hard wing vs soft wing?

Discussion in 'Hydrodynamics and Aerodynamics' started by Slingshot, Apr 11, 2020.

  1. Slingshot
    Joined: Aug 2019
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    Slingshot Junior Member

    Is there a difference in force or efficiency between a hard wing with a mylar surface compared to a soft wing made from dacron or similar material assuming both have the same foil shape and area.

    I have read of a 20% increase in force between a hard wing and regular soft sail. Would a soft wing be in between?
     
  2. tlouth7
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    tlouth7 Senior Member

    If the shape is the same (it won't be) then the only difference is skin friction. Woven material (dacron) will have greater friction than mylar, this will almost certainly result in increased drag overall, though it is possible it could delay separation on the leeward side in a beneficial way.

    By the way force is not really a valid measure, as sailors we care about maximum lift and lift-to-drag ratio (for a given planform, across a range of conditions).
     
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  3. Will Gilmore
    Joined: Aug 2017
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    Will Gilmore Junior Member

    A Force Diagram from a Physics class would use the term 'force' to express lift and friction. These specialized words are just ways of visualizing the particular vectors in the calculation.

    I absolutely agree, same shape, orientation, and size, same forces: lift, etc. pushes/pulls the vessel along.

    I'm placing my bet on the lighter foil.

    -Will (Dragonfly)
     
  4. tlouth7
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    tlouth7 Senior Member

    Sure, but nobody characterises wings by comparing the "force" without some sort of qualifier as to what they are talking about.
     
  5. tspeer
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    tspeer Senior Member

    Under your assumptions, the difference would be seen in the boundary layer development due to the difference in roughness between the Dacron and Mylar surfaces. The smoother Mylar will have laminar flow over a greater distance than the Dacron sail, with its stitched seams and woven texture. This may lead to less lift for the Dacron sail, due to a thicker boundary layer. Or it may lead to higher maximum lift if the turbulent boundary layer delays separation. It depends on the sail shape and the operating conditions.

    I don't know where the 20% figure came from, but I doubt it assumed the same shape for the soft sail and rigid wingsail.

    Here is one comparison I did between a wingmast+soft sail vs a slotted rigid wingsail section. These two sections had very different shapes, but were designed to similar requirements for the same boat. The soft sail, operating at its best operating condition, was the equal of the rigid wingsail. However, the slotted wingsail was much better at off-design conditions. It had a higher maximum lift by virtue of its slotted flap. The soft sail, of course, can be adapted to different operating conditions, so the comparison over the whole range would have to take this into account. But the flow into a sail is constantly changing, and the sail cannot be perfectly trimmed everywhere along its span. So one advantage of the rigid wingsail may be its ability to be far more forgiving than the soft sail.

    If one tries to design a rigid wing without a slot, and one assumes the boundary layer is fully turbulent, it is hard to better the performance of a soft sail. That's because the soft sail is thin, and thickness is detrimental to both maximum lift and minimum drag when the boundary layer is turbulent.
     

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  6. Eric Lundy
    Joined: May 2017
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    Eric Lundy Junior Member

    This thread is close to an answer that i have been searching for. If there is one person who has the knowledge my guess would be Tom Speer does. Here is the question:
    [Wing shape slot vs solid wing]
    Lets assume two wing sails of equal area and aspect ratio. One wing has two symmetrical foils with an ideal slot that can adjust angle twist slot for maximum efficiency. Foils remain symmetrical.

    The second a single element asymmetrical adjustable wing that can adjust camber chord thicknesses etc to maintain maximum efficiency.

    I will assume the wings are equal in most regards: area, aspect ratio, materials, etc

    Question - Dual element symmetrical foils with all adjustments for efficiency. (No changing foil shape) VS Single element adjustable asssymetrical wing with all adjustments for efficiency. (Foil can change shape but no slot) which is better?
    If someone has an answer great, at the same time if someone can point me to the right programs, literature or research that i can find my own answers that would be fantastic too!

    If my question is unclear i will post some drawings.
    Cheers, eric
     
  7. tspeer
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    tspeer Senior Member

    You say the dual element section can't change its shape, but it does change its shape by changing the angle between the two elements. This is not only necessary for performance, but to be able to sail on both tacks.

    It depends on what operating range is of interest. It also depends a great deal on what your assumptions are regarding the boundary layer. Because section design is all about the boundary layer. If it weren't for viscous effects, all sections would have zero profile drag and produce about the same lift.

    A section with a single slotted flap compared to a unitary section of the same thickness, will generally have a minimum drag that is about 15% higher. With careful design, you might get that penalty down to around 5%. The slotted section will have a much higher maximum lift. So which is better depends on whether the low-lift range is important to you, or the high-lift range. For many wingsails, the low lift range is very important, as when the AC72s operated in a depowered mode in San Francisco. Maximum lift may only be important going downwind in light winds.

    If the boundary layer is fully turbulent, then the profile drag is not very sensitive to section shape. It mainly depends on the section thickness. The soft sail rig can probably be thinner than the rigid wingsail, so the soft sail has an advantage here. An asymmetrical wing without a slot will be at a disadvantage compared to the soft sail.

    If the boundary layer has a substantial run of laminar flow, then the shape matters a great deal. Whether one can maintain the laminar flow will depend not only on the shape, but on how smooth the surface is, and what the operating conditions are. When the marketing types put graphics on a wingsail, the thickness of the graphics may be enough to trip a laminar boundary layer, especially if the graphics are near the leading edge. When the craft sails in fog, and the surface is covered with water droplets, it's unlikely that there is any laminar flow to speak of. So while depending on laminar flow can yield big dividends, it is also risky. It can look good on paper, but fail to materialize in practice.

    An adjustable asymmetrical section might be able to maintain a longer run of laminar flow on the lee side than the wingsail with slotted flap, especially in the low lift range. But maybe the unitary section has to be designed for less laminar flow in order to meet a high lift requirement. In that case, the slotted section may not have much of a drag penalty, if any, in the low to moderate lift range.

    So there is no general answer to your question. Some constraints make designing for some characteristics hard, and some are not big issues at all, depending on the requirements. You really need to establish a set of requirements and then try different designs to see how well they meet those requirements. Only then can you answer your question, and then only for that specific set of circumstances. Change the requirements and the answer may be different.

    For the AC72, I tried to achieve a balanced design, with a shape that was capable of maintaining a moderate amount of laminar flow while not suffering greatly if there was no laminar flow. ETNZ evidently aimed for somewhat less laminar flow and was oriented more to the high lift range. Artemis' first wingsail looked to be designed for extensive amounts of laminar flow, but it was destroyed before it had a chance to race against the alternatives. So similar requirements, but three different approaches that depended on how much laminar flow was at stake.

    I was recently comparing an articulated rigid wingsail for a Moth with a soft sail. With the assumption of turbulent flow, I wasn't able to get the rigid wing to outperform the soft sail. Of course, a slotted section wasn't an option there, because of the class rules. Does that mean the soft sail is the best rig for the Moth? Not necessarily. I assumed both rigs had fully turbulent flow because the seams of the soft sail would preclude laminar flow, and I wanted to compare the two on equal terms. But if I'd designed the rigid wing for laminar flow, sailing against the soft sail with turbulent flow, the answer could have been quite different.
     
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  8. Eric Lundy
    Joined: May 2017
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    Eric Lundy Junior Member

    My goodness such a good reply. I appreciate the depth and time you took to answer Tom. I had to read, re read, digest, read it again and feel uninformed, do some research, still feel dumb and now finally I have enough gin and tonics in me to try to ask another question or two.

    Firstly, In terms of the dual element wing changing it's shape: all I meant was the individual elements remain symmetrical, while the overall wing can change the angle between elements. as in the section of an individual element doesn't have it's own flap or deform in a way to change the symmetrical element's shape tack to tack. Too many variables kills an experiment.

    I was completely unaware of turbulent flow vs laminar flow differences in wing design. I can only assume any wing I could produce would make turbulent flow. Your description makes me think that most wings are designed for turbulent flow as even passenger jets have seams and rivets and flaps that I assume would make it turbulent. In this regard I will use a golf ball analogy to ask my next question. The idea behind golf ball dimples is to create a thinner boundary layer to minimize the drag/wake. Another example I remember hearing was an AC boat that had a specially textured hull to create a slippery boundary layer. So the question is: A boundary layer and turbulence are interconnected but the boundary layer is not considered overall turbulent flow, correct? or no...

    I am trying my best and would love to delve into the appropriate research materials to learn more about this stuff on my own. any reading suggestions?

    Also if a design is for turbulent flow, what is the section difference than one designed for laminar flow?
    cheers,
    Eric
     
  9. Will Gilmore
    Joined: Aug 2017
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    Will Gilmore Junior Member

    I'm probably the furthest from an expert here, and I certainly can't claim to follow all that's been written, but there are a few concepts here I've been exposed to and think I have a little bit to contribute.
    This is how I understand surface texturing.
    In the case of a golf ball, the pips are designed to assist in giving a spinning ball lift as it flys through the air. The texturing on the bottom of a boat to assist in water flow isn't meant to cover the entire bottom. By texturing the back side of a curved surface, capillary adhesion is encouraged, resulting in better attachment of the fluid as it follows the exit curves of the bottom. This improves flow where it often breaks from a laminar flow and cavitates or becomes turbulent.

    -Will (Dragonfly)
     
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  10. tlouth7
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    tlouth7 Senior Member

    In general turbulent flow sounds worse than laminar flow, as energy is lost to heat in a turbulent region. However turbulent boundary layers have a useful property. To understand it we should go back to the basics of boundary layers:

    Immediately next to the surface of an object (sail, golf ball, hull, keel)) moving through a fluid (air, water) the fluid 'sticks' to the surface, and so moves along with it. Far away from the surface the fluid is stationary. NB we are thinking in the reference frame of the fluid. Clearly as you approach the surface the fluid has to transition from stationary to moving. The region where this occurs is the boundary layer. In a laminar boundary layer each 'layer' of fluid is moving slightly faster than the adjacent one. This is smooth and low drag. In a turbulent boundary layer there is lots of mixing of fluid, with particles spending some time close to the surface, then moving out into the boundary layer for a bit etc.

    This would not be desirable, until we get to flow separation. This occurs when you have an 'adverse pressure gradient' which roughly corresponds to the downstream half of a convex surface (the rear, leeward side of a sail, or the rear of the golf ball). Basically the fluid doesn't want to cling to the surface but rather to carry on in a straight line. It turns out that a turbulent boundary layer is better at keeping the flow attached. This is good for a golf ball because it reduces the size of the wake and therefore the drag, and it is good for a sail because flow separation is what we call stalling, and means you get no lift on the stalled region of the sail.

    If you can design your sail, foil or aircraft wing such that you can be sure you will get no flow separation then laminar flow can have lower drag, but for the wide range of situations we encounter having turbulent flow is often lower risk.
     
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