How best to build a rigid airship hull?

[Brent Gair]

(My first post here.)

I've been working for YEARS trying to figure out the mechanics of scratchbuilding an airship (a topic which could go into the construction tips forum). I consider myself a fairly accomplished builder. I saw your post about this forum on the Hobbytalk BB. You might have noticed my lengthy post about the 36" foam and fiberglass Spindrift that I'm building. I've also built some big RC ships (water borne) and I've scratchbuilt a few of the 1950's era rocketships (even sold a WHEN WORLDS COLLIDE space ark model to author Jack Hagerty). So I'm not a guy who's easily stymied by a modeling project.

But airship building has me rather stumped. I'm just not sure how to tackle the problem. I've done complex plank-on-frame ship models where that kind of construction is necessary in order to provide interior space. However, that seems like a lot of effort for a vehicle that is completely enclosed and it yields another set of difficulties. And I want something fairly "solid" as I'm not a big fan of fabric covered frames for display models (great for big model airplanes but not ideal for banging around the house). I do a lot of lathe work but that, by itself, yields a smooth surface that won't do for a rigid ship.

I saw a picture of a 10' long model of the MACON in an old book and I've wanted to build a moderately large (maybe 4') ship for years. I only hope I can figure how to do it before I'm REALLY too old.

[Charles Adams]

Aha! Welcome aboard!

I have heard most often that people want to build but don't have sufficient reference. I hope to cure that ill with this site! ;)

I've also thought a lot about the same things you are struggling with. You are obviously a highly experienced modeler. But are you really certain the fabric-covered frame approach is not the ideal/best method?

Carving and plank-on-frame only simulate the underlying shape so they don't yield satisfactory results. I've seen models built this way that are sold and therefore need to be manufactured in quantity. They carve the shape out of wood and then "cheat" by putting little strips of material down the hull to simulate the divisions, then paint over that. It just doesn't look right. Funny, the company that made the now-highly-sought-after Graf vac-form kit even used this method -- and it shows!

Carving from foam yields the same result, only perhaps making it a little easier to "dig out" some of the foam and try to simulate the sagging of the fabric hull cover. But that is a lot of work (yeah, I've tried it!). You could use a hot wire rig to dig out channels in the foam, but good luck being steady enough to make that look right without first building a complex jig.

The best results I have seen have consistently been making a frame and then gluing on a covering. The huge Akron at the Smithsonian was built this way and it looks like the real thing! Matt Hargreaves 9-foot Zeppelin featured in FineScale Modeler had a styrene frame covered in Econokote and it looked very nice. The model of the Hindenburg built by ILM for Indiana Jones and the Last Crusade was also a fabric covered framework and it looked outstanding.

Now it's been years since I've built a balsa frame and then stuck on a covering! I don't remember it being very simple to pull off, but of course there are lots of different choices now other than just tissue and dope. On the advice of a good friend, I plan to experiment with silkspan to see if you can apply thin strips, overlapping enough to bond them to each other, and then hide the seams effectively using dope. I am told this works very well and I do know others have used this method in the past with success.

Of course, it really helps that I can build digital patterns and produce artwork to have parts laser cut. I am designing some "core" structures in light but strong Gatorboard that should make assembling the frame of a large airship model a snap. (Who knows, this could even form the basis of a practical kit!) Of all the various ways to build an airship hull, I just keep coming back to the basics and finding that this old, tried and true method may just very well be the best method.

The only really effective alternative I've been able to come up with is to build a super accurate CG model and then have the hull carved out of foam using a CNC router. But then you may have to spend a lot of time cleaning up the hull to get it just right. Plus you have to mold it and then make castings. Even if you want to vacuum form, you still have to make a suitable master somehow and that means something strong yet porous.

Other methods seem to be a lot more trouble and not a small expense. So, in the end, if you just want a one-off model then the frame method may be the easiest!

My two cents. . . :D

[Brent Gair]

My primary concern is the durability issue. Any sizable model is bound to get moved around or bumped into for any variety of reasons. I can't tell you how many times I've walked through a 24" doorway with a 36" model...sideways! Fabric will eventually end up with a hole or a tear. If the underlying structure is damaged, you have to peel of the covering to get to it. This is the kind of thing that is acceptable as part of the "deal" for big RC model airplanes. Big ugly patches are the price of keeping something flying. But it's not something I'm happy about on display pieces.

Of course, I'm a particulalry klutzy guy and I like to overbuild things because I'm capable of inflicting a lot of damage myself. But I would live in fear for a fabric covered model sitting in the living room just waiting for me to back into with the vacuum cleaner, or having some relative drop by with 6 year-old left unattended for 10 seconds.

One possible idea I'm kicking around is foam construction similar to my Spindrift model. I've found that my bandsaw allows for very accurate cutting of foam. In the Spindrift, I cut individual hull sections and stacked them together on a keel before final shaping. I can envisage a similar idea for a rigid airship: Use something like aluminum tubing to form a central core. Then cut each hull section on a bandsaw (this would allow the shape of the formers to be incorporated into each section as opposed to the smooth round shape from a lathe). Stack the pieces together on the aluminum tubing (having drilled a hole in the center of each foam hull section). Carefully sand to the final taper being careful not to smooth down the shape of the formers.

I would envisage this being covered with a lightweight fiberglass cloth (which conforms well to sharp corners).

But I admit that it all theory (well, PART theory because I have built glass covered foam models). Maybe when I get that Spindrift finished off in a few months, it will be time to test the theory.

[Brent Gair]

As for a more solid model, I would have thought the plank-on-frame approach would have been ideal, seeing as you're looking at a hull with a polygonal cross-section.

Let me say that I'm NOT opposed to plank-on-frame construction as it does have the obvious advantage you mentioned. But it does introduce a host of problems which can make it less than ideal.

I have experience with that method as witnessed by the skeleton of this 60" minesweeper (note: I usually put pictures in my post but, in accordance with policy here, I'm putting in a thumbnail link to larger image...never tried this before so I'm not sure it will work)


http://img204.imageshack.us/img204/7281/wp6wz.th.jpg (http://img204.imageshack.us/my.php?image=wp6wz.jpg)


As I mentioned earlier, I'm concerned with the durability issue if the model is fabric covered. But even wooden planking over the frames has problems. The planking would require extensive support due to the very precise shape of an airship hull. On a smooth-hulled model ship, bumps and "wows" can be filled with bondo and sanded easily. On an airship, every step would have to be taken to minimize that requirement. While certainly possible, this creates a lot of work for a relatively simple looking structure...and I'm getting too old for that.

So I just want to say that while I consider plank-on-frame ONE ALTERNATIVE, I don't consider it to be the obvious or necessarily the best alternative.

[Brent Gair]

BUT, with that said, it's a lot more work to make a proper hull in foam! Still, all you have to make is one half!

I disagree.

I did various plank-on-frame projects for years (airplanes and, as you see above, ships). About two years ago, I started to make the switch to foam because it's less work. The reason it's less work is that the basic construction embodies both the frame and the shape into one task.

I point you to my very detailed Spindrift thread on the Hobbytalk Science Fiction BB where I first learned about this new forum. It's packed with photos showing the stacked foam construction.

With conventional plank-on-frame, each frame is cut and must then be planked for the final shape (yeah, I'm a real Einstein to figure that out!). That planking can be a real S.O.B.! Or, if not planked, the frames must be covered with some kind of fabric.

With foam construction, the frames and the outer shape are, in essence, one step. Each "frame" is a 2" thick piece of foam which carries with it all of the strcture for the exterior.

Let me compare the contruction of my ship (as seen in the earlier thumbnail) to my Spindrift. The ship skeleton as you see it was first assembled from cut and drilled pieces of plywood, it was laboriously planked with wood, sanded to shape, and covered with fiberglass for a smooth hard shell. With the Spindrift, each section was cut from foam, sanded to shape and covered with fiberglass for a smooth hard shell. Notice that for the Spindrift, the planking phase has completely dissappeared.

[Charles Adams]

I hear ya! I think that method is ideal for smooth flowing or organic shapes because it is actually a sculpting technique. I think this would be the ideal technique to make a smooth blimp hull.

Where this runs into problems with rigid airships is because of the precise "faceting" that runs all the way around the hull. These facets must be cut out of the foam, just as the facets of a crystal or diamond must be cut from the underlying block. How will you cut these facets and ensure they are nice and straight?

I posted a possible solution to this problem in the other thread (and I am going to merge this discussion and put it in another section for better organization). :)

[Charles Adams]

Regarding paper models, I think in concept they are close to ideal -- inexpensive, fairly easy to produce, not overly challenging to assemble. But my concern has always been the joints. If you have an advanced model like my Los Angeles which has a hull formed by long thin strips, how on earth can you put those together without the need for some sort of filler? I'm not aware of any filler that is truly suitable for paper.

Assembling an airship from "gores" made of styrene is certainly possible, but this would be a huge amount of work. No matter how careful you were, every joint would require filling. And then you would have to build up the area along the joints of each "facet" in order to simulate the "sag" of the fabric in between the frame supports.

Another alternative I thought of was using thick foil sheet. This would be easy to form over a ribbed framework. But foil is fragile and every time you touch it you would leave a mark. It would have to be buffed with steel wool in order to prime and paint it. Thick foil with an adhesive backing (like BareMetal) is expensive and only available in small sheets, so this method would not be very practical for a large Zeppelin.

It keeps coming back to ribbed frame with fabric or sculpted foam. For a one-off, it's hard to beat a fabric covered frame (or a mylar covered frame). For a model which is intended to be produced, foam sounds ideal for either fiberglass or vacuum formed copies. BUT, with that said, it seems to me at least that it's a lot more work to make a proper hull in foam! Still, all you have to make is one half!

[Gil Russell]

Charles,

Most commercially available scale models of airships are offered as paper models. There general hull assembly is through polysided, truncated cone stacking. Yes, this still gives you a seam problem but this can be rectified by first sealing the assembled hull with a matte sealer followed by application of magic model filler. A light sanding followed by very fine sanding will prep the surface for painting with an undercoat. A final sanding with 1200+ grit will prepare the surface for the spray application of nitrate dope with aluminum powder as used in the originals.

This isn't a purest approach but it will produce an amazingly good hull that is fairly resistant to hangar rash.

-Gil Russell

[Charles Adams]

Interesting! I still haven't tried a paper kit yet, but I want to just to see what it's like.

I like the idea of paper models as a sort of "mockup" or pattern test. Kind of like when an architect is exploring a building design and creates a paper model to show what the concept looks like in three dimensions. I think a paper model that was plain white with black lines representing detail would look really neat. That way you wouldn't need to worry about coloring or edges showing.

It's when you build a model from paper that is meant to look like a "regular" model that problems start. The photos I've seen of paper models, no matter how incredible they are, really show off the fact they are made of paper when you start looking at the edges where parts come together. Even nice coated stock has a "fuzzy" look to the exposed, uncoated edge.

I'm not trying to knock paper, just be realistic about expectations for what it can and can't accomplish. And I continue to be amazed at some of the incredible subjects that people tackle with this medium. Some parts just can't be developed into flat patterns, yet model makers with unlimited ingenuity keep finding ways to stretch the limits. Like making a TOS Enterprise starship out of flat paper patterns! :)

[Charles Adams]

I had a brainstorm about covering hull frameworks I wanted to share. Something in the back of my mind kept saying "foil." But this hardly seemed suitable as a medium for an airship model! But then I thought of this:

Take your wooden framework (of a rigid airship design) and cover it with large pieces of foil. Use the thickest stuff you can find. Glue two layers if necessary to make it durable.
Now glue regular covering material over that. I'm talking tissue or silkspan here (or even fabric for some models).
Coat the whole assembly in diluted white glue rather than dope.This may sound crazy, but there is reasoning behind this train of thought. First, I worked on some crafts as a kid where we created "antiqued metal" sculptures using foil. The foil was glued to a substrate using plain white glue thinned 50/50 with water (I'm talking Elmer's Glue here). It stuck the foil down quite well.

Second, foil would make it very easy to precisely control and simulate the "sagging" of fabric panels between frame members. Foil can be carefully stretched and it will hold its shape when you're done.

What got me started on this was my work on the Los Angeles. You see, on certain Zeps the longitudinal frame members are triangular, and the "point" of the triangle faces outboard. This means that only a thin sliver of frame actually contacts the fabric outer envelope.

Now when you build a model of such a Zep using balsa stringers, your stringers must be turned so a "corner" is facing out in order to maintain a scale appearance. I've never seen anyone do this, which means their model is not accurate in this regard.

But if only a corner of the frame member faces outboard, this leaves almost no surface area to adhere your covering! And the silkspan or tissue + dope method requires the covering to be stuck firmly onto the wood strips. This also requires overlapping which leads to issues of trying to hide a bunch of seams.

Foil could be used as a backing underneath the final tissue or silkspan for a couple different reasons. You can more easily manipulate the foil to adhere to frame members, or literally wrap it around frame members as necessary. Now you have a surface to glue your outer covering to rather than just a bunch of thin little strips.

What if you wrapped the foil around the lengthwise frame members and used it to span every other gore? For those Zeps with triangular frames, you would then create a wide strip of sub-surface to which you could then apply your final covering.

I'm also thinking that with this method, you would not need to use dope. The mixture of white glue and water would effectively adhere the final covering to the underlying foil. This would harden on drying to make a rather tough outer "shell" from the covering. This, I would imagine, would make such a model much less fragile than one with a traditional covering.

Dope is probably somewhat toxic while white glue is completely harmless (not to mention dirt cheap) so I am seeing a lot of potential benefits here. By diluting the white glue with water and brushing it on the final covering, this covering would then harden and become much stronger than a dope-covered surface. Since the model is not meant to fly, the added weight would not be an issue.

In theory, this should give you a covering which was scale-like in appearance, yet not nearly as fragile as a traditional tissue and dope application.

[Charles Adams]

To bring up another potential point of debate, I wanted to mention hull coloring for a moment. Most people paint their models silver I would imagine. But I wanted to point out that paint and coloration on models vs. their real life counterparts is affected by a number of factors that force you to make some adjustments to account for the scale difference.

It is generally accepted that colors should be lighter on the model to more accurately reflect the way the real object would appear from a distance. This takes atmosphere into account. Since airships are particularly huge, as modelers we should pay close attention to this kind of detail.

Also, silver coloring on a real airship would not appear silver when viewed from a distance. At least it would not match the appearance of the vast majority of silver paints and pigments we would traditionally use on our models.

Silver is made by adding tiny little "sparkly" particles of metal to an adhesive binder. On the real airships, they added aluminum powder. This, as you can imagine, was pretty fine. There is a limit to how fine such a powder can be made. So when you use the same basic mixture on a 1/144 or 1/700 model, the color is technically correct, but your model comes out looking like a toy!

I would suggest painting the model grey rather than silver. Then experiment by dusting coats of fine lacquer spray over that to give the model an overall "sheen." The finish should be neither gloss nor matte, but somewhere in between, i.e. satin.

It is the sheen that tells your eyes you are seeing "silver" on a huge object viewed from a great distance. But it is really a shiny grey. That's because you can't actually see the particles that make the color silver from such a distance. But look at silver in your paint sample up close and those particles are plainly visible. This is not to scale! So painting silver on your model is not going to yield an appropriate, scale effect.

Of course, I could be completely wrong about all this. But it seems pretty logical to me. What do you think??? :)

[JoeVanCleave]

First, thank you for this forum; I am a new member. When I was first became interested in airships - in the early 1970's - it was difficult to get information, or talk to others who are knowledgable on the subject. Books became one's lifeline, of which I've built up a good collection.

Regarding how best to build airship hulls, my long-term project - which I only got "serious" about in 2001 - is an indoor RC, electric-propelled rigid. While on a 9-month business assignment away from home, I had the spare time to begin work on the project.

My approach to building a floating model rigid airship hull is as follows:

I began by determining how best to build a framework from balsa stringers (1/8"x1/16" for longitudinals and 1/16" square for lateral frames), with the rectangular panels diagonally braced under tension with prestressed thread, and the mainrings radially braced with threads. I did destructive stress testing on various mainring joint designs, built a gram balance beam scale for weighing extremely light parts, etc. Essentially, I had to build up the fundamental engineering principles for constructing such a model, with the main aim to find the lightest, strongest design for each subassembly.

What I have partially completed thus far is the front half of a Graf Zeppelin hull (not necessarily perfectly to scale, but the hull profile was modelled after the Graf), 6 feet (72") long and 16" diameter at the widest point. Each mainring is spaced 15" apart, with 2 subrings between each mainring, spaced 5" apart.

All of the rectangular panels formed by the intersection of the longitudinals and the ring members are diagonally triangulated with a light polyester/cotton thread, as are all mainring radial bracing wires. All joints use CA glue, rapid cured with CA accelerator to facilitate rapid assembly. I found thread to be an ideal tension member for such a project. It weighs much less than monofilament (which I tried in the early 1990's in a aborted attempt to begin such a project) and, best of all, it absorbs the CA glue into its fibers, so it doesn't easily pop loose. This light thread would break only after the entire framework was destroyed; it virtually impossible, with my equipment, to weigh the thread. The glue joints are more of a weight issue.

I have the hull partially covered, with a portion covered in ultrafilm (from Aircraft Modeller's Supply out of Oregon) and another portion covered in condensor tissue. I am leaning toward the use of condensor tissue as the final covering material, as it proves easier to apply, its final weight is less than the ultrafilm (even very thin plastic films are dense), and can be sprayed with a very light coat of lacquer to gain a bit of strength without too much weight gain.

This hull section is really a test-bed of ideas. The application of the covering material is carefully weighed before and after, and extrapolations made as to final covered weight.

I have flown this half-completed airship hull on a tether line. Yes, it floats, with 84 grams of excess lift. This was using 14" latex balloons inflated inside the gas cell bays, which were not efficiently filling the cylinderical gas bays since these balloons were spherical in shape. I expect over 100 grams of excess lift with properly shaped gas cells.

A completed 12 foot long version would expect to have nearly 200 grams (i.e. almost 1/2 pound) of excess lift, enough for a micro indoor propulsion system.

I have also built a transport "hangar box", six feet long and 34" square, made as a rigid, triangulated frame of 1"x2" lumber, and sheathed on the inside of the frame in white corrogated plastic. A hinged door in the end of the box facilitates safe moving of the hull in and out. It was originally designed for the 16" diameter model to potentially be 24 feet long, composed of four 6-foot long section, that would all fit into the box. The box is very lightweight and easy to transport in my small pickup truck bed.

I am now leaning towards a new design, incorporating a hull that assembles from two sections, each section containing one large call cell, and the fineness ratio more blunt, for more volumetric efficiency. To do this, I will have to revisit all of my stress testing to come up with a hull design that is not radially braced by mainrings, that will have free and clear interior volume, yet still be strong enough.

I'll post some images soon. Thanks for your patience.

Joe

[Stephen Cunha]

I read through this thread today and thought I would mention a couple of my experiences and add a thought about technique and longevity of the model.

I finished a model of the Graf Zeppelin (LZ127) and the Navel airship L3 (LZ24) circa 1970. Both are made out of wood with a Mylar covering. They are static models with the hull made of spruce keel aircraft plywood frames and basswood longitudinals. The frames are cut as rings to allow free movement of air within the hull. It also allows expansion and contraction without warping the frames. After completing the framework including the longitudinals and all mounting points, they were treated with many layers of shellac applied both by brush and aerosol spray. The lonitudnals are about 1/8? square basswood with diagonal splices where joints are made to lengthen them. They are placed in triangular cuts in the frames so they present a sharp edge where the fabric is attached. The fabric is a model airplane Mylar covering with glue on the backside called MonoCoat?. This material is glued on with heat. I started with strips of Mylar heat glued to the flats of the longitudinals and girdled the ship heating the Mylar at each longitudinal as I went around the ship. After the ship was covered I heated the whole craft to shrink the Mylar to a smooth finish. After 36 years the Graf Zeppelin is still flying in my den. The L3 suffered a puncture to the Mylar but was fixed by taking a small patch of Mylar and heat welding it to the area around the puncture, then heat shrinking the patch. After 36 years and moving twice they have passed the test of time and durability. The only drawback I know of is heat. They cannot be kept in strong direct sunlight or areas of extreme heat.

Environmental concerns affect the longevity of models. If the intention is to have a model last for many years, then special attention must be given to the coefficient of expansion of the materials used. Using aluminum tubing with plastic and wood expand and contract differently and will weaken the model if the was not provided for in the design. I have see two professionally built models destroyed when the wooden dowel running through the center of the hull pushed out the ends of the fiberglass envelope.

[Charles Adams]

Welcome aboard! This sounds like a fascinating project. There are so few flying rigid airship models! I have only seen one and that was in England (the R-101).

Your issues for hull construction are completely different from those the rest of us face when building a static model. We only need something practical to construct that looks realistic. You need all that plus it has to weigh as little as possible, be completely hollow, structurally sound, and strong enough to actually fly! Sounds like you have your work cut out for you. . .

:D

:cheers:

[Charles Adams]

Stephen, great points! I have stressed some of this as well in the past. Models need to be kept in a temperature- and humidity-controlled environment. Different materials do expand and contract differently so the best way to avoid problems is to keep things stable. Display models should never be exposed to direct sunlight and they should be kept indoors in a stable environment. The worst place you could put a model indoors is in the garage for any length of time!

[joec]

Hello again,

I haven't visited here for a while. In the meantime I have started an airship model - not a particular one, a modified Hindenberg maybe.

See photo

http://www.rcgroups.com/forums/attachment.php?attachmentid=1490593

It started out as simply a sort of test on a ring and viola it becomes a full blown time consuming project, I'm sure you've all experienced that.

It is about 120" long, 20" diameter, it has 12.5 facets, but that would be expanded to 25 by adding intermediate stringers.

I built the main rings with 3/32" balsa sticks on a table and pulled carpet thread tight across the vertices and glued them down. This resulted in a relatively stiff frame that was understandably prone to out of plane buckling.
Once I had two main rings I cut stringers (3/32"square also) to the length between them 15" or so (length of the curve not straight line) and carefully glued it together. I then cut the facet pieces of the intermediate rings to the correct length and glued them between the stringers, they are longer than what they would be for a cone so the last one in spreads the stringers and forms the bulge rather than a cone. All along I tried to keep it square etc which was tough without a jig. Then I strung carpet thread x bracing thoughout using cloths pins for attachable weights to put a slight tension on all the threads.

I'm a bit too impatient, some of the main rings are out of shape which easily could have been avoided by building them on a pattern, instead I used carefully measured facet pieces and measured across the tips as I added pieces.

Anyway, I've got the main part done, have engine cars and possibly real gasbags to go. The frame as shown is about 5 oz but that is truely bare bones, there are a lot more pieces to add.

Feel free to comment/question/critic, I can take it. It's far from perfect, the thread knots at the main ring joints bug me the most, it would likely look shoddy covered because the bumps would be quite apparent. I prefer to see the interior structure anyway.

Joe (capricorn) Hello Majortomski, I saw your name there.

PS I also built the R80 gondola at 1/16th scale, I'll post photos if I can figure it out.

[chilly]

Hey guys,

Quick question on seams in the covering... as I'm not too sure.

But there had to be seams in the fabric covering of a real rigid right? so having seams show in your model would seem protypical right? as long as they were to scale.

which brings up another question...

How were the fabric panals attached to the frame of an airship? As I think this maybe helpful in the building process.


Oh and an alternative to diluted white glue would be acrylic matte medium... very handy stuff.

-kev

[Charles Adams]

Nice work JoeC!

Chilly, the seams were nearly invisible in most rigids unless you were right up next to the hull. The frame was triangular, with the pointed side facing out. (This is the biggest mistake most people make when building models -- stringers with a wide, flat edge facing out are not accurate to scale.) Fabric strips called gores were laced to the edge of each frame. The strips got within a few inches of the frame and eyelets were inserted at regular intervals. Tough cord was then used to stitch the cover panel to the frame.

After the covering was attached, a sealing strip was applied over the seam. This was just wide enough to cover the exposed lacing and was doped to seal it to the rest of the fabric.

The result is you can't see the seams in most airships when viewed from a distance. Due to the scale of most models, the seams would be rendered invisible and so should not be included at all. You only need to worry about them on very large scale models. The actual pattern of the lacing was pretty much invisible in most cases as well. Sometimes, however, you could make out the cord as a narrow "X" stitch pattern underneath the sealing strips in extreme close-up photos.

[joec]

First, thank you for this forum; I am a new member. When I was first became interested in airships - in the early 1970's - it was difficult to get information, or talk to others who are knowledgable on the subject. Books became one's lifeline, of which I've built up a good collection. . .

Joe

This is bizzaar, my name is Joe too and I am building an airship quite similar to what you've described. I obviously didn't read the entire thread, must have skipped over this part. I'd love to see pictures of it. Now I see the date, 2006, hope Joe is still around, anyone know?

JoeC

[JoeVanCleave]

Hey, that's one neat project, Joe C! I've sent you a series of PM's regarding my project.

I see that you've solved the problem of the tapered contour of the airship hull by the way that the minor ring struts are installed; very clever. As you'll note from my PM, I intend on using a horizontal assembly fixture.

Your model has beafier struts than mine; the longitudinals of mine are 1/16"x1/8", while the ring pieces are 1/16" square. Rather than using carpet thread, I'm using a very light polyester/cotton thread. The tensile strength of this thread, as it contains polyester, is much stronger than it needs to be; the balsa compression members would buckle before the threads break. The cotton in the thread blend helps for CA glue to stick better.

The 72" long section that I've built and partially covered weighs in at 63 grams, partially covered; one section is covered in Ultrafilm, the other in condensor tissue. It's probably overbuilt; plus, the number of glue joints that you use will directly affect the overall weight; it's not the weight of each individual joint, but rather their combined weight, since there are so many joints involved. Also, you'll notice that in assembly of the rings with the longitudinals that you're reglueing over previously assembled joints; the same is true when it's time to add the tension threads - they get glued once again. Hence the idea of the assembly fixture, which may cut down on the overall number of glue joints used.

I like your keel; it looks like a handy way to route control wires for an RC system. Plus, any ballast weight can be moved along the keel for balance. It's also a handy place from which to attach tether lines for mooring and handling.

Thanks for posting this, as it inspires me to 'get with it' on my project.

~Joe

EDIT: Regarding the thread knots at the vertices of the mainrings, what I did on my mainrings is I assembled them on a sheet of foamcore board; I double cross-pinned each facet piece in place, used small squares of polyethelene film under each vertice corner (so the ring wouldn't stick to the foamcore board), and then glued a small, black craft paper diamond gussett piece (about ~1/4" long) onto each joint, such that the CA glue soaked into both adjoining facet pieces plus the paper diamond. I then pinned lengths of thread across opposite corners, using map pins with loops knotted into the ends of each thread, stretching the threads to remove the slack; these were oriented so as to cross over the middle of the paper diamond gussetts at the opposite corners; then a drop of CA glue. All the radial threads were installed and glued into place in this manner. After the glue was cured, I then pulled off all the map pins and trimmed the excess threads back to the corners. Viola, no knots at the corners, and the loads at each corner are distributed through the paper gussets to each facet strut.

My model is overbuilt, since I'm radially bracing every pair of corners (i.e. 16 threads for a 16-sided mainring), rather than bracing for just the vertical loads; but it does make for a rather rigid airframe.

I also discovered CA glue accelerator only after being half-way done with this section of the project. What a time saver this liquid is! Instead of waiting for 5-10 minutes or longer for each glue joint to cure, I can cure them in 10 seconds, and move on.

[joec]

Brent,

That's a beautiful boat hull, I assume the boats finished do you have photo's posted somewhere? Hope your still planning on starting your airship. Looks like it's getting toward fall, lots more activity on the website here. JoeC

[joec]

Joe, You've written a lot in the past day or so, hard to take it all in, if I haven't responded to a particular point I probably just haven't absorbed it yet. I see your glueing technique for the threads is much thought out, I pretty much layed them in with a cloths pin for weight and glued them where I could get them to stay. I've since built a model railroad bridge using 1/8x1/8 balsa and steel fishing leader wire, it's a peculiar model, almost ? shaped so the train running along the top of a wall can curve out and back in to cross the wall head on. Anyway, I drilled holes and glued the wire in with CA, it worked fine except where I had too many diagonals coming in to a joint, which was about half of them, but by the time I finished I had the technique down pretty well. Obviously you just have to offset them as they come in so they don't cross inside the balsa.

The idea of building another one (by JVC) without braced main rings (two halfs front and back) is going to be tricky especially if you make them to nest without even an end frame at the joint. Have you worked much with carbon fiber reinforced plastic? They are very expensive, at least at the hobby store, a bit cheaper on line. The material is very stiff, stiffer than steel, it's lighter than steel too but not as light as balsa. I thought about a laminated ring, balsa core (square roughly because rectangular would be tougher to bend, or even laminated balsa to form the curve) with a thin layer of CRP on the inside and outside faces. If you've got the funds I would say this would yeild a very stong ring with a relatively low weight. Trusses work but they are so much more work, and like you said the deeper it is the more internal volume you loose. That's why I think carbon fiber would be a good bet. I'm sure you are familiar with it but if not go to the hobby store and pick up a piece of it and flex it, it's awesome stuff.

I'm thinking I'll go ahead an get some ultralight weight "polyester" and try my hand at a gasbag. I don't know how much helium costs, I'm getting the idea it's somewhat expensive. I wouldn't be flying it more than once or twice though so I'd probably shell out what it took to fill it. (on another website there are some fellows who make their own hydrogen gas to fill their blimps, apparently it's relatively easy, and cheap) JoeC