The CF prop molding method I cover in this article was not invented by me. In fact,
Michael Henriksen in Denmark (epilot on the Ezone discussion forum) got me started
with this method and I owe him for giving me that start. However, I've made my own
refinements as I have gone along. There are other sources of CF prop molding info
on the web. Team
Klaustrophobia has a how-to on molding props directly on top of a prop, Michael's
inspiration. And, David Fee
has a how-to on prop molding using male and female mold halves.
The illustration below (kindly provided by Chris O'Riley, Chris3D on the discussion
boards) shows the essence of the technique. A clay mold supports a prop. Together
these form the plug. A CF cloth/epoxy layer is placed on the plug and finally a
layer of clear heat shrink is placed on the top, wrapped around the sides and joined
underneath with tape. When this is shrunk tight it acts as the female mold and
results in a smooth surface for the front of the prop.
The illustration below shows a cross-section of all the pieces after the heat shrink
has been shrunk tight. This shows that even if the prop being molded on has a
cross-section that is not constant thickness, the resulting prop will be more or less
constant thickness and undercambered. I have tested CF props I've molded from GWS
props and the undercamber CF props perform virtually identically to the plastic
original. However, this might not always be the case. (Sometimes it's better- Graham)
The other common technique for molding props involves a two-part male/female mold that
squeezes the CF/epoxy between them. The method detailed here has one main advantage,
that is the speed and simplicity of producing the plug and the final propeller. With
a two part mold, the mold building process is much more complex and a lot more care
must be taken when preparing the mold or layup. Otherwise you may end up with two
mold halves glued together, with your lovely carbon prop trapped for eternity.
What do you need and where to get it
A number of suppliers of composites materials are listed in the supplier thread on the
Ezone's
micro forum. I purchased all my supplies from The Composite Store . Not only do they have a wide
variety of materials and tools, but they have excellent technical support staff ready
to answer your questions over the phone.
Materials
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The prop molders paraphernalia
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To get started you need:
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Carbon fibre cloth Cloth. I use 2.4oz (80g/m^2) weight, the lightest available. I
get the bias cut which makes it easier to cut off strips that have the weave at
45-degrees.
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Epoxy resin and hardener. Check what is available in your area. But, the well
known West System epoxies are readily available. Whatever you use it should be the
thin type as used for laminating etc. (I use SP laminating epoxy in the UK -
Graham)
Other supplies:
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Mold release wax, high temp variety is better since you will be heat curing your
prop. Some people use car wax but I have no experience with this.
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Mold release agent (such as a pva release agent), high temp is better. Some people
don't bother with this.
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CF tissue. You do not need this but it is handy for making lighter props by
combining with cloth. I use 0.20oz/yard and 0.50oz/yard as the application
requires.
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Clear shrink tubing or kitchen wrap. I get my heat shrink from Air Dynamics who now have it in sizes up to 4-inches in
diameter.
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Fingernail sandpaper boards in various grits for sanding the props.
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Bakeable clay for supporting the prop while molding. I use Sculpey brand.
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Medicine measuring syringes from your local druggist for measuring resin and
hardener.
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Small disposable brushes for applying epoxy.
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Wax paper for laying CF cloth on while working the epoxy resin into it.
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Acetone to keep the brush in between making props, or you will go through a lot of
brushes.
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A small toaster oven that can be continuously adjustable down to low temperatures
around 54C (130F). I got mine at Wallmart for $20. This is for heat curing the
prop. Some people rig up an insulated box with a light bulb, but for the cost of a
toaster oven I prefer to not risk burning my house down. Some people use their
kitchen oven but this is a really bad idea as some nasty stuff can end up on your
food. (I put mine on top of the radiator - Graham)
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A disposable face mask to wear when sanding the props to finish them off. CF dust
is bad stuff and you don't want it in your lungs.
If you go with the bare minimum of two-part epoxy and a foot of CF cloth and scrounge
or use supplies you already have your initial cost to start making props will be about
$100. If you get the additional nice things like wax, mold release agent, toaster
oven, CF tissue, etc, you are probably looking at an investment of around $170. The
minimum amount of CF cloth the Composite Store sells is one foot, by one yard ? which
will make a tremendous number of micro props.
If you would like to try molding CF props on a smaller scale, Bob Selman Designs has agreed to make a kit
available with all the ingredients needed for the CF prop molding technique presented
in this article, but in smaller quantities. It has CF cloth and two weights of CF
tissue, two-part epoxy resin, bakeable clay, mold wax, mold release agent, clear
shrink tubing, and a craft brush. This is a nice way to try molding your own props
without having to buy considerably more of the ingredients than you need. The kit
should allow making between 10 and 15 props, depending on the size of props you choose
to mold and how stingy you are with the materials.
Controlling Prop Characteristics
One of the advantages of composite structures is the ability to tailor them as
needed. Different regions of the structure can have different properties. For props
this means we can combine different weight (thickness) CF materials in one prop. For
most props I use two layers of cloth. But, if weight is of primary concern I can
combine (as I do for the example prop in this article) one layer of cloth and two
layers of tissue - which are thinner and hence lighter. If I want a very stiff prop I
can use three layers of cloth. Or, I could use two layers of cloth with a third layer
just in the hub and blade root area. For a multi-bladed prop I use an extra layer
where the blades come together at the hub.
We are not limited to varying the CF material in a prop. We can also change the
amount of epoxy resin in the composite. I achieved my ultralight copy of the 16cm
Westechnik prop by individually pressing two wetted out layers of cloth between layers
of paper towels and smooth boards. Repeating this pressing three times with fresh
paper towels each time results in a very "dry" composite structure. Epoxy is heavy
and reducing the amount of it reduces the prop weight. The final prop is not glossy
smooth. But, in tests in my wind tunnel at speeds up to 5mph, or 2.6 meters per
second (moderately faster than normal for the nine gram room flyer it was molded for)
the prop performs virtually identically to the Westechnik original. And, a weight
savings of nearly a gram was achieved from a prop with a final weight of 0.58g
including hub.
Let's Make a Prop Mold
I wanted a prop smaller in diameter than a GWS 5x3.0 for my 13 inch span Fokker
D-VII. Shown in the picture below is the result of trimming one of those props down.
If you look closely at the trimmed prop you can see I've sanded the edges of the hub
region more rounded to allow the CF to have an easier shape to conform to. An
alternate method is to repeatedly apply small beads of thick CA in the corner where
the blade meets the hub till a smooth transition is achieved. Left unsmoothed,
corners like this will be a problem when molding as they will be difficult to force
the cloth down into. I've also used a very fine grit fingernail sandpaper file and
sanded the raised GWS and prop size letters off so the prop doesn't stick there when
molding.
The next step is to embed the prop in some clay that can be baked in the oven. The
clay will support the prop while molding. The clay needs to be reinforced so it is
formed over a spruce stick. These are shown below. There are a number of brands of
clay that will work. Make sure it is the bakeable kind. I use Sculpey brand and get
it at my local craft store. I cut off slices of clay with a hobby knife about equal
to 1/4 of its width. I press these down on the stick so they wrap around and push
together where the ends meet. I wax the back side of the prop with mold release wax
to make it easier to remove the prop from the clay. Then, I place the prop on the
clay, turn the entire thing upside down and press on the table so it forces the prop
into the clay. I cut off thin slices of clay and press on the sides where it needs
more so the clay will taper out at a slight angle from the prop. We want the clay mold
to be wedge shaped and wider at the bottom so the molded prop will come off. I take a
hobby knife and carefully trim off excess clay around the perimeter of the prop (be
careful or you will cut your prop) such that the walls slope outwards. Finally, I
smooth everything with my finger.
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The Skulpey clay needs to be baked at 270 degrees F (130C) for 15 minutes for every
6mm of thickness. However a technique I've used on some of my molds (and recently
verified by Mike Taylor) is to bake the clay with the prop in place at 120 to 150
degrees for about 15 minutes, or longer. When, the clay is fairly rigid, remove it
from the oven, pull the prop off, and then continue baking at the higher 270 degrees.
If you bake initially with the prop in place at 270 degrees it will curl up and ruin
itself. After baking and cooling, I wipe thin CA on the clay which seals the clay
and makes it less likely to flake off in the molding process. I also use CA in the
joint on the bottom around the stick to glue it in place. All this takes almost
longer to write than it takes to make a plug. After your first one they go pretty
quick.
Let's Mold a Prop
Start by having all your materials to hand - epoxy, syringes, small brushes, etc. The
first thing to do is prepare your mold. Take a paper towel with mold release wax on
it and wax the prop and the outside edges of the clay support. Next coat these same
surfaces with mold release agent. I do this with my finger, dipping it in the bottle
and spreading it on. Set the coated mold aside to let the release agent dry. Some
people don't bother with the release agent and some use car wax. You can experiment
and determine what works for you.
Next cut two strips of CF cloth (with the weave at a 45-degree angle) wide enough to
cover your prop and wrap around the sides. Lay them out on a sheet of wax paper. If
you will be using heat shrink tubing, cut a length of it long enough for your prop.
Cut it lengthwise down one side. Attach a strip of clear packing tape (about half the
width of the roll) along one edge. You will use this to rejoin the heat shrink on the
bottom of the prop mold. If you are going to use kitchen wrap then cut a strip of
wrap that will go over the prop mold from one side to the other. You will join this
snugly at the bottom with small pieces of cellophane tape after the CF/epoxy laminate
is in place.
I use small baby food jars to hold my epoxy resin and hardener. First, this is more
convenient. But, if I manage to contaminate the epoxy by putting something in it with
hardener on it, like a syringe, I only ruin a small amount not an entire expensive
quart of resin. The epoxy system I use specifies 2-parts epoxy to 1-part hardener.
To measure this out I use syringes I get for free from my druggist. Label one with an
R and the other with an H for resin and hardener. Keep them in their own separate zip
lock bags so they don't touch each other and start the hardening process. To make a
prop I generally need about 5ml (3/4tsp) of resin and half that in hardener. I mix my
epoxy in another baby food jar with a craft brush. Each time I just mix on top of the
last and it gradually fills up. Pull the amount or resin you need up into the
syringe for resin, then squirt it into the jar for mixing. Put the "R" syringe back
into its own zip lock bag before pulling up the hardener in the "H" syringe. It is
easy to forget and you find yourself using the wrong syringe and contaminating the
hardener with the syringe with resin on it. Mix the epoxy with the small disposable
brush.
Finally, we are ready to start the molding process. Lay out two layers of CF cloth
separately on kitchen wax paper. Or, if you are going to make a lighter prop, perhaps
one layer of cloth and two layers of tissue. I use the disposable craft brush to work
the epoxy into the cloth. Be very gentle. You have such small pieces of cloth that
being a bit too vigorous will move the threads around and leave you with gaps.
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The next step is to get the wetted out cloth arranged on the prop mold. Carefully
pull the layers of wetted out cloth off the wax paper. It wants to stretch, so be
gentle. This can either be done by laying each strip individually on the mold and
then the clear heat shrink on top of that. Or, more recently I've found it easier to
arrange the CF layers on the inside of the slit open heat shrink. Then, I place the
mold upside down on this till I have it positioned correctly, and then I pull up the
sides of the heat shrink and seal with the packing tape already in place. This
variation seems to disturb the individual CF cloth fibers less. Another alternative
would be to wet out the cloth in place on the heat shrink. In this case be careful to
not get too much excess epoxy around the cloth.
After the cloth is encased in heat shrink and sealed up on the bottom with packing
tape, shrink it with your heat gun. Take care not to get too close, and keep the gun
moving at all times. I find that gradually shrinking the bottom and sides first works
best. Shrink the top last, and again gradually. If you leave the gun on one place
too long you can burn a hole in the shrink. Because the heat shrink shrinks so much,
you should be able to end up with a perfectly smooth stretched tight surface over the
prop area.
If you choose to use ki tchen plastic wrap instead of heat shrink, you will have to
put the wetted out cloth on the prop and then plastic wrap over it. Once it is all in
place, take previously prepared short pieces of cellophane tape and carefully join the
kitchen wrap underneath. Start in the middle of the prop and sequentially work your
way out to the prop tips. When you are done you need the kitchen wrap to be wrinkle
free and reasonably tight. You can shrink the kitchen wrap a little with a heat gun.
But, it can't take much heat and it only shrinks marginally. The picture below shows
the prop with the shrunk heat shrink in place. Note the smooth wrinkle free area over
the prop.
Read the directions for curing your epoxy carefully. Most call for curing at room
temperature for 24 hours or so, followed by heat curing at a higher temperature. The
heat curing is a key part of the process and cannot be skipped.
I have found that the longer the heat shrink I get from Airdyn stays in contact with
the CF/Epoxy mixture the more it wants to stick. If I leave it on too long, I cannot
get it off resulting in a layer of heavy heat shrink permanently attached to the front
of the prop, rendering it useless. Over time I've found that I can remove the heat
shrink when the room temperature curing is complete enough that the layup is
semi-rigid, but not completely so. For me this seems to be at about the eight to ten
hour mark. I slit the tape on the bottom of the mold, and carefully start peeling
back the heat shrink at one end of the prop. If the cloth starts coming off with it,
I stop, put the shrink back in place, put some tape to join it together, and shrink it
a bit again. Then, I wait another couple of hours and try again. When, the epoxy is
cured just right, you can pull the heat shrink off leaving behind a slightly tacky
epoxy. The epoxy needs more curing, so leave it for the remainder of the 24 hours, or
whatever is called for.
If you are using the kitchen wrap method, it does not need to be removed early. I
have never had kitchen wrap stick. Simply wait 24 hours and pull it off. So, the
difference is that the shrink wrap is easier to put on, but the kitchen wrap is less
problematic to remove. However, the heat shrink seems to result in a much tighter and
smoother form for making the front of the prop. The picture below shows the prop
after room temperature curing and the removal of the heat shrink.
After the prop is done curing at room temperature for 24 hours, and the heat shrink
has been removed, I place it in my small dedicated toaster oven and heat cure it for
another 12 to 18 hours at 120-degrees F. Check the manufacturer's recommendations for
your epoxy.
After the heat curing is done you can finally remove your molded prop. I usually use
a hobby knife to gradually work the molded CF loose around the edges. This is why you
wax and PVA release the edges of the mold, to make this step easier. It is also why
the clay part of the mold needs to be wider at the bottom. Eventually it comes off and
you have something like the back and front shown in the two pictures below.
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All that remains at this point is the trimming and final sanding. Generally the
excess CF cloth around the edge of the prop can be trimmed off with a pair of
scissors. Repeated use of scissors for this will probably render them useless for any
other task. The trimming is best done looking at the prop from the back side so you
can see the molded edge of the prop. I generally try to get within about 4mm of the
edge. I have a small power belt sander which is very useful for sanding the excess CF
away to very close to the mold line. This results in CF dust, which is supposed to be
one of the worst things you can get in your lungs. I wear a very good dust mask for
this step, and vacuum up the dust with a shop vac when done. Alternately, this can be
done with coarse fingernail sanding boards, but will take longer.
Once the prop is sanded close to the edge of the molding line, switch to fine
fingernail sanding boards. This is where personal preference comes in. You want the
trailing edges of the blades to not have any ridges and to be sanded to a nice fine
taper. For the leading edge a small bit of curve results in a stiffer blade. Too
much, however, results in a poorer performing prop. The hub region is another
troublesome area. If too much material is left wrapping around the edge, it may
affect performance. Also at this point if you have a hub for your prop, drilling and
gluing it on may be useful. This will allow balancing the prop, by sanding a bit more
from the trailing edge or end of the prop on the heavy end. Shown below are the front
and back sides of the finished prop.
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I you have a gearbox like a Kenway that takes a screw straight through the prop,
mounting will be easy. Just drill a hole in the prop the size of the screw and you
are done. If you are going to mount the prop directly on a shaft, you need some sort
of hub. I've made these hubs a few ways now. Michael Henriksen glues a small piece
of ply in the back side of his prop, and then drills through this and the prop and
inserts a length of plastic tube. He reams the tube out tapered so it is a press fit
on a 1mm shaft. I've done similar hubs, but used aluminum tube instead of plastic,
which I could not find in the right size. That's what is shown in the picture below.
The hub is very light and is just glued in place with CA.
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Shown below, left to right are a variety of possible hubs that can be used. First is
the wood washer with a length of aluminum tube glued in it like in the prop above.
Next is a PVC plastic hub turned on a lathe that also allows a press fit on a 1mm
shaft. Third is a Didel prop hub for pushing on the 12t gear on the side of all Didel
spur gears. It has holes molded going sideways for inserting CF rods for individual
blades. But, in this case it can be simply glued on the back side of a molded prop.
And, last, is the Didel hub, but with the front portion turned down on a lathe (Dremel
would probably also work) to create a shoulder. A hole this size is carefully drilled
in the molded prop, and this hub is glued in the hole with the smaller portion
protruding slightly out the front of the prop. This hub works quite well with my copy
of the Wes prop as I have a sharpened piece of 2mm CF rod in the hub of the Wes
original, but protruding slightly from the hub. This results in a molded indentation
on the back of the prop showing exactly where the hole should be drilled.
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The prop I molded for this article used a single layer of CF cloth and two layers of
0.20 oz CF tissue, and a third layer of tissue in the hub region. This prop, complete
with the press fit hub weighed 0.34 grams. The corresponding trimmed and shortened
GWS prop, with the rubber prop adapter weighed 1.41g. This is a big weight savings
for the plane I molded this prop for, my 22g Fokker D-VII. This plane with the prop
on it is shown in the picture below.
And Now For Something Completely Different
I needed a three-bladed prop for a RFFS-100 based profile P40 I was building. No such
prop existed, so I made one as shown in the steps in the photo below.
First, I cut the blades off two GWS 5x4 props. I marked where the three blades would
go on a scrap piece of wood. I drilled a hole in the wood and inserted a dowel to
glue the individual blades to. After sanding the hub areas so they would fit, I glued
them one at a time to the dowel, and then filled in gaps with CA.
Next, I made a three-bladed wooden support for the clay, embedded the prop on this and
the clay. And then molded a prop on it after baking the clay mold. For the heat
shrink I took a length of Airdyn 3-inch heat shrink, and slit it open to get a 6-inch
square. I trimmed this roughly the shape of the 3-bladed prop, plus enough to wrap
around the edges. After the epoxy and CF cloth were in place, I taped the heat shrink
on the bottom as usual, and shrunk it tight.
The rest of the steps were similar to those for a two-bladed prop, except there was
more sanding and balancing took longer. My final version of this prop weighs 0.97g
and is shown in close up below.
Some of the props I've molded.
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They are, top to bottom: the 3-bladed prop, to an ultralight weight copy of a
Westechnik prop, a copy of a wood prop that Matt Keennon CNC
machines for many of his micro planes, a variety of sizes of small GWS props, and
last a copy of the U80 prop - that weighs just 0.24g compared
to the original at 0.66g. In each case, I have been able to tailor the prop for its
intended purpose. The Westechnik copy, for example, in the picture used the "dry"
epoxy technique to make very light CF props while still retaining enough stiffness for
a slow flying room flyer. And finally, making your own props not only allows you to
get exactly the prop you want, but they make nice micro plane gifts for friends around
the holidays.
Mini Article - Applying Tissue Covering To Foam
By Ralph Bradley
During my many years of model building, I have occasionally managed to discover a few
building techniques, or new materials that worked so well I immediately adopted them
as a standard technique. This process for covering foam with tissue turned out to be
one of those "Happy Discoveries" that left me feeling like I had just found a fifty
dollar bill lying on the sidewalk.
I have not done a lot of work with foam, however, when Bob Selman begin offering foam
wing cores in sizes suitable for micro R/C I decided I had to try them. After
assembling the first wing I thought it looked a little bland so I decided to see if I
could add some tissue trim to dress it up a bit. I had the left over foam beds the
cores came in to experiment on so I started looking for a way to attach some tissue
trim stripes. I was hoping to find an adhesive process that would work similar to
clear dope when used to apply tissue to balsa wood. In other words, I needed a foam
safe dope. I begin experimenting with glue stick, thinned with Alcohol, but quickly
concluded that the old standard of using white glue thinned with water looked like the
only realistic option.
Aliphatic Resin is the technical name for the slighly yellow version of woodworkers
white glue (White glue is polyvinyl acetate or PVA and also works.) Titebond is the
brand modelers are most familiar with since it is sold in many hobby shops. Elmer's
Carpenter's Wood Glue is what I found in my local hardware store and seems to be
identical to the Titebond.
When I sat down to add trim stripes to the wing, I was still in the mood to experiment
a little before thinning the white glue with water. I tried thinning some of the
Elmer's with Alcohol. I mixed a sample amount in a small cup and quickly had a mess
that looked more like oil and water. After a few more seconds of stirring the solids
in the glue collected at the bottom of the cup and there was a crystal clear layer on
top that had the consistency of egg whites. It seems that the glue formulation
includes a clear binder material, which the Alcohol had separated. The material at
the bottom of the cup is not needed for covering with tissue and I suspect it
represents the larger part of the glue's weight. I tested 3 or 4 brands of glue both
white and yellow (carpenter's type) and got the same results with each. Once the
clear material has been separated it can be further reduced by adding more Alcohol
until the desired brushing consistency is achieved. I used Isopropyl Alcohol (rubbing
Alcohol, 70% solution) from my local supermarket pharmacy for thinning the glue. My
test samples turned out to be exactly what I was hoping for so I applied the trim
stripes to the wing and allowed them to dry over night.
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Bare pink foam wing with red tissue trim striping.
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To apply a tissue trim stripe, first paint a coat of the clear glue on the foam
surface where the stripe will be positioned. Lay the tissue over the glue. After the
wrinkles are smoothed out and the piece is properly aligned, lightly burnish it down
with your finger, apply a topcoat of the clear glue and set aside to dry.
The first stripes I did turned out so well I decided to remove them and cover the
entire wing with tissue. To strip the tissue, all that is required is wetting the glue
with water. After wetting, the tissue will lift off after a minute or two. When
covering the larger area of the full wing its best to work outward from the center
towards the tips gluing and smoothing a couple inches at a time. As long as the glue
is kept wet (with more glue) the tissue can be lifted and repositioned until all the
wrinkles are worked out. When the tissue has been completely laid down trim the edges
and apply a topcoat of glue to the entire surface. After the wing is covered and dry
it should be sanded very lightly with a well worn piece of 400 or 600 grit paper. The
trim can be applied after sanding. Once the trim has dried sand it lightly also. Be
careful when sanding, as it is very easy to cut through the tissue if you over do it.
To add a bit of waterproofing and give a nice gloss to the finished wing I sprayed on
a coat of clear Krylon acrylic enamel. The Krylon concealed the brush marks in the
glue and evened out the color very nicely.
The tissue covering added a surprising amount of strength. It seemed more like the
wing had been sheeted with balsa rather than just covered with tissue. The tissue
greatly enhanced the appearance as well. The bare wing weighed 3.5 grams. The clear
glue and tissue added 1 gram. The clear Krylon added an additional 0.4-gram bringing
the weight of the finished wing to 4.9 grams. I think that the improvements in
strength and appearance are well worth the extra weight. So far there has not been
any problem with warping. The wing is still warp free today.
One last test I tried was to see how this process would work with printed tissue. The
printer ink is water-soluble and I was concerned that any water in the Alcohol would
cause the ink to run or bleed. I was not sure I could get away with using a 70%
Alcohol solution but it worked fine just as it had on plain unprinted tissue.
So far I have only used this process on foam wings. I think some of the small all foam
models that are being converted to micro R/C would benefit greatly in both strength
and appearance with a "full tissue job". White glue, tissue paper, and Alcohol are
likely to be some of the cheapest covering materials you will ever use. I also enjoy
being able to purchase my covering materials at the local supermarket across the
street.
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Here is the foam wing (below) with full covering and a Krylon clear coat next to
a balsa wing with the traditional tissue and dope finish. The absence of visible
wood grain is the only thing that gives the foam away.
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Mini Article - Simple and Light Rigging Method for "scaleish" models
By Joachim Bergmeyer
This article is about how I have done the rigging of my semi-scale Aeronca C-3. The
rigging is an important part of the original plane's construction and adds a lot to
the look, so I wanted to add this rigging to my model also. Besides that a rigging can
add a lot to the stiffness of a plane without adding much weight and thus helps to
keep the plane light. The Aeronca C-3 has a big aspect ratio, so it is difficult to
keep the wings from warping when made from light 0.8mm balsa. This means that the
rigging has to be made from a material that does not expand much under load
(technically spoken, it needs a high stress modulus), should be light, and should look
like steel wire. The material that I have found is woven fishing line made from
polyester (Dacron™). I have found a 20lb quality in a local fishing supply store
that is coloured steel grey and is quite thin. The weight is almost non-existing. To
cut this material you need a very sharp new blade, it is almost impossible to cut it
with scissors due to its extraordinary strength, and it is very stiff (does not expand
under load) also. An ideal material!
Because this material is so stiff the fixings have to be adjustable, otherwise they
will slack and you are unable to adjust the wing twist. To do this I use a clamping
fixture. I start with a ring cut from a PVC tube, bought as inner Bowden cable from my
LHS. This tube has 2/1mm outer/inner diameter. I cut rings in the length of the
thickness of my sheet material (0.8 or 1 mm) by rolling the tube back and forth under
a sharp knife on my cutting pad.
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Cutting the rings from PVC tubing
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If the ends are not even and parallel then you might need to use some sanding paper
to make them straight.
Then you have to make holes of 2mm diameter into the wing where the rigging goes to
the wing. Use a sharp drill bit and the highest rpm your Dremel or similar tool can
provide to get a clean hole. Then glue the rings into the wing using thin CA.
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PVC ring glued into balsa wing surface
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The surfaces should be flat, otherwise use sanding paper again. Now the holes for the
rigging are protected against the fishing line which would cut the balsa otherwise.
For the fixture points in the fuselage I have used pieces of tube that go all the way
through the fuselage from one side to the other. This way the fixture points can
withstand the stress although I have used Selitac (a flexible insulation foam) for the
fuselage which cannot bear punctual stress easily.
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PVC tubing installed in fuselage.
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To fix the rigging inside the tubes I use pieces from toothpicks. I cut a piece from
the tapered tip of the toothpick that has a slightly bigger diameter than the hole in
the PVC tube.
Then I pull the rigging through the tubes. Hardening the end of the rigging with thin
CA helps a lot to get the thread through the holes. Now you have to hold the model
with one hand, pull the rigging tight with your other hand and push the toothpick tip
into the PVC tube with your third hand. I always find this difficult :-), so this is a
great moment to get a little help from your significant other!
The rigging which is fixed this way is adjustable just by pulling at the threads
(leave a little extra length on the opposite side (pic)), will keep your wing twist in
shape, will even allow you to adjust the dihedral, but just pulls out of the hole if
the stress is too big without damaging the model, for example if you happen to catch
the threads by accident. If you want to keep the rigging forever then you can also fix
it permanently with a drop of thin CA, but obviously you cannot adjust it afterwards.
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Here the securing tooth pics can be seen. In this case the rigging threads are
passing right through the fuselage and are secured at both sides
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The fixing on the wing, the small amount of extra thread can be seen clearly.
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The picture of my Aeronca C-3 shows a model that looks very natural in the air. You
can tell from the dents that this model has been flown frequently over the last indoor
season. It flies so well and stable that it has become my "steady Eddie", the model
that flies when everything else goes wrong.
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Jochen's "Old faithful", every one should have one!
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As you can see in the last picture, the fishing line is also good for hinges. I use
8-loops that I fix with a tiny drop of CA on the outer side where the thread goes
through the balsa. Do not let CA wick into the hinge point itself, or the hinge will
be very stiff! The hinges look quite natural in my opinion.
So there you have it, add a more scale appearance and improve stiffness for little
weight, money or time!
Mini Article - Travels with my camera
By Graham Stabler
It struck me that for a column about micro and indoor models we had not had very
many pictures of models. For that reason I thought I would show a few of the models I
have seen this year at both the Leicester
Aeronutz fly-in and a similar event held at Nottingham. I must admit now that
photo journalism is not my thing and not only don't I take enough pictures they are
often not in focus and without further details. This is something I must fix as I
often see models at these events that just make me grin. I think if that a model
makes you grin when you see it fly or even when stationary then the modeler has
cracked it.
He needs a holiday:
Ray Holiday is a very busy man, he must be as he seems to build models 24 hours a
day. Ray worked for Rolls Royce and apart from the copious amounts of tape he must
have "borrowed" (most of his models are held together with it) he also got a passion
for jets. This model is his ducted fan Vampire that has been flying at Leicester and
Nottingham successfully since early in the season. The model has a 14" span and is
made from carved foam for the fuselage and depron for the wings. Ray makes his own
ducted fans and uses the M20 LV motor (KP00), yogurt pot for blades and thin ply and
lithoplate (thin aluminum sheet used in printing) for the duct. Control of the model
is via the Ztron infrared system which in this case is controlling the throttle and
rudder. Additional info: 145mAh LiPoly cells used for power, 1" duct with blades set
to 45 degrees in hub.
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Ray's tiny ducted fan Vampire
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So that was a standard boring model f
Jun 03, 2003, 11:44 PM
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