Suitable dimensions of ribs and longitudinals (skin on frame velomobile)

Discussion in 'Wooden Boat Building and Restoration' started by fairing guy, Feb 23, 2020.

  1. fairing guy
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    fairing guy Junior Member

    I want information on suitable dimensions of ribs and longitudinals for a skin on frame velomobile shell (essentially an upside down canoe).

    How how little width/thickness can you go with strips of wood before they are impractical for steaming and conforming to form geodesic shapes?

    I will use this information to estimate weight and cost and determine if this is viable.

    Example that used pvc pipes. PVC heavy.
    [​IMG]

    Example of design
    [​IMG]

    Thanks for the benefit of your knowledge and experience.
     
  2. bajansailor
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    bajansailor Marine Surveyor

  3. Rumars
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    Rumars Senior Member

  4. wet feet
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    wet feet Senior Member

    I have seen such a vehicle,but didn't take too much notice of the details.I don't know if the attached image files are of any help.
     

    Attached Files:

  5. fallguy
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    fallguy Senior Member

    You need to calculate the maximum allowable deflection of a simple beam. Then determine the size of your wood. The force applied is probably the wind force.

    Once you determine the minimum wood size, you plan all the pieces in spreadsheet and determine the weight of each design.

    The fairest design SOF is all skeleton and no skin, basically and very heavy. Obviously, you don't want that, but understanding limits is important. The least fair design is all skin and ultralight, but also undesirable (basically a square box is least fair, but probably lightest).

    What is so special about carbon, is the tradeoff between fair and heavy sort of vanishes.
     
  6. fairing guy
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    fairing guy Junior Member

    Looks very interesting. Yes I am looking to build something similar.

    Throwing out some calculations, 761 grams for 45 meters of 6mm x 10mm balsa. Balsa at 281.92 kg per cubic meter.

    I will check local timber suppliers here in Australia for quotes on timber prices and get any recommendations for suitable timber.
     
  7. Rumars
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    Rumars Senior Member

    It's not like you have a lot of choice in woods. Balsa, paulownia (kiri), obeche, red cedar, pine. Kiri beeing the most likely to be cheap and available.
     
  8. fairing guy
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    fairing guy Junior Member

    $1.55 per linear meter at 6mm x 20mm. That sounds reasonable.
     
  9. fallguy
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    fallguy Senior Member

    you don't need 20mm lumber for that

    did you calculate the load for maximum spans?
     
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  10. fairing guy
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    fairing guy Junior Member

    I would not know how.
     
  11. Rumars
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    Rumars Senior Member

    Look at the posted photo for clues. You see 3mm ply box bulkheads, 12-14mm square stringers and 3mm veneer basketing to supporting the unreinforced plastic skin.
    The whole fairing is a system and it's weight must be calculated for the whole structure considering the materials, loads and performance. You could for example ditch the stringers and increase basketing thickness, or ditch the basketing and use a reinforced skin. Performance will vary, the basketing emulates a fair surface better then a multichine stringers only construction but less then a monocoque shell.
     
  12. Barry
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    Barry Senior Member

    Your original build ribs were out of a round profile and that is pretty much needed to get the compound curves that you desire. With a rod you can bend a single piece over many planes and directions AND have a continuous
    smooth line where the fabric hits the top side of the profile. It will be HARD ( but not impossible) to replicate in thin, wide strips to produce a fair surface. Of course mild curves can be made as evidenced by cedar strip canoes and the like.

    Regarding the loading calculations: you can go on Engineering Tool Box as a site and find calculators that will determine stresses etc for simple beams in various types of loading, uniform, variable, multi point, single load etc. plus
    you have to consider what type of end conditions that you face, ie simple beam, one end fixed, two end fixed, ( and with fixed ends you need to be able to determine if the "fixed" portion of the structure is actually strong enough to make
    the beam act as fixed.
    For simplicity's sale. Normally you would use a simple beam with a point load as a basis for analysis of the size that you need to achieve your strength and deflection goal as usually, at least in this type of structure, this will produce numbers
    that are the "worst case) scenario. If you calculate loads on a uniform loading basis and hit something that applied a single force mid point in a span, you may exceed the strength of the beam
    If say Boeing was designing this, they would more than likely factor in/design around the pre-tension that will be in the skin if it is a Dacron shrink wrap surface. Perhaps not if the dope coated Dacron loses its elasticity when coated with whatever you
    are going to coat it with.

    As an extremely simple analysis\

    You determine the force that the beam might see in a single force at the midpoint of the beam, using the formulas to determine the appropriate stress. Apply some factor of safety that you can live with. As no one will likely die if a beam
    fails you could go down say 1.2, which give you a 20% factor of safety. Then you can pick a round profile, ie diameter and max tensile strength, that will satisfy your max load. Because this cross section will be small,
    you will then have to back to the calculators and determine the deflection that you load will result in and see if you can live with this.
    A round cross section is not the best for strength to weight basis, but it will bend better than most other profiles, without steam to get to what you want to accomplish

    I would do a search for something like "wood aircraft wings with fabric covering" "ultralight airplanes" and so on as they will have examples of what you are trying to achieve

    If your bend shapes were developable, ie able to bend in the form that you need, then an I- beam or channel would be better choices to carry the same load and result in less weight.

    It is difficult to answer your questions adequately unless the factor of safety is that you want to use, the max load that you might face, the type of wood that you can get, or plastic round tubing, etc

    I would look at round hollow plastic tubing of sorts but that may introduce issues of producing a joint with the frames as the contact area is small

    I am curious as to why you have chosen a blunt front leading edge. This does not appear to be a low drag entry. A conical point of some variety would be better, and you should find a way to ventilate the back side of this
    surface with atmospheric pressure air to inhibit a drag on the back of the surface.

    If you chose to use wood for the frames, and do not want to go through the work of steam forming members, you can take thin wide strips, shape the first because they will be easy to bend, then glue on another strip on top of it
    and get the curves that you wan
     
    Last edited: Feb 27, 2020

  13. messabout
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    messabout Senior Member

    Fairing Guy, the pictures show curvatures that are not difficult to make with appropriate stringer thickness...........But what is the most practical thickness to use? That all depends on how much abuse the skin and its supporting structure is expected to endure.

    A structure like that could be substantial enough when using 9mm thick by about 20 mm wide strips. A stick of that sort will bend along curves, like the ones in the pictures, without having to resort to steam boxes and other annoyances. For curves more severe think in terms of reducing the thickness to something like 6 ,7, or 8 mm.

    To assess the stiffness of a stick of wood, or any other material, you can get a general idea by using your handheld calculator. Stiffness is a function of the third power of the thickness and the first power of the width. For example; a 9mm thickness will have a relative stiffness value of 9 x 9x 9 = 729...a 7mm thickness will have a stiffness value of 343 a 4.5 mm thickness will have a relative value of 91. You can easily see that doubling the thickness from 4.5 mm to 9 mm will increase the stiffness .......729/91 = a little bit more than eight times the stiffness.

    None of that tells you how much abuse the structure can endure. . We could get into a whole mess of calculations to evaluate impact strengths and I will not go into that in order to maintain our collective sanity. It should be obvious that the stiffer the stringers the more impact it can withstand. On the other hand it is clear that the total weight of the vehicle is a matter of concern. If your aim is to establish a record terminal speed then the weight is not so much a factor. If on the other hand you are interested in acceleration, then weight is indeed a big deal. On a short race track acceleration is a principal factor. If on a very large track, or very long straight course, then not so much. See Newtons second law in which F=Ma

    The distance between supporting structures for the stringers becomes an item of importance. Think of the stringer as a beam like that in a building or bridge. If you put a load on the stringer half way between the supporting structure ( here we are talking about bulkheads), it will bend some amount. Structural engineers call the bending amount; deflection. For a point loaded beam the deflection depends on the load and the stiffness of the stringer or beam. That is a calculable amount no matter what the material is. The common equation looks like this........................... PL^3/48 EI The letter I is actually the Mass moment of inertia of the beam itself. Never mind that part just think in terms of stiffness factor. The E is the elastic modulous or the relative strength of the material, in this case wood. Just ignore that for now. The letter P is the pressure or the load at the center of the beam. The letter L is the distance between supporting members (bulk heads) Notice that that is the biggest of the several variables. The distance between bulk heads is raised to the third power. It all boils down to keeping the distance between the beam supports as small as is practical. ................................SO you see that there are so many variable here that it will be a better deal to simply experiment with stringer thickness and bulkhead spacing.

    If you have a functioning table saw you can easily rip up some stringer bits and mess around with the spacing of the supporting elements. You can probably do that in less time than it might take to understand the clumsy explanation that I have tried to convey here.

    Best of luck with the project. If I was about half my age I'd be all in with the idea of building a similar human powered vehicle.
     
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