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Thread: built up wood spars

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    built up wood spars

    Does anyone have any specifications on how to determine the materials sizes, for making built up spars like the Mini-Max and others does.
    looking to build my own spars, and want to make sure I get them sized right.
    Building them for the Light Sport area, not the ultra lights.


    Robert

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    Re: built up wood spars

    At least some of the information you need can be found in NACA ANC-18. For instance, there is information about different species of wood you can use, how many stiffeners are necessary when using a plywood shear web, etc. Maybe more, since I haven't looked at all of it. You can find ANC-18 at: http://www.westcoastpiet.com/construction.htm I don't know if they use the same scan, but you might also find it at the NTRS server or the Magic NACA Archive, or maybe even the FAA.

    ANC-18 is an older publication, so there may be a number of suitable adhesives or other methods not covered. For instance, it's possible to use Styrofoam or similar extruded polystyrene foam as a shear web in lower stressed areas and as an aid to assembly in more highly stressed ones. I don't know how much it's been used in full scale yet, but you can also wrap Kevlar around spars at points where there may be forces pushing the spar caps apart. And probably other tricks I don't know about.

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    Re: built up wood spars

    Quote Originally Posted by Robert Dingus View Post
    Does anyone have any specifications on how to determine the materials sizes, for making built up spars like the Mini-Max and others does.
    looking to build my own spars, and want to make sure I get them sized right.
    Building them for the Light Sport area, not the ultra lights.


    Robert
    What are the shear, bending, and torsion loads at each station along the spar? You can't size a spar without knowing the loads.
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    Re: built up wood spars

    Quote Originally Posted by Robert Dingus View Post
    Does anyone have any specifications on how to determine the materials sizes, for making built up spars like the Mini-Max and others does...
    This is a place where I'd recommend that you learn to use the method of composite sections (not to be confused with composite materials) to evaluate the moment of inertia (usually denoted I) of a cross section of a beam. Once you know the maximum tensile or compression strength of your material, and you know the depth of the beam, then you can easily determine how much I you need. My go-to reference for this stuff is Machinery's Handbook, a good solid nuts-and-bolts engineering guide optimized for fabricators and others who have to actually make things.

    Being able to play around with the actual values for I and stresses and stuff is really handy. I sometimes use it to develop new designs, but more often to reverse-engineer existing designs to determine what sort of assumptions went into them.
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    Re: built up wood spars

    BoKu, what edition of the handbook do you have? Are there any editions to avoid?

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    Re: built up wood spars

    Quote Originally Posted by plncraze View Post
    BoKu, what edition of the handbook do you have? Are there any editions to avoid?
    It's not here right now, so I can't check. But any edition from the last 25 years or so should be fine. Beam theory hasn't changed much in the last century.
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    Re: built up wood spars

    ok, no easy answers that is ok. I do have ANC-18 plus Bruhn and Stress without Tears, so looking at those today.
    I have looked at the Beaujon charts in that small book, titled how to build ultralights, it gives a chart but no math about how the chart was made.

    my next build is based on an existing Kit Fox type clone welded fuselage, in 4130 steel. So I have a solid base to build from, however there are no wings with this, as it was originally a kit from a company that died a while ago. there were no plans with dimensions or accurate templates etc.
    I am basing my wings on this similar to the Piper Cub making them in all wood, as that is the most cost effective route for me at this time.

    I cant afford solid spruce spars, so a built up spar is more economical, and I have accurate templates for almost any airfoil I want to use.
    basing my build on the light sport category of 1320 gross, this puts me at the 3.8 G rating for standard safety factors.

    I want to size the spars 2 of them, to be only as strong as necessary for the 3.8 g ratings to keep it as light as possible as well.
    target weight is roughly 750 pounds wet minus pilot and passenger.

    Robert

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    Re: built up wood spars

    If the weight is doubled then you need to double up the spars.
    If the weight is tripled then you need to triple the spars. Assuming same depth (spar height).

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    Re: built up wood spars

    Quote Originally Posted by Robert Dingus View Post
    I am basing my wings on this similar to the Piper Cub making them in all wood, as that is the most cost effective route for me at this time.

    I cant afford solid spruce spars, so a built up spar is more economical
    If you're designing it from scratch anyway you should consider carbon fiber. The stuff is so much stronger than wood that if you put two equal strength beams next to each other the carbon one looks ridiculously skinny compared to the wooden one. Hand laid carbon fiber may also be cheaper than aircraft grade spruce for the same strength part.
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    Re: built up wood spars

    The usual method for calculating wooden spars in bending is the Prager method. It is very well developped in the book by Vallat, but unfortunately in French only. I found the original article in English but it is a bit harder to read.

    https://www.flightglobal.com/FlightP...-2-%200754.PDF
    https://www.flightglobal.com/FlightP...-2-%200755.PDF
    https://www.flightglobal.com/FlightP...-2-%200756.PDF

    This gives you the allowable bending moment for any given wooden spar section.

    So first you need to know what bending moment is applied on your spar. This load calculation is described in the first chapters of the Bruhn if I remember correctly. Other keywords you may look into are the Schrenk approximation for the distribution of lift on the wings, or the german distribution (at least it is called that in France) in case you have a wing strut. I believe the latter is the one described by Bruhn.

    After the size of the caps is checked with this method, remember you also need to check the strength of the glue bonds between caps and webs, and the shear strength and buckling strength of the webs. All necessary data for the plywood webs calculation can be found in ANC-18.

    I don't think getting into any more detail would be useful on an internet forum, there is a good deal of reading needed. But with this done you should have a good understanding of what to do and probably some more specific questions to ask. So have a good reading !

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    Re: built up wood spars

    Quote Originally Posted by Robert Dingus View Post
    ok, no easy answers that is ok. I do have ANC-18 plus Bruhn and Stress without Tears, so looking at those today.
    Those books are for later stages of the design process. As Topaz mentioned you have to know the loads before you can design an appropriate structure. Something like Hiscocks' "Design of light aircraft" or Evans' "Light plane designers handbook" might be good to study.

    The LAA has some good basic info on their web site:

    http://www.lightaircraftassociation....ft_design.html
    http://www.lightaircraftassociation....ry_design.html
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    Re: built up wood spars

    Quote Originally Posted by Norman View Post
    If you're designing it from scratch anyway you should consider carbon fiber. The stuff is so much stronger than wood that if you put two equal strength beams next to each other the carbon one looks ridiculously skinny compared to the wooden one. Hand laid carbon fiber may also be cheaper than aircraft grade spruce for the same strength part.
    If it's a strut braced wing that's not terribly long, carbon fiber may not be worth the extra trouble. I remember seeing the front spar on a strut braced, two passenger high wing light plane. Maybe it was an Aeronca Champ or something. It must have been something like 1 inch x 6 inches. A solid spar of that size, 30 feet long, might weigh 37 lbs if you don't taper it. Tapered, it ought to weigh less than half that amount. Of course, a carbon spar can be lighter, but by how much? Another way to save weight on the spar caps is to use a thicker airfoil than you'd see on a Cub.

    Don't skimp on the design. If you copy a wing from a reputable airplane that's as least as fast and heavy as yours, that might save some work. At least if you don't change anything, including the way it's mounted. But if it was me, I'd still look at the design carefully, to make sure it's up to the task. If weight is really important, though, it may be best to design your own.

    Just to get a ball park figure (NOT FOR ACTUAL USE IN YOUR DESIGN), the top spar cap on a Sky Pup is 1/4" X 4 inch at the root. The airfoil is very thick at the root, over 11 inches. The gross weight is 400 lbs, so if your design spans the same 31 feet, has a very slight taper, and is a cantilever wing you'd need 3.3 times as much cross section, or actually a little more as you'd lose a bit of that 11+ inches. Of course, if you use an airfoil that's only 8 inches thick, you'll need more for that too. You could reduce the required cross section by using stronger wood. Douglas fir is often used, and ANC-18 has other possible woods to use. One thing about using heavier, stronger wood is that, since the cap is thinner, it averages further from the neutral axis. Anyway, if the top spar cap of a Sky Pup was made from wood weighing 30 lbs per cubic foot, it would weigh something like 4 lbs. (!) So our hypothetical plane's cap might weigh 13 lbs or so. And the bottom cap is only 3/16". So perhaps 23 lbs for the pair, if you have the thick root airfoil.

    Keep in mind that the Sky Pup plans allow you to use different kinds of wood, so with Sitka spruce or well chosen douglas fir, you can probably use less. Also keep in mind that this is only for purposes of estimating, very roughly, how much your spar caps will weigh. Also keep in mind that plywood is needed front and back at high stress points like the wing joiners. So you'll have to throw in more weight to account for that.

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    Re: built up wood spars

    P.S. The caps can be much lighter with wing struts.

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    Re: built up wood spars

    Extruded I beam spars are available from Carlson Aircraft. Various sizes.

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    Re: built up wood spars

    Yes, Carlson was my first stop, to look at he cost for spars, that is the fastest way to build the wings, but not the most cost effective. for the roughly 450.00 it will cost me before I drive up and get them. I am still deciding what way to go, I have more time than money. Those spars would make it lighter, and there are several sizes to choose from.

    Robert

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