2 blade rigid rotor

[discap]

once again I will prove my ignorance of all things Helicopter. Is it theoretically possible to build a 2 blade ridged rotor system. What I am envisioning is the teeter and feather replaced by the same type of plates used on a multi bladed ridged system.

I have been thinking about this for a while and cannot come up with a reason that it would not work. Is the amount of flapping in a 2 bladed system too much? Seems like it would eliminate mast bumping and tail amputations.

Please educate me.

Bill.

[Ascend Charlie]

Feathering will always have to be there, to allow us to change collective pitch, and to get differential cyclic pitch to move the aircraft in the chosen direction. I believe there were a couple of attempts to remove feathering bearings, and just twist the blades to get it, but apparently that was not successful.

2-blade teetering systems exist because they are simple and CHEAP. The CHEAP side of it makes them the accepted starting point for most low-time people, and they are generally the ones to have problems with it.

Next step up is 3-bladed articulated systems, allowing the blades to shuffle around the plane with the lead-lag forces. Then add more blades as desired. Articulated hinges can be replaced with rubber/ plastics as in the AS350 series.

The most rigid is the head of the BO-105 and BK-117, no flapping or lead-lag hinges at all. Flapping is allowed by the flexible blades, not by hinges.

Early experiments with rigid systems caused crashes - the experimenters couldn't see why their models would fly but the real things wouldn't. Then they saw that the models used bamboo blades which allowed flapping - so they devised the teetering head to allow flapping, and away they went.

[gulliBell]

Isn't the 214ST a 2 bladed rigid? Excuse my ignorance, we did operate them, but I never got to fly one. They had air-conditioning so the Nationals had first pick at flying them. The expats got to enjoy 42 degC at 4000' in a 212 with the windows open. Well and truly oven roasted by the time we arrived at the other end.

[discap]

From my understanding even the rigid heads all for feathering without a dedicated hinge. So that doesn’t explain why it can’t be done.

Maybe you need hydraulics in order the activate the feathering.

[GrayHorizonsHeli]

i think its considered an elastomeric head. but i'm not endorsed, so I could be wrong

[discap]

An elastomeric bearing simple replaces the other bearings in the feathering hinge.

[Ascend Charlie]

It had a head a bit like on the 412, a flat plastic yoke instead of a tubular metal one, better able to cope with lead/lag forces.

[SASless]

Excuse my ignorance, we did operate them, but I never got to fly one. They had air-conditioning so the Nationals had first pick at flying them.

Your curiosity fail you?

Never walked up to one and take a look at it to see what she was made of while you were quaffing cold bottles of water between flights?

Of course you were probably isolated at Tanijiib and never got to where the 214ST's were located.

[jayteeto]

The 135 Head is so rigid it doesn’t exist

[wrench1]

Isn't the 214ST a 2 bladed rigid?.
Semi-rigid. It's more a mutant version between a 222 and 212 head with elastomers, plastic blades, and inflight blade tracking.

[discap]

Nick Lappos are you out there listening. I was hoping you might chime in on this one.

[JackJones]

The 412 head is basically 2 x 2 bladed hubs bolted one on top of the other, so if you took the second "head" away, and beefed up the remaining blades that could work - but you now lose the offset hinge advantage in the head, so controlling the machine will be quite different.

[Robbiee]

2-blade teetering systems exist because they are simple and CHEAP. The CHEAP side of it makes them the accepted starting point for most low-time people, and they are generally the ones to have problems with it.
.

CHEAP never crossed my mind when I was admiring a Bell 222 at Heliexpo.

[Ascend Charlie]

Cheaper than a Bell 430...

[gulliBell]

...Of course you were probably isolated at Tanijiib and never got to where the 214ST's were located.

Right on the money. And the 214ST's were fenced off at RT, and as you know, climbing fences in that part of the world is ill advised.

[wrench1]

From my understanding even the rigid heads all for feathering without a dedicated hinge.
By definition, a rigid rotor includes feathering but not flap or lead-lag movement. Could a 2 blade rigid system be built? Sure, but what would you do with it. The ride would be terrible. And with any type of useful load, I would imagine the bending forces at the hub/mast/xsmn would require those components to be pretty substantial in strength/weight/size.

[[email protected]]

Discap - have a look at this description of the 212 head and see where you could replace the hinges with plates http://www.thecontrols.ca/References/PTM/212%20PTM09%20Main%20Rotor.pdf as wrench1 says, I don't think it would be very practical

[haihio]

If I’m not mistaken the ec135 does not have feathering bearings and pitch change is achieved through twisting of the blades.

[wrench1]

If I’m not mistaken the ec135 does not have feathering bearings and pitch change is achieved through twisting of the blades.
The 135 M/R system is bearingless and hingeless. But feathering and flap (to a limited extent) takes place through a "flex-beam" mounted inside the hollow blade cuff. Lead/lag is handled by elastomerics mounted externally on the blade cuff.

[Washeduprotorgypsy]

Now if everyone just stopped goofing off in shop and math class , or on the computer they could of built the Hiller XH -44 in their dads' garage at the age of 19, just like Stanley Hiller. ....or so the story goes.

The "original" Sikorsky ABC concept got its start in Stanley's garage, with a two blade rigid counterrotating head. There are clips on YouTube. Besides the maple seed, It actually probably gots its start with the turn of the century "Wright brothers" types with their steel tube and fabric, hopping contraptions. Scratching their heads , not knowing that without the teetering hinge the rotor will roll to equality vs flapping to equality. Stanley wasn't preoccupied with the two legged birds so he had this all figured out and made a counter rotating head to cancel things out. I can't seem to find the tale of vibration in the story but would guess from the UH12 rotor quickly superseding the xh -44 's enroute to mass production that it lurked not far beyond translation.

Apart from the vibration caused by the inevitable poor bedfellows of coning and flapping without an underslung hingeline ( or multi blade head that is called "rigid" but accommodates some lead lag) I would think the stability and gust response on a single rotor design would make the Boelkow 105 seem a docile beast in comparison. Lockheed anticipated this on a model prototype in the 50's building a rigid two blade that was stabilized by linking it to a gyroscope ring atop the rotorhead. Needless to say they quickly abandoned it due to vibration in favor of 3 and 4 blade heads in the lead up to the Cheyenne project.

I say follow Stanley's lead and build a model RC helicopter refit to a rigid two blade head slowed down to 300 to 500 rotor rpm. I'd advise motorcycle gear and hockey equipment. Happy Tinkering

[wrench1]

Seems like it would eliminate mast bumping and tail amputations.
Maybe an updated version of the single-blade BO-103 would also reduce those issues. Provided you could add another seat, and a windscreen to keep the bugs off the crews white shirts.

[discap]

Thanks for all the input, but I still don’t see a reason that this cannot be done. A teetering rotor head has no lead lag (at least not active movement). All of the rigid systems either twist the plates or allow flex in the blades to account for feathering and flapping. Why can’t these be carried over to the 2 blade system.

It it seems to me that a flexible blade would not be any harder/more expensive than a rigid blade. It also seeems that a stack of plates would be more cost effective than a teetering mechanism.

I hate being slow here, but’s what am I missing?

it just occurred to me that I never introduced myself here. I have posted on other sites but this one seemed to have more technical gurus.

I am a low time pilot (4000 hrs of combined fixed and rotor). I have never been paid to fly, so all on my own nickel. I am an engineer and I built and fly a Safari Experimental. I have developed several balancing techniques/devices for the ship so that thes are in line with most comparable certified ships.

thanks

bill

[IFMU]

The problem with the rigid rotor is dissymmetry of lift. Juan de la Cierva figured this out.
https://en.m.wikipedia.org/wiki/Juan_de_la_Cierva
So can you crank in enough cyclic pitch to overcome this dissymmetry? Let's say you could. For a US rotation helicopter you need a bunch of right cyclic as you start going forward. But if you allow a bunch of left cyclic you give a tool to the pilot to get in a lot of trouble. And then what about sideward and rearward flight? I guess the pilot needs some of that addional travel to account for it. But how do you limit his authority in the down flap direction when he changes mode of flight?
Flapping does all this for us and it works as a bias summed with the collective control. The stick does move laterally with airspeed but not really all that much that we can't live with it.

[whoknows idont]

The problem with the rigid rotor is dissymmetry of lift. Juan de la Cierva figured this out.
https://en.m.wikipedia.org/wiki/Juan_de_la_Cierva
So can you crank in enough cyclic pitch to overcome this dissymmetry? Let's say you could. For a US rotation helicopter you need a bunch of right cyclic as you start going forward. But if you allow a bunch of left cyclic you give a tool to the pilot to get in a lot of trouble. And then what about sideward and rearward flight? I guess the pilot needs some of that addional travel to account for it. But how do you limit his authority in the down flap direction when he changes mode of flight?
Flapping does all this for us and it works as a bias summed with the collective control. The stick does move laterally with airspeed but not really all that much that we can't live with it.

Obviously discap was not asking about a truly rigid rotor but rather about a hingeless head. All the issues you are explaining are solved on hingeless systems, such as the 105. Now the question remains why this hasn't been realized with a two blade system yet. I've been asking myself the same question and so far I don't see any convincing answers on this thread. I may be wrong but I suspect that helicopter designs have been evolving towards higher number of blades solely due to the obvious reasons, not because of the elimination of the hinges.
For example the evolution from 212 to 412, I don't think they put on the two extra blades because they wanted to go hingeless. They probably wanted to develop an overall more modern rotor system, with all its benefits (if these elastomeric bearings were such a good idea is a different question).

[chopjock]

Presumably a Bo105 would still fly if two blades were removed... I heard someone did it on a 500C model.

[[email protected]]

If you can relieve the stresses that occur from flapping and coning by using a hinge (mechanical or elastomeric) why would you go to all the trouble of engineering a complex solution to a problem you have already solved?

You have to make a blade now that bends and twists enough to absorb all the periodic lift changes without transmitting them back to the mast and fuselage as well as making a control system to input that twisting when you want it to happen. Good luck, but why bother?

[Ascend Charlie]

So can you crank in enough cyclic pitch to overcome this dissymmetry? Let's say you could. For a US rotation helicopter you need a bunch of right cyclic as you start going forward.
IFMU, the phase lag from dissymmetry of lift results in the blade reaching its peak of travel at the front of the disc, so you need forward cyclic to stop it, not right cyclic.

A rigid rotor would not have underslinging, so the coriolis effect would cause some extra stresses on the blades/grips/head, which a teetering head minimises.

[Jelico]

Like the original poster I too have always wondered why there hasn't been more development in a two bladed rotor head that is either articulated or rigid. To my simple brain it seems like it would be a good functional system - easy to hangar, cheaper to build (less blades), potentially more efficient? Maybe not cheaper than an underslung system but surely cheaper than a 3/4 blade setup? A two bladed helicopter that you couldn't mast bump sounds like a pretty good setup to me?

I see RC helicopters seem to be set up this way as well, obviously different scale of loads but surely it proves the concept?

[Ascend Charlie]

2-blade articulated? Sorry, it won't work. If there is lead/lag in the head, then the coriolis effect will put both blades onto one side of the disc and the imbalance will destroy the aircraft very quickly.

With 3 or more, the blades can shuffle around a bit and even it out, but 2 blades must be forced to stay opposite each other.

[Washeduprotorgypsy]

I see RC helicopters seem to be set up this way as well, obviously different scale of loads but surely it proves the concept?

I don't think the practical advantages of ease of storage , safety and ruggedness of a small rigid 2 blade trainer are lost on the manufacturers. Especially the "boutique" ultralight helicopters that seem to crop up every year or two in Italy or Eastern Europe. But they are competing with the likes, handling and ride of a Cabri or S300. If it rides like a jitterbug/buckboard and needs a functioning SCAS to make them manageable as the RC ones do, maybe it's just easier to deal with the third blade.

Interesting that we don't see helicopters beyond the light/intermediate twin class marketed as "rigid". They all revert to fully articulated. Might the same phenomenon of scale be at work as we progress from rigid head rc toys to full size underslung teetering machines?

[discap]

WhoKnows thank you, yes “hingeless “ is a better descriptor that “rigid”. Flapping would be absorbed by the blades/plates. Feathering by twisting the plates. Crab this would conceptually be a simpler system having fewer parts than a teetering system. The advantage would be no mast bumping. The control system should be identical to standard controls.

As as I work through this the force required to induce feathering (and therefore control) is he only issue I can see.

[whoknows idont]

Have you given mast moment some thought?

[[email protected]]

Good point - you get rid of the minimal threat of mast bumping and replace it with the sorts of forces that would snap the rotor mast instead - unless it was massively engineered.

Plates might be conceptually simpler but how will you make them flexible enough to flap and feather yet strong enough to resist aerodynamic back forces?

[discap]

Ok I have already admitted that I am ignorant, trying to fix that. Why would the forces on the mast be different on a 2 blade system than on a multiple blade system that uses plates (hingeless) for those same functions? As I understand it the blades and plates on the hingeless systems absorb those forces, am I wrong here?

Thanks again for all the responses

Bill

[Ascend Charlie]

A teetering system has zero offset. The aircraft hangs underneath a single point of suspension. A bit like a "T" with the vertical part being the mast, and the horizontal bit being the disc. There is a simple loose bolt attaching the mast to the disc. The disc can tilt without really moving the mast. it doesn't have a Moment.

When the disc is tilted by cyclic input, the disc then pulls on the top of the mast, and the fuselage follows a little later, like a bucket of water under a handle. Get the bucket moving, then suddenly stop the handle, and watch the bucket continue forward but then swinging up because its suspension point has stopped. The water doesn't spill, but there is delay between the movements of the handle, and the bucket following. This is zero offset. In the hover, you can waggle the cyclic like crazy, but the fuselage doesn't do much.

With a fully rigid system, the T no longer has a bolt or hinge. the blades are attached at the ends of the T, and are a bit flexible. Move the horizontal disc by tilting it, and immediately the mast is forced to follow. This can cause some big stresses. The disc now has a Moment over the mast, and depending on how far out from the mast are the blade attachments, the moment generated will be more or less. This is the amount of Offset.

In the air, it means that the aircraft is very responsive, and in fact for the BO105 and BK117, they are aerobatic, and can generate -1g safely. (A teetering head must maintain +1g to be safe.) The downside is that the fuselage follows the disc very rapidly, making the ride a bit upsetting for the passengers if the pilot is at all aggressive.

On the ground, just moving the cyclic can exceed the mast's safe limits, so there is usually a Mast Moment Indicator fitted to reduce the chances of breaking something. Slope landings get a bit tricky.

[AMDEC]

once again I will prove my ignorance of all things Helicopter. Is it theoretically possible to build a 2 blade ridged rotor system. What I am envisioning is the teeter and feather replaced by the same type of plates used on a multi bladed ridged system.

I have been thinking about this for a while and cannot come up with a reason that it would not work. Is the amount of flapping in a 2 bladed system too much? Seems like it would eliminate mast bumping and tail amputations.

Please educate me.

Bill.

Do not think about it. It would be a perfect shaker.
The flapping motion of the blade induces some vertical force at the flapping hinge that directly varies with the flapping angle. If you sum the contributions of the different blades, it gives you a constant moment in the fixed frame... as soon as you have 3 blades or more.
With a two-bladed rotor, the vertical force on both blades is opposed (when one blade is up, the other is down). This makes a pure moment at the hub center, which varies cyclically, like the flapping. The only way to get rid of it is to have no flapping hinge offset. All 2-bladed rotors are see-saw rotors.

[discap]

Ok after the last two posts I finally get it (I told y’all I was slow). I knew I came to the right place

thanks

bill

[AAKEE]

Good posts from Ascend Charlie and AMDEC!

To add to the reasons why there is no rigid two-bladers:
The rotor head on a rigid/hingeless rotor will be exposed to very high forces. The MBB BO1XX rotor head is made from titanium which is expensive.
The mast will be exposed to high bending moment, an would also need to be made from expensive materials.
To be sure that the mast not suffer from mast moments exceeding the limits there might need to be a mast moment indicator, which also doesnt seem like a cheap solution.
As for the BO’s we know they cannot be flown without hydraulics due to high forces comming from the rigid rotor system to the cyclic and collective, making the hydraulics vital for the safety. It has to be two hydraulic system with a system ensuring that if one system fails it will not compomise the remaining hyd system ability to let the pilot control the helo. Not cheap either.

I would say that the main reason to build helos with two rotor blades is simplicity to keep the price and operating costs down.
The two blades solution doesnt mix well with the above requirements for a rigid system.

[whoknows idont]

Do not think about it. It would be a perfect shaker.
The flapping motion of the blade induces some vertical force at the flapping hinge that directly varies with the flapping angle. If you sum the contributions of the different blades, it gives you a constant moment in the fixed frame... as soon as you have 3 blades or more.
With a two-bladed rotor, the vertical force on both blades is opposed (when one blade is up, the other is down). This makes a pure moment at the hub center, which varies cyclically, like the flapping. The only way to get rid of it is to have no flapping hinge offset. All 2-bladed rotors are see-saw rotors.

Finally a reasonable and satisfying explanation to the original question! Thank you, Sir (or M'am)!