Airfield Models - How To

Make Cowling Cut-outs

May 03, 2015



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Airfield Models (http://www.airfieldmodels.com/)How to Make Cowling Cut-outs

In my early days of model-building I could be counted on to botch the cut-outs in the cowl of every model I built.  I was doing things the most difficult way imaginable.  And that was even after I could build very nice wood structures which made the poorly cut cowling stand out even more.

Sometime back in the 80's a friend was visiting my shop and shaking his head while watching me hack away at another cowl.  He asked, "Why are you doing things the hard way?"

''Because it's not art if I'm not bleeding?'

He went on to describe the method shown below.  I immediately had an, 'I'm a freakin' moron!' moment but then smiled knowing my model-building life had just become multitudes easier for the rest of my life.

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Making the Cowl Cut-outs

The engine should be mounted before you begin.  It is very important the cowl matches the engine and not the other way around.  If you mount the cowl and then match the engine to it then the thrust line will probably be fairly far off.

After the engine is mounted take careful measurements and mount the cowl.  If I can disassemble the engine to a point where the engine can remain mounted with the cowl slid over it then that's what I do.

Normally engine disassembly means removing the muffler, needle valve or the entire carburetor and possibly the engine head.  The cowl is aligned properly and then securely taped in place.

Whatever mounting holes are drilled and the cowl is screwed in place.  Hopefully it all comes out aligned properly.

Now remove the cowl and you're ready to get started.

There are normally up to four engine cut-outs that must be made.

  • Muffler
  • Main needle
  • Idle needle
  • Glow plug or possibly the entire engine head

Both needles must be accessible in order to adjust the engine.

The main needle may be long enough to reach past the cowl.  If not then most needle valves have provisions to mount an extension so the needle can be adjusted with the cowl in place.

The idle needle normally won't extend beyond the cowl and does not have a built-in way to extend it.  Most commonly the idle needle is adjusted by putting the proper driver through a hole in the cowl.  If this is the case then do not make idle adjustments with the engine running unless you like bloody knuckles.

In all cases, the needle cut-out is usually just a hole drilled large enough to pass the driver or for the needle to pass through the cowl without touching it.

If the engine head interferes with the cowl then a larger cut-out must be made to clear the cowl.  If the head is fully enclosed then you can use a remote glow plug adapter or cut a hole in the cowl to attach the glow igniter.

Except for scale models with fully enclosed mufflers, almost all cowls need a cut-out for the muffler.  This article demonstrates how to make the muffler cut-out.

Other cut-outs may be necessary for cooling air to enter and exit the cowl.  Many times the cowl is molded in a way that makes it obvious where to make these cut-outs and no template is necessary.  Other times you may need to make templates from patterns on the plans.

Normally all cut-outs are made before the cowl is finished.  In this case I'm making cut-outs in a pre-finished cowl for an ARF.

Make cardstock templates and tape them in place over the engine. Mount the engine to the aircraft.  Make a template for each cut-out you plan to make.

Always leave plenty of clearance around items that extend past the cowl.  If anything touches the cowl it can cause all kinds of problems such as excess vibration, inconsistent engine runs or a cracked cowl.

Align each template and tape in place as shown.

Remove the engine and mount the cowl.  Do not remove the template. Remove the engine and mount the cowl.  Do not remove the template.
Mark the cut-out on the cowl. Use the template to mark the cowl.  If the cowl is unfinished then a fine point Sharpie marker works well.

If the cowl is finished then you might want to use something that can be removed easily without damaging the finish.

My plan was to cut away the opening to the outer edge of the marker so the marks would go away during the process.

Remove the cowl and drill holes around the inside perimeter of the cut-out. Remove the cowl.

Drill lots of holes around the inside perimeter of the opening.

Connect the holes with an X-Acto knife or a cutter in a Dremel.

Don't use a method that you can't control or you may have a mis-cut which will either be ugly or require you to make the cut-out even larger to hide the mistake.

Finish the cut-out its final size and a neat appearance. Finish the cut-out using whatever method is appropriate.  A piece of sandpaper wrapped around a dowel works well but takes a while.

If you use a Dremel then a garnite bit is an excellent choice for two reasons.  It is very unlikely to suddenly cut into the fiberglass as a high-speed steel cutter might and it makes the edge much smoother than cutters.

Use a piece of fine sandpaper wrapped around your finger to finish sand the edge smooth.

The cut-out centers nicely around the muffler. Along with being a muffler exit, this cut-out doubles as a cooling air exit.

This setup requires the muffler to be off the engine when the cowl is attached.  The muffler is then attached to the engine and tightened.

I don't have this model any more and I honestly can't remember what method I used to tighten the muffler.  But I do remember that it could be tightened securely so there may have been a cut-out underneath the cowl that allowed a wrench to be inserted.

 
 

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How to Make a Fiberglass Cowl
How To Install Robart Hinge Points

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Copyright 2006-2009 Paul K. Johnson