2 blade rigid rotor
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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
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
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.
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.
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.
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.
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).
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?
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?
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.
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.
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?
I see RC helicopters seem to be set up this way as well, obviously different scale of loads but surely it proves the concept?
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.
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.
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I see RC helicopters seem to be set up this way as well, obviously different scale of loads but surely it proves the concept?
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?
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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.
As as I work through this the force required to induce feathering (and therefore control) is he only issue I can see.
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?
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?
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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
Thanks again for all the responses
Bill
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.
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.
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Vibration issue
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.
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.
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.
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.
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.
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.
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.
