offset this by making an aluminium hub root block that was deeper and which accomodated the alignment pin.I´ll have a think about it and get back to you when it comes to me.Cheers,Nick.

[asra]

From here it looks like the fix would be to cut the top leaf back to the same lenth as the bottom one and then just hang the rotor blades off of the centre one if its strong enough. That would at least cure the bolt shear problem.[]Another thought may be to just use one big leaf[?]Flying the right side up in Canada

[asra]

Thanks for all the helpfull advice fellers. I intend to monitor the "shear problem" by checking the torque on the 5/16" teeter block bolts, and later pulling them out to check for damage, as they will be more vulnerable than the 7/16" bolts in the pitch blocks.John EvansThink logically and do things well, think laterally and do things better.

[Brian]

Question. Why not just substitute steel for alluminium for the hub bar ?? Yes, it will be heavier but are there any other factors to consider ??Brian

[asra]

I think some helicopter tail rotors use multiple steel shims (flex bars) as hub bars. Using several steel bars could be resonably light and very flexable with the advantage that if a crack starts or one bar cracks completely, it may be found before the other bars develop cracks.