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V-bottom float design article

By Luis Lopez,2014-05-05 21:59
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V-bottom float design article

Foam Floats By Alasdair

    Sutherland

    Making Floats

    I have had both failure and success with floatplanes, but now I wonder how at first I managed to make it seem difficult. Using floats of the correct size and shape is a great start but, if mounted at the wrong angle, they can turn a landplane into a high speed boat. Extra power can make up for some deficiencies it is not the whole answer. I had my 66" span trainer weighing over six pounds take off quite easily and fly on an Irvine 25.

    Over the years I have deliberately tested some of the limits of float size, float shape, step position and mounting angle, and chanced to overcome a few pitfalls on the way. Floats of my design are easy to make and work very well, and though this is not the only way to make successful floats, it is a good starting point for keen would-be waterplane fliers. What do floats do?

    The floats on a floatplane perform several functions. They

    1. support the weight of the aircraft at rest.

    2. allow it to taxi forward slowly in displacement mode, under control, while lifting the prop

    clear of oncoming waves.

    3. lift it onto the planing surface and allow it to plane at take off speed across the water

    surface.

    4. let it rotate to an angle which will allow it to take off.

    Suppose you want to design floats to covert an existing landplane model, how do you make them fit for the above purposes?

    3030

    Figure 1 Float Design Nomogram

    400by Alasdair Sutherland56

    202030025

    6020052020504

    153.54015100

    10104example993.58030883601010772.59940668820330557715

    2.52066model weightfloat X-sectfloatfloatprop tofloatbefore(width x depth)depthlengthwidthCG distconversion(ins)(squ ins)(ins)(ins)(ins)(ozs)2LWBDAX

396439458.doc.doc Page 1 of 14 Copyright ? 2007 Alasdair Sutherland

    Foam Floats By Alasdair Sutherland

    Dimensions

    The total length of the float is 2.5 times the distance from the prop to the aircraft’s CG, which makes sure the front of the floats are far enough ahead of the prop to provide buoyancy and hydrodynamic lift well forward. Using the example on the nomogram in Figure 1, if the prop is 13” ahead of the CG mark that on scale A and read off the minimum float length of

    32.5” on scale L. I will round up the length to 33”.

    The volume of each float should provide just enough buoyancy to support the weight of the model, which should mean that each float will be half submerged. Dividing the weight of the model by the density of water (0.6 oz. per cubic inch) and doubling the answer will give the "block volume", i.e. length time’s width time’s depth, of each float. However on Figure 1

    you simply take the weight of the model in ounces before conversion, i.e. as a landplane, and mark it on scale W. (I have allowed for 15% weight gain).

    In the example, I marked a starting weight of 96 oz., then a straight line from the 13” mark on scale A through the 96 mark on scale W and continued to scale X gives the width x

    depth of my float as 11 square inches. Another line through this point, from scale B to scale D will give a suitable combination of breadth and depth for the float.

    I generally swing the line up a little on the B scale for wide shallow floats on slow models and down for narrow deeper floats on fast models, but check you will end up with a decent depth at the rear end. Since I already have 3.5” wide templates, a depth of 3.2" will do just fine, though I could have made them 33" long, 3.2" wide and 3.5" deep.

    Float Shape

    propA/C

    CG1050 (max)40(min)

    2

    48 (max)2527

    100

    Figure 2

     The basic profile of the float is shown in Figure 2, which also gives approximate dimensions of various parts of the float as a percentage of the overall length. At least 10% (up to 15%) of the length should be ahead of the prop, to lift the prop clear of the waves as it taxies forward.

    The underside is gently curved at the bow (picking up on nautical terminology here) for about a quarter of the length to lift the float as it moves forward. Then there is the flat portion, parallel to the top surface, on which it planes, back to the step

    Plan the step about 2% of the length aft of the CG, a step depth of about 1% of length, and slope the portion aft of the step up at 5 or 6 degrees to allow room to rotate for take off. Balsa sheet will be used at front and back, and I’ll come to the sizes I use later.

    There should be no problem if you make the floats a little longer than needed, but make sure that the step is just behind the aircraft’s CG. If you need to make them shorter 396439458.doc.doc Page 2 of 14 Copyright ? 2007 Alasdair Sutherland

    Foam Floats By Alasdair Sutherland

    because your foam block, balsa sheet or veneer is too short then you can take an inch or two off the rear end.

    Cross-Section

     The simplest floats to make have a flat bottom and the attachment points mounted on the outside like an afterthought. For your early attempts these are recommended for their ease of construction (and repair) and they perform very well. Types A and C in Figure 3 will not look out of place on a trainer or sport model.

     I will also present a more deluxe approach to float construction in which the hard points for fixing are internal and the bottoms of the floats have a Vee shape like the ones used on full size floatplanes, Figure 3 B and D. I am not convinced that they perform any better but they look better. Once you are convinced that you like floatplanes and that they can last for several seasons you may prefer to make the deluxe models.

    ABCD

    Figure 3

     In Figure 3 you will see the four different sections. The flat sided options, A and B, are best suited to built up construction from sheet balsa or liteply. They are built upside down on the building board as a box, just like building a fuselage. You start with the float top, stick on a number of formers, then add the sides and bottom.

     The floats with rounded top and sides, Figure 3 C and D, are meant for foam/veneer construction. It is easy to cut the rounded shape from foam, though I always have a flat portion on top for the mountings, and veneer wraps around the top and sides in one piece.

    I consider it essential to provide the front half with spray strips on the bottom edges as shown, to control the spray pattern. Without them water sprays up from the edges of the floats and gets into the prop, slowing the engine and sometimes stopping it altogether. Spray strips, in deflecting the water down and away from the prop, also help to lift the float onto the planing position as it gains speed.

    Box Floats

     Floats can be built from sheet wood just like a fuselage. I suggest 1/8” liteply for the top, which is initially pinned down to the building board. If you want to use hidden fixing blocks glue pieces of ?” birchply in appropriate positions inside the top piece now. Then cut a set of formers, I suggest five formers placing one at each end, one at the step and one near each fixing position. The formers are all the same shape at their top but obviously have different depths, so cut the deepest first, the one at the step, and each of the others can be copied, with its depth reduced according to its position.

     Glue the formers in position inside the top panel, and then add the sides. If making V-bottom floats I suggest a central keel of 1/16” ply to support the bottom. Forward of the step 396439458.doc.doc Page 3 of 14 Copyright ? 2007 Alasdair Sutherland

    Foam Floats By Alasdair Sutherland

    the bottom may either be sheeted with 1/16” plywood, or sheeted with 1/8” balsa, which is easy to cut and bend, or thin plywood (1/32” birchply or 1/16” liteply). Aft of the step use

    balsa.

     When the float is built lift it from the board, trim the front face and add a piece of 1/2" balsa to the front and round it off to blend with the shape. Round off the top corners as well but leave the lower corners and the step and rear edges square.

Spray Strips

     The spray strips are important. They help keep water out of the prop, improve the lift from the float and make it easier to get the floats planing on the step. I made them from 3 laminations of 1/16" balsa (to get round the curves) glued on with cyano then carved to shape. The outside is blended with the sides first and then the inside is cut to form the triangular spray strip as shown in Figure 4.

    or, made fromspray stripcarved from1/32" birchply3 layers of

    balsa strip

    Figure 4

Covering and Finishing

     A paint finish would be adequate for 1/8” liteply, though thin glasscloth applied with epoxy resin will make them more durable. The bottom ahead of the step will certainly benefit from a layer (or two) of glasscloth.

    If your floats are made of 1/8” balsa you will need to cover them to bind the grain of

    the wood. Tissue applied over sanding sealer and doped then painted will be adequate, but thin glasscloth, either the 25g/sq.m. or 50g/sq.m. grade, applied with laminating epoxy will be stronger and tougher, and probably no heavier. Apply it all over, including over the spray srtips. It looks very attractive left natural with the wood grain showing, or it can be painted. Forward of the step, cover the bottom with a layer of heavier glasscloth (100g/sq.m.) and resin, or two or three layers of the light stuff.

    A strip of carbon fibre tow on the lower edges of the spray strips will harden them against beaching damage. Alternatively a strip of metal, wire or umbrella spoke, will protect the edges of the spray strips.

    Mounting

     Unless you fitted plywood fixing blocks inside the floats you must glue them on top now. Decide where the fixings will be and prepare two blocks for each float, ?” by 2” of ?” birchply to take the screws. Also cut a strip 5/8” wide of 1/8” liteply which extends from in

    396439458.doc.doc Page 4 of 14 Copyright ? 2007 Alasdair Sutherland

Foam Floats By Alasdair

Sutherland

    front of the forward block to behind the rear one. Glue on the liteply first then the two blocks of ?” birchply.

     Make up two frames as shown in Figure 5 using the same size of piano wire (music wire in the USA) you would use for an undercarriage. Saddle clamp the wires to the floats and to the fuselage, and don’t forget the diagonal wire which prevents fore and aft movement. The horizontal tie bar between the floats to keep them parallel has been found to be necessary. Without this the model is apt to lunge suddenly to one side during take off.

    Rig the tops of the floats parallel to the aircraft's datum line, which is usually parallel to the tailplane. Conveniently, the bottom planing surface is also horizontal which helps with alignment. The prop disc should be an inch, or preferably two, above the tops of the floats. The spacing between the centrelines of the floats should be 25% to 30% of the wingspan for lateral stability. Alternative mountings will be described later.

    2 wire framesthispropshapeclearance 2"

    float top parallel to tail

    Figure 5

    Simple Foam Floats

     All my early floats were of the simple flat bottomed veneered foam type. They are dead easy to make and the method lends itself to making batches of them, so get some friends together and make a pile of floats. They will be simple flat bottomed floats made of white foam and covered with obeche veneer as illustrated in Figure 6.

    standard float1/4" plyblocks

    1/8" liteplystrip

    white foamveneer

    spray strips

    Figure 6

    396439458.doc.doc Page 5 of 14 Copyright ? 2007 Alasdair Sutherland

    Foam Floats By Alasdair Sutherland

    The mounting plates will be simply stuck on top and spray strips on the lower edges will make them quite effective. The length width and depth, the step position are calculated from Figures 1 and 2. You will need some templates for foam cutting, made in pairs from either Formica plastic laminate or thin metal (I find the wire burns notches into plywood). Formica is much easier to work and lasts long enough for the amateur.

    Make a pair of "F" templates for the front, and a pair of "R" templates to cut the rear and the step, and a pair of section templates like those in Figure 7. My "F" templates are 3.9" deep to make best use of 4" foam and the "R" templates are 3.5" deep in the corner to give a 0.4" deep step, and they slope down at 6 degrees. Using these two part templates floats of varying lengths may be cut for different models. Cut the U-shaped templates carefully to achieve a symmetrical float shape, the width and depth you want, and mark them as a mirror imaged pair, with a 20 mm wide flat bit around the top middle (point 5), which will make mounting easier.

    F

    Pair of section1919

    templates2828

    Figure 77733464565R

    Start with a block of foam the length of the float less 1/2" at the front and 1/8" at the back. Mark the step position you have calculated, measuring from the back end of the float. The advantage of cutting flat bottomed floats is that the pair, or even half a dozen pairs, can be cut at once.

    Start by drawing round the U-shaped template on each end of the foam block, and then check that when the templates are positioned on either side you will get a sensible area of foam to stick on the front and back balsa pieces. All fitting in well? Let’s start cutting.

    Fix one of the R templates with the step at the marked position firmly on each side of the foam block. I use nails pushed through closely fitting holes with a rotating motion so that they pierce the foam rather than crushing it. Cut from the back up to the step, pause, and lift vertically out against the template to form the step. Position the front F templates and cut from the front to a bit past the step position and lift the wire away.

    Now you need the pair of "U" shaped templates cut to give you the required width and depth and your chosen shape of cross section. Position these firmly at either end of the foam block and, with as taut a bow as possible (to minimise drag) cut down around and up to end up with a perfect foam float. Reposition the U templates and cut another (and another and another).

    Covering

    Take some time to admire your foam cutting and decide how to finish them. My suggestion is to glue 1/8" balsa on the rear, sand flush, and veneer the bottom cross-grained. Sand the edges flush, and then glue 1/8” balsa in the step position. Sand that flush and veneer the bottom ahead of the step (cross-grained again) and sand the edges flush with the foam. 396439458.doc.doc Page 6 of 14 Copyright ? 2007 Alasdair Sutherland

    Foam Floats By Alasdair Sutherland

    Now prepare a piece of veneer big enough to wrap around the top and sides. Draw a centreline before applying the contact glue and, when the glue is ready, lay the float upside down along the centreline. Press it down firmly and then rock it first one way then the other sticking the veneer progressively further down the sides until it is wrapped all around in one piece. Trim the veneer flush aft of the step, but leave 6mm protruding between the front and the step, then add a piece of 1/2" balsa to the front and round it off to blend with the shape. Spray Strips

    Spray strips are important, and you have almost made them already. Run a little epoxy thickened with micro balloons into the corner between the bottom and side veneers. Glasscloth the bottom with one thick or two or more layers of thin cloth and epoxy resin, and add a little more resin and filler to the corner. Glasscloth the top and sides with one piece of light glasscloth. When the resin has set trim off the excess glasscloth and trim the spray strips to about 4 or 5mm. Metal umbrella spokes, which consist of a small U-shaped section of hard steel, epoxied onto the edge of the spray rails are very effective in reducing beaching damage. Mounting

    Glue a strip of liteply 5/8” wide and about half the length of the float right along the

    centre of the float top. On top of this glue two blocks of 1/4” birchply, 2” long by 1/2” wide,

    at the positions where you want to screw on the float attachments. The reason for this arrangement is that in the event of a severe landing (OK, a crash) the 1/4” ply is likely to break

    away, possibly pulling a layer of liteply with it. Repair is easy and the floats themselves should, with luck, remain intact and waterproof.

    I have found that it is often convenient to use the main gear of a tail-dragger, whether high or low wing, as the front attachment. The main undercarriage is designed to take horizontal as well as vertical forces when fitted with wheels, so all you need to add is a light attachment aft of the wing, as illustrated in Figure 8.

    The axles can be saddle clamped onto the 1/4" plywood plates, and the addition of a horizontal bar between the two floats to keep them parallel is necessary. You can use 1/8" piano wire or 1/8" x 3/8" strip aluminium for the rear support and again I recommend a horizontal tie between the floats and corner braces to stiffen the whole thing up.

    The whole thing is arranged to get the step about ?” to ?” aft of the CG and the top of the float parallel to the aircraft's datum line, which is usually parallel to the tailplane. Conveniently the bottom planing surface is also horizontal which helps with alignment. The prop disc should be about an inch or two above the top of the floats. The spacing between the centrelines of the floats should be 25% to 30% of the wingspan.

    Figure 8

    extra rear supportfloat top parallel2" clearanceto tail

396439458.doc.doc Page 7 of 14 Copyright ? 2007 Alasdair Sutherland

    Foam Floats By Alasdair Sutherland

    An alternative mounting is an "N" structure made of piano wire or aluminium strip consisting of two identical frames connected by a diagonal cross-brace for rigidity as described with the box floats, Figure 5.

    Refinement

    Water rudders are an optional extra on these floats. A mounting for them can be stuck on the back end, and the rudders controlled by a snake from the rudder servo, or a separate servo connected to the rudder channel by a Y-lead. I have not found water rudders necessary on high wing models because they weathercock into wind anyway and turn over if subjected to much crosswind when taxiing.

    On low wingers one or two water rudders controlled by bowden cable (snakes) can be quite effective. Plywood mounting plates could be mounted inside the floats, recessed into the foam or the built up structure to improve the appearance.

    Deluxe Floats

    Full size floats always have a V-shaped bottom to cushion the impact of landing, and the reinforcement for float attachment is built in to the internal structure. Floats with a Vee shaped bottom and the mounting blocks internal certainly look better, and more like scale floats, than the simple ones described before. Though they take twice as much work to make, many modellers will think it worthwhile.

    I wanted more scale-like floats with a shallow V at the step, with the V angle increasing at the bow, and I wanted to use my original templates. The basic size and profile of the floats is unchanged; only the cross section is different, as illustrated in Figure 9. Note the provision of spray rails again, to deflect the spray downwards away from the prop, and a small central keel.

    1/4" ply

    strip

    deluxe float

    Figure 9

Templates

     Using Formica or thin alloy, make a pair of "F" templates for the front, and a pair of "R" templates to cut the rear and the step, and a pair of section templates just like those used for the simple foam floats and illustrated in Figure 7. My "F" templates are 3.9" deep to make best use of 4" foam and the "R" templates are 3.5" deep in the corner to give a 0.4" deep step, 396439458.doc.doc Page 8 of 14 Copyright ? 2007 Alasdair Sutherland

Foam Floats By Alasdair

    Sutherland

    and they slope down at 5 degrees. You can use these two part templates to make floats of varying lengths for different models. Cut the U-shaped templates carefully to achieve a symmetrical float shape, the width and depth you want, and mark them as a mirror imaged pair, with a 20 mm wide flat bit around the top middle (point 5), which will make mounting easier. Cutting

     Start with blocks of foam the length of the float less 1/2" at the front and 1/8" at the rear, and a bit wider than the finished float. I used 125 mm wide blocks and the angles worked out well. Using the U-shaped templates mark the section on front and back ends. Mark the step position, allowing for the 1/8" balsa rear facing. Lay a broad and a narrow melamine faced chipboard shelf, each the standard 19 mm thick, on the work table as in Figures 10 and 11 such that the foam fits against one edge of the lower shelf and the upper shelf fits snugly against the other side of the foam. Now one of the "F" templates rests on the work table and the other rests on the upper shelf. Move the high template 50 mm (2 inches) FORWARD relative to the low one (Figure 11). Keeping the 50 mm offset move both templates fore and aft, checking where the wire will cut, to achieve a front section 20 to 25 mm deep, which will be easily faired in with 1/2" balsa.

    hot wire

    foamshelf 2

    shelf 1

    work table

    Figure 10

     Mark the optimum template positions for future reference, and make sure they are held snugly against the foam. Now cut from the front to an inch past the step position and lift the wire out. (I made a special one handed bow a foot long for these narrow cuts. It is so much easier than using an unwieldy four foot bow.) Move the templates and shelves around as shown in Figure 12 to cut the other side.

396439458.doc.doc Page 9 of 14 Copyright ? 2007 Alasdair Sutherland

Foam Floats By Alasdair

    Sutherland

    cut back

    to herestep markhigher template

    foam

    lower template

    shelf 2

    shelf 1

    work table

    Figure 11

    hot wire

    Figure 12

    foamshelf 2

    shelf 1

    work table

     The rear end aft of the step can also be cut with a V-bottom. Position the "R" templates as shown in Figure 13, one on the table and one on the high shelf, but both with the step lined up with your marked step position. Cut from the aft end to the step, pause, and lift the wire vertically against the template to form the step. Then reverse the shelves and templates again to cut the other side of the V.

    step mark

    cut

    pausecutshelf 2shelf 1

    work table

    Figure 13

     It is probably best to put the offcuts back on for the next bit, so re-assemble the block and place the U-shaped templates on either end as in Figure 14, making certain that the peak of the Vee is dead centre as in Figure 15. With minimal drag in the wire, to ensure an accurate cut, and keeping both ends carefully synchronised, start above the 1 mark and progress slowly 396439458.doc.doc Page 10 of 14 Copyright ? 2007 Alasdair Sutherland

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