Tri-hinge and lead lag
 

For a long time I have wondered how the tri-hinge head design on the R22 is able to work without lead/lag hinges.
The search engine here does'nt seem to work for me, I have searched for years and still don't have a clue.

Can anyone suggest why the Robinson can use flapping hinges and not need lead/lag hinges as would be needed on other designs?

No need to tell me "they are coning hinges not flapping hinges". I read the patent, still no answer. Shawn Coyles book does'nt explain it either.

thanks
slowrotor

R22 has 'rigid in-plane' but free to 'teeter and cone' type head. ie 'tri' hinge. The lead-lag is just absorbed. A robinson design decision. Feel free to correct me if i'm wrong.

So, you are saying tri-hinge design does not do anything for inplane loads,
the inplane lead/lag loads are absorbed near the root of the blade.

Lead/Lag hinges are needed to compensate for geometric imbalances (coriolis effect) while flapping. because the R22 is rigid in plane it maintains even centre of mass on both blades, therefore neither blade wants to accelerate relative to the other, hense no need for lead lag hinges. Tri-hinge design is to undersling the load which gives head ability to teeter.

That's ballpark my level of knowledge explaination. apologies for any inaccuracies in terminology :\

NB

Hope this helps
 

Slowrotor,

Lead lag hinges are only potentially required if the rotor can generate roll/pitch moments. The moment causes assymetrical blade flexure, so some blades "cone" more while others "cone" less. This asymmetrical "coning" would sets up lead lag forces if not allowed to hinge. Bearingless designs rely on the lead-lag stiffness being high enough to resist in flight resonances.

Since the R22 is teetering both blades flex the same amount so only the cone angle changes. Cyclic input causes the whole rotor to change it's rotation plane. The only real effect is a momentary increase Nr when collective forces cone angle increase.

Mart

slowrotor,

The following is basically what the other guys are saying.

As you probably know, there was a considerable amount of discussion on PPRuNe about the Robinson rotorhead back when Lu was around. The problem is that there is just about too much stuff to dig through.

I also tried to find some of the related work on my site, without much luck. A couple of the pages being; http://www.unicopter.com/0385.html and http://www.unicopter.com/B185.html#Robinson

After all the PPRuNe discussions etc., etc. this was my take.

• Teetering rotors such as Bell and Robinson do not need lead-lag hinges because the in-plane motions are quite minimal and the long masts absorb them.
• The outer two hinges act as coning hinges in that they allowed Robinson to manufacture lighter blades.
• Calculations were done and it is extremely unlikely that these two outer hinges would ever flap (ie. the angle and its sign would be different between the two). This is because of the strong centrifugal effect.
• The Kaman synchropters also have teetering rotors with delta-3. They have lead-lag hinges, but it appears to be because of the interaction between the two rotors.

Dave

Dunno about Robbies specifically, but for Bell:

the underslung design of the head puts the rotor’s pivot point more into line with the blade’s normal C of G when in flight, and makes it more stable:

http://www.electrocution.com/precone.jpg

If it were otherwise, when the blade flapped up, the C of G would be more inboard:

http://www.electrocution.com/precone1.jpg

In this way, most of the bending stresses in the lead-lag plane caused by coriolis effect and geometric imbalance when flapping are relieved (that’s from Bell - the exam answer is: preconing relieves bending loads in the plane of rotation). Preconing is the setting of a blade angle to a slight incline when the helicopter is level - on the Bell 206, it is 2½°. The centre of mass stays about the same distance from the mast after the rotor is tilted.

Phil

OK, so the theory as described by Dave and Graviman is that the R22 blades do not flap about the coning hinges individually, they flap as a unit about the teeter hinge as any other Bell two blade would. So the coning hinges would act as flapping hinges only if the teeter hinge was welded firm, but as long as the rotor is free to pivot on the teeter hinge the coning hinges are held tight.

Phil,
The 206 has fixed undersling of course, but at low Rrpm with a heavy load the 2.5 degree undersling would be less than needed. It is my understanding that for this case the blade is made extra strong to absorb the additional lead/lag load.

Therefore, the R22 would have some lead /lag strain also when the coning angle is higher than the fixed design undersling. But the R22 blade does'nt look very strong to me in the lead/lag plane as compared with a Bell. And some of the blades have failed at the root fitting attachment.
The coning hinges relieve the blade bending strain, but I think the design has some problems with with in-plane lead/lag fatigue loads.
The tri-hinge design does have some appeal, I wonder if others will use it now that the patent has expired.

Thanks for your help.

slowrotor, Now you're starting to sound like Lu. ;)


Look at the picture halfway done this page. http://www.unicopter.com/B185.html#Robinson

Dave

Thanks Phil. Removing lead-lag forces by underslinging hadn't occured to me.

Slowrotor, teetering for variable RPM is only possible if you consider Dave's CVJ hub - i'm sure he'll link it. This design would neatly remove both lead-lag forces and the "universal joint" joint forces at the hub.

Even for rigids the dynamics can become tricky, due to flexing modes coinciding with aerodynamic loads at certain RPMs. I've only scaped at the surface of blade flexural dynamics, to improve my understanding, but am realising more and more why S-69 ABC chose a 2 speed strategy (thanks Nick).

Mart

Dave,

Well yes, I thought of Lu. But I am not going to worry about something like the 18 phase angle. I have flown the R22, it flies just fine. But I have also seen the pictures from the ATSB that show the fractured blade.

Robinson eliminated the bending loads with the invention of the tri-hinge, thats good.
If someone could eliminate the lead/lag loads with a more advanced invention, that would be even better.

I may use the tri-hinge.
or that A-160 hummingbird rotor head if I can ever figure how it works.

slowrotor,

Sorry, the post was not clear enough. Like you, Lu was also concerned about the in-plane wear in the coning (flapping) hinges of a few R-22's. He had a picture showing elongation, of the pivot hole in the hub as I recall.
.

For the pleasure of ranting http://www.unicopter.com/Rant.gif;

The A-160 hummingbird rotor and the tri-hinge rotor appear to be at opposite ends of the spectrum. IMO, the A-160 rotor is extremely rigid whereas the tri-hinge/delta-3 rotor is very 'loose'.

It strongly appears that the main cause of lead/lag is cyclical Coriolis. This cyclical Coriolis is due to a misalignment between the axis of the tip-path-plane and the axis of the mast.

The [Constant Velocity Joint & Hub Spring Rotor] and the ['Absolutely' Rigid Rotor] are intended to keep the axis of the tip-path-plane and the axis of the mast in alignment, and thereby reduce the lead/leg to such a small level that the rotor assembly can absorb it.

If interested, Flight Dynamics - General - Lead/Lag covers this subject.

Dave

Dave,
Thats interesting. (the coning hinge wear)

To change the subject... Dave I was looking on your website (nice data base)for your list of engineering books... could not find the list. Are there any books that would help with helo mechanical engineering? (detail design of the mast loads etc.)
I am ready to build the mast and transmission assembly but not sure if I will do the design by eyeball engineering or if a more accurate means are available.
I want to build a test stand first like Igor did. No need to design the rotor now, I will do that later.

slowrotor,

Wow. Not an easy question.

This is the list of books.
__________________________

'Machinery's Handbook' is an obvious one, which you probably have.

'Helicopter Theory' by Wayne Johnson has over 1,000 pages of mathematics and it is very economically priced.

A Preview of an Analytical Guide to Helicopter Transmission Design ~ 56 page paper by Kaman,1972 ~ was available on the Internet.

'Mechanical Design and Description ~ Rotary Wing Aircraft Handbooks and History '~ Vol, 8 ~ Dated 1954 from the United States Department of Commerce - Office of Technical Services ~ is a good simple book but is possibly not available.
_____________________

I found that the greatest problem was trying to determine what the life and safety factors should be for rotorcraft. When working on the SynchroLite, I even partially reverse-engineered the transmissions of the Mini-500 and the Ultrasport.

The little SynchroLite was taken to the point of building the craft. You should be able to dig a reasonable amount of information out of its web pages, but remember that they are my, unqualifed, calculations.

Dave

Thanks Dave,
I have Helicopter Theory and will try to find some of the others on your list.
I have "Principles of helicopter engineering" by Jacob Shapiro 1955, pretty good book for me as I am using mostly old technology.
Wish I had that book by Webb Schuetzow, I sent him a check about 30 years ago for the book and never got it. I think he died about that same time.

I better get the Machinery Handbook, don't have that.
I think I am mostly on my own, as the majority of helicopter engineering has been for very large machines.

Bill

Specifically, see "Considerations of Blade/Hub Geometry" in Chapter 32 of Ray Prouty's "Even More Helicopter Aerodynamics" (ref RWP4 on Dave's comprehensive list above).

Machinery's Handbook is fascinating in it's own right; I highly recommend the large-type version if your eyes are no longer young!