Cutting balsa airplane parts by hand

Here’s a plan that I bought from RCM in the 1990s, the Guppy.  It’s a really cool plane with no ailerons, but there are flaps.  I built it with flaps and ailerons, and it was a really great flying airplane.  Then I mounted it on floats and flew it like that for a while.  I highly recommend building this model.  The plan says to use a 30 size engine, but the one I built 15 years ago flew great with an OS 26 Surpass, like the one sitting on my table now.

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I’ll start by making a set of ribs.  Sometimes you get the rib outline from the fuselage side view, but this plan shows an empty wing profile there instead.  Complete wing rib templates are in the upper right area, as shown in this photo.

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Typically, a set of ribs is made in two steps.  First a master rib is made, and then the rest of the ribs are made from the master rib.

MAKING THE MASTER RIB

The first step in this process is to get a piece of paper to trace the rib outline.  I have a box of used office paper for this purpose.

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The wing rib outline is traced with a pencil.  Note that it is traced on the inside of the wing sheeting, to make a rib the correct size.

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Sorry about the dirty fingernails. I changed the oil in my wife’s van today.

Next, the traced template is cut out with scissors.  It doesn’t have to be super accurate, so don’t worry about it.

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Here’s a photo of the template, placed over the outline on the plan.  You can see that it’s a fair representation, but not super perfect.

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The ribs of this plane are cut out of 3/32″ sheet.  The template is placed on a piece of balsa, and the outline is traced with a ballpoint pen.

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Next, the rib is cut with a knife.  Don’t worry about getting it perfect.  Theoretically the part should be cut on the inside of the line, but if you try to do that you will inevitably cut it too small in one or more spots.

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When the part is placed on the plan, it’s obviously too big.  You can clearly see that the rib is covering the wing sheeting in the drawing.

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The rib will need to be sanded to make it fit correctly.

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Sand it in one area until that part of the rib fits inside the line.

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Then sand the next segment, and recheck the fit.  Sand a little bit more, and check again.  Keep doing this until the entire rib is the correct size and shape.

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MAKING THE REST OF THE RIBS

Now you need a whole stack of these.  For most planes there will be two or more specific rib outlines.  In the case of the Guppy some ribs go full length, some are short to make room for the flaps, and some are just different because they go in the rounded wing tip.  When cutting ribs for this type of wing, it makes sense to cut the biggest ones first, then trim the master rib for use as a template for the smaller ones.