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Thread OP
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Discussion
Ribbed wing vs different types of laminated wings
Hello guys,
what's your opinion on Strength and Weight vs a traditionell laminating method (Glasfibre/Balsa) vs foam with brownkraftpaper vs a rib construction ? Cheers Nik |
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I generally favor traditional full scale- like model building methods for models, especially scale models. It is usually the lightest, most practical, and quite achievable with most readily available materials. However, I can appreciate the need, for sometimes looking a little farther afield when it comes using foam and composites to facilitate certain building needs. I hate to use the term " short- cut", but there is that aspect, too.
For instance, certain vintage aircraft had ply covered wings etc. Balsa is often the go to material for sheeting. However, I can appreciate that a thin(more scale-like) paper/resin composite skin, over a balsa frame might be a more practical alternative, not only in weight and labor savings but more economical, too. Also, when a constant chord wing has MANY closely space ribs, it may not be prudent to cut the ribs from sheet, but a foam wing with false ribs added for detail might just be the right thing to do. When one gets into higher tech fabrication aspects, such as vac bagging and or more exotic materials, etc. it soon , for me anyway, becomes too involved for my by building situation. I tend to lose interest, if I perceive any building technic is too specialized for my immediate use etc. |
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Last edited by packardpursuit; Oct 02, 2019 at 11:03 AM.
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Epoxy and fiberglass is typically uses some manner of vacuum bagging setup to get the best strength with least weight. It does great but it needs some extra equipment. All in all though folks that do a lot of vacuum bagging say it's just as fast or faster to make as doing a built up wing. And while there's more gear and materials to assemble at first if you commit your shop to these items and learn to hotwire the cores and do the bagging I think that it is no more complicated than any other method. Just different. And done well it can be quite light. Not as light as a carefully designed and constructed built up wing but certainly very light. Just look at the very low weights on DLG's that are primarily vacuum bagged.
The kraft paper or newsprint bonded to foam is not going to be as strong as the epoxy and fiberglass. But on thicker wing sections it is certainly a viable option. Generally not as smooth a finish though since the paper forms to the surface texture of the foam core. But done well on thicker airfoils this is a super easy and cheap method for making a rough and tumble sport model that is quite tough and reasonably light. If you want a nice surface for a fancy looking finish though it is not the way to go. A built up wing can be light or heavy depending on the designer and how well you pick the wood. So compared to the other methods I'd say that it can require more experience and care. There is also some skill and a keen eye for shapes when you carve and sand the leading and trailing edges to shape and how they transition to any wing tip shapes other than the very simple ones. So there again more skill and judgement needed to get a really nice looking job. Or if the design is a fairly simple one it can be fairly easy to do. It is also a very classic method with classic results. Strength wise to resist flying loads any of the methods is just fine. And to some extent it depends on what sort of method you prefer and what sort of results you want. |
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Thread OP
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I purchased a HotWireCutter (yesterday), gonna practice a little and use my gained experience for something like Cessna 1,2Meter.
A HoWireCut wingcore (left and right) would cost me 60bucks plus shipping here in Germany, plus the additionell cost for vacuumpump, bags, tubes and so on (probably in 150bucks range or more) plus uncertainty if everthing is gonna go well which is unlikely because I am a Noob when it comes to this building technique. Getting a wing build would cost me 350-500bucks. A ribbed wing would cost me 80-100bucks plus covering film. You can see the model I like to build the wing for in this video.
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Each wing design and construction method has to consider what the models required flight parameters and loads are.
There isn't really one type of construction that suits all. For lightweight gentle scale like flying I prefer a built up Depron foam wing, very similar to a balsa built up construction, plus I like big models, (up to 100" span), and can't afford that much balsa . That system works for me, but would probably be unsuitable for model expected to survive high 'G' loads and stressful aerobatics. The more you build and try different methods, the greater your knowledge of what works for you. Ray. |
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One point I haven't seen discussed is repairing a damaged wing.
Built-up wings (especially those covered with heat-shrink plastic) are fairly easy to repair. Foam wings can be much harder to repair, especially those covered with paper and glue, CD |
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Thread OP
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When it comes to scratch building, trying something different can be a big part of the learning curve, ( I nearly said 'big part of the fun', but we all know that a lot of frustration and swearing can't really be classed as 'fun', .... or can it ? )
Build what you want. . |
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Thread OP
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I did some research and found out that ribbed wings are lighter if build and designed well.
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Wings have two main (and about a thousand million gazillion minor) load cases. First is bending. That's caused by the weight of the fuselage and downward "lift" of the empenage being carried by the wing. Considering that most models can have short bursts of 150+G loads, it's quite a demand. Bending is counteracted by wing stiffness. E (Young's modulus) multiplied by I (Moment of inertia of the cross section). You can't do much about E as it is set by the material properties of your balsa, foam, carbon fiber, etc. However, the I term is where you, as a brilliant aircraft structural designer, have the most control. Generally, I increases as mass moves away from the neutral axis. The neutral axis is the line through the cross section that undergoes no compression or tension during bending. It's also the maximum shear line. So, moving materials away from the neutral axis is best. That means a sheeted wing with a very thin spar will be, in general, lighter than a built up wing with ribs and spars. It ends up being a semi monocoque wing design and is very strong and light. This is all based on selecting the correct skin thickness and spar thickness to handle the load. You can usually get away with a 1/16 sheeted wing with a 1/16 thick sheet spar and have a stronger and lighter wing than one with no sheeting, two 3/8 square spar caps, a 1/8 inch shear web between them, and a sub spar or whatever other pieces needed to attain desired strength. A couple "in between" solutions are the D box wing and wings with stringers as opposed to sheeting. The next major load is torsion. No airfoil known to man operates at zero CMalpha through all flight regimes. Things like the Clark Y and other flat bottom airfoils can develop extremely high torsion stresses due to aerodynamic forces. Torsion creates shear flow in the wing cross section. Sheeted wings are the absolute best answer in this manner. A D box type wing section is a far second. Open structure, well, you better be really good at covering. Think about the Sig FourStar wing before covering. It twists like a wet noodle. After covering the skin (plastic or fabric, etc) takes the shear load and passes it to the ribs. Compare that to a fully sheeted wing. It's stiff as a board (or plank LOL) in torsion before covering. So, all that is to say, you can use your noggin and a pencil (or Excel spreadsheet) to design a wing that is optimized for your specific needs. As further comparison, look at the wing structure for a full scale Pipe Pawnee. Fabric covered open structure with a relatively massive spar, sub spars, etc. Great flying bird, but pretty heavy structure. Then look at a Cessna Ag Wagon. Both planes designed to carry nearly 500 lbs of extra weight (or better in some cases) and the AG Wagon, if I recall correctly, has 0.032" wing skins and 0.063 spar webs. It's also notable that there is a metal wing STC for a Pawnee, but no rag wing STC for an Ag Wagon. Fun times. |
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But it is a far cry from fullsize to model size. Smaller models can be built to very light structures that easily resist flight loads with almost no materials. I'm talking about stuff at the extreme other end of the spectrum. Models like indoor F3P, some of the even lighter indoor RC, and most smaller free flight designs. These are all structures that would not be anywhere near as light if made in skinned foam.
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What the heck, I will throw my 2 cents worth in. As you can probably tell by my avitar I am into fairly high performance planes. I have built many speed and race planes, along with several pattern designs. I build as light as possible always. It has been my experience over the past few decades that I can build a built up balsa rib and spar wing and a similar balsa sheeted foam wing and end up with comparable strength and very similar weight. The major difference to me is the foam wing can be completed in much less time. I have a friend that cuts my wing cores for me, so sheeted foam wins due to the convenience factor. If cores are not readily available, a suitable wing can be built using conventional techniques, it just takes me longer.
Scott |
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Compared to the average 40 size trainer, it was a feather and considerably stronger. As for indoor rubber powered models, I would still support my ascertation, if balsa sheeting was made available in three micron thickness! LOL. The strength to weight ratio of balsa is pretty amazing, especially compared to mylar or polyester sheet (MonoKote). But, yes, availability of materials, suitability of materials, construction methods, etc all factor into the model designer's decisions. My whole point is that intuition doesn't make good aircraft. It takes a healthy amount of midnight oil and #2 lead to make something optimal. It's not incredibly hard and doesn't take a team of 50 engineers to analyze every single glue joint, but a few hours spent in an Excel workbook could make any given model several dozen percentage points lighter with the same strength capability. |
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Some minor disagreement.. Paper on foam Does work well and the quality of the skinned Surface is directly proportional to the fineness of the core cutting. Garbage in = garbage out.
Besides it can be easily sanded and does Not demand the tedious / heavy weave filling of laminated (as opposed to Bagged with Mylar sheets) glass cloth. Nuances to every technique. |
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