Touching Pee-pees in the Dark and Adventures in Airplane Bonding

Our first really big pane is about to come off of the line. I guess it is easy to see that I was slacking. Not really, just got side-lined by a bunch of stuff.

Today, I wanted to talk about the bonding experiences. We are pretty good with the cheap and cheerful 60min and 90min epoxies available from Loctite. In general, they are worthy of their cost. For the most part:

•  they get it done
•  are easy to clean
• decent finish that is sandable, but micro-balloons help immensely
• and wet chemistry does not rip the skin off of your hands

 The other side of the coin is that :

• they are not super hard
•  sometimes leave a sticky surface which needs hand washing a bunch of times to get the surface clean
• cheap means heavy
• in the warm, humid air of Florida sometimes the pot runs really fast.

It is exciting to see it melt through two cups and run epoxy all over your surface and drip down your leg. The steam is cute on a  100% humidity afternoon. Around here, it is not uncommon to see rising off of your sneakers, but it is exciting to see rising off of your freshly-laid skin. It still burns your skin ... boiling water is hot... If the cup gets to be hot to the touch, or the epoxy runs really fast then you only have a few more minutes before the epoxy fixes.

We have some other adhesives and layup chemistries used by the "pros". Ok, yikes. There are enough flames, warning symbols and ventilation requirements that we honesty did not have the facilities to use them. However, some basic tests show that the stuff from Aircraft Spruce is really hard and easy to sand. It stuck to everything , was hard to get off of skin, and 10:4 is not an easy ratio to eyeball. So it needs a scale and  a fan to pull air away. I can only say it must be best with vacuum-bagged molds and infusion systems.

I would suggest using either, just know what you are getting into. With any bonding system, rough your bond area with steel wool, clean your area with a clean cloth and alcohol. Cleaning with alcohol is to pick up any grease, dust, grime and hair. Test your bond between scrap pieces of material.

Hairspray Trick Addendum

If you are reading more about using hairspray to pre-wet rc models before they are glasses...

This works great with balsa models. It seems to be a pretty reproducible effect on wood because the wood absorbs some of the hairspray. The wood keeps the spray wetter so it does not dry out as fast. Fast forward to a foam model. Foam does not absorb any liquid, relatively speaking. Raw hairspray dries really quickly. This is a problem, we have found in many wrecked sets of cloth that if the hairspray polymerizes past a certain (not discussed in this article) point, the epoxy will not wet the surface.

What it does do is bunch up the glass when you spread the resin. Pretty quickly you can see that something is wrong because it just does not behave. You cannot straighten out the new wrinkles fast enough to get in front of it. The weight of the resin does not hold down the old wrinkles,and you make a mess. If this starts happening, we just yank off the damaged glass and toss it. Yeah, it seems wrong at the time, but we have no idea how to clean it. Always looking for hints on that one.

After the glass is tossed, we roller the wet surface of the model to spread the remnant epoxy. That is just to help fill in a pockets or spread any runs. It just air-dries after that, just leave it. When it is dry, remeasure your glass and do it again.

All in all, the hairspray trick rocks, on wooden models. Not so good on foamies.

Remember try to do your epoxy work in warm or hot conditions at the lowest allowable humidities. Yesterday, a thunderstorm snuck up on us and we had a very energetic pot of Loctite 60min epoxy. It fumed and melted several  plastic cups and unbound the wax on a paper coffee cup in the span of 1min. Finally, it melted several plastic shopping bags while we ran cold water over it.

Basic chemistry class says, take the heat away from exothermic (heat generating) reactions to slow or reduce them. Live with a chemist and you know these things.

Practical Composites: Fiberglass Over Wood

We are putting together some good pieces for everyone see how we are doing it. The big tip that we can tell everyone is hair spray. It is too funny to think of either of us ever sing hairspray. One has been bald for half of his life, and I have never been a big coiffure kind of guy.

The pictures that I am including are of a model that is 57" (143cm) x 6" (15cm)  wide and 4" (10cm) tall.

                       

The glass sued is standard s-glass, 2oz per yd glass from Aircraft Spruce . Our experience shows for fuselages, two plies is more than enough for fuselages. Make sure that your plies are pig enough to cover all of the piece and hang over about an inch (2-3cm). More than that makes it hard to support the piece, the fabric keeps pulling away from the edge  or it gets tangled.

Apply the hairspray liberally to wet the surface of the cloth, then use a foam roller to spread the spray and wet all of the glass. It takes a few minutes to work the whole surface. When it dries, it will feel more like paper than silk.

Having learned the hard way. You can let these dry for 15min or so, until they are not tacky to touch. However, do NOT let them dry for lunch and a trip to the hobby shop. The hair spray dries too much and coats the glass so that the resin does not stick. That is bad, it will just wreck your finish and everything will look all orange peel or like bubble city.

I hope this helps with your projects.

More building on a budget tips

I have been speaking with many people recently about how they build their uav's and larger-scale working aircraft. We were going down the balsa road, which is not so bad. It makes a pretty stiff structure with relatively inexpensive materials. However balsa is graded and strong, light wood is not easy to find nor is it cheap. You can easily run up $300-500/board foot. That is a lot of balsa, I know especially in 3" strips. However, you get the idea that this is not a cheap way to go.

We have been using kiln stabilized wood and it is making a decent product. There have been some issues balancing the stresses in the airframe. Traditional methods more or less work to relax them. My consultations with other people in the community leads me to believe that there may be better construction techniques. The system that we have uses balsa and then fiber glass on top. Our main issue is that balsa takes so long to lay up. To do a good job, it could take upwards of 3-4 weeks of concerted effort to get everything pinned and glued, sanded and filled, sealed and prepped. It is a big job, especially on bigger models with large spans.

OMG the twist, you will make lots of little weights and hooks to untwist your larger pieces as well. In some ways it is a terrifying process for the beginner. It is not hard to do, it can be terrible to do well.Blah blah, no whining. The others use mostly styrofoam cores covered in composites. I have been experimenting this weekend with it too. I have found that you can have a pretty good product with almost no invested time. My realization is that the cheap epoxies available from the hardware store, are easy to handle, water washable, but are way to soft-cured. For these applications, an epoxy chemistry that cures with a hard surface is better than a softer one.

I assume that epoxy chemistries with harder fixing strengths are less easy to handle. We have made several layups and this really is the case. In Florida, it is hard to get away from the humidity and large swings in it. So we lay up outside and bring the layups inside the air-conditioned house. That seems to help even out the handling properties.

The following pictures are using LocTite 60min cure epoxy, 80 oz./yd glass cloth and 3mm depron foam. There are many relief cuts in the foam to help conform to the shape of the fuselage. The pieces are masking taped together, a foam-safe CA glue would work too, but I was trying to limit the number of seams that needed to be joined. Most of the tape is to enforce the shape of the fuselage, not to hold the piece together. Both halves of the fuselage took about 2h to sheet and the glassing took about 20min for one person. It is not perfect, but it is an experiment. My illustrious helpers would assist on the final version for sure.

This model is about 5' (150cm) long and averages 6" (15cm) in diameter.

More Practical Composites

After several weeks of analysis, I came to the conclusion that using a glass or carbon fiber reinforced balsa composite is one of the best ways for us to get the weight out. Our other attempts were plagued with uneven resin densities. We have good luck matching allowables for 2-4 layer lay ups, beyond 5-6 layers we start to diverge pretty badly.

The best results that we have seen is with 60min epoxy and making sure that each layer is well wrung out before it is let cure at normal room temperature, 77F. As the humidity goes over 80% the pots react differently. The biggest issue for the divergence we think is the number of pots of epoxy used. Something happens with the weight and density between the pots.

Make sure that when your layups are done that you have at least a 2-3 layer pot. If you need a much thicker lay up, I would make sure that you make enough for everything. Also it is an issue with the humidity in the room. Do not add alcohol to the resin when you mix in the hardener. If the mixture is missed, it will just ruin the pot. At worst, it will react exothermically and create large amounts of bubbles both make a mess of the part.

Adventures in Composites

The big thing that we learned in this episode is how to manage holes or penetrations. We made a male mold and were trying to decide how to mark and spot where we wanted to make penetrations called for in the fuselage.Marking the fabric was not particularly accurate, but it did work. The real answer that we saw was to make a potrusion where the penetration was.

It is easier to cut off a piece that sticks out past the piece that you want than it is to dig out material where you do not want it. Our design has four or so penetrations into the fuselage. Additional wedges of blue insulation were hacked to fit into the holes. They were not made to perfectly match, but were close. So you could easily see what you wanted to keep and cut.

Another interesting thing that we learned was the relative softness of home-owner classes of epoxy. Be aware of this. Y, is that any feature that looks liou can make a mess of your piece if you heat it too much to melt the wax. You have to pull the piece free form the molds and set them back down once the part is free. Putting it back down to the mold really helped the part retain its shape. As we pulled our parts off, we were concerned that they would not be square when we put the parts back together.

Due to some communications issues, there were some delaminations in areas that would have been better had they not occured. These anomalies were healed by forcing a needle into the void and using a syringe to fill it with epoxy. Epoxy is viscous, be aware that it is tough to dispense through a narrow needle. Voids were filled until it appeared to be full visually, pockets were no longer hollow appearing.

Molds seem to be effected strongly by Murphy's Law. Everything that could have gone wrong with our parts did. Our shape has several bays. Bays are hollows where square sides would be best. The bay that we took the time to box in with plywood came out well. The bays that had no boxing in had delaminations and other issues such as corner delaminations, non-square lay ups and twists where pieces hung on something and were not carefully managed back into place. In a bay, is the only area that we had any mold damage. It was due to pieces of cloth almost wrapping a narrow piece of mold. Had it been boxed in, the pieces would not have been able to wrap the way they did and pin the mold into place. This damage was minimal and was fixed with filling the cracks with glue and for the most part was a non-issue.

As If You Thought I Have Not Been Working

So I have been trying to get some interesting pictures of the layup of the Big Baby fuselage. However, the pictures that I do have are not as demonstrative as I would like. I guess I will have to get down there and take some myself. All in all, the mold concept worked and the internal, integral structure also seems to be pretty successful.

Our idea, is to mold a certain amount of internal structure into our composite molds. The interest is to reduce production time of aircraft to the limit of curing and finishing time. More advanced resins could be used in the future. Our experiments are with normal 60 and 90min set air-cure epoxies. We also used a male mold to reduce our total investment. Surface finish is less than important to see for these experiments. They are about the viability of integral structure composite lay ups. I have said that last bit a few times, but I have received several comments about this.

The next step is to relief the wings so that they make a pocket, lift system that will effectively create a similar cross-section to the metal equivalent airframe.

Adventures in Composites: A Bright Idea

So after many months of gnawing on how to reduce the construction time of uavs and model aircraft, we finally decided to watch a million youtube movies. Sating ourselves on anything that had anything to do with composite lay ups we set off onto our own project. The first version of this idea is a proof of concept experiment rather than a real product. Aircraft Spruce sent us some great almost silken 2.5oz/yd^2 fiberglass.

This is an experiment in how to create multi-cell,internal structure in composite structures. If an airframe just needs a few reinforcements, that would go a long way to reducing the production costs of a unit model. Having built several models now with varying levels of success. It is clear, the longer it takes to produce a model, the more likely it will have issues. It could be as simple as being heavy, due to liberal adhesive use. Or just general issues such as fiber breakage or delaminations due to mis drilling or toughness issues in large unsupported panels.

After watching many others' techniques and lots of research into how successful airframes are created. It is about reducing the handling of each piece. The best paint jobs are ruined by finger prints, finishes scratched, just because they had the most chances to get ruined before delivery. Our concept centered around simple low curvature lay ups, that could be lain individual. Then stiffeners installed before the final layers are put down. Each mold would then be set to cure. Final assembly, would be put in place in the planes perpendicular to the molds to join the sides together and to increase the technical stiffness of the airframe.

Our design required a center web and some easy radii transitioning each of the perpendicular webs. We sectioned our fuselage to have three dorsal access bays. The fuselage was then bisected so that we had two symmetric halves and three continuous doors. Empennages and noses are difficult to build and to shape. There is always an issue with the symmetry or the strain in the materials during the curing process. Careful attention was paid to each of these areas to reduce the known issues that had ruined some of our previous projects.

Our split molds were then mounted to three inch styrofoam wall insulation. Not the ball stuff, this is the small,closed-cell material with wood glue. Everything was painted several times with latex enamel. The enamel did not stick during the first few coats. It probably has to do with the static charge that built up during the work. There was an eventual critical coating area that finally whetted the surface. Once the surface finally whetted, the coats proceeded quickly.

We found several movies about hand-lain composites that kind of tipped us off on how not to complicate our chemistry. They all had a few things in common, latex paint and paste wax as the sealant and release agents. After fourth coat of latex paint, Johnson's paste wax was applied liberally and hand-buffed to a shine. Each mold and the foam several inches (3-4") from the model was covered and buffed.

Then it was all let rest for a day.

Adventures in Composites: Two Bogans and the Art of the Lay... Up

After a year of sitting, our polyester resin had spoiled. So we contacted some people and they said you could do it with just about any medium cure time epoxy. Out into the proper disposal container for petro-chemical waste. Yeah, I went there. Off to the local hardware shop for some epoxy. We bought several batches of 60 min epoxy. Some of the information that our friends told us suggested that we could use isopropyl alcohol to help keep the viscosity of the epoxy down as it is laid up.

Information like this is when it gets exciting. We found out a few things. You can paint the alcohol onto a layer to help flow the epoxy and to reduce the bubbles and flatten wrinkles. I would suggest that pour some alcohol into a cup and paint it on where you need it. Each layup will be different so it is hard to say where you would have issues.

There were some bubble generators in the seam where the semi-mold met the backing. This is probably pretty obvious, but it was due to training air under the resin laden cloth. Our mold also had several regions with nearly perpendicular sections which were hard to wet. In areas like that, I would suggest liberally pre-painting. Be careful not to pool the resin in the bottom since that is not where you want the part to be strong. Another observation was to make sure that when each layer is put down, both people should work in the same direction. Do not work the resin from the nose and the tail. It makes wrinkles and other defects.

It is very important to make sure that each layer has as much of the resin out, or at least even distributed as possible. This means that all of the bubbles, wrinkles and puddles need to be worked toward the edge of the layup. A roller and brushes are the weapons of choice for this work. The handle edges are great for wringing out excess resin from tight radii.