L-39 Wingtips

I’ve been working on these wingtips for a few months now, just delivered them today.  The customer is Minh Jet in Hollister, California.  Minh had some aero engineers come up with some custom tips for the L-39 Albatross, which is what his shop primarily services.  He found that they decreased the stick forces on the ailerons, increased the climb rate and made made it more efficient.  Since his tips were hand shaped, he came to me to digitize his existing tips so that I could make production molds and parts.  This is the original tip that Minh and his crew made by hand:

As you can see, they have a double winglet configuration.  I digitized the last few inches of the wing and the tip using my FARO arm.  If you look at the full res version of this picture (click on it) you can see all the data points I captured for both the airfoil sections, and the tips:

The surface model for the tip was made with T-Splines.  For me, this was my “point of no return” project with T-Splines.  I will never attempt anything like this just using standard Rhino commands again, because it was around this project that T-Splines seemed to truly click in my brain, and the whole process became very intuitive and fun.  This was the first time I knew what the topology of my T-Spline surface should look like before I started it.  The ability to create high quality surfaces with T-Splines, especially in the blend regions, is just wonderful.  And the fact that I could make changes to one part of the model and those changes were seamlessly integrated into the surrounding geometry allowed me to work in ways I’ve never done before.  So, when the customer informed me that the wing I had scanned was a very rare variant of the L-39 wing, and that we should tweak our molds so that they fit a more widely available wing, I was able to do so quickly and easily – and at the very end of the surface modeling process.   What would have been a nightmare of model rebuilding took only about an hour to accomplish.  At the end of surface modeling, my master surface looked like this:

The silver ring at the front is the tip lens.  In this case, the lenses were supplied by the customer.  I had to digitize those separately and do the best I could to match the surface of the tips with the lenses.  Again, T-Splines V3 came to my rescue here.  Since the tip lenses were hand shaped, not surprisingly the left and the right were off significantly – around 3/16″ or so in some spots.  V3 allows you to isolate certain regions of symmetrical models, and so I was able to tweak the fit of the master surfaces to each lens, all while maintaining the symmetry of the remainder of the model.  Again, this allowed me to work in a completely different flow from previous projects – I could work on any portion of the model at any time.  Of course this also means you have to have some discipline and keep yourself from just tweaking your model endlessly….perhaps something I can work on in the future?  Anyhow, once I had the master surface models, I broke down the surface into manufacturable components – in this case it took seven molds to make the exterior surface.  The components are all joined with a double joggle, which is a classic method of joining composite structures.  It make a very strong joint – just as strong if not stronger than the surrounding surface.  Here is the top and bottom components with the double joggle:

The areas that are inset around the seams are for the 2 layers of carbon fiber that will get applied after the halves are joined.  I don’t typically model that since it’s done by hand and has no bearing on the mold.  After the surfaces get broken up, the molds must be designed.  Here is the mold surface model for the top main components:

Then the molds get CNC machined on a 3-axis mill.  For this project, I used a mix of Renshape 2025 and 15 lb./cubic foot polyurethane tooling foam.  Here is the top main mold, which was made from the Renshape:

So what happens between the machined foam and say this finished part here?

Well, I’m not really going to say – this is a process I have developed over the past few years and so I’m keeping pretty quiet about the particulars.  The important things to know are that the molds show absolutely no wear after use – other than that created by the occasional clumsiness of yours truly- which can be fixed with wood filler.  There is no resin left on the foam, and the life cycle of the mold is for all intents and purposes infinite.  With the Renshape, the surface finish is close to, but not quite what you would get out of a Class A mold.  That is – it’s pretty shiny, the FORM is perfect, the finish just takes a few minutes to knock the very slight texture off with a sanding block.  Since nearly all my parts get painted, they have to get the shine sanded off them before primer anyhow, and so from a production standpoint this does not add any labor to the process.  So, once I’ve got one mold, it’s just a matter of cranking out the rest of them and making the parts.  Here is all the sub assemblies for one tip ready for joining:

I designed a joining fixture in Rhino and had a local metal fabrication shop cut it out of plywood on their CNC waterjet.

Water jetted plywood is my go-to solution for jigging these days – it’s cheap, it’s accurate and it’s fast – doesn’t get much better than that!  I cleco-ed all the sub assemblies together in the jig and then bonded it all together with Hysol 9430:

After that it was all about adding the two layers of carbon fiber to the areas around the seams, and then bodyworking those seams.  I also installed the tip light lenses and a bracket to hold the tip light.  Here’s a bunch of pictures before delivery:

I should mention that the flared surface along the inboard edge is for alignment purposes when the customer goes to install the tips – that part, and roughly 1″ of material will be removed during installation.  I drove them over to Minh Jet in Hollister today and delivered them to the customer.  We slid them in place on the wing and of course took lots of photos:

This plane isn’t slated to fly until at least late this year, so it will be a while before these tips see flight, but it seems that Minh has been getting interest from several customers who already have L-39’s in getting a set of their own.  So, with any luck, I’ll be cranking out more of these over the coming months – fingers crossed!  If by some chance you have an L-39 and want a set, these are sold exclusively through Minh Jet, so drop Minh a line at 408-829-1855.

3 Responses to “L-39 Wingtips”
  1. parker bankston says:

    very nice– thanx for sharing some of the design and tooling up process…. plz notify me of other projects u are willing to share — i am currently constructing a small boat here in Costa Rica using a simple CNC machine and learning the process as we go…. parker bankston

  2. George Bye says:

    What results did you find in flight test?

    • George – I just did the manufacture on these, but my understanding is that the greatest benefits are in improved rate of climb and reduced stall speed. The reduced stall speed seems to be the most important thing to the folks flying them, as they are on shorter fields. If you want to know more (and more specifics) you should give Minh a call, he’s got all the data. -Sky

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