HUB BARS ON GYRO'S FOOD FOR THOUGHT ONLY

[Des Garvin]

I have been thinking about the hub bar failures of late as to the differences in the rotor lengths verses hub bar thickness.PLEASE NOTE THIS IS NOT TECHNICAL FACT OR ENGINEERING INFORMATION.On the smaller gyro's the hub bars are made out of 19mm thick by 63 to 65 mm wide material. Then along comes the longer blades where the thickness of the hub bar material is 25mm thick and the width is still the same 63 to 65mm.Question now that the thickness has been increased by 33.33 % so MAYBE the width of the hub bar should increase by 33.33% also.PLEASE NOTE : I AM NOT AN ENGINEER, I AM JUST THINKING ALOUD TO PREMOTE DISCUSSION IN THIS AREA ONLY WHERE, BETTER QUALIFIED PEOPLE CAN GIVE QUALIFIED OPINIONS.Let's not let personality differences enter this disscusion as we as a group of same interest people may find an answer to a on going incident there by saving a life of a fellow Gyro pilot.NOTE; I HATE PAIN.[] Des Garvin

[mrford61]

Des,I read somewhere recently that in regards to the stresses on a hub bar that to double the thickness of the bar multiplies the strength X 2. To double the width of the bar multiplies the strength X 8. Also read that the "right" resonance will crack the bar regardless of the thickness / width.I have no idea either but wonder if it is a case of getting back to basic,s.Mark.

[asra]

I will give a simple explanation........................for example, take a wooden ruler in both hands with the measurements looking up at you and see how eazy it is to bend it downwards, now turn it 90 degrees so you are looking at the edge, and now try and bend it downwards.This simple engineering principle is used in all construction works, their called "I" and "H" channels, and don't forget the old well proven Rafter and Beam, all the best. Pete Barsden

[asra]

Rest assured that this matter is receiving consideration right now, an AD will be issued shortly.There are a lot of factors involved in the present situation. I am not an engineer, but I do know that the forces involved can be calculated, and the fatigue characteristics of various materials are well known.The effect of holes drilled in various places are well known, the correct position of the coning angle bend near the middle of the hub bar is well known.The flexibility of the rotor head mounting to relieve the stress of the 2 per rev rotor shake on the hub bar is well known, yet we have gone to a stiffer 2"X21/2" mast, this is particularly stiff in the diagonal direction in which the shake occurs.The effects of large starters without soft starts giving the hub bar a sledge hammer blow on start up, and their mass and position are less well known.Mark, I think you are right, we have to go back to basics with the appropriate engineering calculations and ensure that good engineering principles are inherent in the designs.Yes we have a problem, but it should be seen as an opportunity to bring the design of this vital component up to scratch.That's the end of my disjointed ramblings.John EvansThink logically and do things well, think laterally and do things better.

[asra]

should i be woried about a laminated 2,2 by 1 mast with the pre rotator located down the mast with a shaft driving through two uni joints to the hub or was the builder onto somthing???what if i have the wrong resonance???

[asra]

Sounds like good design to me!It seems that the 2" X 21/2" mast is "stronger" than laminating 2 2"X 1", the assumption being that stronger is better. This would seem to ignore the need for the mast to be supple.John EvansThink logically and do things well, think laterally and do things better.

[Tim]

Though the 2.5" X 2" is definately stiffer than a 2" X 2" mast, in most of our cases the 2.5" X 2" masts are considerably longer than the "older" 2" X 2", probably more than making up for the difference in elasticity.It is a fact that the smaller mast is lighter and supporting more weight/squ", so it is possible that it would tune to a higher more destructive frequency.I too am not an engineer, but even I can see that there are many combinations of forces in play here.Personally I am quite happy flying on a 2" X 2" mast on a light machine, but I would feel much more secure with a 2.5" X 2" mast on our current crop of heavier machines.Tim McClure

[asra]

I know it's not fashionable to talk of RAF having done anything right, but I'm starting to think their "magic donut" mast would be a good way to go. Using rubber to allow the required movement, as is done in helicopters with teetering rotors, seems a lot more rational to me, than relying on the 2" X 21/2" masts to flex and avoid the lead/lag reversals that may be at least one of the causes of our problem.John EvansThink logically and do things well, think laterally and do things better.

[birdy]

If your blades are tuned to the machine, you don't need to bother.Ignorance is bliss............but only till you realise you were.You can always get the answer you want, if you ask enough experts.

[asra]

John,I have fitted a rubber damper system to my last two Aircraft, this goes back approx nine (9) years, and as early as thursday I have been working on a similar system that can be retro fitted to the standard Aircraft, will keep you informed on the progress. All the bestPete Barsden

[asra]

Pete, Look forward to hearing progress on this project.John EvansThink logically and do things well, think laterally and do things better.

[asra]

I'm glad YOU said that Birdy ! and not me ! cuz i was going to say the same thing ! Bob..." Momm'a alwayse told me , Son the impossable is only a little bit Harder... and ya know I do believe She was RIGHT ! "

[asra]

Gentlemen,There seems to be a misconception of the reason that I "floated" the head in a rubber damper system, it had nothing to do with the flight envelope, doing the bending moment instead of the mast, trying to absorb stick shake etc, the reason is as follows............... many years ago a in Australia a blade broke away in flight (fatality) and was attributed to the Gyroplane being frequently taxied out to the strip over rough ground and thereby placing large stresses on the rotor system, the damper system was an earlier idea to try and absorb some shock in the rotor system because of the perceived problem above, all the bestPete Barsden

[birdy]

?????????I didn't think that sorta 'shock' would worry the blades at all, I thought it as the emormous stress put on the center section [bar and straps] of the blades from 'bouncing' blades while taxieing would do the most damage.Rubber bushing wouldn't make the slightest difference on a bouncing blade, but centrifical force dose.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,Correct, thats exactly what I said, the Rotor "system", this includes Bar, Tangs, and Roots of the blades (that being the perceived problem that was mentioned above), and whilst taxing over rough ground with the blades being tied down or without the blades being brought up to enough speed,Pete Barsden