Giles G-200 – Gear Leg Fairing Tooling Design

Making the overall shape for the gear leg fairings, as I described here, is really about 1/3 of the way to making the molds.  Granted, from my perspective, it’s the most fun part of the design process, but there is a lot of work that goes into taking that macro shape and turning it into g-code to machine your molds with. The first step is to figure out what material you’ll be using to make your part, and how much of it.  In this case, we decided on 3 layers of 5.7 ounce bidirectional carbon fiber.  The intersection fairings at both the wheel pants and cowling will also be made with 3 layers of carbon.  So, how thick is 3 layers of 5.7 oz carbon?  Well…..that depends.  Are you going to be laying it up without a vacuum bag?  With a vacuum bag?  If you’ll be doing it under vacuum, how much vacuum do you anticipate getting?  All of these factors affect the final thickness of the molded part, and so you have to have this all figured out before you take your overall shape and turn it into a mold.  If you layup your part without a vacuum bag, you can expect your laminate to be AT LEAST 50% thicker than if you lay it up under full vacuum.  Say you only get around 15 inches of mercury with your vacuum setup, instead of a full 30, then you can expect your laminate to be around 25% thicker.  We typically get around 28 inches of mercury, and have found that we typically get around 0.007″ of thickness per layer.  So, if this is your first time doing this, I would say the first thing you should do is go layup a test panel using your setup – take say 4-6 layers of material, and lay it up as a flat sheet.  Let it cure, measure how thick your final laminate is, and then calculate your thickness per ply.  This is absolutely critical to keeping the amount of filler (and the associated weight) off your final product, and also to minimizing the amount of labor your have to spend finishing off your parts.  Believe me, there is something extremely gratifying about making parts that are engineered down to the material thickness – the extra effort is well worth it.  All up, we know that we can design around a thickness of 0.021″, since that’s three layers at 0.007″ per layer.  So here we have our gear leg fairing as it was lofted:

The next question is – how will this thing be held in place?  Well, I decided to mimic the Van’s Aircraft style of gear leg fairings, and put a piano hinge along the trailing edge to hold it together in the back.  So that will keep it from splitting open in flight, but what about keeping it secured?  Well, the intersection fairings at both the top and bottom will capture about 1″ of the gear leg fairing, and each will be secured with 4 #8 countersunk screws.  The gear leg fairings must therefore duck underneath the intersection fairings.  This offset is called a joggle, and when you’re designing composite structures, you’ll need to get very comfortable making joggles.  Okay, so we know we want this gear leg fairing to joggle under the intersection fairings for about an inch or so, and that these intersection fairings will also be around 0.021″ thick.  So our surfaces at the top and bottom will look like this:

Hey! What’s that surface that got tacked onto the back there?  Well, remember how I said the back would be joined with a piano hinge?  It’s kind of a pain to get a surface like this all nicely aligned, if it’s trimmed to the end of the part.  So, what I did was create a flange on each side, that can be used to hold the fairing together when the piano hinge is installed.  After installation of the hinge, that section will get trimmed off.  It’s things like this that make your life much easier on the fabrication side.  The next question to ask is – how many pieces do I need to make, in order to achieve this shape?  Well, I could have gone crazy with inflation bladders and making a closed mold, but I figured for a one off, let’s keep it simple, relatively speaking.  So, just by looking at this shape, I know we need to make this out of two parts, which will get joined together.  Where should we put the seam?  Well, my rule of thumb is to always put these seams where there isn’t a whole lot going on, shape wise.  That will generally make them disappear with much less bodywork.  You might be tempted to put the seam right at the leading edge, but to me that’s the worst place to put it.  Much better would be bring it back a bit.  Believe me on this – putting your seams on your leading edges is a bad idea.  So, here is a shot of the overlap between the outboard fairing section, and the inboard section:

Granted, it’s a bit tough to see what’s going on here, but what we’ve got is one surface ducking under the other.  The offset it 0.021″.  Okay, we’re done right?  Well….no not quite.  Think about the intersection fairings.  Certainly, we are going to have to make those out of more than one piece, which will be joined by a joggle, right?  So… that joggle…..the thickness of the surface will be DOUBLE what it is.  So, here’s what I did:

If you look carefully, you can see that the joggle at the end of the part is offset and additional 0.021″, to account for the thickness of the joggle for the intersection fairing.  This way, your intersection fairing fits perfectly with your gear leg fairing, at every point.  I like to keep my design files separate from my mold/tooling files, and so at this point, I spit these surfaces out into their own Rhino file.  Okay, so after you spit these things out, you’re ready to make toolpaths, right?  Nope. Not even close.  If you were to make toolpaths off these surfaces your mold would end at the end of the part.  With composites, it’s far far easier to make your part slightly oversize, and then trim it back.  Believe me, never try to make your part the exact size, you’ll come to regret that later.  Okay, so we need to extend these surfaces around the perimeter, but we also want to make a nice clear cut line, so we know where to trim to.  Well, if you have a very simple part, or a 5 axis mill, you could use an engraving bit to make a scribe line.  I don’t have either of those for this project, so here’s what I do – I extend the surface (usually by using Sweep1 after placing lines along the edge of the part) by 1/2-1″, and I make a roughly 20 degree break between the part surface and the flange.  Here’s what that looks like:

The 20 degree break in the surface is not enough to mess up the laminate at the end of the part, and it shows up bright as day in the finished part.  In fact, I’ve come to prefer this method, as you can never lose your scribe line with filler or sanding.  You always know where the trim line is.  So, I do this 20 degree beveled extension around the entire part, then mirror the whole thing, and join them together.  For the inboard fairing mold, that looks like this:

That’s a whole lot more complicated than the plain airfoil shape we started with.  But wait, there’s more!  The other half – that half wraps around the leading edge, right?  With a 3 axis mill, the easiest way to make that mold is with two pats that get joined together to make one mold.  So here’s what I did:

I made my 20 degree beveled extension around the whole perimeter, and then added a mating flange along the leading edge.  I then split the mold along that mating flange, copied the flange and flipped it over:

Voila!  You’ve got two pieces that are easily made on a 3 axis mill, and then joined together to make one mold.  Typically on something like this, I’ll use dowels to pin the two parts together.  Now, you can finally make your g-code.  In this case, I’ve used madCAM.  The best way to finish off a part like this, in my opinion, is with a zig-zag toolpath.  This keeps all your tool direction changes OFF your part surface – all your hard stops and changes happen at the end of the part, which is where your excess trim flange is.  This keeps things smooth as butter where you want them – in the middle of the part.  Here’s a shot of the g-code for the finishing pass:

Nice and tidy!  As I’m sure you’ve realized now, going from macro shape to toolpath is no small amount of work.  But the payoff for this work is parts that fit PERFECTLY, happy customers and very little frustration.  Seems like a good payoff to me!

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