Airfield Models - Fuselage Construction Example

Jigging and Dry-Fitting a Model Airplane Fuselage

May 03, 2015



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Airfield Models (http://www.airfieldmodels.com/)Jigging and Dry-Fitting a Model Aircraft Fuselage

To ensure there will not be any significant problems during assembly it is important to always dry-fit parts.  To that extent you should fit as many parts as possible.

A straight fuselage is as important as a straight wing.  Many kit instructions are very helpful in providing us with instructions that do not give any insight into building a straight fuselage.

For example, the formers may be centered and the tail post may be centered, but each side of the fuselage may curve differently than the other.  In essence what is created in this case is a semi-symmetrical fuselage.  More about this problem in Part 6.

This fuselage is being built using my Magnetic Building System.

 
 
The inner fuselage sides are completed and ready to be joined together. The completed fuselage sides on the board ready for assembly.
Always dry-fit everything possible to eliminate problems. Formers and other components that will be used during the dry fitting.  Always fit anything possible so that potential problems can be corrected before parts are glued in place.

It is much easier to eliminate problems before parts are glued together permanently.

Select a former to start with and jig it to the board per the plans. Draw a centerline on your build board to assemble the fuselage over.

Pick one of the formers in the wing saddle area to begin with.  This is the former to the rear of the wing saddle.  It is erected perpendicular to the board and square to the centerline.

Use pins, clamps, weight or whatever is necessary to hold things together.

Most kits have a former at the leading edge and trailing edge of the wing.  For most of my planes I build a former to split the wing saddle area into two compartments.

This former simplifies keeping the receiver and battery pack in place.  Of course on larger models, the wing saddle area is so cavernous that this will not work the components need to be strapped down in some fashion.  This model is small enough that I can fill unused areas with scraps of latex foam rubber.

Note that this former is made from two cross-grain laminations of 1/8" balsa.

No problems with this part.  It fits just as it should. The rear face of the above former.  Notice that it butts against the servo rail supports.  Also note the throttle pushrod guide hole and the cut-out at the bottom of the former to pass servo wires to the receiver/battery compartment.

Be sure to smooth this edge carefully to prevent it from sawing through the wires especially if the former is plywood.

Check the radio installation during dry-fitting. Details of the radio installation are here.  The servos rails are ready to drop in.  All seems well and no further adjustments are necessary thanks to careful planning.
Pull the tail together over the centerline.  Ensure the fuselage sides are perpendicular to the board for their entire length. The tail is aligned over the centerline and clamped.  Even though the formers are centered and the tail post aligns perfectly over the centerline, the fuselage is not straight aft of the wing.  Reasons this can occur:
  • The fuselage sides are not identical in length
  • The sides vary in hardness
  • One side is ahead of the other

The result of any of these conditions is a fuselage that is not symmetrical.

Check that both sides align with each other.  A line drawn from a point on one fuselage side to the same point on the other side should be perpendicular to the centerline.

Note the tapered tail-post piece.  It will be cut at the end of the fuselage construction to allow passage of the stabilizer.

Check the fuel tank installation by actually installing it.  In this case there is no tank hatch so the radio compartment is the only way in or out without performing surgery.

Note that there is room all around the tank for foam rubber.  The selected tank will allow 12-15 minutes flights.

Ensure that the fuel lines pass through the firewall without kinking or excessive strain. Careful measurements taken while building the firewall ensure there will be no problems.  You can see the tank is not strained in any way.  The fuel lines pass easily through the firewall without radical bends or kinks.

The tank was blocked up to the correct height for the dry-fitting.  Latex foam will support the tank when it is permanently installed.

The front of the tank is pressed against the blind nuts.  This is bad because it is possible for one of the engine mount bolts to puncture the tank.  The fix to this is three-fold:

  • A piece of foam rubber will be attached to the forward part of the tank to prevent the tank from directly contacting the firewall.

  • The bolts will be cut to so they do not extend past the blind nuts.

  • Do not dive the airplane into the ground or fly it into walls or trees.

Pull the nose together over the centerline.  All formers should be checked during the dry-fitting. The former directly behind the nose ring.  It is best to install this former with the engine installed.  However, this engine will be side-mounted and it requires a large cut-out in the fuselage side.

I chose not to do this because the cut-out will be so large that it is likely the fuselage would warp during construction due to the sides not bending equally.

I have decided to place my bet on my building accuracy and trust that the spinner will line up with the nose ring when assembly is completed.  This is a risk because the chance of having a perfect match are about nil.  That is why I left the ring slightly over-size.  I doubt it will be off by a significant amount.

 
 

Before You Glue Anything...

Again, this was a dry-fitting.  No glue was used during any of these steps.  Now is the last chance to do things while they are easy to do.

Have you...

  • Drilled holes in formers for the pushrods to pass through?

  • Cut pushrod exits in the fuselage side(s)?

  • Drawn all necessary alignment marks?

  • Drilled all holes in the firewall for the engine mount, fuel lines, nose gear mount and throttle pushrod?  Are other holes necessary for items such as on-board glow-drivers?

  • Checked that the tank can be installed?

  • Ensured that the fuel tank does not interfere with the throttle pushrod?  Have you trial-fitted the throttle pushrod?

  • Trial-fitted the nose-gear steering pushrod?

  • Planned for an antenna exit?

  • Planned for a switch and charge-mount location?

  • Ensured that nothing in the fuselage will interfere with a center-mounted aileron servo?

Take care of all these items now because they become exponentially more difficult after the fuselage sides are joined.

At this point I will disassemble the fuselage and put it together again exactly in the order shown on this page.  This time I will use glue. I knew I was forgetting something.

The next installment of this article will continue after the assembly shown here is glued up..

 
 

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Next

Making the Fuselage Formers
How to Build a Straight Fuselage

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Copyright 2003 Paul K. Johnson