Semi-rigid vs semi-articulated

[Sikorsky]

What is the difference between a semi-articulated and a semi-rigid rotorhead , if any??

[Nick Lappos]

Those labels are confusing and only semi-useful!

If the rotors are mounted to a plate that is hinged at the mast, so the whole disk teeters around that hinge, and no bending forces are imposed on the mast during those teeterings, it is a teetering rotor, sometimes called a semi-rigid rotor. This is a classic first-generation rotor head, invented by Art Young in the mid 1940's It is easy to make, easy to maintain, and relatively light. It is also a very marginal design for the high-end work helicopters have to do, mostly because it has a relatively small maneuver envelope, and does not provide consistent control in low g or sideslipping flight. For very light training helicopters and home builts, it is a very nice rotorhead, as long as its limitations are respected. (As an aside, the rotors on the V-22 Osprey are basically teetering rotors, with the 3-bladed rotor head gimbal-ring being atached to the mast by bearings).

A semi-articulated rotor is again a bit of a stretch, but the Robinson design could be said to be one, where the blades flap from a hinge that is a bit away from the mast, but the whole rotor head also tilts. this allows each blade to relieve its forces at the head, but also lets the whole head tilt. It has most of the limitations of the teetering rotor above, but the head can be lighter because the flapping of each blade prevents the bending from being carried by the rotor head, thus it can be lighter. Also, the blades move more freely, so the controls should feel quicker and more responsive than a true teetering rotor.

[vaqueroaero]

At the risk of correcting the master I was told that the 'flapping' hinges on the R22 head are not flapping hinges, but are actually coning hinges. This came from someone way up there in the lofty heights of Robinson management.

[Nick Lappos]

Vaqueroaero,
Thanks for the correction, has anyone got a pointer to a Robbie rotorhead sketch so I can more fully understand that?

[handyandyuk]

I believe the difference is that the MR blades are prevented from moving below their 'resting' attitude by the droop stops on their hinges, yet are not prevented from rising and thus coning.

If I'm wrong please correct me..... I'm here to learn from you guys!

Andy:D

[Dave Jackson]

Sikorsky,

"A semi-rigid main rotor is always a 2 bladed rotor system. It gets its name from the fact that it does not have a lead-lag hinge." This is basically the same as what Nick said.

The following is an interesting story about a semi-articulated rotor, which had three blades. I suppose that the hub was called "semi-articulated" because the blades did not have lead/lag hinges.
Semi-articulated rotor (http://www.aviationtoday.com/reports/rotorwing/previous/0601/0601flightdyn.htm)


Nick,

It appears that semi-rigid and semi-articulated may just be two ways of refering to the same thing. :confused: The following two web pages may be of value.

Pictures of rotor hubs; including four of the Robinson hubs. (http://www.b-domke.de/AviationImages/Rotorhead.html)

Technical information on the Robinson R-22 rotor. (http://www.unicopter.com/B185.html#Robinson)

Dave J.

[Letsby Avenue]

An articulated head has a flapping hinge, A feathering hinge, and a hinge that allows lead and lag.

A semi-articulated head (eg: the gazelle) has a flapping hinge, a feathering hinge but lead and lag is taken up by a psuedo drag plane damper.

A semi rigid head (eg: the lynx) has a solid titanium head the extension arms of which can absorb lead, lag and flap but it still retains a feathering hinge for pitch.

A fully rigid head will have no hinges whatsover and all movement is absorbed by rigid extension arms to the head.

As an aside - there is no such thing a coning hinge as the only thing that cones is the blade which would do so with or without a flapping hinge as Mr Cierva discovered in the 30s (or didn't as the case may be)

IMHO of course


:cool: :cool: :cool:

[Irlandés]

Letsby Avenue,
I'm not sure if you're definitions are on the money. I was under the impression that a fully rigid rotorhead (e.g. BO105) was perfectly entitled to have a feathering hinge without running the risk of losing its 'fully rigid' description. And also if you maintain that there is no such thing as a coning hinge, then could you explain what is the design purpose of the two offset hinges on the R22?

Just from a semantic point of view I don't think there are any fully rigid rotor heads. Is it not a bit of a misnomer as all rotor systems (hingeless or otherwise) have elasticity to varying degrees?

Irlandés

[Letsby Avenue]

Not being an avid R22 fan I can't comment on their 'offset hinges' but they sound like Delta3 hinges which act as flapping hinges but due to their offset cut across the blade reduce flap to a manageable degree. Most helicopters achieve this D3 by offsetting the control arm instead. There is of course every chance that I am talking b*ll***s

:D

[Irlandés]

Letsby Ave,
whether or not what you just said is b*ll***s, I'll let you decide! ;) But I will say that the 'offset hinges' on the R22 are neither Delta 3 hinges nor flapping hinges. There is a delta 3 angle designed into R22 rotor head (18 degrees - see Dave Jackson's link) but there is no Delta 3 hinge per se (unlike the tail rotor). They are in fact principally coning hinges although whether or not there is a small element of flapping to back up the teetering hinge as I think Nick is suggesting I will not venture an opinion. My brain cells only do so much work on a sunday morning before they go on strike! :D

As an aside, if the changing of a blade's pitch angle due to flapping is called 'pitch-flap coupling' (because of Delta 3 angle), what do you call the changing of a blade's AOA due to flapping? AOA-flap coupling?

That's it! Brain cells on strike! :)

Ciao!

Irlandés

[Letsby Avenue]

Yep - I can confirm I was talking tosh - since the R22 has a two blade teetering head there is no need for flapping hinges.

DOH!:confused:

[Barannfin]

It was always my understanding that the r-22s "coning hinges" (whatever u want to call them they are still there) served mainly to lighten the rotor system. The logic being that the blades didn't need to be as heavy because they didnt need to take all the coning loads, the same went for the head. And these things being lighter allowed for lighter transmission and associated systems. Right??


One quick question, in that link that Dave Jackson posted i noticed what looked like some electrical connections on the top of the Mi-2 head. Are these for an anti-ice system or something?
thanks.

[Dave Jackson]

Irlandés,


You're right. There aren't any absolutely rigid rotors; but the guys in this picture (http://avia.russian.ee/vertigo/hiller_x-2-235-r.html) must have felt that they came close. :D :D

[Sikorsky]

Thanks for the good anwsers, it's a great help ;)

Sikorsky

[3top]

Hi Dave J.,

I read the explanation for the Robinson 18 deg offset somewhere but can´t find it anymore - maybe it was on your site. It is time to read it up again, maybe this time it´ll sink in and stay,:D.
Would you have a direct link to the article? It explained how Uncle Frank got to 18 deg, something being 17 deg the other 19, so to split he put it to 18.......

Thanks in advance,

:D 3top

PS: .....or anyone else knows the place I am talking about?:)

[Dave Jackson]

Hi 3top,

You can try a search on ~ Posting by Frank Robinson on www,pprune.org ~ Nov. 29, 2000.

~ or ~

A copy of it can be found by going to web page http://www.UniCopter.com/B185.html#Robinson and then scrolling down until you see;
R22 ROTOR SYSTEM ~ Posting by Frank Robinson on www,pprune.org ~ Nov. 29, 2000.

Dave J.

[3top]

Thanks Dave!

That was exactly it! Here a place with some nice pics and explanations but nothing like the details on the report on your site:

http://www.cybercom.net/~copters/mech/mr_semi.html



:D 3top