Sunday, April 25, 2010

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.

Sunday, April 11, 2010

Building on a Budget

So we are working out our system to build giant-scale planes on a budget. We build mostly our own designs. It is far more challenging to wing it. I have to say that building from forms is pretty challenging, and worth it for those of you who like to do it their way.

One suggestion that I have for you guys is, lattice. Maybe that is not an obvious description. We have the most luck building complicated curvatures, when we lay out a regularly spaced net of 1/16th inch (1.5mm) balsa strips x 1/4" (6mm) . The spacing is what ever makes sense for the model. I mean, you will need to work out some of this for yourself. We find that wings are good with 6"(15cm) pieces cord-wise and 20- 25% spacings span-wise. That way there is a reasonable pitch and still a good amount of coverage. Onto this, you glue your actual skin down.  This helps reduce the actual curvatures involved, but also allows for the skin to be supported without the need of a foam internal block. Use wood glue if you are working on a Styrofoam base or CA if you are working on a compatible base. CA will eat Styrofoam, even the vapors will destroy it.

We get all of our balsa from local suppliers. Our favorite supplier is .

Thursday, April 8, 2010

Trip to the SPIE Defense Sensing Show

I went to the SPIE Defense Sensing Conference this week. It was a nicely arranged affair in the Marriot World Center in Orlando. They insisted that you park several miles away and use one of the shuttle buses to get to the conference, that worked out well. The buses were often enough that you were not forced to wait forever to go either way and they gave you granola bars and water for the trouble. Don't they know, engineers get bagels, factories get doughnuts? You can't make them think...

After perusing the technical book area, which was pretty well stocked with an array of SPIE documents and textbooks,, if you had the cash. I was pretty interested in their IR and Spectroscopy handbooks, but they were pricey. If anyone has a heart, I would gladly accept a torrent or a photocopy :)

We spoke with several vendors in the cavernous exhibition area. It was clear that in many ways we were just not up to the task of doing business there. I also noticed that people feeding out of the government trough were particularly mislead in the value of things. How can someone in good faith try to sell tank windows to a normal person? Especially when they cost $30k+ to start in bulk. We saw some neat optics stuff. To be honest, I was not even sure what you did with it. Even as slowly as we fly, a FLIR with 300 lines, 90deg range and 30Hz is going to be hard to use unless the goal is just to get an image.

All in all, a good trip. A little humility is a good thing.

Thursday, April 1, 2010

Big Baby Developments... Daddy's Building a Brand New Plane

I am not ready to reveal the Big Baby just yet. It is going pretty well, symmetric and no smeared blood or anything. No blood this late in a project is a big relief.

The airframe for the next version will be a bit different. This time, I was careful to build the skin by wetting and pinning the skin to as closely as possible match the model curvature. Besides the 10s of pins that were used, the technique was pretty effective. 

Use masking tape along the down side of the board so that there is a basic joint between the pieces. Then just use a wet sponge to wet the downside of each of the planks. Lay the plank onto the surface of the model to be copied. Work from one end and pin along the edge every inch or so about a 1/10th of an inch from the edge each time. Do this for one inch rows along the plank. Depending on the curvature being copied you may need more pins or to pin in specific directions to keep the balsa layer in contact with the curve.

After the part has dried, overnight or after a few hours, adjust the pins to fit the planks as closely as possible to the curve. The planks may have to be rewetted to relax them into place. Then use a foam-safe (in my case) glue in the joints. The tape will be removed before you install internal structure. Once it is dried and the seams are glued. You will want to remove the pins and the part from the form. I cut simple cross-sections to support the structure. Then used CA glue to glue the seams and to bond the internal structure to the skin.