Dodgy Hand Launch
Hi micro fans. In this issue, we will have mixture of new technology and old technology with a modern twist. The feature article is by Gordon Johnson who you will all know has written several articles including last months feature on actuator torque. This month he describes his adventures with carbon fibre and shows how he makes carbon propellors using existing plastic propellers as a mold This is not a new technique but Gordon gives a good recipe for wanna be prop molders to follow. I can tell you from experience that molding parts from carbon can be an excellent experience and also make some other parts that will be covered in future episodes of IS. Some outlay is required as is care and thought but surprisingly professional results can be achieved. Best of all is when you impress yourself. As with all good "how to's" Gordon presents a specific method but the general points about release agents, wetting out, carbon handling and compression of the matrix are all transferable to other molding ideas such as making sheet, tube, spars etc. Go on have a go. Gordon has even arranged for a small carbon kit to be made available so you don't have to buy enough carbon cloth for a wedding dress to get started.
I should make a small disclaimer here. Obviously propellers spin around and can do so at a good speed. If you do decide to have a go then it must be at your own risk. That said our micro models are the ideal place to learn the art as the forces involved are not so big, as long as you take sensible precautions such as not standing over the prop and check the condition before use a carbon prop should be as safe as a bought plastic one.
Next up is some old technology brought up to date. Ralph Bradley shows his method for applying tissue to his latest model. That's the old technology but wait, it's a foam model and Ralph has made his own foam safe dope. What's more he has used an inkjet to print on to the tissue somewhat like last weeks method for printing on to balsa, this is very popular amongst the freeflight boys and results show why.
Joachim Bergmeyer then presents his simple method for rigging one of his scalish foam models. He uses a high tech fishing line to improve both the appearance and the flying qualities of an already nice model. Jochen apologies for the condition of the model as he insists on flying it, who would have thought it.
Finally to increase the model count for the column I present a few pics that I have taken on my travels. They include pager powered models, micro ducted fans and even a few rubber powered ornithopters.
Feature Article - Molding Carbon Fiber Props
By Gordon Johnson
Why might you want to mold your own super sexy carbon fiber props. Well, in a word, they are super sexy. But there are other reasons. Most important is the weight saving. The CF prop I molded on top of a GWS prop for this article weighs 0.34g with hub compared to 1.41g for the original with a rubber insert hub. On a plane weighing 22 grams this is a 5% reduction in all up weight. For lighter planes the relative savings will be even more dramatic. The other main reason for molding your own is to get exactly what you want. The beautiful Westechnik CF props start at 16cm (6in) diameter. So, for a semi-scale micro plane a Wes prop is out of the question. If you want a small lightweight CF prop you are pretty much going to have to make your own. As I show in this article, you can trim down an existing plastic prop to the size you want and then mold off that. The final reason to make your own is you have complete control over the materials and how much of each is used. A 16cm Wes prop weighs about 1.4g and works nicely with a 4mm Didel pager geared 21:1. But for a 10g room flyer plane this represents one of the largest single components by weight. I have been able to cut the weight to 0.58g by copying the Wes prop using an extremely dry epoxy lay up. This is a huge reduction in weight over the commercial CF prop. (Note, since I did this Wes has come out with a lighter version of this prop that weighs 0.85g).
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.
The prop molders paraphernalia
To get started you need:
- 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.
- 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)
- 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.
- Mold release agent (such as a pva release agent), high temp is better. Some people don't bother with this.
- 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.
- 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.
- Fingernail sandpaper boards in various grits for sanding the props.
- Bakeable clay for supporting the prop while molding. I use Sculpey brand.
- Medicine measuring syringes from your local druggist for measuring resin and hardener.
- Small disposable brushes for applying epoxy.
- Wax paper for laying CF cloth on while working the epoxy resin into it.
- Acetone to keep the brush in between making props, or you will go through a lot of brushes.
- 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)
- 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.
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.
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.
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.
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.
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.
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.
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.
Bare pink foam wing with red tissue trim striping.
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.
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.
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.
Cutting the rings from PVC tubing
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.
PVC ring glued into balsa wing surface
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.
PVC tubing installed in fuselage.
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.
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
The fixing on the wing, the small amount of extra thread can be seen clearly.
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.
Jochen's "Old faithful", every one should have one!
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.
|Ray's tiny ducted fan Vampire|
So that was a standard boring model for Ray, now for something completely different yet similar. This is Ray's first attempt at using a 6mm pager motor in a 0.6" duct. The model is free flight and does fly but only marginally, it is built very lightly from 2mm wallfoam (thin expanded polystyrene sheet) and the wings actually curve upwards in flight. Ray is again using a 145mAh cell and is driving the motor from a DC-DC converter as pager motors tend to need higher voltage to get decent power from them.
I hope to cover this new class of model in more detail next month but as they were one of the things that created most interest at the flying "field" I couldn't leave them out. These models use radio gear salvaged from tiny radio controlled cars that many of you will have seen on sale over the Christmas period. These tiny cars were developed by Tomy and are called BitChargers and so we often call these models Bit Flyers. This is really out of respect as most use the cheaper clones that have sprung up.
The first two Bit Flyers are made by Geoff Raygada. Geoff has not used the motor that comes with the car but rather decided to use a larger motor. This allows him to build slightly larger models that he prefers. The biplane pictured flies very well and the lovely open top job just requires a little more power and I am sure will be flying well next season. Both models are throttle/rudder and use 145mAh LiPolymer cells for power.
The next model built by Brian Springall is at the other end of the scale, it uses a 6mm pager motor, 7:1 Didel gearing and a single 40mAh LiPolymer cell. With a wing span of 13" and a weight of 13g the model flies very nicely. Brian uses the receiver board as is so the prop production (out of a yogurt pot) had to be done very carefully as did the trimming to get the most out of the limited power. Brian may upgrade his receiver with a mosfet to get a little bit more juice and that will only add to this lovely flyer. You can also see on the left hand side of the picture the box he has used to rehouse his car transmitter. A hinged video box, takes 4 cells nicely and is great for maintainance.
Last but not least is a cute little Fike by Don Surtees pictured as an illegal alien at the indoor freeflight nationals held at Nottingham. Again Don has gone for a larger motor and has used 1mm/0.5mm Depron foam for his construction. In case you are wondering the lack of cheeks on the engine cowling is due to the late finishing of the model. Midnight the previous night! Details: 145mAh Lipoly cell, modified Bitcharger clone RX, M20HV (2.5ohm) motor direct drive with a grey Potensky 65mm prop. Wingspan is about 13" and it has an AUW of 22.43g which Don thinks is heavy, I think that's pretty good. Flights have had mixed success with some trim problems leading to left hand wing dropping. Don also mentioned his finishing technique, he sprays mat Humbrol enamel paint for the base coat and then carefully uses a Staedtler pigment liner pen for the lettering as it doesn't come off on over excited fingers.
Old and new
Next are a few models from the other Gordon, Gordon Brown. Strangely this Gordon also has a passion for carbon propellers and has made many for us Aeronutz, transforming quite a few models I might add. His method which uses a two part mold will be featured in a future issue of IS.
On the left we have Gordon's Bristol M1.C still using old technology, 2*110mAh NiMh, M20 LV motor, 4.75:1 homemade gearbox and home made 123*85mm carbon prop based on a Gunther. It was made with a single layer of 200gsm cloth for those who are now in the know. Construction is white foam for the fuselage (turned and then routed hollow) and depron for the flying surfaces, it climbs at an incredible angle considering this is a pre lithium model. The model weighs 24g and is controlled by Ztron IR.
The next model along is his blue foam Bearcat. This model again uses the M20 LV, a homemade gear box and prop but this time has the added performance injection of a LiPoly battery. It looks quite ominous in flight and makes you want to duck for cover, especially as Gordon is finding his actuators a little weak. A consequence of using high resistance actuators as required with the Ztron system. Vitals: 15.3" WS, 24g AUW, power train as per the Bristol.
Finally is Gordon's "Pink Thing" (I'm saying nothing) a test bed model for a 7mm Namiki pager. The model uses a 145mAh LiPoly, an IndoorFlyer DC-DC converter to supply 5v to the motor , a 4.75:1 gear ratio (gears from a union micro servo) and another homemade carbon prop (92X82 1 layer 200g or two layers 100g). Construction is carved pink foam and Depron and performance is excellent, a real pylon racer. This model also uses the Ztron IR system so popular at Aeronutz events. If you recognize the model it is because it is a scaled up version of a Gunther FF model.
Free as a bird
Not electric but more than worth a mention are the ornithopter kits John (wow that's small) Mack was flying at Leicester. John normally designs his own ornithopters (and I would have pictures if not for flat batteries) but bought these two beauties as kits. The larger one on the left has a wonderfully prehistoric flap to it and a higher aspect ratio than most O's something John is keen to explore, it is called a Flapping Flyer and is available from Sams models. The other design is a double flapper or should I say triple as the tail also flaps! The mechanism is fantastic and the picture does not do justice to the action. This model is called the Dragonfly 1 and is designed by Youhei Takatani of Japan. You can buy many of his designs on his website. Johns words on the Dragonfly 1: "My first Dragonfly 1 flew away, but the kit thoughtfully provided enough materials for more than one model and included alternative feather quills for the LE spars, which were used in the model you saw. It has been a very consistent flyer, with indoor durations exceeding 60 seconds." Flew away? Wow! Enough materials for two, double wow. When John refers to feather quills he means it too. An excellent material for the job by evolution.
John's main project (read obsession) is actually the production of a scale dragonfly, 1:1 scale at that. He has done it but will not be satisfied until he can get 15 seconds duration reliably. John will hopefully be presenting some of the techniques he uses in future mini's assuming the powers that be don't mind the lack of electrons ;-)
I'm posting this again, in this thread, as promised above.
The carbon fiber prop molding kit I mentioned in the Inside Story is now available from Bob Selman. There is sufficient matterial to make about fourteen CF props of the 5-inch size. More detail and picturs are on Bob's site at the above link.
An improvement since the article was written was we found a heat shrink tubing that does not stick to the prop when it is waxed and taken off within 24 hours. This is a nice development.
BTW, I have no financial interest in this kit. I worked on its development with Bob simply to make a kit of the materials needed available in smaller quantities so more people might try the technique.
If you have questions, please post them in the [url=http://www.rcgroups.com/forums/showthread.php?s=&threadid=129052] BSD CF Prop Thread and I'll answer them there.