Gyrocopter rotor instead of canvas sail ?

Discussion in 'Hydrodynamics and Aerodynamics' started by CocoonCruisers, Mar 22, 2019.

  1. CocoonCruisers
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    CocoonCruisers Junior Member

    Yes, i was assuming that. The 'whole rotor / overall feathering' was already mentionned a few posts back.
    It would be a great way to slow the rotor, and making this a default behaviour would obviously make the the boat much safer.

    Yet i thought that one might *also* want to be able to lock the *blades* at zero lift or in fully feathered state, just to avoid self-start of the rotor when docked or stopped.
    It's probably easy to achieve in a hub that is already designed to let the blades work both ways (Otherwise i suppose we'd rely on a brake or on straps to lock the rotor).
     
    Last edited: Apr 3, 2019
  2. CocoonCruisers
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    CocoonCruisers Junior Member

    Still thinking about tlouth7's bi-stable rotor from post #58 :
    If i think of this, for cruising scale and above, with
    - a hub with a ring of bevel gears allowing synchronized 180° rotation of the symmetric blades (= from fully feathered one way to fully feathered the other way around),
    - the blades axis' arranged so that the blades always tend to turn their leading edges into the wind,
    - a simple device (some kind of lock) that limits rotation to the useful pitch (+6° and -6° for examle) when in operation,
    - but releases the pitch automatically in case of overload/overspeed so that the blades will feather,
    - plus ideally a way to trigger the feathering manually as well,
    - a little starter/generator used for rotor start, electricity generation, final braking, and to turn the rotor to a parking position where it can be locked. (No clutch needed, electronics should be enough).

    then i think we'd get a fairly safe system with
    - overload protection
    - redundancy for depowering, through the feathering plus the possibility to steer or turn the rotor perpendicular to the wind
    - reasonably simple tacking and gybing,
    - simple controls
    - same behaviour / same height of the rotary sail's center of effort on both tacks
    - self-tacking, if this is deemed desirable, OR a behaviour where the rotor turns out of the wind when the sheet is released, OR neutral behaviour (Pick one. It only depends on rotor hub offset from the mast axis)
    - relatively simple structure because we could avoid large mast rotation angles, let alone continuous mast rotation. (Rotation needed might be as little as 60° on a fast platform able to build apparent wind when broad reaching)

    The only part i don't like about this approach remains the reduced efficiency of a symmetrical rotor (less efficient foil section, and cannot use twist, cf post 51).
     
    Last edited: Apr 5, 2019
  3. tlouth7
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    tlouth7 Senior Member

    Now that I have convinced myself of the how, I turn to the question of why. What is it that a gyro-yacht would offer that makes up for its complexity? The advantage of an autogyro over a fixed wing aircraft is that it does not stall at high angles of attack, allowing flight at very low forward speeds. Is this relevant to a boat where we can rotate the rotor to any desired angle of attack? Potentially it is better than a conventional sloop dead downwind if it is more stable?

    The main advantage that I can see would be if the gyro offered a better lift-to-drag ratio than a sloop rig for equal thrust and heeling moment (or conversely the same L/D ratio for lower heeling). I do not know whether this is achievable in practice.
     
  4. Angélique
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    Angélique aka Angel (only by name)

    They also need the long times in that many ports to store the then surplus captured wind and solar energy in their batteries and H2 tanks, to be able to safely complete the next section of the journey, hence they have planned 6 years for the 2½ months trip. And I'm still not sure if they don't have the ability to connect to shore power in the many ports they visit, and if they won't do so to cover some energy shortage, and store it for the next lap.
     
    Last edited: Apr 5, 2019
  5. CocoonCruisers
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    CocoonCruisers Junior Member

    I'd say no: Yes we'd probably use this functionality since it's available. But sails also do fine at high angles of attack and in light winds, so i can't see an advantage in this respect.

    Is this relevant for a performance rig ? Why would you want to go dead downwind ? VMG beam reaching & gybing will always be faster.

    Neither do i at this stage. I'd love to but remain stuck so far. Appropriate sizing could probably be loosely infered from Gyrocopter data and pictures. But eyeballing the CE height of a rotor in twisted wind with a gradient isn't quite trivial, and there are two cases to consider, depending if the lower or the upper rotor blade goes against the wind.

    - Stock helicopter panel codes won't lead anywhere: Helos don't operate in twisted flows.
    - CFD at the high speeds of the blades, and with the appropriate twist is certainly possible, but complex enough to require a lot of work and core-hours (maybe tspeers approach in CFD sail trim optimization https://www.boatdesign.net/threads/cfd-sail-trim-optimization.51720/page-5#post-711306 could do the trick)

    - If seeking exact data, it might actually be a nice student project to bring into the Twisted Flow Wind Tunnel in Auckland - BLUR http://www.blur.se/2012/02/05/twisted-flow-wind-tunnel-in-auckland/

    - RC modelling and racing against a good sloop rig should be reasonably easy since stock components are available. I've updated post #57 ( Gyrocopter rotor instead of canvas sail ? https://www.boatdesign.net/threads/gyrocopter-rotor-instead-of-canvas-sail.62003/page-4#post-851356 ) with a proposition.
     
    Last edited: Apr 7, 2019
  6. dinoa
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    dinoa Senior Member

    Reality check point. I think there is no need to ponder this level of complexity now. If you go with a variable pitch, rigid rotor with symmetrical airfoil you might as well go all the way and connect it to a prop. Like a wind turbine, it will handle wind gradients and abuse by brute strength, complexity, weight and cost. A teetering hinge rotor of say 24' will weigh in at about 70lbs and cost $2,500 which would probably be an order of magnitude lee than the turbine. A 12 degree twist would be beneficial but would provide only marginal benefits. Most gyro blades are untwisted. Pitch would be set at about 2.5 degrees from zero lift line. Add to this a hub $800, prerotator/generator maybe $1000 and you have something you can attach to a rotating mast to experiment with.

    Hope this doesn't dismay.
     
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  7. Squidly-Diddly
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    Squidly-Diddly Senior Member

    really? I'm thinking about 2 mtn-bike type simple, light disc- brakes would bring it to stop in about 3-4 seconds, roughly judging how quickly they can bring a 300lb rider from 30mph to stop in routine operation. And that is a simple squeeze.

    My understanding is once stopped it will be effectively "reefed" and not catching too much air, whether its feathered or not.

    I'm thinking easy "reefing" would be a selling point. It would be easy to rig a "dead man switch" or a "panic button" to reef.
     
  8. AlexanderSahlin
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    AlexanderSahlin Junior Member

    Autogyro-sails have a few advantages over other sails: 1) the blades sweep over a disc that can be up to 50 times the total blade-area, 2) by adjusting the rotor-disc's angle of incidence relative the wind, the rotor-speed, and hence total aerodynamic force, can vary in a very wide range. 3) The total centre of effort is always very close to the hub (provided you use a teetering hub for a 2-bladed rotor or a hub with similar function for a 3-bladed rotor).
    I "invented" the autogyro-rig some 20 years ago, but after I tested it as a skate-sail I learnt about Lord Brabazon and a few others who tested it long before me. The attached photos, by Jens Österlund, show my skate-sailing rotor. Total blade-area 0.3 sqm. Swept rotor-disc 14 sqm. It was very easy to control, the total force was of the same order as for my regular 8.8 sqm. skate-sail. It was a little slower than my regular sail, but I was still sailing approx. 2 times the windspeed and made good VMG. It is very hard to stall the rotor -if the disc's angle of incidence increases the rotor-speed just increase in a fraction of a second. At this size and with this configuration it is quite easy to stop it, but if you think a simple disc-brake will do in a larger size, I recommend you calculate the energy in the rotor. For a large size rotor I have considered air-brakes at the tip. Leading out water in the blades to a forward-pointing outlet near the tip can also be very efficient, but maybe dangerous if the 600+ knot (this is what you get at 300 knot tip-speed) water-jet is not diffused immediately in the air.
    However, I learnt that you get an oscillation 2 times the rotor's frequecy for the 2-bladed configuration, because the lower blade has higher kinetic energy, and hence higher centrifugal force, than the upper blade. For a gyrocopter with horizontal rotor this is not a problem. With 3 or more blades the forces balance each other also when the blades move a little faster in the lower part of the disc. After the 2-bladed rotor I made a 3-bladed, that rotated much smoother, but the blades have too low torsional stiffness, and wrong C.G., so the torsional flutter comes already at some 100 to 120 knot tip-speed. But with some trimming-weight along the blade's leading edge, I expect smooth operation up to at least 180 knot tip-speed, or 400 N total lift for this rotor.
    Bild 2019-05-10 kl. 14.47.jpg Bild 2019-05-10 kl. 14.47 #2.jpg
     
    Last edited: May 13, 2019
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  9. dinoa
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    dinoa Senior Member

    amazing
     
  10. CocoonCruisers
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    CocoonCruisers Junior Member

    Awesome Alexander !
    I'm about as impressed with how the experiment works out, as with you managing to survive it.
    What was the disc-area of your 3-blade rotor ?
    Could you operate this with almost vertical rotor at speed ? Or is it always sailed close to horizontal like in the pics, more like a kite ? Or do you only need it to lift it's own weight ?
    How noisy were these rigs ?
    Is it safe to infer that the L/D of your rig more or less matched your sail's then ?
    If so, what kind of sail did you use, something simple, or rather high-tech ?
     
    Last edited: May 29, 2019
  11. CocoonCruisers
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    CocoonCruisers Junior Member

    Sorry for answering late, been away sailing!

    Thank you Dinoa, no, of course it doesn't dismay, it is super helpful. I'd been around friends' expensive (think 100 000 $ or so in Europe) 3-axis ultralight motorgliders, and was assuming about 3 times higher costs because of their various repair horrorstories. Sure makes sense to look at more down-to-earth kit.

    However, i don't have a suitable platform (large beachcat ?). Even at this 'reasonable' size there's a pretty strong free-standing mast to build, and the controls need to be set up in a safe way. That would still amount to at least 10 000 $ and a couple months of work. At this size, the contraption would already be a little dangerous to RC (what if it gets out of control?), and i wouldn't know where to find a really safe spot aboard for a driver either.

    So there may still be some merits to the small-scale RC approach (~2000$, 1 month work or so).
     
    Last edited: May 29, 2019
  12. AlexanderSahlin
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    AlexanderSahlin Junior Member

    On my pictures the rotor may look almost horizontal, but like a kite it is most efficient when you get it as vertical as possible. The limit in the skate-sailing experiment was when the blade-tips tocuched the ice. I had some light tocuches, which it survived, but you can certainly destroy a rotor by crashing it on the ice or on the water. My skate-sailing rotor was very light, just some 3 kg, so lifting it's weight was no problem. It may look almost horizontal in the pictures, but I estimate that it is just 30° from vertical in the lower picture ( because the vertical projection of the 2 m long mast is about 1 m there). These pictures were taken on my first or second outing. Later I dared to fly the rotor more vertical, which worked fine.
    The noise from my rotors is on the same level as you hear when flying a glider. Not as much noise as from a helicopter. But this certainly depends on the tip-speed. My tip-speed has always been below 140 knots. Helicopters use to have much higher tip-speed.
    Unfortunately I have no hard data from comparisons with traditional skate-sails. But my estimated VMG of the same order as the windspeed indicates a total L/D for sailer+rotor above 2. The L/D for the rotor alone is certainly much higher. Here is a link from a clip on me skate-sailing with my regular skate-sail. When we have measured VB/VG max (=L/D), we have got values at 3 or slightly above at best.
    My 3-bladed rotor has the same disc-area as the 2-bladed.
    You consider testing in model-scale. I think it is a good idea, but one problem for such rotors is that the chord-Reynolds number for the blade will be very low. For my skate-sailing rotor the Re is below 200 000. An even lower Re will give a lower L/D. So I recommend reasonably large model-rotors. At least 2 m disc-diameter.
     
  13. Doug Lord
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    Doug Lord Flight Ready

    Alexander, that is really terrific! Thanks for posting....
     

  14. CocoonCruisers
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    CocoonCruisers Junior Member

    Had to let this settle a bit.

    Fascinating discussion, but i ended up abandoning the idea of building a model:
    - Feasibility and efficiency roughly on par with a classic sailing rig have already been demonstrated by Alexander Sahlin's and Lord Brabazon's bold experiments.
    - At a model scale for which suitable stock parts are available (around 1m), the deterioration of rotor L/D due to the low Reynolds numbers is at least 30%, compared with 'beachcat' or 'cruising' scale. This precludes meaningful direct comparisons with a sail rig on an identical model (Classic sails are optimised for low reynolds numbers, so they suffer far less from the downscaling).
    - L/D ratios a bit above a classic sailing rig seem within reach, but not without careful design engineering of the whole system - quick&dirty won't do.

    As for potential of the idea, my conclusions at the various scales are as follows:
    - At drone scale, it would be easy to use a more efficient wingmast, rather than a rotor in autorotation.
    - At drone scale, the precise positioning possibilities (with a rotor/prop linkage) could be attractive, but speed against the wind would be very low, and the high complexity seems unrealistic for unassisted remote usage. A solar+battery electric drone with the same positioning capability would win any time, and something with a simpler sailing rig may do just as well if the positioning constraints aren't all that tight.
    - Beachcat or cruising scales (probably in autorotation, without linkage to prop, since no micro-positioning is needed), are obliviated by security and noise/vibration concerns.
    - I do see potential of a rotor in autorotation for sail-assist at cargo scale. The main benefits would be
    - more durability than canvas sails for this 'industrial' usage
    - better upwind capability than a flettner,
    - fairly safe and simple depowering *for such scales* (swept area > 1000 m2) via blade pitch adjustment,
    - small wind loads + much lower rig height than an equivalent classic sailing rig at rest.​

    Experimental development is unrealistic and obsolete at cargo scale. Simulation for such rigs is already possible, but still quite expensive. It will become easy within the next 5 years. Appropriate CFD, structural engineering and manufacturing capabilities and access to capital are all readily available at major wind power manufacturers, and they will remain more credible for this project than the marine folks. So i'll hope for these to have a go at it at some point.

    Thank you so much for your thoughtful contributions !
     
    Last edited: Sep 17, 2019
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