Rivets and Panel Lines

[Bullethead]

Gruppen-

Back when I was a practicing engineer, I worked for what was then called Vought Aircraft building new planes, and later for what was then called Chrysler Technologies Aviation Systems repairing, repainting, and modifying old planes.  As such, I've had the opportunity to examine a lot of planes in detail, in various stages of the lifespans.  So I'd like to share my observations on the subjects of panel lines and rivets, which seem to give a lot of people trouble when they're making skins.  The art of making stressed-skin airplanes hasn't changed significantly since the 1930s, so my info is valid for most WW2 skins .

One of the most important steps toward doing the structural part of a skin right is, IMHO, to get a detailed reference of the plane you're skinning.  You want to know how it was buit, so you can draw the appropriate types of structures in the correct places.  Then, I find that it works best to make the skin in layers, starting with the aircraft structure, then applying the paint in slightly transparent layers to allow the structure to show through where it should.  This method has the added advantage of "thinning" the paint colors so they look better at the scale of the AH plane then just applying the full-strength color.

This post will be about the lines between fixed skin panels.  I'll cover removable panels and fasteners later.

In general, there are 2 types of skin panels on aircraft:  fixed and movable.  Fixed panels are part of the aircraft's structure and are permanently attached with rivet-type fasteners or spotwelds to the underlying structure.  Movable panels, OTOH, comprise things like engine cowlings, landing gear doors, inspection panels, etc.  These are attached with a variety of latches, screws, and hinges.  Thus, these 2 types of panel appear very differently on the aircraft, so should be drawn differently on the skin.

Fixed panels make up the vast majority of the exterior of any stressed-skin aircraft.  Where the edges of adjacent fixed panels meet, the usual practice is to butt them together to form a smooth exterior surface.  However, you always leave a small gap between the panel edges, about 1/8" wide, to allow for thermal expansion and the flexing of the structure in flight.  This gap is then filled with a type of caulking material (sealant), both to smooth out the gap and to keep water from getting under the edges of the panels.  

On some aircraft, the edges of some skin panels overlap.  This can be done with the overlap on the inside (such as on the 109's aft fuselage) or outside (usually not common except for doubler plates).  In the former case, the appearance differs very little from a butt joint (most 109 model kits totally exaggerate the lines on the aft fuselage).  In the latter case, you have at least 1 panel edge standing proud of the net aircraft surface, so need to give it a 3D appearance, the amount of which depends on the thickness of the overlapping panel.  But these are special cases--research the structure of the plane you're skinning to find any such things.  When in doubt, assume a butt joint.

Land-based planes spend almost all their lives out in the weather, either flying or sitting around the airfield.  In addition, they are subjected to constant flexing in flight and to wide temperature fluxuations during flights to high alt.  These forces change the appearance of fixed panel lines over the life of the aircraft by impacting the sealant in the panel joints.  To begin with, the sealant has a slight meniscus to its surface and is flexible like rubber.  Over time, it dries out, puckers up above the level of the panels, and cracks along the length of the panel joint, eventually falling out in whole or in part.  As it cracks and falls out, oil and dirt accumulate in the resulting spaces.  This has the following effects on appearance:

On painted planes, the fixed panel butt joints are nearly invisible to start with.  Usually, they appear as faint, thin lines of slightly lighter color, due to the light catching part of the meniscus surface.  They are almost impossible to see from any distance except in certain lighting conditions.  As the sealant puckers up, you get a slight 3D raised effect, with the joint slightly darker than the base color on one side and slightly lighter on the other.  This is visible at longer distances than the original condition.  Finally, as the sealant cracks and the dirt gets in, the panel lines become even more visible as dark lines of varying width.  Often on planes of medium age/usage, the same panel line will have different areas along its length with all these variations in appearance.  On old, hard-worked planes, the panel lines will be almost all be visible and fairly dark.  However, the width of these dark lines should still be less than those around removable panels (or they should be a lighter color if of the same width) because you're still only talking about a 1/8", shallow gap.  

Note, however, that sealant lasts much longer on the lower surfaces because these are hardly ever exposed to the sun.  OTOH, the lower surface accumulates more general dirt and grime due to a) landing gear spary, b) all the drain openings, and c) the laziness of anybody washing the plane.  Thus, lower surface panel lines are often more noticeable than those on top, especially because the bottom of the plane is usually painted in a light color.  For instance, when the lower sealant still has its original meniscus while the upper sealant is puckering up, grime will accumulate in the lower meniscus and make a somewhat darker line on the light lower paint, while the upper panel lines have a faint 3D look.  

On unpainted planes, the overall visibility of the fixed panel lines doesn't change much over time.  This is because the lines are usually pretty visible to start with, due to the sealant often being a dark color compared to the bare aluminum.  There is thus little change as dirt gets into the cracks and gaps.  The only major appearance change is if the dirt is a noticeably different color than the sealant.  In such a case, the panel line would vary in color along its length (especially on the lower surfaces), with some areas remaining blackish while others being the color of the local dirt.

A final note on fixed panels:  they should almost never be given paint chipping effects to make it appear like they've been removed and reinstalled several times.  These panels provide much of the plane's structural strength and are only removed when major repairs or total replacement of the skin or the underlying structure is required.  When these panels are removed, the rest of the plane will sag and get out of alignment, and thus not fly right afterwards, unless it is very carefully and accurately supported.  This is usually beyond the capabilities of a field unit (plus field units often don't have the replacement parts).  Thus, such work is usually performed at upper-echelon repair depots, who often do a lot of minor repairs and touch-ups on the rest of the plane while they're at it.  These facilities also usually repaint the plane afterwards.  Thus, usually the plane would return to the field unit looking like new, at least in the area of the major repair.

All the above are, of course, generalizations.  Weathering effects varied by theater, amount of hard use, and the quality of the various types of sealant used by the various combatants.  Or even its availability.  I'm sure at some point nearly everybody quit using sealant, at least temporarily, due to shortages or in the interest of getting planes to the field quicker.  Especially if the planes had a combat life expectancy of only a few weeks.

[Bullethead]

Lines around panels that can move may or may not look different from those of fixed panels.  The main reason for this is the intended frequency of the panel being moved.  The more often the panel is supposed to move, the more different its borders look from those of a fixed panel.  This is largely a function of the type and number of fasteners holding down the panel.

Panels that move a lot, such as cowlings, gear doors, ammo bay doors, etc., usually have few (if any) fasteners around their edges.  This allows a lot of wiggle room, so that as the panel and the rest of the plane deform gradually over time, the edges of the moveable panel get more out of plane with the rest of the surface.  However, such panels usually have some form of rubber bead around their edges to fill the gaps, so the resulting mismatches are at least partially accounted for.  Still, these should be the most visible panel lines on the plane, wider and/or darker than those between other types of panels.  Either bigger gaps or more of a 3D effect due to different heights.

Often panels of this type have a hinge along one edge.  Hinges make no gap between panels per se so these lines appear much lighter and narrower than the gaps around the other edges of the same panel.  Actually drawing the hinge itself might not be possible, but it's possible to fake it with dotted lines of the right colors.  Hinges may be mostly flush with the surface or protrude above or below it--research helps here--so might or might not need some 3D effect.

On such panels, there are usually fewer paint chips than on the surrounding fixed panels.  This is because the ground crew leans or stands on the fixed panels once the removeable panel is gone.  Paint chips on the removeable panels are most common around the edges, and around the fasteners or latches that hold them down.  Especially if panel is screwed down or has dzus fasteners, where misplaced tools can chip the paint.

Some panels are intended only to be moved for periodic inspection and maintenance.  For example, the panels to gain access to the tailwheel strut and various control linkages.  These panels are usually screwed down or have dzus fasteners on all edges.  Often, these panel lines are quite visible, either because there is no bead around the edge of the removeable panel, or because the removeable panel lies completely above the surrounding surface.  In the latter case, you frequencly see arcs of paint scratches on the surrounding area, resulting from the groundcrew removing all but 1 fastener, then pivoting the panel out of the way instead of removing it completely.

Finally, some panels, such as most hand holes on the lower surface of the wing, aren't intended to be removed except for major maintenance or periodic overhauls.  While these panels are usually screwed down or have dzus fasteners, they are also sealed to the surrounding structure just like fixed panels.  As such, their edges usually look just like those of fixed panels in the same area.  Sometimes there are raised doublers around such panels, however, the lines of which will have a 3D effect.  

These "mostly fixed" panels hardly ever have any paint chipping.  For the vast majority of the time, they're just another part of the surrounding surface, so get only whatever random scatter of chipping you have on the surface as a whole.  However, occasionally it's necessary to open these panels in the field.  When this happens, you get a lot of chipping because the groundcrew has to dig out the sealant and this takes a lot of paint with it.  Then, when the panel is replaced, it will probably be sealed back up but not repainted in the field.  This results in sharp black panel edges from the sealant, surrounded by a lot of a lot of paint chipping.

[Bullethead]

For the purposes of making skins, fasteners can be divided into 2 main types:  flush and protruding.  Flush fasteners are usually very hard to see at all (unless they're removeable), whereas protruding fasteners are always highly visible.  Research is important so you know what type of fastener the plane you're skinning had in which locations.  In general, however, most planes designed before the war had mostly protruding fasteners, whereas later planes mostly had flush fasteners.  But even so, planes with mostly flush fasteners almost always had some protruding fasteners here and there.

Rules of thumb for fastener placement if you don't have detailed drawings:  Normally, permanent fasteners (rivets, huck bolts, hi-loks, etc.) are spaced about 1" apart along rows.  When put in parallel rows, the rows are about 1/2-3/4" apart.  Double (or more) rows are usually only seen where a skin panel touches a major structural memember (usually on its edges), single rows being seen elsewhere and out near the extremeties where the major members have tapered down a lot.  Removeable fasteners, such as dzus fasterners and nutplate screws on removeable panels, are usually spaced about 1.5-2" apart and have heads ranging from 1/4-1/2" in diameter.  

On painted planes, permanent flush fasteners (countersunk rivets, etc.) are mostly visible only as very slight discontinuities that catch a little light or shadow compared to the surrounding surface.  Thus, if shown at all, they should be spots that are very slightly lighter or darker than the surrounding paint.  On a fixed skin panel, flush permanent fasteners are usually most visible around the edges of the panel.  This is because the main structure is usually less malleable than the skin panel, so the panel often dimples around the fasteners.  OTOH, in the middle of the panel where it crosses over light structure, either the structure bends to match the skin, or was shimmed to it in sub-assembly, and the fasteners are basically invisible except by close inspection from only a couple feet away.  However, the tighter the curve of the skin, the more visible the flush fasteners will be, regardless of where they are on the panel.  But in all cases on painted planes, flush fasteners are not nearly as visible as even newly sealed panel lines.  Also, flush fasteners on painted planes don't really become more visible with age.  So in general, it's safe to ignore flush fasteners on painted planes, except for perhaps some pixels of very slightly different color around the edges of panels or where the skin is highly curved.

On unpainted planes, flush fasteners are always more visible than on painted planes.  Besides any dimpling and skin curve effects mentioned above, the fastener heads' metal grain orientation will be different from that of the surrounding skin, so catch the light differently.  In addition, flush fasteners usually become more visible as the plane ages, because the flat head of the rivet gets more out of plane with the skin panel, which in turn allows grime to collect.  On painted planes, the paint usually flexes enough to smooth this effect out.

Fasteners with protruding heads are always very visible due to the 3D effect they produce.  In general, protruding rivets are more visible than even larger protruding bolts because rivets usually have tall, domed heads whereas bolts usually have low, flat heads.  On planes with predominantly flush fasteners, there will still usually be areas of protruding heads near the extremities and scattered, individual protruding heads elsewhere.  This is because the use of flush fasteners requires enough skin thickness to make a countersunk hole.  Usually, the skin thickness decreases toward the extremeties, often becoming too thin to use flush fasterners.  In addition, various underlying structural joints might require the odd protruding fastener in the middle of a wide expanse of flush rivets.   Also, during assembly of the plane at the factory, if the workers really screw up a hole, they might have to install an oversized protruding fastener.  Finally, hasty field repairs often use blind fasteners out of necessity, which always protrude a bit.  Thus, bullet patch plates should usually have protruding heads, plus a raised edge all around.

Removeable fasteners are usually flush but are also usually quite visible on both painted and unpainted planes.  To begin with, they've usually got heads about 3/8" wide that have some sort of screwdriver slot in them that catches the light and shadow easily.  On painted planes, use of the fastener quickly chips the paint off, so they're really visible then.  And on all planes, the more the fastener is used, the more it ceases to seat fully.  So except on "mostly fixed" removeable panels, the fasteners should be very visible, regardless of the presence of paint.