Big Peripheral Rotor-Bearing, practical? 1

[Key200]

Hi everyone, this is my first post here, so please feel free to move this to another section if this is an inappropriate one, Im really not sure whether this question belongs more to rotorcrafts or mechanical engineering section.
I am by no means a huge expert in absolutely all rotorcrafts´ aerodynamics and mechanics. However I am fairly knowledgeable in these areas and have also built many RC models to test various ideas and concepts.

My question here is related to the possibility or impossibility of a certain diameter-big peripheral rotor-bearing in a theoretical helicopter design.
The issue I am trying to investigate to see if this is a practical flying machine or not, lies thus in the design of this big peripheral bearing itself! My first thoughts: Heavy, Complex, really power consuming, unreliable and fail ready. But am I cynical?

To make an assumption, consider this theoretical helicopter:
Very low discloading, total diameter of 30ft, the peripheral blades are 4ft in length each, about 10 or more blades total running around a circular body. Rotor rpm is low: 50-70rpm. Gross weight 600lbs. Structural stresses should thus be relatively low I think.

The Bearing issue: ( check the attached pictures below. Sorry about the crude drawings, my 3D skills would take more time to remember them again)

http://postimg.org/image/4fc5qhkst/

http://postimg.org/image/ip3zihy5b/full/

A 30 ft diameter ring makes the bearing rail/way. Each SINGLE blade-outer-tip has two ordinary ball bearings on top and bottom, one bearing travels on the top surface of the red ring and the other bearing travels on the bottom surface of the red ring.


(you probably realized I am after true Absolutely rigid rotors)

So my question to you which I would greatly appreciate is your opinion on whether this giant radial bearing concept is impractical or possible? I mean, the disc loading is
supposedly low, rotor rpm is low. So the stresses should be low(?). These are the main reason for me thinking this idea could have any hope. Is this big radial bearing still not a practical idea? If so, why, how, and what could be a better alternative from your opinion?

Thanks for your time
Best Regards
K.A

 

[IRstuff]

I think the added complexity, weight, power requirements, etc., all conspire against something like this.

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[3DDave]

The closer the bearing is to the center of rotation the lower its speed. The only advantage to greater diameter is greater moment resistance, such as found in cranes and other overhung loads.

 

[Key200]

Thanks for the reples. Indeed I agree too about eh added weight and complexity. Yes 3DDave, the whole reason behind this idea is to have very rigid rotors without having very high stresses point(like blade root) or very heavy rotorblades. Such a bearing seems to me the only method of having very rigid and light rotors

IRstuff, I wonder however one thing:

If we were to compare two designs:

One is an ordinary helicopter in a sense that it has a central traditional bearing BUT the RED fixed DISC & RING structure is still there attached so there is a completely unnecessary and completely useless weight added to the craft(I know, but please bear with me, I am trying to illustrate something)

The second design is simply adding ball bearings on the tip of the rotor-blades and thus is the one just as described in my Opening post.

The apparent difference between the two (everything else being equal is the addition of ball bearings only apparently), is the weight .

Now with the two designs having the exact same fixed disc and ring body structure, do you still think that the second design(with bearings at bladetips as in OP) is still heavier or more complex and far more prone to failure?
You see my point? I am interesting in to know what to consider in the addition of blade tip bearings that slide around a smooth ring.

Regards
K.A

 

[GregLocock]

if nothing else you'll run into the problem that flapping blades were designed to eliminate.

Cheers

Greg Locock


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[3DDave]

Here's a basic rule for exploring engineering design concepts: If there's an idea that seems really good and no one uses it, what is the reason no one does? While it's possible for anyone to have some breakthrough idea, it isn't likely that the person with the new idea is unaware of the limitations that have prevented others from making that same breakthrough.

The first clue is that the small diameter bearing at the periphery of the fan will have the circumference of the fan to cover. Figure 500rpm on a 10 foot blade with a 1 inch bearing. What rpm will that bearing be turning? If you are thinking of sliding friction then you can figure out how much power will be absorbed in that friction and how hot they will get until they can dissipate at steady state.

Second clue is how much more will the peripheral bearing that has increased stiffness weigh over centrifugally stiffened conventional blades? I'm guessing a 10 inch by 15 inch by .125 inch wall aluminum tube. It also needs to have a blade-like structure to hold it to the helicopter body against gyroscopic loads on the rotor blades, so include that as well.

A free-body diagram will go far in determining why this is not a popular idea.

It is also related to why peripheral bearing aren't typically used in bicycle and automobile rims rather than the compact central bearing.

PS - did anyone else get 120,00rpm for the example? That's a lot for a bearing, particularly one taking the side-load from the weight of a helicopter.

 

[MikeHalloran]

Assuming that the outer ring levitates itself for the moment, rollers under and over the blade will change vertical position as the cyclic or collective pitch changes. Sure, you can add a swivel yoke and a couple more bearings to each blade tip, again at some cost in weight.

The rotation speed of the rotor is usually chosen to limit the tip speed to subsonic at max rotor rpm and max translational speed. Arbitrarily reducing the rotor speed, say in the interest of noise abatement, also reduces the available lift, and even with turbine engines, real helicopters have to struggle to fly.

You can't just ignore the weight issue, because in aircraft, weight is of paramount importance to every part.
That in turn is why aircraft structures are designed to the highest stress that they can tolerate; all the mass that you don't absolutely need must be trimmed off; otherwise, the aircraft can't leave the ground.








Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

Looking at the images and specs linked in the OP (30ft dia rotor, 60rpm, 600lb GW) it appears the blades only extend about 5ft outboard of the center disc. With a 600lb GW and 8-12 blades, the out-of-plane moments at each blade root would be fairly modest. Thus it would seem more weight efficient to use the conventional rigid rotor approach of attaching the blades at their root end using a pair of preloaded rolling element feather bearings. Making a very stiff 5ft rotor blade is not difficult with modern composite materials.

3DDave points out the most obvious issue with this concept, which is the dN number the bearings will operate at. If the bearings follow a circumferential path of 30ft diameter at 60rpm, that would be a surface velocity of 67,858 in/min. If the OD of the bearing roller was 1" then the bearing speed would be 21,600rpm and the bearing dN would be over 300K. A dN that high would require the use of a recirculating oil lube system. The bearing rollers would need to be preloaded at the contact with the track surface to prevent skidding. Also the track surface would need to be surrounded by a tightly sealed housing to prevent the rollers being damaged by rolling over even tiny debris particles.

The one suggestion I would make to you is rather than using a pair of rollers to constrain your blade tip to the outer track surface, why not see if you can get a pad type air bearing or foil bearing arrangement to work. Your surface speeds are high and your normal forces are low. Plus the air bearing would not require any lubrication, and would be relatively insensitive to debris since there is no contact with the track surface during operation.

 

[Key200]

Thanks for you inputs everyone,. The whole point of this was originally to make the rotors as rigid as possible, without having to make the rotors very heavy carrying this huge inertia that is metallurgically/structurally very complex and dangerous IMO. My line of thinking was rather having the rotating structure light and the fixed structure heavy than the opposite. I see now however that bearing dN, lubrication and tight sealing are what really potentially stopping this idea..

I will think more about this

 

[3DDave]

Thanks in turn - it's an interesting thought experiment to look into the why-nots.