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Stressed Skin Technique
Reproducing the stressed skin effect in scale aircraft models
To make aircraft lighter, manufacturers use the lightest possible materials, tailored to specific applications. The skin of an aircraft is often very thin to reduce weight. The challenge of welding such thin films is enormous.
In addition, some aircraft bodies are made of aluminum, which has a relatively low heat resistance. The welding process generates a large amount of heat, which is clearly unsuitable for aircraft with aluminum bodies.
The world’s most advanced commercial aircraft make extensive use of composite materials. These materials can also be damaged by welding. The connection between different materials must of course be ensured. Riveting is more stable and reliable connection
What do they have in common? They are sandwiched with structural details. To understand what a stressed skin is, let’s start by talking about these structural details. As you know, every aircraft structure is essentially a skeleton covered by a skin. The skeleton parts are generally spars, ribs, and chords. These structural elements provide resistance to bending and tensile loads, but almost no shear strength. If one were to use only ribs and ribs and still aim for puncture resistance, the weight of the resulting design would be prohibitively high. That’s where the skin comes into play. The skin is not just a thin sheet of metal nailed along the ribs and chords. It has to be prestressed – or at least tightly secured in place, which. which means that the rivet holes are calculated to be drilled in lines slightly smaller than those along the chords and ribs. As a result, the skin has to be somewhat stretched to fit into place. When you do this to all the panels of a wing or fuselage, the skin is pre-tensioned, just like a guitar string. The result is a much stronger structure than we would get if the skin were not pre-tensioned. Not all panels in an aircraft are pre-tensioned, but you can bet that no panel is loosely attached. Because it is so thin, without the pre-tensioning it will only add weight and no additional strength to the structures. So much so that fuselages don’t have spars, they’re just strings, ribs and skin… Therefore, strings, ribs and occasionally grids are the areas along which the skin panes are riveted. It’s not hard to recognize them on an airplane drawing. Now, what exactly is the stressed skin effect? It’s a scale model representation of the relaxation of a skin panel due to use and loads. At this point it’s important to understand that the wing (or any other structural part) sustains forces (lift, landings, maneuvers) during operation. In this case, bending is an example of forces exerted on the structure. Lift loads tend to bend the wing upwards. This causes compression of the upper wings and adhesion to the lower wing panels. Since the compressive force of the skin is minimal, it can buckle outward, creating a bubble in the area inside the perimeter of each panel’s lines (buckling is – strictly speaking – not necessarily what is happening here, so the term is used loosely):The effect can occur with a few panels or many consecutive panels. It all depends on the design of each particular aircraft. After the load is reduced to normal levels, it is expected that the skin will return to its flat shape. However, after hundreds of repeated actions, small permanent deformations will appear, either in the skin, rivets or surrounding structures, preventing the skin from returning to its original shape. Understand that this is completely normal to a certain extent and that a buckled frame does not necessarily mean that it has failed. Do not confuse this with the severely deformed skin found in poorly repaired areas or with severely wrinkled panels found in refurbished (non-flying) warbirds which always tend to use the strongest original components. It is also worth noting that the manufacturing processes play a significant role here and many aircraft leave the factory already showing the effect. I mean that the riveting process itself can (and generally does) produce the effect as the lines of rivets pull the leather into the ribs and laces, leaving sequences of bulges that are easily visible. However, if the skin panel is no longer stretched as originally designed due to use (has loosened over time), then it is no longer a pre-tensioned structure as desired. . Here are some photos showing the effect on real aircraft.
Get ready
So, where do you start to replicate the effect on your model? If you want a shortcut, panel lines are definitely a good place to start, but you have to remember that panel lines, as the name suggests, are the lines that define the perimeter of the skin panels. Along with these, there are many other rivet lines within each skin panel. If we look at a cross-section spanning many ribs, what you see is this:
So, to be precise, you will benefit from good quality drawings of your aircraft. These drawings help you know where the rivet lines should go, in addition to the panel lines. Start doing some research. You don’t want (shouldn’t) do the effect on the entire airframe. Study photos of the real thing. Choose the areas that are most prone to showing the effect and take notes, if possible with the help of a reliable drawing. Once you are done, mark the areas that need to be worked on using a pencil or a small CD accessory marker (be careful, clean it afterwards) over the parts: Once you have established where the dents are going to go, the first thing you need to do is quickly remove the material.
1) A scalpel with a round blade
2) Some tape preferably Dymo
3) A pen on top, the CD marker
4) 1400,1200 grit wet sandpaper
5) A Proxxon or Dremel tool with a polishing attachment, use a cotton disc. Do not use the felt type! see the pictures! A fine-grained polishing paste would also be good. I have bought 3M automotive polishing paste, very fine
6) A pair of compasses with two needles for correct measurement
7) A ruler of foil
8) And the tool “Rosie the Riveter” for scale models In scale 1/72 1/48 or 1/32, Depending on the kit you are working on.
https://eshop.gmodelart.com/product/professional-transport-case/
A curved hobby knife can be used as a rapier. It is important that it is a curved tool, otherwise you will create risks that are difficult to remove later. Start lightly and then increase the force, but do not overdo it – a few passes are enough.
Getting Started
Draw all the rivet lines on the part. Measure on one side only and use your judgment to place the Dymo tape in the correct position. This will give a better result as if you were measuring on two sides.
If you are working on the wings, start at the bottom to practice and always do both wings at the same time (on line on the right… same line on the left). This will make them look the same and you can’t forget anything.
With the ruler you measure everything. Always use the lines on the board as orientation points. If you see four rivet lines between two rows of panels, measure the distance between the lines on the board and divide it by four. Then you know the distance to mark with the pair of compasses. Use the Dymo tape as a guide for your pen. All the lines are drawn. The part in the middle that seems a bit messy didn’t have a strong leather effect because the rivet lines are very close together. Use your scalpel with a round blade to make small marks from the sink between the lines, first the long ones and then the short ones. Just run your scalpel along the lines. you’ll get a feel for it soon. I repeat this should be done with gentle strokes. That’s it. Note that it’s very important not to follow your intention of making the lines that go from the front to the back of the wing in one pass. It might be less work, but it wouldn’t give the desired look. Now sand everything wet with 1200, 1400, or 1600 grid wet sandpaper. Don’t use lower grades. It will work just fine. To check if there are any large scratches left, you can polish the surface with your Dremel tool. After a few seconds you will think your plastic has turned to metal. At this point it would be a good idea to go over it with some polish before sanding it with your Dremel tool. A gentle pass helps to identify any unwanted scratches. Any remaining scratches can be cleaned up with the latex polishing tool that comes with the proxxon. (I’m sure there are similar products for all kinds of mini drills). Now measure again and pull all the rivets into the recesses with the “Rosie the Riveter” guided along the Dymo tape. If you worked accurately from the beginning on the rivet lines they should all be at the deepest point of the dents. After making all the rivets, I sanded again with sandpaper and polished it again to check the result with a low speed on the Dremel.. To see the results. You can use India ink spread liberally on the treated surface, let it dry and remove it with an old shirt tightly wrapped around my finger. This leaves ink in the recessed areas, allowing you to check your results. You can apply a coat of primer, the part is ready for painting and generally, I do not use the method on the lower wings, but on selected areas of the fuselage and on the upper surface, depending on the subject. This technique has an even better effect. If you combine it with the “Natural Metal Finish” technique (also known as “NMF”), which we will talk about in a new article in more detail.
It is also worth mentioning that we can cause different impacts on the surface of a fighter. On external fuel tanks, or on the engine ring. Mainly on those that were launched from aircraft carrier catapults during WW2, as well as on abandoned shipwrecks. This technique can be applied to all old and large airplanes. Except for modern, fourth and fifth generation ones that have completely new structural elements and manufacturing specifications
What is required is a beautiful stressed aircraft effect and not a single scratch. You do not need any Mr Surfacer and you do not need to be afraid that your rivets will come off because you did the stressed skin effect after nailing. It is worth the time to observe drawings and photographs.
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