Gyrocopter rotor instead of canvas sail ?

Assuming a relative wind speed of 15 knots and a rotor tip speed of 300 knots, the advancing blade would see 315 knots while the retreating blade would see 285 knots airflow. The difference in lift over the rotor disc would cause a rolling moment on a reach in addition to straining the blade roots. This would require a massive rotor, while a teetering hinge would allow the rotor to see-saw to equalize lift over the disc. The angle of attack of the advancing blade would be less than the angle of attack of the retreating blade. To an observer at the stern looking forward at the disc edgewise, the disc would be canted relative to the rotation axis. While there are other types of blade articulation that compensate for dis-symmetry of lift, the 2 bladed teetering rotor is by far the simplest.

 

I understand that a rotor without teetering hinge would see asymmetrical lift, and that this results in a rolling moment. However given that the boat is already dealing with a significant rolling moment (due to the entire rotor pulling on the top of a mast) this is less of an issue than for an autogyro. I suppose that the oscillating lift induced stresses in the root would promote fatigue failure, but I think they would be much less than the oscillating stresses due to gravity (not an issue for approx. horizontal autogyro blades).

I still don't like the idea of tacking by swinging the rotor all the way through the wind, because during the middle of the tack it will be providing a great deal of drag downwind which will slow the boat, potentially preventing a successful manoeuvre.

 

Then a feathering blade with symmetrical airfoil and heavy construction would be the alternative.

 

High blade angle of attack stalls the blade. Normally the inner retreating portion of the disc is stalled. By reverse flow in that context I meant reverse flow over the disc or relative to the rotor tip path plane. Heeling could be reduced by having the advancing blade on the lower part of the disc on both tacks in a feathering blade rotor.

 

That wouldn't be mechanically complicated at all. lol.

 

Having a feathering *symmetrical* rotor would certainly be attractive: It solves the depowering challenge, and halves the rig swing angle .
However, i fear that a lot of efficiencyy would be lost, not only because symmetrical airfoils are less efficient, but also because the symmetrical approach is not compatible with the usual blade twist allowing to even out the lift across the rotor disc. (Not sure if increasing the blade root chord could help, while remaining efficient. And it would increase windage at rest.)

Unless going for the symmetric rotor, we'd see slightly odd, though probably not unmanageable behaviour if the whole rotor is tacked:
- On one tack, the blades would go against the wind on the high side, on the other tack on the low side.
- So the rotary sail's center of effort would be at the different heights.
- Forces on the whole rotor would be a tad lower too for the latter (wind speed gradient + slower apparent flow on the upper part of rotor).
But since a rotor on a rolling mono sounds like a receipe for disaster in anything except really fair weather, i'd simply assume that we go for a multihull with massive reserve buoyancy and decide that this problem can be neglected.

How great to see someone come up with real-world ballpark angles and all these clear pics !
What i don't understand so far is what kind of angle (relative to the hull) we'd want for downwind VMG sailing, analogous to a gennaker.
(I'm pretty sure that we could go faster than with the 90° approach (rotor used as a large obstacle to the slow tailwind), and perhaps faster than with the 'geared' approach too: since there are no rollerbearings on the ocean, a boat won't come anywhere near the impressive speed too windspeed ratios of the Blackbird landyacht.)

I tried to think along comparable 'self-feathering' lines for a proa, which wouldn't need the whole 180° rotation, and would also be an easy way to keep the 'fast' side of the rotor up where the wind is.
With two rudders at each end, more or less balanced but in opposite directions, one could probably achieve a let-everything-go-behaviour where
- the upwind rudder will turn perpendicular to the flow, producing huge amounts of drag like a kind of sea anchor, and
- the rest of the boat 'hangs' downwind of said rudder, effectively turning the rotor out of the wind.

 

Agree that makes sense. Of course they should be able to find many places where the message is still fresh enough to hit the news, and where nobody will care about the french ballast in the renewables discussion, or worry much about the sponsors.

 

For a fixed pitch rotor, which necessarily always presents the same face to the wind, what mechanism would you use to control AoA? Basically you need to be able to control mast rotation without any end stops. Perhaps a continuous line system similar to some genoa furling setups?

 

Swashplate helo head on stereoids, but quite feasible at RC scale as far as i can infer from these crazy '3D flying' videos:

 

A worm drive ?
(The 1933 boat seems to use a long lever)
(Hydraulic cylinder with a linkage like on an excavator bucket, allowing to control a 180° turn ?)

 

But this won't be at RC scales is it? And also hinged/pivoting tangental to the hub axis... If you don't think what you suggested will be an engineering PITA then sorry but...

 

I do, i do

(I was just trying to point out that if someone is willing to build a model, easily available stock components would also allow to test out many of the more ambitious ideas around this thread.

EDIT: An aerobatics RC helo head is just 200$ or so on helidirect.com who also have great blades* in sizes that match IOM rc boat rig sizes. That would allow a comparison with good soft sails on a fast** stock platform like this: One metre Foiling Multi hulls - New Zealand Radio Yachting Association Inc. http://www.nzradioyachtingassociation.co.nz/2017/09/05/one-metre-foiling-multi-hulls/ , by Ian Holt and Dave Burke, known from the thread below : (*: these blades are all asymetric and twisted, so the boat could only sail really well on one tack. But for a Proof of Concept, who cares ? **A high-speed platform is desirable because it will be more sensitive to pointing ability, and exacerbate the differences for beam reaching as well.)

Unfortunately, as far as i'm concerned, the rotary sail will remain a thought experiment:
- I'm too short on time&skill to build such a model within reasonable time (i'd be happy to contribute a bit to the cost though, if someone else moves ahead), and
- I don't currently have adequate funds, nor nearly enough aero knowledge to bring it to life, or even simulate it thoroughly at cruising scale (Cruising is my usual focus, thus all the comments at cruise scale. But the design i'm working on is low-tech compared with what we're discussing here),
- let alone at the drone or cargo scales and reliabilities that could actually turn out useful.

I'd be glad to see anyone doing it!
(Still somewhat surprised that nobody came up with a KO argument against the experiment yet )

 

My point was that you need to be able to do more than a 180° turn, you need continuous rotation. Consider doing a 720° turn of the boat. If you always have the same face of the rotor facing towards the wind then the rotor has to swing twice round (relative to the boat). You could tack the "wrong" way with the rotor backed as you pass through the wind, but then there is a risk of it stopping.

For a "feathering" rotor you don't need to be able to continuously adjust the pitch (as per helicopter collective control), it only needs to be at the two limits. Given that, could it not be set up to switch automatically as the rotor plane passes through the wind? If so there would be no need for a complex mechanism (e.g. swashplate).

 

Ok, got it now, thanks! Fixed pitch rotor needs continuous mast rotation.
I like the simplicity of your continuous furler, but i also like the auto-locking of the worm drive.
(EDIT: continuous furler matches the self-feathering mast/rotor like described before, whereas worm drive could provide safety in arrangements where we would rather want to keep the mast/rotor angle under firm control at all times.)

Oh yeah, just like a Maxprop. Now that is a simplification!
Such a mechanism would allow the rotor to work in both directions, but it would not provide the auto-feathering we know from boats.
It should be easy enough to add some kind of zero pitch lock though.

 

I was thinking that you would feather the whole rotor plane such that it swings up to zero AoA. I figured you could achieve this overall feathering by having the rotor hub aft of the mast, so that the thrust acts to rotate the rotor disk. This is much like how autogyros have a torque tube. The nice thing about this is that you could use a sheet and thus tack (and gybe) just like a normal yacht. There is even a fixed leading and trailing edge (of the rotor disk).

By contrast a fixed pitch rotor is more like a square rigger, with wind blowing onto only one face, but leading and trailing edges (of the disk) switching. Obviously that means easy gybes, but makes tacking more complex.