Airfield Models - Fuselage Construction Example

Building a Straight Model Airplane Fuselage

May 03, 2015



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Airfield Models (http://www.airfieldmodels.com/)Building a Straight Model Aircraft Fuselage

All the formers are glued in place and the tail post is glued together.  In theory, the fuselage should already be straight.  I have never known that to be true, however, and I will show you why.

The main reason being that the two sides never bend exactly the same no matter how careful you are or how carefully you select your wood.

Models built from kits have it even worse.  I have had kits where one side was so much harder than the other that there is no way a straight fuselage could have been built with the kit pieces.

If you buy a kit like this I suggest you return it for a replacement.  If you decide to keep the kit then you should replace the sides with wood that matches within reason.

 
 
The fuselage is jigged to the building board with the formers centered and perpendicular to the centerline. Even with all this jigging I can guarantee that the fuselage is not symmetrical.  There is nothing between the rear of the wing saddle and the tail post to ensure the fuselage is straight, square and symmetrical.  I am going to fix that.
The sides do not make proper contact with the fixtures in areas where there are no formers to support the fuselage sides. In the next two images all the uprights are centered around the fuselage centerline.  However, you will notice that fuselage sides are not square against the uprights.  I could just go ahead and glue in cross-braces and add the decking.  The airplane would fly fine.

I am not satisfied with that though.  The amount of effort to get the fuselage straight is negligible compared to the improved trim characteristics of the finished model.

More problem areas with the aft fuselage. The left side of the fuselage.  The fuselage is not even touching the lower jig.  It is hard to tell in these images, but the gap at the top of the lower jig is over 1/8".
Use clamps to hold the fuselage sides to the fixtures.

The fix is to brace the fuselage against the jig using whatever means works best.  In this case I am using magnets at the bottom of the fuselage sides and reversible clamps (X-Acto) at the top of the fuselage to spread it apart and push it against the jigs.

Now I am confident that the fuselage is actually straight.  The problem is keeping it that way.

Cross-braces are fit in the aft fuselage to help keep it straight.  The cross braces also support the decking. Cross braces will be used to strengthen the aft end of the fuselage as well as support the decking.  Here I am using 1/4" square balsa.

Starting at the forward-most cross-member, mark a piece using a razor.  The idea is to make a cut-line, not to cut through the balsa.  I pushed the razor in farther than usual so it would stay in place for the photo.

Initially cut the braces slightly over-size. I make cuts slightly to the outside of the where the end of the finished piece will be.  This gives me a little margin when sanding the pieces to fit.
Pin the top and bottom braces together and shape them at the same time. Pin a second cross-member to the piece you just marked.  Cut and sand them at the same time to make two identical cross-braces.
Use a razor saw to remove most of the excess. I use a razor saw to cut outside the line I marked with the razor.
Sand the ends to a perfect fit.  If the cross-braces end up too short, do not use them.  Make a new set. This sanding block is weary and needs new paper.  However, there were a few spots good enough to finish the ends of the cross members.

Do not try to make the cross-member a perfect fit on your first attempt.  You will be guessing at the angle to match it to the fuselage and you want the piece to be a little long while getting the angle right.  Once the angle is correct, maintain it while sanding the members to the proper length.

The idea here is to sand, fit, sand, fit, sand, fit, etc. until it is perfect.  If the piece ends up too short then use it for the next shorter cross-member and start over.

One set of cross members for this fuselage took me four attempts to get it right.

After a couple attempts a set that fits well is completed. This is a good fit.  Using gussets would not be a bad idea I would certainly use them if this were open framework.  However, the top and bottom decking will be glued to the cross members so no additional reinforcement is necessary.
All the cross-braces being test-fit. All the cross members have been fitted and are ready to be glued in place.
Glue in the bottom braces first. Starting from the rear of the fuselage, glue in all the bottom cross-members first.  Be sure they are perpendicular to fuselage centerline and are a good fit.

Also be sure they are flat against the board.

When gluing end-grain wood, always use the double-gluing technique.  More information about double-gluing is here.

Glue in the top braces. When gluing in the upper cross-members I usually eye-ball them to align them with the lower cross-members.  They should either be flush with the top of the fuselage sides or slightly proud so that they can be sanded flush.

You do not want them too low because it will cause the deck sheeting to bow in.

At this point the fuselage is straight.  The forward fuselage is straight due to the several formers and the small distance between them.

The aft fuselage is straight mainly due to the jigs.  If the fuselage was to be pulled from the board at this point it would remain fairly straight, but to guarantee it, the rear decking must be glued on.

As you can see I have replaced the taller jig pieces (see first image at top of page) with shorter ones so that I can sand the upper aft fuselage and then add the decking.

Glue in the landing gear mounting plate while the fuselage is jigged to the building board. Shoulder and high-wing aircraft have fuselage mounted landing gear.  If possible, add the mounting system while the fuselage is still jigged to the board.

Here I have cut a scrap of balsa to wedge the triangle stock reinforcement against the fuselage sides and landing gear plate.

 
 

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Jigging and Dry-Fitting the Fuselage
Adding the Upper and Lower Decking

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Copyright 2003 Paul K. Johnson