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Model Building Tips - General Construction Techniques

May 05, 2015

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Airfield Models ( and General Model Construction Techniques

Model building is about problem-solving.  I have a few philosophies that have worked well for me.  For example, I never put glue on a part until I am sure I can put it where I want it how I want it.  I find a way to make sure it's straight and will stay straight while the glue is drying (and hopefully remain straight afterward).

What this means is making some type of jig that I know is accurate rather than trusting my eyeballs to get things lined up properly.  It comes very naturally to me now, but it wasn't always.

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  • Have a system that can hold as many pieces in place as possible without using glue.  You'll build a much better aircraft if you can see what the problems are going to be before you glue parts together.

    Parts clamped securely while glue is drying.

    Also see

  • Do not attempt new techniques directly on your model without practicing on something other than the model first.  Get in the habit of performing tests.

For example, when you're finishing your model, finish a couple scraps of sheet wood at the same time.  Then if you want to find out if a paint is compatible you can try it on one of the scraps first.  The same goes for any building technique you have never used before.  Unless it's something you are confident that you can do properly the first time then practice on something other than your model.

  • Always cut the longest sticks first.  What I mean is cut the longest structural pieces such as spars and longerons before cutting shorter pieces such as cross-braces.  Manufacturers normally give you enough extra wood in a kit to make a mistake or two.  If you cut all the cross-braces first you may end up with a bunch of scraps and have nothing long enough for the wing spars or longerons.  Additionally, you should always use the shortest piece that's big enough for the job.

  • Be sure to remove pins, magnets or anything else that shouldn't be there before you add the sheeting that will completely enclose an area.  I once built a magnet into a wing and had to perform surgery to get it back out.  Until I announced it here nobody ever knew about it but I still felt pretty stupid at the time.

  • If a part looks straight, it might be... or it might not be.  If it does not look straight it's not.  It's easy to get caught up in measuring the wing attachment to the fuselage to make sure it's on straight but once you think you have it perfect don't drill the bolt holes.  Instead stand the plane on it is nose with light coming straight at it so shadows don't play tricks.

    Now stand back about fifteen feet and take a good look at it.  If it looks good and it measured ok, then it's probably straight.  If it looks wrong, then something probably is even if it seemed to measure out OK.  Check your measurements again.  It might not be the wing that is wrong, but the fuselage.  This philosophy does not only apply to wings.

  • Don't assume the plans are perfect.  Paper expands and contracts with the weather.  Lines may not be straight and may not be spaced correctly.  It's probably less likely that two symmetrical drawings (such as wings) are not drawn the same with the advent of CAD drawn plans but it's still a possibility.

  • Generally speaking, any time I build two identical parts, I build the first and then build the second upside down over the same drawing or directly over the previously built part.  It is more important that an aircraft is symmetrical than it matches the plan exactly.  The best case is to build some kind of jig that you can leave in place after building the first part.  Then the second part is built in the same jig and you end up with two very closely matching components.

  • Because paper shrinks, stretches and warps, when it comes time to join the two fuselage sides, I never do it directly over the plan.  Draw a vertical centerline on every former.  Actually measure the former don't just lay it over the plan and transfer the line.  Draw a line on your building surface with perpendicular lines at every former station and other known points.

Line up the formers on the centerline and pin them in place.  When you draw the fuselage sides together, assemble it with clamps and go back and measure everything a couple times to ensure that all is straight and true.  A warped fuselage is very difficult to correct and your plane will never fly right.  It is worth taking the time to build it straight in the first place.

Also see

  • If the firewall has right thrust and the fuselage is not parallel (in top view) from the firewall back to the next former, then the centerline of the firewall is not actually centered on the firewall.  I will let you think about that one. Hint:  Drawing a picture with the sides extremely angled will make it obvious.

Also see

  • Always add a chordwise grain cap to the end of the sheet balsa control surfaces.  This will help prevent the sheet from cupping.  I also cap surfaces built up from sticks because it looks neater and covering does not adhere well to the end grain wood.

  • I use Carpenter's glue as the primary adhesive in my construction.  It gives me plenty of time to work which is important in some cases such as attaching the leading edge sheeting on a D-Tube wing.  Normally there are cap strips aft of the sheeting.  The sheeting and cap strips meet at the center of the main spar.

    First I use a piece of masking tape to mask off the aft half of the main spar.  Run a bead of glue along the spar.  Place the sheeting on the spar and insert a pin about every 6" or so to get started.   (Rocket City Pin Clamps are very useful in these situations).

    Then I go back and add a pin between each of these pins (every 3").  Then add another pin in between those so that pins end up being space about every 1-1/2".  Use as many pins as necessary to ensure the entire sheet is held firmly to the spar.

    The idea is to get the sheeting glued along the entire length at first so that the glue is not setting up at one end of the wing while putting pins in the other end.  Now remove the masking tape and use a scrap piece of balsa with a square edge to clean up any excess glue on the aft edge of the sheeting.

    All this to dry thoroughly before gluing the sheet to the ribs.

    The next step is to add glue to the ribs and sub-leading edge if there is one.  Start at the center of the wing panel and pin the sheeting to the rib at the mid-point between the leading edge and the spar.  If you think the sheeting will not bend without cracking then use a wet sponge to dampen the outside of the sheeting to make it bend more easily.

    I run my palm along the sheeting from the spar to the leading edge to make sure the sheeting is actually being glued to the rib before putting the pins in.  Pin the sheeting in place working your way from the center towards each end of the panel.  Use additional pins as necessary.

    Lastly pin the sheeting to the sub-leading edge starting at the center and working towards the ends of the panel as above.  The carpenter's glue gives plenty of time to get all this done but you do have to keep going once you start.

  • Measure from the same base point to each part when laying out multiple parts such as ribs.  Don't measure from rib to rib.  If your measurement is off a little at each point then the entire wing well be off by the sum of the errors.

    In other words if your measurement is off by 1/32" inch for each of 16 ribs then the first rib is off by 1/32", the second by 1/16" and so on for a grand total of 1/2".  If each panel is off in a different direction then the panels can differ by as much as 1".

    I realize that's extreme but it gets the point across.  Instead of measuring from rib to rib measure each rib station from the wing root.  Then the most the wing can be off is by the 1/32" (in this example) and the most the two panels can be off is 1/16".

  • Extend lines before measuring for greater accuracy.If you're like me you have a maximum error tolerance when measuring with various tools.  This tolerance is a percentage of the measuring tool I am using.

    For example, I can measure to about 1/2 of the smallest division I can read on a ruler.  For me that's about 1/64".  Anything closer together than that makes my eyes start vibrating and my brain shuts down.

Whenever measuring points, try to measure them as far apart as possible.  No one can transfer a point from a ruler to a piece of wood with 100% accuracy.

By moving the points farther apart, the error becomes negligible.  In other words, if you draw a 6" line that is off by 1/64" and you draw a 12" line that is off by 1/64" then the 12" line is twice as accurate.

For example, let's say you have a former that is 4" wide.  When you draw a centerline on it, you could measure 2" from one edge at both the top and the bottom of the former.  If you extend the line and then measure, the resulting centerline should be more accurate.

Normally I will push the part against a straight surface and then measure from the surface rather than the part.  I can measure as far apart as the surface is wide.

  • Because of the error tolerance I mentioned, I use a drill bit that is smaller than the finished hole initially if hole placement is critical.  I might be off by 1/32" if I use a 1/4" bit, but only 1/64" if I use a 1/16" bit.  After the hole is drilled, it guides the end sized bit and results in more accurate hole placement.

  • Whenever I am building something flat on a board that has straight edges (wings, flying surfaces, etc.) I pin one of my heavy straight edges or a level to the board and push one of the outline pieces (leading or trailing edge) against it.  This virtually guarantees a straight surface and less sanding to true things up.  If possible I will pin down a straight edge at both the leading and trailing edge.  If I am building two identical surfaces then I will leave the straight edges down and build the second part upside down in the same location.

  • Do not try to make a part an exact fit if you can glue it on oversize and easily sand it flush later.

  • If you are building a two-piece elevator that is joined by a U-shaped piece of music wire it's easier to bend the wire and drill holes to match than the other way around.

  • Anyone who has ever installed radio equipment inside an R/C aircraft knows that sometimes it gets cramped inside the fuselage.  I have gotten some pretty deep scratches from various components inside the fuselage and finally decided to do something about it.  First, any time I make a Z-bend, I make sure to deburr and round off the end of the wire.

    For throttle/nose-wheel steering cables, I melt solder into the ends of the cable and then use a cutoff wheel in my moto-tool to smooth the sharp edges.  I knock off the edges of cutouts in formers with fine sandpaper so that I do not get cut up by plywood splinters and I round off the edges of wing bolt blocks before I glue them in.  Now when I put my hand inside the fuselage, all I feel is nice, smooth parts.

  • I use a countersink to chamfer the edges of fuel line feed holes in the firewall on both sides of the firewall to prevent sharp plywood edges from cutting the fuel line.  I use fine sandpaper to finish it off.

  • Do not be afraid to scratch build a part if you do not like the kit part.  Likewise, don't be afraid to modify a kit if you have a better way.  If a kit rib needs to be thicker for some modification you are making (retracts, for example), you can use the original part to trace a new part but leave it slightly oversize.  Put some masking tape on the neighboring ribs and sand it to shape once it is in place.

  • Just because the wing fits in the saddle perfectly does not mean it's straight.   Measure and trim or shim as necessary until every part of the aircraft is lined up perfectly.  Don't trust your eyes.  Set up something solid that will give consistent measurements.  The airplane needs to be firmly held in place the entire time.  If you move the plane you have to start measuring all over again.

  • Always use shear webs between spars if the wing is not fully sheeted.  Add them to sheeted wings as well if you think the spars may not keep the wing from twisting.  Webbing will stiffen a wing panel without adding appreciable weight.  On wings that have no sheeting webs will do more to resist torsion loads than any other component other than the covering.

  • There is a big difference between dented wood and gouged wood.  To remove a dent from wood you can wet the area slightly and then run a hot iron over it.  To prevent water from running all over the wood fold up a paper towel and then wet the towel.  Squeeze out most of the water and then press it down onto the wood.

    Another method is to put white vinegar on the dent.  You can use any kind of vinegar but dark vinegars will stain the wood.

    Gouges can be handled the same way to bring the wood up as much as possible but a gouge means that the wood grain was cut and it will not magically be repaired.  You can strengthen the area if needed by putting some type of glue in the gouge.  In most cases you will need to use a filler to completely smooth the gouged area.  I like to use Ambroid glue and micro-balloons for small areas because it dries quickly, is light and sands easily.

  • Hinging is a tedious process.  Learn to do it properly on scraps of wood.   There are a lot of gizmos available that help or hinder the process.  None of them make it painless no matter what the manufacturer says.  Hinges that are located and glued in properly will significantly improve the aircraft.

  • If you use Robart Hinge Points, you can make the holes before you ever build the control surfaces.  I use a drill press and they come out right every time.  All I do is align the fixed portion to the moving portion of the control surface and then draw a fine line with a marker at each hinge location.

    When I build the control surface, I place dowel stubs in the holes so the parts stay aligned properly while I build.   It is much faster and easier with less chance for error than any other method I know of.

  • I always glue hinges into one surface at a time and let the glue cure before attaching the surface.  It is very difficult to keep the hinge line straight when trying to glue the hinge to both surfaces at the same time.

    I use slow drying epoxy that has been heated slightly to allow it flow better.  Even using slow-setting epoxy, I usually have barely enough time to get all the hinges glued into each surface before it starts to set up.  This is a job that you do not want to rush.

  • When I am between projects, I spend the occasional evening making things that I use frequently.  For example, I have gusset patterns for 3/16", 1/4" and 5/16" stick structures (built-up fuselage).  I will make a copy of the pattern and attach it to a piece of 1/64" or 1/32" plywood and cut out a baggy full of them.  Then they are ready when I need them.

    These gussets are designed to be cut out with an X-Acto knife, so unless you need the extra strength of 1/32" gussets and want to torture yourself, I suggest using 1/64" ply for most purposes.

  • Make small blocks to slide over the throttle cable housing.Another thing I like to make is routed balsa blocks to glue to the fuselage sides to guide the outer throttle housing.  They look sort of like landing gear blocks, but smaller and made from balsa.

    I make the blocks from 3/8" x 1/4" balsa and then glue a piece of 1/16" balsa over them.  I cut the blocks to about 1/2" long and slide them over the housing.

    After the housing is in place, I glue the blocks to the fuselage using CA.  The reason I do this is because I normally set up the throttle servo so the arm is about 1/8" away from the fuselage side.

    These blocks space the cable from the fuselage side and anchor it in as many spots as necessary.  It is fast, neat and solid.



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Copyright 2002 Paul K. Johnson