Hub bar design

[asra]

The subject of hub bars is an old topic but I am raising it again now because there are a few question marks after reading the various posts on the forums, the ATSB report for the Charleville coroner in 2006 and knowing what I have put my hub bars through.http://www.atsb.gov.au/publications/...01.pdfFracture controlFor all critical components (i.e. those components, if they were to fail, would resultin the loss of the aircraft), the aircraft engineer typically implements a design-levelplan to address the risk of failure by fracture. The ‘fracture control plan’ as such,will examine of the possible failure mechanisms and establish controls to limit theinfluence of each. In the case of dynamically loaded structures such as the hub bar,the plan may require the design to call for strong materials and large sections tolower the net stresses to a level where fatigue is unlikely to occur for theforeseeable life of the item. Alternately, ‘life-limiting’ may be implemented toensure that critical components are removed from service before a pre-establishedfatigue life is exhausted. Periodic non-destructive inspection also finds applicationin monitoring the performance of a structure during its service life to ensure thatcracking is detected and the component retired before the cracking reaches criticalsize.It was not known whether the designs of rotorcraft that have sustained recent hubbar cracking and failures had incorporated a fracture control plan for the hub bar orother flight-critical components.The inspection procedure called by AD 05/2005 was a surface and bolt-hole eddycurrentinspection technique (ECI), backed up with fluorescent liquid penetrantinspection (FPI). The inspection intervals specified by the AD were related to thetype and operation of the rotorcraft, with two-seat and single-seat type approvedgyrocopters and single-seat gyrocopters involved in high-energy manoeuvres1requiring inspection at 100 hourly intervals after 500 hours service, and hub barreplacement after 800 hours. Single-seat gyrocopters used for sport and recreationonly were required to have hub bar inspections carried out at 100 hourly intervalsafter 800 hours service and hub bar replacement after 1000 hours.The methodology employed by the ASRA in determining the inspection intervalsand retirement lives of the gyrocopter hub bars was not documented.Safety advisory noticeSAN20060016The Australian Transport Safety Bureau advises owners, operators, maintainers andmanufacturers of autogyro rotorcraft (gyrocopters) of the potential for prematurefailure of the rotor hub bar or related components as a result of fatigue crackingdeveloping from bolted connections within the hub bar assembly.http://www.courts.qld.gov.au/Czislow...pdfCharleville coroner(b) That CASA fund an investigation into gyroplane hub-barlrotor blademanufacture, and in-service inspection and maintenance procedureswith a view to eliminating failures and accidents involving thesecomponents."Given that the hub bar failures to date have resulted at least in part,fiom damage induced during, or as a result of work carried out on therotorcraf it may be prudent for a responsible body to examine firther thecircumstances surrounding the known instances of cracking and failure.Aspects such as the requirements for tightening and checking the tightnessof hub bar blots should be examined, as should the operational andmaintenance history of those rotorcraft known to have sustained hub barcracking.An evaluation of the hub bar design may also be applicable to investigatethe suitability of the various hub bar and rotorcraji designs for high loadaerial work activities such as mustering.The ATSB considers that this work cold appropriately be conducted by.design and continuing airworthiness engineersfiom the aviationindustry ".There is a lot on the forums about looking after rotors and how this may be related to hub bar failure. I have an AK bar with 2000 hours and another with 1500 hours and this is how I treated them. For a start I always used an electric pre-rotator without starting the rotors by hand. If the strip was rough I would get the mains off as soon as possible to stop the machine from taking a hammering. For a few years I had the offset in the toque tube reduced to a point where changes in lift on rough days could not be felt through the stick. This made it easier to pull out of a steep, full power dive into the scrub and get enough inertia in the rotors to clear the trees in a near vertical climb. I would also come in fast behind a mob, pull up hard, chop the throttle and then a bit of forward stick so the gyro was climbing on the inertia in the rotors.

[Adrian S]

Peter,All good comments regarding hub bars. During my time as the ASRA Tech Manager I undertook a reasonable amount of research before we implemented the general Service life limitations you see in the present AD.Its my belief that the present AD has gone a long way to limit the number of U/S

[Brian]

Both Rob Patroney and Jeff Henley Smith were trialling different idea in hub bars. The last hub bar I received from Rob had more coning angle built into it and Jeff gave me a thicker hub bar to try but obviously, the hours without cracking are the proof of the pudding although, having said that, you and many others have clocked up big hours on hub bars, some I believe have been in excess of 3000 hours without trouble.I"m no techy but I reckon plastics [or type of] could prove to be the answer for a number of reasons. I use the term " plastic" loosely as there are a number of materials around now that have taken the place of steel and aluminum in the industrial world with great success. BTW, the Patroneys with the increased coning angle are I believe an improvement over past hub bars.

[ajm]

Should we be looking at the newer hub bar designs, as ELA etc instead of trying to perserver with old tecknology. Keep in mind, weights increase but so do ideas and tecknology.

[Brian]

Should we be looking at the newer hub bar designs, as ELA etc instead of trying to perserver with old tecknology. Keep in mind, weights increase but so do ideas and tecknology.Personally I"m not sure, not that I believe the Magni /MT/ ELA dont have the answer but have they really done enough hours to prove the point. I know Adrian has

[asra]

I agree about the composites and I like the fact that if stress is kept below a certain level they have an infinite fatigue life, but I think it will be awhile before hi-tech composites filter down to gyro level in terms of pricing. Although most composites have a better strength to weight ratio than aluminium, the composite rotors that are available now are heavier than aluminium rotors. The heavy rotors are very smooth in turbulence but they limit manuverability. The good thing about gyros is that they can be configured with a high inertia rotor for smooth cross country flying, or like I do with mustering where my rotor choice is a compromise between smooth flying in turbulence and agility when working stock, to a recreational machine that is taken out to throw around for an hour or so with a light rotor. There is a number of different rotors available all with different flying characteristics. The only weak point is the hub bars and they are failing at the bolt up points. In looking up information on a couple of two blade helicopters that developed cracks in the hub section, I found that one had a slight grind mark where the crack started and this was called a stress riser. The other had a slight ridge that had not been removed from the forging and this was also called a stress riser. With hub bars I think the bolt holes and the sudden change in cross section where the blade straps and pitch blocks and

[asra]

G"day All, I"m new to this place, but looking forward to becoming a regular attendee. I"m an Army aircraft technician by trade, with about 6 years experience maintaining Black Hawk helicopters, and am now in my final year of my Bachelor of Aeronautical Engineering.

[asra]

Hello Steve, its good to see this happening. Please keep us posted as to progress.Peter

[GraemeMonro]

Steve, your most welcome here and we are all looking forward to your findings.It"s not your fault who you are related too, we will not hold anything against you for this.Graeme.

[Greg Mitchell]

Welcome Steve,Mitch.

[birdy]

and sharing with your experience, knowledge and suggestions....This is good.Can never have too many brite brains on any subject.

[Mark Horan]

Steve,I notice you are in Canberra, If you need anything, I am Less than an hour away in Braidwood.Regs,Mark.

[Ken Watson]

What: like a leg of lamb or a good steak. Or maybe even a comfy bed. Ken

[Mark Horan]

Yeh,somethin like that!Mark.

[asra]

Thanks everyone for the warm welcome.