DDWFTTW - Directly Downwind Faster Than The Wind

Discussion in 'Propulsion' started by Guest625101138, Jan 4, 2009.

  1. Guest625101138

    Guest625101138 Previous Member

    Inferring a prop L/D from that figure is nonsense - no relevance at all. The 2.85 means the gear ratio, ground to air, is a little over 1:0.65. Also, as the acceleration in the bottom range was poor, the lowest system efficiency would not be much higher than 65%.

    This value of efficiency is not easy to achieve at low rotational speed.

    The efficiency improves as it goes faster but with fixed gear, the ratio to wind speed cannot be altered. It is fixed by the selected gear ratio by design. With CVT, variable pitch or push start it would be possible to take advantage of the higher efficiency at higher speed although the power handling ability of the drive needs to be up to it.

    If the gear ratio was 1:0.5 it would accelerate faster but then the top speed would be a little under 2X windspeed.

    So all you can infer from the windspeed ratio is the chosen gear ratio. Forget about sails and think of it as a propeller driven by the wheels. If you have trouble with that then the winch and string analogy is better than anything to do with sails.
     
  2. Guest625101138

    Guest625101138 Previous Member

    Yes. That is true. For some individuals NO explanation works. Some are still looking for the battery.

    I think Achimedes is credited with saying something like "give me a place to stand and with a lever I will move the world". Here it is give me the right gear ratio and I will go faster than the wind down wind - simple gearing. If the ratio is way too high it will go up wind. If it is just too high it will lock up. If it is way too low it will be a lower wind speed ratio than it could be. It is mostly about the GEARING.
     
  3. sirclicksalot
    Joined: Jun 2010
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    sirclicksalot Junior Member

    Yes and no. There is a formula for a theoretical max velocity for a foil with no other losses:

    Vs/Vt = sin(gamma) * (L/D) - cos(gamma)

    for which the maximum is

    Vs/Vt = SQRT(1+(L/D)^2)

    at gamma = 90 + arccot(L/D) but of course with the cart prop the equivalents for "velocity of the foil" and gamma are interesting if not problematic.
     
  4. spork

    spork Previous Member

    Sorry for the delayed response. I just got back from a paragliding weekend in the mountains.

    Your observation of the entire cart behaving as a turbine, despite the fact that the rotor is clearly acting as a prop, is a good one. And it's one we've reflected on a number of times when discussing the prop/turbine issue. It's yet another way this simple device brings up some very interesting questions (such as whether a sail on a sailboat acts as prop or turbine).



    I assume you're talking about engineering realities, rather than theoretical possibilities - yes?


    In that case I'm feeling pretty good. On our very first runs we had fixed pitch, no CVT, and got pretty close to 3X windspeed.
     
  5. spork

    spork Previous Member

    But it's not quite that simple. The accepted definition for prop efficiency indicates an efficiency of 0.0 for a prop on a plane sitting still on the runway. Clearly it's doing work, but not effective work on the airplane. If you wanted a house-fan, you'd find it worked just fine.

    This is true in practice, but in theory the two are identical. It's literally just a matter of scale. Consider two sailboats sailing around the world. Their sails form a propeller with a ludicrously large hub.

    Comparing the prop to a sail is both accurate and instructive - though it seems ridiculous in terms of scale.

    Well... it's about both gearing and efficiency. I can make the cart go ANY multiple of the wind speed by simply choosing the right gearing - except for one detail... the efficiency has to be ludicrously high when I start shooting for speed multiples in the 4X range.
     
  6. Guest625101138

    Guest625101138 Previous Member

    To me engineering realities and theoretical possibilities are the same. If they are not then the theory is limited or the engineering is flawed.

    You did well to get to 2.85X in the first iteration. I have not gone through the analysis of where the efficiency is lowest but it will be very difficult to get better than 70% in the low speed range with your scale.

    Increase the gearing and push start it you could get maybe 4X.
     
  7. Guest625101138

    Guest625101138 Previous Member

    I was referring to the whole cart system regarding the efficiency. As soon as the prop turns it is doing work and, with the wheels connected to the prop, the cart has to move when the prop is turning.

    The prop may not be producing the bulk of the work initially but once you start going faster than the wind it needs to be providing all of the required thrust. This is around where the overall system efficiency is lowest and what will be limiting your gearing and potential cart to wind speed ratio.
     
  8. spork

    spork Previous Member

    I disagree. There are many engineering realities today that were quite impossible 30 years ago - but the theory hasn't changed.
     
  9. Guest625101138

    Guest625101138 Previous Member

    The theory along with engineering is constantly evolving. Have a look at how the theory of matter has evolved over the last 30 years and how it is still evolving. The matter has not changed but our understanding of it has changed.

    Theory is a model or construct of the "real" world. That construct and understanding is constantly evolving as is engineering that applies it.

    Some years ago there was a common threory that the earth was flat. Now that we can get beyond it we can see that it is cose to a shpere. Some theorised that it was a shere before it was actually observed.

    So theory and engineering are constantly evolving.

    The situation here is that the theory of lifting foils needs to be applied to designing a propeller for a cart but the key to undertsanding is the simple relationship for power being equal to force X velocity and mechanical advantage through gearing.

    As your cart picks up speed the prop slip reduces so the advance ratio approaches 1. Basically the prop threads its way through the air as if it was a screw in a tube. The tube just happens to be moving over the ground and the wheels are turning the screw so it can move faster than the tube.

    Thinking about sails and lifting foils confuses the understanding. The fact that it is a propeller just introduces some small losses compared with the screw in the thread analogy.

    So if you want a simple theory for DDWFTTW consider a screw geared to the ground working its way along the moving tube. Forget about sails and even propellers.
     
  10. spork

    spork Previous Member

    This seems like the extremely long way around the question. Are you suggesting that it will never be possible to make a vehicle that goes directly downwind faster than 4x to 5X, or are you saying that our current engineering methods won't allow it? If you're saying it will never be possible, you're wrong. I understand full well that theory evolves, and I think you understand that that's not what we're talking about. I think you understand that we're talking about F = M x A (at low speeds so there's no need to bring dMV/dT into it) and work = force x distance. We can use this simple theory to determine that there is no theoretical limit to the speed of the cart relative to the speed of the wind (again, keeping the cart well below mach). That's entirely different from the question of just how efficient a bearing or gear we can make. I have a hard time believing you're not understanding this distinction.

    We don't have to talk about sails and lifting foils if that confuses you. It doesn't confuse me.

    Who on earth are you talking to!? I know you're not new here.
     
  11. sirclicksalot
    Joined: Jun 2010
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    sirclicksalot Junior Member

    No, not exactly new; he started the thread.

    I think Rick's missives are directed at me and you wandered into the middle of a pointless p*ssing contest (with predictable results - check your trouser leg;-). Being a sailor I am most comfortable understanding via foils and extrapolating to props by admittedly messy integrations across cylindrical cross sections; I assume that being an expert with props Rick is comfortable working directly with the prop equations and avoiding the mess.

    I know that at the root I agree with Rick: the limit is reached when the incoming prop-relative windspeed is too high for the prop to push on it (Rick's comfort zone), which is about the same as, but a little more complicated than, the blade foils on average reaching their optimum angle of attack = cot(optimum L/D) (my comfort zone).

    [Btw Rick: what is the name for that in prop jargon i.e. the other end of performance from stalling? It's like the prop is spinning but can't do anything because the far field velocity is the same as, or greater than, the - another prop term I don't know - screw speed?]

    And I have to admit I am a little ADHD and am having fun getting attention by poking Rick with a stick, like this:

    Me the foil boy sez:
    Code:
     Cart
     Velocity
     Vector 
     Wrt
     Ground
          ^<--___
    Blade |      ---___
      |   |            ---___ Blade Velocity Vector Wrt Ground
      v   |                  ---___
      \   |                        ---___
       \  <-------------------------------O
        \      Blade Velocity Vector wrt Cart
       ^ \
       |
       |
       True
       Wind
       Velocity
       Vector
    
    Rick the prop cop sez:
    [​IMG]
     
  12. ThinAirDesigns
    Joined: Dec 2008
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    ThinAirDesigns Senior Member

    Wow -- just wow.

    The theoretical possiblities of the space elevator are perfectly sound. The engineering realities *currently* dictate that it's not constructable. That's about as far from "the same" as you can get.

    JB
     
  13. Guest625101138

    Guest625101138 Previous Member

    Tell me what the the theory of the space elevator is.
     
  14. ThinAirDesigns
    Joined: Dec 2008
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    Location: USA

    ThinAirDesigns Senior Member

    http://en.wikipedia.org/wiki/Space_elevator

    "Current technology is not capable of manufacturing practical engineering materials that are sufficiently strong and light to build an Earth-based space elevator. Recent conceptualizations for a space elevator are notable in their plans to use carbon nanotube or boron nitride nanotube based materials as the tensile element in the tether design, since the measured strength of microscopic carbon nanotubes appears great enough to make this theoretically possible"

    (But don't get stuck on this particular example as there are an infinite number of other theoretically sound solutions just waiting for engineering realities to catch up -- to keep in in the DDWFTTW realm, just go back to the two mirrored tacking ice-boats connected by a long telescoping pole with the captains chair/controls in the middle going DDW)

    JB
     

  15. spork

    spork Previous Member

    Yes - I particularly enjoyed Harold's argument that this proves nothing because "where could you ever get such a pole?" :D
     
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