6

When building a 5 feet wingspan UAV, what building techniques:

  • used on smaller aircraft can't scale to aircraft of this size?
  • are best used on aircraft of this size?

Ideally, I want a simple graphic that tells me approximately what techniques are best suited to my requirements. Heres an MS paint mockup:

Graph of wingspan vs mass showing appropriate techniques as shaded sections of graph

  • I think some more information would be useful here, for example what you need the aircraft to do, what flight characteristics you want etc. – Drones and Whatnot Apr 18 at 11:12
  • Lets keep it simple: No aerobatics, Reasonable flight speed ect. Lets say this is for the average beginner who wants to build big, but doesn't know how to. – Krish Apr 18 at 11:28
  • I’m more knowledgeable about multirotors than planes, however FliteTest releases plans for their planes, and I’d imagine modifying them to suit your purpose would work well. AFAIK most planes are made of foamboard, which is a good place to start, though I strongly recommend giving RamyRC’s most recent A350 build a look. Obviously his methods are above and beyond what most people will do, for example his custom moulds and carbon fibre, but watching him might give you some ideas about what is possible. – Drones and Whatnot Apr 18 at 11:32
  • I'l check that out, thanks – Krish Apr 18 at 11:34
  • Many of mine are mostly Duct Tape and Glue now - you get get quite big with those! – Kralc Apr 18 at 12:32
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While I'm far from knowledgeable enough to give you a chart like you requested, I can do a general overview of the methods commonly used in RC modelling for airframe fabrication.

I must begin with this: Almost all of these are applicable to an aircraft of about any size. It's even hard to tell which is better, they're all just different. So, without further ado, the list:

  • Extruded polymer foam. This is what most commercially manufactured models are made of. The manufacturing process involves CNC-machined molds, special extruding machinery, and a lot of money spent upfront. However, the models can be of any shape imaginable, are quite durable, light (although not the lightest) have excellent aerodynamics, and the unit cost is cheap once you get going. The load-bearing parts are usually reinforced/stiffened with carbon fiber inserts, although wooden reinforcements are not uncommon. There are several types of foam that can be used, each with different characteristics.
  • Hotwired polymer foam. This is the DIY method of making foam planes, although some commercially-available planes use it as well. It involves a resistive wire which is heated by a current and is used to cut a block of foam into the desired shape using a bow or other device that keeps the wire under tension. As with extruded foam planes, load-bearing sections are usually reinforced with carbon fiber for stiffness. The same types of foam can be used as with extrusion. The available shapes are limited by what can be achieved with straight cuts, although complex shapes can be created by gluing simpler sections together. See this video for an overview of the process. Sometimes (most of the time?) only the wing airfoil is wire-cut, while the rest of the airplane is fabricated using other methods. Planes made using this method are often covered with heatshrink film to add stiffness and durability.
  • Folded flat materials. Airplanes can be made from a variety of sheeted materials by cutting, creasing and folding them into shapes which are then glued together. The materials include (but are not limited to) foamboard (foam between two layers of paper/thin cardboard), depron (flat sheets of foam), corrugated cardboard or even paper. The very simplest models can even be made without folding, just by cutting the required outline out of a flat sheet, while the advanced builds feature smooth curved surfaces. While usually not especially sleek-looking, airplanes made using this method fly surprisingly well and are very durable! Most of this comes from the fact that they are also extremely light for their size, though. In any case, a foamboard (or cardboard, or depron) plane is very easy to build or fix and requires minimal tools (knife, ruler, glue) to do so. This type of construction is extremely popular in the RC community due to the simplicity and cheap price. The most prominent proponent of folded models is Flite Test, who publish free plans for all of their models. There are also countless plans for folded planes on various RC forums.
  • Wood and film. This is the "oldschool" method. It is very similar to the methods used for construction of early full-scale aircraft, and involves building a wooden "skeleton" for the plane out of precisely-cut thin pieces of wood (usually balsa) and covering it with a special heat-shrink film to form the surface of the airfoil. This method takes a lot of time and work, and involves cutting and gluing together hundreds of wooden parts. The method creates light, strong and stiff airframes, which can nonetheless be brittle if hit the wrong way in a crash and time-consuming to fix. While less common nowadays, this construction method remains moderately popular, with a great variety of plans available online.
  • Glass and carbon fiber composites. While carbon tubes, strips and sheets are used extensively together with all of the construction methods discussed above to add mechanical strength and stiffness in critical points to a weaker or more flexible material, some airframes are constructed entirely out of composites. This is arguably the most high-tech and definitely the most high-performance (if done properly) construction method of the ones discussed here. It is also the most expensive one, per unit, though it doesn't come anywhere near extruded foam in terms of upfront setup costs and thus is within reach of hobbyists who want to make only a small number of copies of their design. Creating a glass or carbon fiber airframe requires making a mold of the desired shape, carefully laying up the fiber, soaking (or impregnating) it with resin and curing under pressure, usually with a vacuum pump, to make sure that the absolute minimum of that resin actually stays in the part. Then comes sanding, polishing, and assembly of the parts. Carbon fiber, when properly done, is extremely strong yet light, and is used in high-performance thermal gliders and other planes which must withstand extreme forces while being as light as possible. While strong, such planes are often hard to fix when they do get broken (at least without compromising their original strength or lightness). Glass fiber, though somewhat outdated, is also still a popular choice for larger planes, as it is significantly less expensive and easier to fix. Sometimes DIY molded glass fiber reinforcements can be found on planes constructed from other materials, and usually made by laying the fiber directly on top of the fuselage, eliminating the need for a mold.
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