Rigid & Articulated Heads

[Vfrpilotpb]

I wonder if someone might explain the flying difference's between the two types of head that are availabel on current Helis, and do these require a different sort of ability in flying when switching from one type to the other?

[Shawn Coyle]

You won't notice any particular change required in the way you fly, but the rigid head (found only in the BO-105 / BK117 in the civil market as far as I know) will have much more 'crisp' response to control inputs. The down side to that is that the ride quality in turbulence suffers from being a bit uncomfortable for some people.
One major difference you would note is that off-level landings in a rigid head require that the cyclic not be moved, and that you pay attention to the mast bending moment gage. (easy to do off-level landings though, just a different technique)
Coming from a teetering rotor background? You'll notice that it is easier to hover precisely in either one, but at the slight expense of having to pay more attention to the machine in the cruise in some conditions.
At high altitude, with both (non-teetering) rotor head types you can expect some cross-coupling of control inputs due to Locke number effects - i.e. a pitch input will show up as a slight roll input as well.

[Dave_Jackson]

Shawn,

Your post raises a related question.

You, and others, have mentioned that "The down side to [stiffer rotors] is that the ride quality in turbulence suffers from being a bit uncomfortable for some people.". Others have mentioned that an advantage of the helicopter over a fix-wing craft is that the ride quality in turbulence is better.

Can anyone comment about the ride quality of helicopters with stiff rotors versus that of fixed-wing planes?

[Vfrpilotpb]

Thank you Shawn, yes I come from Teetering heads(could be a good name for a group), I noticed quiet a lot more precision when going from the B206 to the Gazelle and even more precision still on the 109, but was eager to know if the rigid head made a lot of difference, just as an aside why have the makers gone on to produce Rigid heads, are they less bothersome for servicing in the field or are they more robust, when being thrown about by people in a hurry!

[Ascend Charlie]

Dave -
Because the rigid head of the BK has such a long virtual flapping arm, the control power for the pilot is huge - make a movement in the stick, and you have an almost instant response with fuselage attitude. Not like a teetering head, where you make an input, the disc responds, and the fuselage then follows a couple of days later.

Conversely, if turbulence makes a blade move, that movement goes straight to the cabin. Responsiveness does not necessarily equal comfort.

If you want a BK, get one with CSAS and autopilot to damp some of the liveliness out.

[Jcooper]

Hey shawn, do the EC 135s have a rigid head? Ive just seen a few pictures but I couldnt find any hinges on it. Just wondering...

[BlenderPilot]

The Bell 412 has a rigid rotor head according to what the Bell/FSI manual says.

Sure doesn't behave like a rigid rotor head (no looping capabilites), but it is the most stable helicopter I have flown, with the HP's of course.

[Anti-Torque]

ahh the b412 is an interesting one! I don't see how they can call it a rigid head when basically its 2 semi-rigids. Nothing like the old semi-rigid systems to make your butt numb!

[Droopy]

Jcooper - yes the EC135 does indeed have a rigid head, and it's very precise; combined with a particularly powerful tail rotor it will hold a high level hover steady as a rock. Turbulence is an issue however, sometimes to such an extent that one might even need to consider the head when choosing, say, a corporate machine.

[RW-1]

Can anyone comment about the ride quality of helicopters with stiff rotors versus that of fixed-wing planes?

Dave,

For the most part, (I would believe) any heli is better in turbulance for "ride" than fixed wing, for the simple fact that most of the turbulence the blades feel is not transferred to the fuselage. (Of course staying within the heli's limitations for said turbulence, etc.)

I wonder if Shawn would agree, certainly down here in SoFla, I'd rather be in a heli in thermal activity than be bounced around in a Cessna ... ;)

[Thomas coupling]

When we fly the 355 it soaks the vibes up in turbulence. The 135 however, is a different beast. It translates virtually all movement thru to the cabin. There have been occasions when I have had to turn back because the 135 was shaking too much, when the Squirrel would have simply wobbled a little!
The pucker factor is well and truly alive in the 135 in "tham thar hills" :\

The converse, however: +3.5g and -1.5g:ok: If only we could explore those limits now and again:ooh:

[Vfrpilotpb]

Hi TC,

Are you saying the 135 becomes so uncomfortable or is it approaching the extent of its envelope?:oh:

[Shawn Coyle]

First of all, the only 'rigid' heads that I know of are in the BO-105, BK 117 and Lynx. All others have a version of an articulated head, regardless of what FSI says about th 412 in their stuff. I have also seen a text that says that the AS-350 AStar and Twinstar have rigid rotor heads- not true. The elastomeric bearings in the rotor heads are hinges.
What to call the rotor heads of the Bell 430, MD900/902 and Ec135 is another story- they do not have the high hinge offset that the 'rigid' rotors have, but also do not incorporate any moving parts. I'd think someone would come up with a unique name for them.

Off Level Landings in 'Rigid Rotors'
The preferred method for the Lynx and BO-105 is after the uphill skid is in contact with the ground, leave the cyclic in the middle and lower the collective. You are controlling rolling moment about the uphill skid and the collective is a bigger control than lateral cyclic. Lateral cyclic will induce quite large moments in the mast that eat into the fatigue life. FOr the BO/BK, they put in a mast moment gage. FOr the EX-135, there is a similar gage, but it is buried in the electronic display.

[Letsby Avenue]

The Lynx is a semi-rigid rotor as it has a feathering hinge..

[Hilico]

Right, naive question time: by 'feathering hinge' I assume you mean the one that allows the change of blade pitch (as opposed to flap or drag).

Quite obviously every ship has a means of changing the pitch of the blades. Does this mean that a true rigid rotor relies on twisting the root of the blade to get the change, and the blade is bolted 'rigidly' to the head? Or have I misinterpreted 'feathering'?

[Jcooper]

Feathering, I have always been taught, is the hinge that allows the pitch of the blades to be changed. I only know (or I should think know) of the kman helicopters that do not have a feathering hinge. I believe they change the attitude of the helicopter by the "trim tab" on the blade as it is controlled by the pilot like an airleron. If I'm wrong (great great possibility) please tell me. Also is the kman heads not known as fully articulated, semi rigid, or rigid and if not what is it called?

[Dave_Jackson]

Jcooper,

You're correct, the feathering hinge is the pitch change hinge and the Kaman helicopters do not have them. Interestingly, the Kaman rotors do have teetering hinges and they have lead/lag hinges.

Their head is referred to as a teetering hub, but this does not fully describe it.


`````````````````________________________


Another helicopter with Rigid Rotors: :D :D

Here's a picture of a helicopter with rigid rotors that was take in Switzerland last week.

http://www.UniCopter.com/Temporary/Intermeshing_RC.gif


Rigid rotor helicopter added

[Vfrpilotpb]

Dave,.... thats a Tennis Court!!:D

[Thomas coupling]

C'mon Shawn,

Don't sit on the fence, the EC135 is either a rigid or it isn't:confused: No moving parts makes it a rigid as far as ECD is concerned.
It would have been an MBB 108 if the Krauts had their way, but the Froggies put paid to that. The head is a new generation MBB 105 head.

The MMI [mast moment indicator] is in the shape of an LED barograph running from green to red for mast bending.

Irrelevant in the air of course.......or is it?

VFR: No I didn't mean the EC135 was reaching its envelope limits - far from it. It reaches its passenger comfort limit quite quickly at times though :ooh:

[Dave_Jackson]

Vfrpilotpb,
Dave,.... that's a Tennis Court!!
True. The device is really a robot, which is holding four tennis rackets. It's playing a one hundred set match against a ball machine. :D

________________


I lie. :oh: It's not a robot, and they're not playing a match.
Honestly, it's a helicopter. It's using the ball machine to practice it's tennis racket effect. (http://www.UniCopter.com/B329.html#Tennis_Racket_Effect)

[RW-1]

I like it Dave, a model of a Kaman intermeshing?

(I have a concept 30 SX myself ...)

[Dave_Jackson]

RW-1,

Charles Kaman might shudder to hear; "... a model of a Kaman intermeshing?" :D
This model, with its rigid hubs and two blades per rotor, will experience (or has experienced) very high vibration during forward flight.


Trivia,

A close-up of the hubs and swashplates is interesting. They have set the phase lag at approximatly 45-degrees. If the craft had basic teetering rotors, the phase-lag would be 90-degrees. If they were able to produce 'absolutely' rigid rotors, the phase lag would be a theoretical 0-degrees.

[Vfrpilotpb]

Dave,

How is steering achieved with this Kaman type rotor arrangement?

[Avnx EO]

Jumping back a few items in this thread.....

Since it was mentioned, The main rotor system in the Bell 412 is actually a "soft-in-plane" rigid rotor, flex-beam type. (straight out of product data) I'm not a rotors guy, but I've been told that being "soft-in-plane" makes it behave more like an articulated rotor than a rigid rotor. It's technically called a rigid rotor because it doesn't really have a hinge like an articulated, it instead bends the material that attaches it to the mast to create almost the same effect. ..... if that helps.

My understanding is the fully articulated has the best ride - which makes sense since the blades are able to move with the most independence from the aircraft. A soft-in-plane rigid rotor is very close and differs only by the fact that there's no physical hinge.

The teetering rotor is next, as far as pure ride goes. The 2-per-rev can still get you though, but systems like the nodal beam suspension on the 206L-4 tranny correct for that, and gets the ride about as good as you can get with a teetering rotor.

BO-105s and the like, with true rigid rotors, are akin to sports cars with stiff, or almost non-existent suspensions. They can do fantastic maneuvers, but the price you pay is a rough ride. Every bump that goes into the rotor gets translated into the airframe. Every gust that lifts a blade gets translated into the airframe, etc. But I tell you it’s wild to watch them stand on their nose as they take off. On a rigid, when you tilt the disc... the whole world tilts with you.

Nick Lappos used to have some diagrams explaining all this stuff on his website but my link to it doesn’t work anymore.

[Dave_Jackson]

Vfrpilotpb,

The model helicopter has conventional swashplates and feathering (pitch) hinges. Their current method of steering is very close to that of the Kamans. Here's an excerpt from the e-mail accompanying the picture.
Helicopter seen in a fair in Switzerland sunday 7th of september. It has 5 hp, rigid rotors and it use both ciclic for roll, ciclic for fore and aft, opposite ciclic for yaw. I have spoken with its builders and they said that they used this control sistem to avoid the reversal yaw problem, they also told me that they thought to use difference in collective for roll, (in R.C. models it's easy to try different mix control by changing electronical parameters) but thay haven't tried yet because it works well as it is.