Elipetical Birds mouth mast

I like the tape trick. Great advice.

 

Anyone thought to reinforce this with glass rovings inside and out and then glass a few socks over it? I'm wondering about a 60 ft mast until I can afford a decent alloy one.

 

Fiberglassing the outside if fairly common. The inside would be a lot of work and extra weight without contributing much to the strength of the mast though.

 

Adding a number of substantial spar caps should be immensely strong.

 

What do you mean by "spar cap"?

 

http://www.rcuniverse.com/forum/attachment.php?attachmentid=1656464&d=1375583891

 

Make a number of narrow spar caps maybe six of, and about an inch wide. Pull them tightly into tensile during the layup. Could machine a gap for that layup, or lay them in the birdsmouth overlap.

 

Internal longitudinal stringers (spar caps) will stiffen the mast, but all but defeat the low weight aspect, unless these were incorporated into the scantlings work up in the design. The only time these offer some real benefit, is on wing style of masts. A typical oval wouldn't need these. Fabrics on the inside are possible but not especially practical to install. Fabrics on the outside are for abrasion protection. Stave layout, taper and wall thickness will be the determining factors once you've worked up loading on diameter parameters. Adding things just doesn't make a lot of sense, except for swallowtails at the hard points, which also need to be well designed (I use 10:1 or 12:1 tapers on these). The real benefit of a birdsmouth is for the novice to build a relatively light weight, economical spar. Carbon, 'glass and other "additions", just increase the complexity and costs associated with them, so maybe a different spar building technique should be considered instead.

 

Thank you, but I am very surprised to hear this. Can you explain further how bracing the mast longitudinally with very strong glass unidirectional has little or no merit? Surely the extra strength from the glass in tensile is the entire point of any reinforcement?

 

When designing the scantlings for a mast, as suggested above, employing similar physical property in materials has obvious benefits. Using materials with widely disparaging properties in the assembly isn't a logical route, mostly for obvious engineering reasons. Could the mast be designed from the start with these longitudinals, yeah, though not entirely necessary, particularly when conventional approaches solve the same set of concerns, without hard points and risers rearing up, increasing costs and assembly (dramatically). You can build relatively small spars with the birdsmouth method, that'll rival aluminum extrusions and some composites, but (again) no one looking for a high end, light weight and stiff spar is looking at birdsmouth. It's basically a backyard building method, that does a far job compaired other wooden building methods, for reasonable cost/stiffness ratio.

Now wing masts can employ some of the methods you've proposed, but this is a wholly different animal and requires a different engineering approuch. Plywood and longitudinals are often employed in these spars, because of loading and weight requirements, though they generally don't do the best, compaired to other approaches.

The OP's origional request and ideas are in serious doubt, mostly because he's not done the necessary math and engineering. His staves have poor bond line lengths and orientation is also not ideal. Lastly in regard to the OP, he admits he has no idea what this mast can take, making one question the very premise. In this concern, can oval and elliptical sections be employed, yep they sure can and I posted the way I've done it, but without the math and common sense engineering in terms of bond lines and internal support, pretty much meaningless.

I design and build several birdsmouth spars each year, including freestanding masts and wings. In an ideal structure, the mast or spar will literally explode into a thousand bits of formerly spar stuff, when it reaches the top of it's loading profile. Having much stiffer, stronger internal elements is a waste of money and materials, as this extra (everything) is just along for the ride, until the weaker portions of the structure fail, in a lot of cases not even participating much, until the very end of the loading scale.

 

Thank you.

Are you saying that the wood core would fail in shear and/or compression before the added glass elements in tensile had served even a small percentage of their purpose?

 

If a wooden mast has the adequate strength and other properties for the purpose, the fiberglass is not necessary. As a surface finish for abrasion it can be justified. I think that at the partners it can be beneficial.

 

From my understanding of most composites: when materials of differing stiffness and elongation are used, the resulting structure does not add to the strength as one material will break before the other. However the structural elements combined can produce a stronger overall effect given the correct engineering. Like in a cedar strip canoe, the fiberglass/cedar/fiberglass sandwich are differing materials but the cedar when used as a core material to separate the layers of stiff glass make for a lightweight strong panel. Where you are engineering the stiff glass into a stronger "beam". The cedar adds less to the overall strength but is the shear web to keep the layers separate. I am not a math scientist but in general that is the gist I think.

 

Agree completely. It is a long-established principle of cored FRP construction, that the reinforcements far exceed the engineering capability of the core - now relegated to shear-web duty.

This is where I am struggling with the previous remarks - to the effect that the (extremely strong) assembly of spar caps in tension add 'stiffness', but contribute little or nothing strengthwise, and cause a large cost in the 'weight' department. I am at a loss with that statement.

Agree completely.

Of course in the case of the fibre-reinforced birdsmouth mast, the moment perpendicular to the mast is large due to the short arm (narrow mast) so the forces are large and the extension under load in tensile (of the FRP) must be microscopic to maintain stiffness, but a framework of Unidirectional Glass excels in this respect.

I ask all this, because I cannot afford a 60ft alloy mast for an upcoming project, and I must either use something I made from wood and glass, or sit on my hands for a year or two. I know that everyones' advice here is going to be the latter, but I'm cheap, impatient, and an innovator.

 

I did a little research on laminated archery bows once. The differing materials in this case had an effect on a bows ability to withstand breaking. The layer on the massively tensioned outer side of the bow being uniform and somewhat elastic would help keep the underlying hardwood from the initial splintering as the tension side of the bow is usually the side of failure. So I do not discount entirely of there being a small bit of a benefit over abrasion resistance when glassing a large spar. It just won't be pure addition.
When I have thought about building a BM mast I was thinking of making it just as a light core(cedar) non removable mandrel and wrapping and epoxying it fully in another material that would be the actual load bearing material. Carbon is expensive and fiberglass is still not super cheap. ...lumber strap webbing came to mind as my first experiment... weave it around the wood core, tension it and then epoxy it... number of layers depending on the size of the mast.
Once I went down that mental rabbit hole I thought... just buy a aluminum mast or build the pure wooden mast strong enough.