Airfield Models - Fuselage Construction Example

Installing the Radio (Flight Pack) in a Model Airplane Fuselage

May 03, 2015

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Airfield Models ( the Radio (Flight Pack) in a Model Aircraft Fuselage

Although this is the final article in this series, installing the radio is not the last step.  I begin working on the radio installation as soon as possible and continue to work on it throughout construction.  As you can see in the photos below, I work on the radio at various stages of fuselage assembly.

I have learned that installing the onboard radio as early as possible not only speeds up the installation but makes the installation simpler with better pushrod alignment.

This installation is a little out of the ordinary because the model will have pull-pull controls on the rudder and elevator.  The fuselage is not wide enough to allow these two servos to sit side-by-side.  Therefore I mounted the servos inline.

The rudder servo is forward of and higher than the elevator servo.  This setup requires three rails instead of two.  When using normal pushrods I can usually mount three servos (rudder, elevator and throttle) side-by-side on a pair of rails.

The pre-cut servo rail supports and servo rails. I use rails rather than plywood trays.  Trays require support rails anyway and are heavier.  Trays are thin and do not give servo screws much wood to thread in to.

These rails are cut from motor mount stock.  The center rail is 3/16" x 1/2" with a 3/16" square step glued on.  The fore and aft rails are both 3/16" square.

These rails are plenty strong and the mount is very light.  Rigidity is also important because if the servo mounts can flex then play is introduced into the setup.

The rail supports are 1/8" balsa.

The servo rail supports are located so that the bottoms of the servos do not contact the fuselage. This is how the servo rail supports will be glued to the inside of the fuselage.  Notice that the bottom of the support is 1/4" above the bottom of the fuselage.  This is to take into account the 1/4" longerons that will be glued to the inside of each fuselage half.
The servos mounted to the rails. The servos are pre-mounted on the rails.  The rails are over-length so I will have to be careful when trimming them to ensure the mounting holes line up.

To simplify this I line up one end of the rails before mounting the servos.  Now I just have to trim the opposite end of each rail and they will remain in proper alignment.

In this image the front of the fuselage is to the right.  The elevator servo is mounted to the lower left.  The throttle servo is mounted to the lower right.

The servo rail supports are located so that the bottoms of the servos do not contact the fuselage.

Here you can see the rail supports sitting on scraps of 1/4" square balsa.  The longerons are taken into account when cutting the rail supports so that the bottom of the servo is close to the bottom of the fuselage without touching it.

The only part of the servo that should touch any part of the structure are the grommets.  If any other portion of the servo contacts the structure then vibration will be transmitted directly to the servo.

A method used to accurately locate the pushrod exits in the fuselage. Locate the pushrod exits.  Pull-pull cables will be used but the method is the same for pushrods.  I often see pushrods that bind where they exit the fuselage and they do not align properly with the control horn.  Normally the builder cuts an ugly, gaping hole or bends the pushrod to relieve the binding.

Temporarily assemble the fuselage sides and formers.  Pull the tail end together.  A straight edge is lined up over the hole in the servo arm that the pushrod will be connected to.  The machinist's square represents the fore-aft and side-to-side location of the control horn.

The triangle is located where the straightedge intersects the outside of the fuselage.  This is the fore-aft location of the center of the pushrod exit.

A purpose-made tool for cutting angled holes in soft balsa. Drill or cut the pushrod exits.  I have a purpose-made tool for cutting pushrod exits.
The pull-pull cable exits. Here are the same holes viewed from the inside of the fuselage.  Note the vertical support has been relieved.  The cables may saw through it which would not cause a problem structurally, but it would cause the cable to slacken.
Short pieces of nylon tubing are glued into the pull-pull exits. A 2" piece of inner NyRod is glued in these holes using thin CA.  The outside of the NyRod is trimmed and sanded flush with the fuselage side.

Again, this is for pull-pull cables.  For a normal pushrod, there would be one slot and that is it.  The inner NyRod pieces would not be used.

If using NyRods then the outer would be glued in the exit and trimmed flush with the outside of the fuselage.  It should run all the way back to the radio compartment with cross-braces of about 1/8" x 1/2" bracing the NyRod about every 3".

Measure the distance of the servo arm from the fuselage side.  Transfer the measurement to the former in front of the servo. In this image the servo rails are not glued in place yet.  The lower servo is the throttle servo.  A hole must be drilled in the former immediately in front of it to pass the throttle pushrod.  The former is represented by the two lines in the lower right-hand corner.

A mistake that is often made is lining up pushrod holes in formers with the servo when the arm is centered.  That only works if the servo is far enough from the former to prevent binding when the servo arm is rotated to the point where the arm is closest to the former.

I line up holes in formers with the servo arm rotated to where it is closest to the former as shown here.  Measure the distance of the hole from the fuselage side.  This will be the distance to the center of the pushrod hole in the former.

Measure the height of the servo arm from the bottom of the fuselage. The vertical location of the hole is measured from the bottom of the fuselage.  In this case a Z-bend will be used on the servo end of the pushrod.  It will enter the arm from the top.  That means the hole in the former should be 2" from the bottom of the fuselage.

If a clevis is used then the hole will be centered on the servo arm.  In this case the measurement is 1-29/32".

Drill a hole in the former at the intersection of the two measurements. The X-Y location of the hole is transferred to the former and then drilled.  You can see that it lines up very well.  It will not align perfectly when the servo is at center, but it is close enough that it will not be a problem.  The further from center that the servo arm moves, the better the alignment.

If the hole lines up with the servo arm at center, then it binds everywhere else - especially at extreme throws (full throttle and idle).  Binding at idle in particular is a problem because that is where the most precise throttle control is needed.

Make small blocks to slide over the throttle cable housing. I use my table saw to cut a 1/8" square groove in a stick and then cap it with some scrap.  The stick is then cut into pieces about 1/2" to 3/4" long and used to brace the housing for the throttle pushrod.

These grooves are easily cut with a hobby knife or a piece of brass tube.  You can use light, soft wood - strength is not an issue here.

The throttle cable is slid into place in the fuselage. Slide as many of these pieces as necessary over the outer housing to brace it.  Allow the housing to find its own path as much as possible.  When it must curve for any reason, make the curves large to prevent binding.
Locate the blocks so that the throttle cable has large bends when bends are necessary to prevent binding. Apply glue to the blocks and clamp them in place.

The excess housing will be trimmed off about 1/8" from the firewall because the throttle for the engine used in this aircraft is behind the engine.

However, I normally allow about 1" of excess to help keep the pushrod stiff.



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Copyright 2003 Paul K. Johnson