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Re: [AWES] Re: Exploring carbon stiffening for the KiteGen kite?

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  • dave santos
    The urgent problem KiteGen has is achieving reliable low-drag retraction by pulling in on one wing tip. This must work flawlessly thousands of times in a row,
    Message 1 of 4 , Mar 16, 2012
      The urgent problem KiteGen has is achieving reliable low-drag retraction by pulling in on one wing tip. This must work flawlessly thousands of times in a row, but they are likely currently unable to do more than a few cycles before fouling. Their hope seems to be that this new carbon skeleton kite will lay flat to retract more stably.
       
      Any conventional power kite tends to roll and foul itself in the KiteGen retract mode, especially LEIs with a built-in C-arch geometry. Normal ram-air parafoils dragged sideways lose pressure in a messy way (squirting air laterally), which does not help with stability, and then they have to reinflate, which can easily hang-up. Valved parafoils might work OK, but you would still need the leading wingtip to be a flyable pod "aircraft" able to resist roll disturbance. Another limitation on the KiteGen retract mode is that multi-line bridles are not practical, so they are stuck with a two line C-Kite.
       
      This is the sort problem high-dollar commercial ventures have a hard time publicly admitting, but they must privately warn investors, or face criminal legal jeopardy. This is why you have to sign NDAs with most "elite" VC's, ti hide weakness, not so much because they have a secret-sauce recipe to protect. Massimo claimed to have some eight or so launch and retract methods as "Plan Bs", but he is running thru them (the blowers were a bust, a quadrocopter test was rumored, and the ungainly stem may end up as the snout of a "White Elephant"). The gamble on early scale-up is bold, but very unforgiving.
       
      A secondary goal of KiteGen's carbon battened kite is to get a thinner foil section for higher performance, but such performance makes control challenges even more acute. A spanwise carbon spar does not make for a better large kite due to cubic-mass scaling penalty. You get a wing like a hang-glider, but due to the added weight, and higher minimum speed, you have to claw harder in light air, on the edge of falling out of the sky, just to stay up. The 1970's Flexifoil has a leading edge spar, but it obviously never scaled, and is one of the hardest kites to fly ever sold. The lighter C-Kite can hold its own operationally, as well as being cheaper and more robust, so it has thrived more. The latest ram-air race kites are so hot, with more progress to come, that carbon-based contenders may never quite catch up.
       
      Adding carbon weight and cost to a large AWE kite is easily self-defeating. Airbeams, and especially ram-air, are dominant structural principles by inherent advantages. One can add thin carbon whiskers to inflated structure, just as sled kites are made, and get "tensairity" synergism, but the small-whisker format represents the effective scaling ceiling. Battens are also an effective use of small spar structure, to damp out membrane flutter, and that is the function of the ribs in the KiteGen carbon kite concept. They may get in new trouble if the carbon kite glide speed exceeds the reel-in speed.
       
      Note to Brian: AoA is not by itself much of a predictor. At very low Re a high AoA can even be the highest L/D. AoA varies all along a highly engineered wing due to changes in foil section and washout. Some foils even operate at essentially zero AoA by just a curved upper surface. Virtually all aircraft vary AoA in flight, using high AoA to develop max lift at higher drag or to slow down. The wing root chord is often used for AoA measurement, but this only weakly predicts overall L/D. An optimized wingtip often even has a decided negative AoA to help cancel the induced drag of excessive wingtip votex generation. 
          
    • blturner3
      I don t know what a LEI is. If you will pardon the criticism, this list often uses jargon level initials without explanation. That substantially lowers the
      Message 2 of 4 , Mar 17, 2012
        I don't know what a LEI is. If you will pardon the criticism, this list often uses jargon level initials without explanation. That substantially lowers the readability for new users.

        Yes, AoA does only weakly predict overall L/D but that was enough for the illustration. Because having airflow across bumps caused by ribs wether it's inflated or just flight pressures leads to an airfoil cross section that is less than ideal. So the ribs would ideally run with the local airflow over the wing. What I can't tell you is if that is worth the trouble. My gut tells me yes because it is mostly a one time expense.

        I don't see stabilizing the sideslip as any more of a challenge than any other reel in method I have seen. The only possibly unsolvable problem is the added cost of the high speed reel in motors that still are efficient generators. You can't leave optimal generator design by much before the cost is huge in lost efficiency. Then you have to add a second reel in motor that costs money.

        The kite forms that have emerged as best for their various applications is not an automatic prediction of them being best for AWE. AWE can tolerate slower turns than kite surfing. The oversized sidewalls of a C kite provide maneuverability that I don't think AWE needs at the cost of more wetted surface and other forms of drag. Inflated, thin wall spars are a maintenance point for a 24/7 machine. Part of that dreaded complexity but not a showstopper.

        I think you are excessively concerned about low wind performance. When you do groundgen you can launch different kites for different conditions. Also because of the positive effects of scale up.

        Carbon, used properly, does not add weight and can easily pay for it's cost. Makani and others tends to use it everywhere much like the proverbial kid with a hammer. I used to do this till I went back over some of my designs and realized it.

        Composite compression load structures are much heavier than air based compression structures or pure tension structures. Yes "tensairity" is a good thing, but it's not the only thing.

        Cool to here KiteGen though of a quadcopter for launch. I personally think that would work.

        Brian

        --- In AirborneWindEnergy@yahoogroups.com, dave santos <santos137@...> wrote:
        >
        > The urgent problem KiteGen has is achieving reliable low-drag retraction by pulling in on one wing tip. This must work flawlessly thousands of times in a row, but they are likely currently unable to do more than a few cycles before fouling. Their hope seems to be that this new carbon skeleton kite will lay flat to retract more stably.
        >  
        > Any conventional power kite tends to roll and foul itself in the KiteGen retract mode, especially LEIs with a built-in C-arch geometry. Normal ram-air parafoils dragged sideways lose pressure in a messy way (squirting air laterally), which does not help with stability, and then they have to reinflate, which can easily hang-up. Valved parafoils might work OK, but you would still need the leading wingtip to be a flyable pod "aircraft" able to resist roll disturbance. Another limitation on the KiteGen retract mode is that multi-line bridles are not practical, so they are stuck with a two line C-Kite.
        >  
        > This is the sort problem high-dollar commercial ventures have a hard time publicly admitting, but they must privately warn investors, or face criminal legal jeopardy. This is why you have to sign NDAs with most "elite" VC's, ti hide weakness, not so much because they have a secret-sauce recipe to protect. Massimo claimed to have some eight or so launch and retract methods as "Plan Bs", but he is running thru them (the blowers were a bust, a quadrocopter test was rumored, and the ungainly stem may end up as the snout of a "White Elephant"). The gamble on early scale-up is bold, but very unforgiving.
        >  
        > A secondary goal of KiteGen's carbon battened kite is to get a thinner foil section for higher performance, but such performance makes control challenges even more acute. A spanwise carbon spar does not make for a better large kite due to cubic-mass scaling penalty. You get a wing like a hang-glider, but due to the added weight, and higher minimum speed, you have to claw harder in light air, on the edge of falling out of the sky, just to stay up. The 1970's Flexifoil has a leading edge spar, but it obviously never scaled, and is one of the hardest kites to fly ever sold. The lighter C-Kite can hold its own operationally, as well as being cheaper and more robust, so it has thrived more. The latest ram-air race kites are so hot, with more progress to come, that carbon-based contenders may never quite catch up.
        >  
        > Adding carbon weight and cost to a large AWE kite is easily self-defeating. Airbeams, and especially ram-air, are dominant structural principles by inherent advantages. One can add thin carbon whiskers to inflated structure, just as sled kites are made, and get "tensairity" synergism, but the small-whisker format represents the effective scaling ceiling. Battens are also an effective use of small spar structure, to damp out membrane flutter, and that is the function of the ribs in the KiteGen carbon kite concept. They may get in new trouble if the carbon kite glide speed exceeds the reel-in speed.
        >  
        > Note to Brian: AoA is not by itself much of a predictor. At very low Re a high AoA can even be the highest L/D. AoA varies all along a highly engineered wing due to changes in foil section and washout. Some foils even operate at essentially zero AoA by just a curved upper surface. Virtually all aircraft vary AoA in flight, using high AoA to develop max lift at higher drag or to slow down. The wing root chord is often used for AoA measurement, but this only weakly predicts overall L/D. An optimized wingtip often even has a decided negative AoA to help cancel the induced drag of excessive wingtip votex generation.     
        >
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