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tech:foils [2012/10/17 07:51] – phil_aldersontech:foils [2020/12/09 19:21] (current) – external edit 127.0.0.1
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 The planform and section shape of the foil can make a big difference to the drag and efficiency. The selection of this is dependent on the use to which the foil is intended i.e. rudder or centreboard for more ideas about the shape see [[tech:foil_design]]. The planform and section shape of the foil can make a big difference to the drag and efficiency. The selection of this is dependent on the use to which the foil is intended i.e. rudder or centreboard for more ideas about the shape see [[tech:foil_design]].
  
-{{tech:foil-planform.jpg?250}} +
-{{tech:foil-profile.jpg?250}}+
  
 =====The Core===== =====The Core=====
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 ====Wooden Cores==== ====Wooden Cores====
 The first steps away from the monolithic wood core was to use a low density but strong wood such as Western red Ceader. This can still be worthwhile as for a single project small quantities of wood may be more available than small quantities of high density foam, and many people find wood more pleasant to work with. The first steps away from the monolithic wood core was to use a low density but strong wood such as Western red Ceader. This can still be worthwhile as for a single project small quantities of wood may be more available than small quantities of high density foam, and many people find wood more pleasant to work with.
-{{boats:0698-20060807b.jpg?250}}+ 
 +{{ boats:0698-20060807b.jpg?250 }} 
 To build a Wooden foil prepare an even thickness blank from timber strips approx. 50mm wide. Quarter sawn strips are preferred as they are stronger. Organise the strips so that alternate ones are turned end for end the grain should run in different directions to reduce the warping of the timber. Warping can happen long after the foil is fully coated and complete, particularly if moisture gets into the foil through damage to the skins or pivot holes. To build a Wooden foil prepare an even thickness blank from timber strips approx. 50mm wide. Quarter sawn strips are preferred as they are stronger. Organise the strips so that alternate ones are turned end for end the grain should run in different directions to reduce the warping of the timber. Warping can happen long after the foil is fully coated and complete, particularly if moisture gets into the foil through damage to the skins or pivot holes.
  
-The timber strips should be bonded together with epoxy and clamped to a flat surface. When cured plane the blank to an even thickness. Cut out the profile of the blade, but make it 5mm smaller than the intended finished size at trailing edge. (Apart from anything else this ensures that minor damage doesn't expose a wood core) Shape the foil, allowing about 1.5mm undersize for the laminate, fairing and painting. Make the trailing edge as sharp as you can, because the fibres will overlap here to create the true trailing edge.+The timber strips should be bonded together with epoxy and clamped to a flat surface. When cured plane the blank to an even thickness. Cut out the profile of the blade, but make it 5mm smaller than the intended finished size at trailing edge. (Apart from anything else this ensures that minor damage doesn't expose a wood core) Shape the foil, allowing about 1.5mm undersized for the laminate, fairing and painting. Make the trailing edge as sharp as you can, because the fibres will overlap here to create the true trailing edge.
  
 ===Plywood=== ===Plywood===
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 One of the most common failure modes with light weight foam foils is for the core to crush against the sharp bottom edge of the boat, this takes the skin on the compression side out of column and quickly leads to a broken foil, which is the reason for using either the higher density core, or dual density core.  One of the most common failure modes with light weight foam foils is for the core to crush against the sharp bottom edge of the boat, this takes the skin on the compression side out of column and quickly leads to a broken foil, which is the reason for using either the higher density core, or dual density core. 
  
-{{*tech:cb-core-20111210.jpg?250}}+{{ *tech:cb-core-20111210.jpg?250 }}
  
 Foam is typically sold in quite large sheets, of different densities, and thickness. It is generally possible to get a sheet of the required thickness, sufficient to make several foils. However if this is not cost effective, you can bond two thinner sheets together. The foam resists shear forces from one side of the foil to the other so if using two thin sheets the bonding between the layers is important for the final strength of the foil. Foam is typically sold in quite large sheets, of different densities, and thickness. It is generally possible to get a sheet of the required thickness, sufficient to make several foils. However if this is not cost effective, you can bond two thinner sheets together. The foam resists shear forces from one side of the foil to the other so if using two thin sheets the bonding between the layers is important for the final strength of the foil.
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 A router can be used to cut grooves along the length of the blank at the required depth for the position along the chord. The contours for each depth are marked onto the foil, and groves cut with the router following the appropriate contour. When sanding it is easy to see if you are approaching the required depth as the grooves will start to disappear.  A router can be used to cut grooves along the length of the blank at the required depth for the position along the chord. The contours for each depth are marked onto the foil, and groves cut with the router following the appropriate contour. When sanding it is easy to see if you are approaching the required depth as the grooves will start to disappear. 
 Once the core is rough sanded the final sanding should be done with a flexible long board to avoid bumps and hollows. Once the core is rough sanded the final sanding should be done with a flexible long board to avoid bumps and hollows.
-{{*:tech:cb-contour-20120708.jpg?250}} + 
-{{*:tech:cb-contour-20120708.jpg?250}} + 
-{{*:tech:cb-part-routed20111126.jpg?250}} +{{*:tech:cb-contour-20120708.jpg?250|Routing contours printed on paper and glued to core}} 
-{{*:tech:cb-shaped-core-20111210.jpg?250}}+{{*:tech:cb-part-routed-20111126.jpg?250|A Partially routed foam centerboard core}} 
 +{{*:tech:cb-shaped-core-20111210.jpg?250|Core after routing and shaping ready for layup}} 
  
 ====CNC Machining==== ====CNC Machining====
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 To get an accurate shape the blank needs to be held solidly. If you mill the foam away to a thin edge, then it is likely that the blank will distort under the cutting head, ruining the shape. Small concave shapes are also hard to cut due to the shape of the cutting head. For this reason CNC machines may be more suited to making plugs, or moulds, rather than the finished foam core. To get an accurate shape the blank needs to be held solidly. If you mill the foam away to a thin edge, then it is likely that the blank will distort under the cutting head, ruining the shape. Small concave shapes are also hard to cut due to the shape of the cutting head. For this reason CNC machines may be more suited to making plugs, or moulds, rather than the finished foam core.
  
-{{tech:foil-plug.jpg}}+
  
  
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 It is also possible to build a foil in a mould, this is typically done with two half shape moulds for the skins, these are then bonded together around a roughly shaped core. There is a large time investment required to build the moulds so this technique is good for building a large number of foils of the same shape, rather than prototyping a new shape. Particular care needs to be paid to the join between the two half skins to avoid splitting. An internal flange could be used, packing could be used on the leading edge to allow you to laminate a strip of cloth on the outside of the blade, and then fair it in. If the foil is built in a mould then expanding epoxy foam can be used to form the core of the blade. There have been some L shaped T-foil rudder moulds made, these use two different Ls for the rudder and top surface of the lifting foil, and a separate flat mould for the lower surface of the lifting foil. This allows you to make the joint smaller, which should reduce drag. It is also possible to build a foil in a mould, this is typically done with two half shape moulds for the skins, these are then bonded together around a roughly shaped core. There is a large time investment required to build the moulds so this technique is good for building a large number of foils of the same shape, rather than prototyping a new shape. Particular care needs to be paid to the join between the two half skins to avoid splitting. An internal flange could be used, packing could be used on the leading edge to allow you to laminate a strip of cloth on the outside of the blade, and then fair it in. If the foil is built in a mould then expanding epoxy foam can be used to form the core of the blade. There have been some L shaped T-foil rudder moulds made, these use two different Ls for the rudder and top surface of the lifting foil, and a separate flat mould for the lower surface of the lifting foil. This allows you to make the joint smaller, which should reduce drag.
  
-{{tech:T-foil_shell-20110111.jpg?250}} + 
-{{tech:T-foil _shell-20110111.jpg?250}} + 
-{{*:tech:t-foil-mould-20120630a.jpg?250}}+{{ *:tech:t-foil-mould-20120630a.jpg?250 }} 
  
  
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 Skins of unidirectional carbon and woven carbon cloth, will give lowest weight and maximum stiffness. Start with layers of 200 g/m² unidirectional carbon, one layer over all of the foil, plus a second layer of 200 g/m² unidirectional carbon over the top half, and a third layer of unidirectional carbon, approx.150mm wide, over top part of the foil, extending approx.100mm past the bottom of hull when the foil is right down. Then add one layer of 200 g/m² woven carbon cloth, and finally a layer of 86 g/m² glass cloth, both covering all of the foil. If you wish you can use white pigment in the top layer to enable you to produce a white foil without painting it. Skins of unidirectional carbon and woven carbon cloth, will give lowest weight and maximum stiffness. Start with layers of 200 g/m² unidirectional carbon, one layer over all of the foil, plus a second layer of 200 g/m² unidirectional carbon over the top half, and a third layer of unidirectional carbon, approx.150mm wide, over top part of the foil, extending approx.100mm past the bottom of hull when the foil is right down. Then add one layer of 200 g/m² woven carbon cloth, and finally a layer of 86 g/m² glass cloth, both covering all of the foil. If you wish you can use white pigment in the top layer to enable you to produce a white foil without painting it.
  
-{{boats:2692-20070211d.jpg?250}}+{{ *:boats:2692-20070211d.jpg?250 |Laminated foil ready for trimming and finishing}}
  
 =====Laminating it===== =====Laminating it=====
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 If you have the facilities to vacuum  bag your foil then this can help to consolidate the laminate and remove excess resin, see the [[*:tech:vac_bag|Vac Bag]] how-to for more information. If you have the facilities to vacuum  bag your foil then this can help to consolidate the laminate and remove excess resin, see the [[*:tech:vac_bag|Vac Bag]] how-to for more information.
  
-{{*:tech:t-foil-mould-20120630b.jpg?250}}+{{ *:tech:t-foil-mould20120630b.jpg?250 |}}
  
 =====Finish===== =====Finish=====
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  • Last modified: 2020/12/09 19:15
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