How to Build an RC Jet – Part 6

Finishing Fiberglass – Let’s finish that dorsal!


Last time we finished up construction of our rudder/dorsal assemblies and built our offset rudder hinge.  So, now it’s time to clean these assemblies all up and make them as ready as they can be to be installed onto the fuselage.  The installation of the dorsal onto the fuselage will occur once all of the construction of the fuse is finished however.  The primary reason for this is so that all work on the fuselage can be done without the dorsal being in the way as the fuselage is rotated around while it’s worked on.

Primer – Sand…repeat, repeat, repeat…
With the dorsal and rudders being glassed (see Part 5 for that discussion), the first item of business is to start the primering process.  The objective for finishing fiberglass here is two-fold:

  1. To fill in the weave of the cloth and create a smooth surface finish
  2. To find any imperfections that need additional attention and filling

To fill in the cloth weave, the process I use is to simply apply a coat of primer, sand it down, and repeat until the surface is smooth.  Another method is to apply a second coat of finishing resin which works well also, but requires quite a bit of sanding and can add weight.  Regardless, more than 1 coat of primer is usually required to get a fully smooth surface anyhow so we can cut to the chase with the single resin layer and just use the primer for filling.

buld an rc jet - skyray primerThe primer I use is EVERCOAT which is available through my folks at Jet Hangar.  Applying the first coat, I apply it down fairly wet and then let it dry and repeat on the other side.  Once dry, using a sanding block with 180 grit sand paper applied to it, I sand the primer away back down to the cloth layer.  Though you can hand sand it using sanding  paper only, the reason for using a sanding block is that this evens out the surface as the layers of primer are added.  While sanding, there are areas of primer that won’t sand away completely which is to be expected, that’s the primer doing it’s job of filling!  Once sanded down, more primer is applied and this process repeats until the surface is smooth.  Typically it will take about 2-4 coats.

buld an rc jet - skyray 41

Once sufficiently smooth, any areas that couldn’t be smoothed out with the primer are individually filled using a glazing putty.  I prefer 3M Acryl-White Puttyas it fills nicely, sands well and gives a smooth surface.  However, this is for small surface imperfections.  Any deep imperfections require Icing filler to fill as the glazing putty doesn’t have sufficient body to hold shape for deep imperfections.

Details, details…
To finish off the dorsal/rudder assembly, there are a few details necessary.  First off, the Skyray has a very definitive fuel dump mast at the top of the rudder with 4 slots in it where the fuel would exhaust through.  To make this part, I designed the two ends (radial slotted base and tip with open slots) in Rhino and had them 3D printed out of SLA.  Those two ends were then glued into a 1/4″ diameter aluminum tube of the appropriate length.  The result is a very convincing and robust part.  The whole part could have been 3D printed and I looked at doing that.  However, I was looking for a light weight solution and wanted to avoid printing a solid part for that reason (I did have a single piece hollow part printed of the entire mast, but it warped). The mast will be final installed onto the dorsal just prior to gluing the dorsal on.

The other details necessary (in my mind) are the scale actuator linkages that are within the slots for the hinges on the full size airplane.  These were also designed in Rhino and 3D printed.  They were printed as a single part, however the CAD model was toleranced sufficiently such that after being printed the rod parts were in fact moveable. This way, they move as the rudder moves and it didn’t require anything additional to do so.  This is the beauty of a very high resolution 3D print!  It saved me lots of time and heartache trying to hand craft something.  You can see the results compared to their full scale counterparts below.

For comparison, here are some pictures of the full size counter part.  This is the look we are trying to achieve…without the bird pooh of course!

So, at this point, the dorsal is as finished as it will be before installing it onto the fuselage.  We still need to run the lighting for the solid white navigation light in the dorsal, but that can come at anytime prior to installing the dorsal onto the fuselage.  Also, we still need to final install the rudders too.  Again, that will occur just prior to dorsal installation on the fuse.  From here we will set aside the dorsal a while as we start to work in on the fuselage (fuse has to be done to setup the wing installation).  So, enjoy your rest my friend, we will see you again soon…


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