Mast Building tips
P.S. this it is not a new page but still cut and paste from old pages, news has to come on mast page, i am talking about it with an aeronautic engineer.
There are many ways to build quite easily a mast (practically a tube) provided you have the correct size for the material you intend to use.
We will consider only those methods usefoul to the homebuilder as we don't intend to make a mast production line in our backyard.
Two are the methods : Moldeless and molded method.
First we weill talk a bit of the moldeless method. This is a easy method that uses effectively a mold made of foam that remains inside the mast. The first problem that arises is chooosing the foam. I think that it has no very critical structural loads as in hull sandwich panels, so we can go using PolyUrethane Foam, this foam is provided in blocks or even liquid ready to mix and cure. While the blocks can be easily glued and shaped, the liquid foam requires a female mold (so what about lay up directly the fiber in epoxy resin?) ,remember that this foam expands itself various times before it become solid, by this way the foam acquires the mold shape becoming a male mold. By using solid sheets of foam you have two problems: the first is how to glue something like 20-30 strips of foam, the second is how to shape these long new piece of foam.
Generally epoxy resin is used to glue the pieces of foam while the foam itself is easily sanded by hand with common sand paper even if a more accurate shaping method (wood templates?) would be favourable not only for aestethical purpouses but also for aerodynamical ones, especially if you are building a tapered wingmast ecc...
However one the biggest problem to me is the fragility of the foam itself prior to laying of the fiber-resin. How to maintain straight a so long piece of foam? How to handle without breaking it ? And not last how to vacuum bag (consolidation of the fiber with resin is a crucial point in mast making) this foam without it collapses under the pressure?
I think it could be a good idea to shape the foam into two pieces (longitudinal cut along the major axis, fore to aft), this will give a flat surface on the foam, that could be fearless laied on long long table made of wood. You could shape the foam, and on the same place without moving the foam, lay-up the first half, wait for it cures and then start the second half. Once you have the two halfs you can glue them in one piece and then lay an external ply of fiber that goes around the mast to tighten the glue joint.
The fragility of the foam under vaccum bag pressure remains, even if i suppose that the large flat support, would provide an improved resistance.
We have to remember the points where fittings (forestay, backstay, trampoline wires ecc...) will goes. It is very important that you design exactly before the exact position of the backplates, they could be made of plywood or from steel plates, even of reinforced fiber, more important is the fact that if make the mast from two pieces, you can have access to the part before you glues the two parts together. This approach even gives the possibility to make a tunnel where wires can go, i think it is not easy to rise a 12 m luff without a wire that pull up the sail.
The molded method is that one i prefer (female mold), yes it takes more time before you start building the mast, but once the mold is builded it gives you the possibility to wait and think for the correct method you are going to use and for the correct lay up, it gives you the possibility to add more fiber on the inside without pain for the aestethical appereance etc..
Let me start saying that a wingmast (lot of surface) wrapped on a male mold is a very critical problems as it is hard to deattach from its mandrel, even if you have used a lot of wax you are not trying to pull up the thing from a mold like as it is possible for hulls etc..
Generally on a male mold is required a cut along the cured mast to deattach first the mast and make it possible to pull away the mold from the inside.
There are also the problem linked to the structural resistance of the mold, how to roll a long and heavy piece of wood?, how to envelope the whole thing without rising (and therefore bending) the mold with the fresh resin on?
So i have the idea that two male or female half molds are easier to build and to handle. It is possible to choose two different
types of half mold, the first types of mold make easier to lay up the fiber, but it leaves less surface for gluing the two sections, the second type allows to have on the half mast a larger area for gluing the two parts, but it make a little bit harder to lay up the fiber becouse of the tight curve. Again the first type can be used in both male or female types of mold, the second type only on male mold becouse of it would be very difficoult to lay the fiber within the tight sections of the mold.
Let me say that professional mast builder uses
filament winding in smaller mast, but they uses in bigger mast type 2
female molds. This allow to make the two mold joined together an autoclave where
high pressures and temperatures can be reached.
We will not use an autoclave, and for certain we are not professional composite workers, so i think that a male mold is easier to build and to work with. Reamains to choose the type of mold, i believe that the second type is to be choosen as it ensures (to me) a better resistence when the tube is closed and a lot of bidirectional and unidirectional cloth is laied up and vacum bagged on both sides. The main shear loads are on transversal direction, so they can spread along entire, not cutted or glued after curing, leading and trailing edge, while compressive loads are on the same axis of the cloth you are used to close the tube. Consider that you don't need to lay up all the fiber on the sides of each half mast, a sort of tapered lay up should be used, as the gluing fiber that we are going to use to close the tube can add (on a single lay up, primary bond in itself) good compressive strength to oppose to the bending moment of the sail.
when closing the tube, different ways can be choosen. It is possible to make a half mast enter into the other half, overlapping by this way the edges, this can be glued with epoxy/microballon resin, rivets can be added etc... After gluing the edges, a little bit of fairing is required to make the overlapped joint disappear with epoxy/microballon resin, and then one can start adding fiber on both sides. It isusefoul not only to keep parts together but also to improve lateral compressive strength.