Collapsible Flettner Rotor Project

Thanks Jeremy.
If I use a Savonius S rotor, I'll have to flip it. If it's a cylinder I can simply reverse it.

max rpm needed will be 400 rpm in 20 kt wind. I have located a 10 inch bicycle hub motor with 3 speed controller 200, 300, 400 rpm. The rotor wiil be as lite as possible. Mostly foam.
Would you expect much gyroscopic force flipping at these rpms?

I believe it would not be too difficult to build a pm motor same size as diameter of rotor, slow speed and high torque.

 

Yes, the precessional forces will probably be significant.

Try lifting a bike wheel off the ground, spin it to 400 rpm and then turn the handlebars, that will give you a feel for the sort of forces you're dealing with. Reducing the rotor weight will help, but the forces are proportional to the square of rpm and only linearly proportional to the mass, so although reducing weight helps, reducing rotor speed has a much greater effect.

I've no experience with the Savonius rotor, but I thought that the lift/drag ratio was a lot lower than for a Magnus/Flettner rotor? I may be wrong, but one of the attractions of the Magnus/Flettner rotor is it's high L/D, which is significantly greater than normal boat sails, albeit with some practical drawbacks, like the lack of useful propulsion with the wind dead astern and the need to reverse the rotor direction when tacking or gybing.

 

There isn't much data using an S rotor for Magnus drive. Some of this data says it produces a superior force over the smooth Flettner cylinder. I intend to try both.

i'll have to cut power prior to flipping rotor. Do-able.

I plan to experiment with rotors and several other propulsion schemes. Even if perfected, I would use the rotor as auxiliary drive. I don't expect universal panacea from the rotor.

 

I was referring to the cost and complexity of a mechanism which retracts the magnus into savonius shape.

Now, it's all your project, ok. But if was mine, I would go for the simplest solution posible. A circular cylinder (magnus rotor) on an unstayed internal mast, just like Jeremy suggests, and as your original idea was. A flipping savonius rotor will introduce a whole range of new problems, constructive and handling complexity and cost. Think of future maintainance, corrosion and parts replacement issues too, not just about the initial cost.

Cheers

 

Hey Yo, check this page too (if you didn't already): http://kellycontroller.com/brushless-hub-motors-c-21_62.html
They have 4.5 kW motors with a brake and waterproof connectors for $480. 80% efficiency over a wide range of input currents.


Looks like a real bargain, do you think you can make anything like that for less?
Cheers

 

No I can't build for those prices. Thanks. I think the hub motor in screen grab below is interesting and comes with controllers.

For the rotor and support I have a simpler more rigid idea to solve some of the recently mentioned snags.
I'll post after doing the artwork.
Fortunately at sea I can usually demonstrate what I want done, or it's already a familiar task to my crew.
Nearest experience prior to this forum, trying to explain what I see in my mind, would be when ordering parts without catalog numbers. I included credit card info which aided understanding my gibberish, I think.
A picture is worth a 1000 words and since I'm not very good at explaining with just words, a picture probably saves me 10 posts explaining my explanations of the initial post.
Artwork and credit card to follow.

took a bunch of tries to upload the pic.

 

here is the 1st sketch.
instead of one tall rotor, two half sized, 8 ft tall rotors. Mounted in a rigid frame, both ends of rotor spinning on bearings.
My Albns are 8 1/2 ft beam. rotors can stow athwartship as pictured.
Frame also handy for rigging a sun shield tarp, important in Florida.

The frame tilts up vertical to port or starboard. Chinese junks had their masts next to the bukwarks instead of on centerline.

I'm thinking the rotors can be erected NOT on centerline. When erect, frame is braced in position with aluminum poles to opposite side.

The option of raising the rotor frame on either side, is the same as flipping the rotors upside down for use with Savonius rotors and thus reversing the buckets.

 

This is crazy talk.

This project has really truly gone off the 'rails' now.

Savonius rotors provide no lift as such, they just spin and generate electricity,

 

Rwatson, a savoius rotor will actually create a lift too, but will do it at a considerably higher drag expense. See my post #279 and the attached document with L-D graphs.

But very little lift and drag data is available around for savonius rotors, because their primary goal is the creation of torque and power. It makes the prediction of a savonius rotor for this particular use very difficult.

Cheers

 

I am working on the sketch of an idea for a simple mechanism for collapsing a Savonius rotor into a cylinder.
Then the rotor can be motor driven Flettner cyiinder for propulsion, and wind driven Savonius generator, both, but Not simultaneously.

Also I want to TRY the Savonius as a Magnus propulsion rotor.
I think KNOWING actual motive performance data of a motor driven S rotor, would have value, if nothing more than counseling other experimeters not to waste their money.

All the experiments of Magnus effect on S rotors I have found on the net, are wind powered. The blades or cups are revolving at less than wind speed.
Pure cylinders also do not have much lift at less than wind speed either. Correct?

If I spin the S rotor twice the wind speed, we may see a lot different L/D.
In wind generators the cups are spinning slower than the wind. The cups work something like sails and resist the wind and capture pressure in the cup interiors, thus driven.
Not a very scientific explanation.

Powering the S rotor in the same direction it would normally turn by the wind, but at 2X revolutions, the cup interiors are in the lee of the airflow and so contain a partial vacuum.

Trying to understand Magnus effect.

On the quadrant of the Flettner rotor revolving contrary to the wind, the surface airflow would be slowed.
Even more so on the S rotor because of the resistance on backside of cup protrusions?

On the opposite quadrant the airflow is accelerated by the spin. Again even more on an S rotor, because of the partial vacuum?

Worth finding out, don't you think?

 

The two half height rotors in the frame. Any recommendations as to separation distance?
Could they be in close proximity?

 

If you think of it along those lines, you will again arrive to the conclusion that the S rotor will necessarily have a higher drag than the F rotor. If you manage to create a simple and reliable mechanism which will make the S/F shape transformation, you might end up with a contraption for both upwind and downwind beats. Might be worth trying.

Earlier in this thread somebody has suggested you (if I wasn't hallucinating) to make a scale model first, and then test it by fixing it on your car and driving it at a controlled speed. I think that it is a good suggestion and that you should do it before committing your resources to a full-size project with an unknown outcome.

Cheers

 

Several papers I had read in these days (thanks to your project ) indicate s/D=0.15 as the optimum separation. If there is a central shaft, then the "s" distance is intended as the distance between blades minus the diameter of the shaft.

But these papers all deal with S rotor as a power-generating device (hence, drag-based), meaning that the said separation is very probably not the optimum one for your intended use of S rotor as a lift-generation device.

So, I'd say that s/D might be a starting point, but it might not be the optimum for you. A field-testing is necessary.

P.S.
then there is this type of S-rotor, with spiral blades: https://www.mdpi.com/1996-1073/6/6/...vf_IdHUYKt5_0VHzA&sig2=6vLC-PYJZVqepw6v8eJQmA which appears to exploits less drag force and more lift force. Kind of a blend of a savonius and flettner concept.

P.P.S.:
This arrangement of the blades (3-stage): http://old.uk.fme.vutbr.cz/kestazeni/vzkavn/savonius/48_9.pdf seems to give more power and much lower vibratory problems than a single-stage S rotor, because it creates three out-of phase vortex trains.

 

Thanks daiquiri.


Lots of ideas to think about.

here is a freehand sketch of a cylinder to S rotor mechanism.
Pardon the obvious dimension errors. I tried doing it in sketchup and wasted 20 hours, so drew it by eye. Several times! After awhile "good enough" attitude dampened my enthusiasm.

The cylinder could be kept closed with some straps or belts around it.

the S rotor would need brackets and pins to keep it open, I think.
carbon shaft arrows might make long lite weight pins through several ribs.

OR! since the S rotor motor always rotates one direction, motor torque could keep buckets open, if designed to do so.
If that were done, then reversing the motor would likely close the rotor to cylinder configuration, and it could be strapped securely, for rotation both directions as Flettner cylinder.

The concept is multiple ribs hinging on one offset corner. The ribs of course have a 1/3 arc cylinder surface attached.

Various degrees of OPEN need to be tried.

 

The rotor frame erected port side, windward side.

I drew aluminum triangle trusses comprising the frame.

This would certainly be sturdy but maybe over kill. Advice on an alternate frame with adequate strength but less windage appreciated.
Found a source for 4 inch square truss weighs 60 lbs for 20 feet. Smallest triangle truss located is 8.5 inch and weighs twice the 4 inch square.

the interior axels of rotors intended to be triangle trusses.

Oh, in addition to my cabin being a wind scoop (until it's closed in), the windshield is built of two safetyglass and aluminum framed hatches that open batwing saloon door style. MORE ventilation.