Airfoils: You Are Doing it Wrong

To be fair, I was at least doing it wrong-ish for many years, or at least, not as right as I could be. Allow me to explain – and yes, I’m dusting off this blog after many years of inactivity. I figured I would get things going again by going back to the very beginning, and pass along some of my updated thinking as it pertains to drawing airfoils in NURBS. In case you haven’t read it, here is my original post on drawing airfoils in NURBS. While I feel that was an improvement over how people approached drawing airfoils in the past, in time I’ve found better and more efficient ways of drawing them. By “better” I mean smoother, and more conducive to modeling the things that ATTACH themselves to wings – namely wing tips and wing/body fairings. The method described in that post is the best approach for doing airfoils – IF you are using T-Splines, which I was at the time. I don’t use T-Splines anymore for my work, and so what I found is that there are better ways of drawing airfoils, when you do NOT have the constraint placed upon you that the whole thing shall be one degree 3 curve.  The purpose of this post is not to be  Rhino3D step by step – in fact this is probably not a great post for beginners – rather this is aimed at folks who already have a decent grasp of NURBS modeling.  To a great extent the information in this post is platform agnostic- any NURBS modeling package where you can easily control point count and degree of your curves will work. So, without further ado……


These are the raw points for a NACA 23012 airfoil. I chose it simply because it’s still in use today on lots of aircraft – namely lots of Cessna business jets. Now, people think that airfoils are complicated, but really they are quite simple. They are the addition of two mathematical curves – a camber deflection, and a thickness distribution. Allow me to explain graphically.


Airfoil ordinate files have top and bottom sets of points. Connect those points with a straight line, and then run a smooth curve though the mid points of those lines, and you have the camber line for the airfoil. So, that’s your camber deflection. Somewhere there exists a formula for the thickness distribution for the airfoil – that is what determines how far from the camber line each set of points is.  So, you have a thickness distribution, deflected along a camber line. Simple. Right? NOW! Here’s where I’m going to make perhaps the most important point of this post. YOU ARE NOT LOOKING AT “THE AIRFOIL.” YOU ARE LOOKING AT A DUMBED DOWN COPY OF THE AIRFOIL. See, go back to the basics of what an airfoil is – the summation of two mathematical curves, right? How smooth are those curves? INFINITELY SMOOTH. How many points do they have? INFINITE POINTS. Does the data above appear to be either 1) Infinitely smooth or 2) Infinitely defined? Heck no! At best, it’s a copy of the original article. Even worse – the entire system that we use to store airfoils predates the existence of NURBS modeling packages, or even really NURBS math. We are trying to store smooth mathematical functions as degree 1 polylines. We are – in the most literal sense – sending our data back to the 1920’s for archival. Seriously – this method of storing airfoils as ordinate points in text files is pretty much as old as aviation itself, and has not been updated. Why are we surprised when this does not work very well for surface modeling. Why do we keep doing it this way? How is this still a thing? Seriously. This is nuts. Need proof? Here’s how most people make airfoils – they simply run an InterpCrv through all those points and call it done. I outlined why this is a bad idea in my last post on the subject, but here’s what you get when you do that with the NACA 23012:


That’s actually not as horrendous as most conversions – it’s not awful, but that’s about the best thing I can say about it. Especially when you turn up the scaling on the back section of the airfoil, you can see it has some weird artifacts:


Again, the best thing I can say is, “it’s not horrible.” And, when you go to actually make things that attach to a wing lofted with this curve, you’re going to propagate those curvature wobbles into the resultant surfaces. Better to make a new airfoil from scratch. Trust me, any time you invest in creating nice smooth airfoils will pay off huge on the back end.

I want you to think of what the Platonic ideal airfoil would look like, especially in terms of it being smooth, and how the curvature graphs would look. This whole post revolves around two very simple tools – point editing/manipulating degree and point count, and the use of curvature graphs to analyze our work. So, imagine in your head what this ideal airfoil would look like, especially the curvature combs. They would peak at the leading edge, and then very smoothly fade out as you travel aft, right? The NACA 23012 is a little odd in that with that drooped nose, it ALMOST creates a flat spot on the bottom, about 15% in from the nose. You can see the curvature dips, rises, and then fades. So, in this case, we would want that feature to be there too of course, but we want our airfoil to be smoother, and ideally with as few points as possible. That ideal airfoil, in your head, should look like this, right?


Right? Nice and smooth, it peaks at the leading edge and then gradually fades. When I turn up the scaling on the back end, we get this:


See, we sill get the required curvature dip along the bottom surface, but everything is SMOOTH. Notice how nice and constant the curvature at the back of the airfoil is. This is what an airfoil is SUPPOSED to look like, right? We are now looking at something that is far closer to what the “real” airfoil is, are we not? This is not a copy of a copy. How close is it to the original data? I scaled this up to 60″ in chord length (in the ballpark for a GA plane) and was able to fit my curves to the original data to within 0.005″. What do my curves look like? There’s four of them – two top and two bottom. Here’s the top and bottom “main” curves, with the points turned on:




Each is a degree 5 curve with 9 points – so, NOT single span, but not so many that you cannot point edit the curve, or any surface created from the curve. Here are the nose curves, which have the break between them at the leading edge point:


How did I create these curves? I started with rebuilding the back sections, and point edited them to fit. There’s no magic here – notice there are more points where there is more curvature, and fewer points where there is less. So, place your points accordingly. Then, point edit to get them to fit your data. Add points if you need more control. Check your new curve against your original points, and always always always check with curvature combs. Seriously, just put a curvature comb up on your curve as you work on it, and leave it up till you’re done. Then, when you’re happy with the main top and bottom curves, create your nose pieces just by using whatever is clever – in this case I used BlendCrv, point editing, and curve matching tools. Again, keep asking yourself, “Is this what the curvature graph for an airfoil looks like?” Keep asking yourself that question until the answer is “yes.”

I’m always blown away in my work how many terribly drawn airfoils I see, because really when you get down to it, making proper airfoils doesn’t take fancy or expensive software. Really, it’s just rebuilding curves, point editing them, and constantly checking your curvature graphs. But, you can’t really sell that as a software package, so we get all sorts of “smoothing” plugins and automatic airfoil conversion plugins. Trust me, there is no purpose to those tools, they will only serve to make your life harder. Rebuild. Point Edit. Check your curvature. Done.


Published with permission from School Street Design Company Blog. Source.

Using Rare Earth Magnets to Join Composite Structures

You know what I think is a little silly?  Using clecoes to join composite structures.  I mean, sure, they work, but unlike when you are working with sheet metal, at the end of it you have a bunch of useless holes that have to get filled in.  Also, multiple insertions/removals of the cleco tends to open up the hole, reducing the grip of the cleco.  So I started thinking about a better way of doing it, and I think I’ve come up with a workable solution – rare earth magnets.  I’ve used them on radio controlled aircraft in the past, and have always been amazed by their small size and large holding power.  So, I bought some of these 5/16″X3/16″ N42 disc magnets from K&J Magnetics.  On the inside of joggle for the Giles 200 gear leg fairings I put down some electrical tape.  I then placed a disc magnet every 1.5″ along the length of the joggle.

The fairing half is on a magnetic building board that I use for building balsa model airplanes, this was very handy because it kept all the magnets in place while I laid them out.  Tucker and Walnut, faithful four legged shop assistants supervise from below:

Okay, really, they’re just watching for that cat that walks by the window from time to time.  Anyhow, moving on – I then broke out the hot glue gun, and just surrounded each magnet with a little glue.

All we’re going for here is keeping them from falling off the tape once we slide this thing off the table.  Then, I simply clamped the two halves together at the edges.  After that, I just took one magnet at a time, placed it relatively close to where I thoughts a magnet was.  Once I felt it pull, I just let go, and the magnet snapped into place.

Well that was easy!  The “hole layout” was less time consuming than if I was using drills/clecos, and more tolerant of edge distance.  Once I bond these together and the resin has cured I’ll simply slide the exterior magnets off one by one, and then pull off the piece of electrical tape that the interior magnets are glued to.  Easy!  Obviously, this will only work in non-blind applications, but it sure seems like a nice method to me.  If your alignment is super critical, I would still recommend using a few holes/clecoes just to make indexing them together fool proof.

Published with permission from Better Living Through CNC.

The Method Part II – Demolding the Parts

Following up on my previous post on my method of making high quality, re-usable tooling with CNC machined foam and Stretchelon bagging film, here’s how things tend to go on the demold side.  First, turn off the pumps, and then remove all the secondary fabrics/films that are bagging the part:

I’ve left the peel ply on here, because the parts are still pretty darn soft, but everything else has gone into the trash.  Now, simply get some air underneath the layer of Stretchelon covering the mold, and then pull the parts up, along with the film:

Now, just simply peel the Stretchelon off the parts – if you’ve used mold release, it should come off very easily more or less in one piece:

Presto!  And the molds are totally unharmed – same as they were when we started.  Here’s a closeup of the surface texture of the finished part:

Yes, a bit of texture, but like I said before, a few swipes with some 80 grit to prepare for primer and the texture just vanishes.  Or, if you really want a smoother texture, just get more dense tooling foam.

Published with permission from Better Living Through CNC.

Creating High Quality, Reusable Molds with CNC Machined Foam and Stretchelon Bagging Film – aka “The Method”

This post is really the culmination of a few years of research, research that I’ve decided to set loose into the world.  We call this “The Method,” or sometimes “The Stretchelon Trick” and it’s my primary method of creating tooling for composite parts these days.  As  an example, I’m using the Giles 200 gear leg fairings that I describe in depth in this post here.   You might read that post and think “well that’s all well and good, but what the heck are you going to make those molds out of that’s cost effective?”  The answer is medium density polyurethane tooling foam, normally 15 lb. density foam.  For these gear legs, the depth of the mold is about 1.5″, so I used 2″ thick foam.  A 2″X48″X96″ block of tooling foam is about $500, and this particular mold is about 23″X40″, so the material cost is a fairly small fraction of that.  Here’s the machined foam of the gear leg fairings, cut yesterday:

Okay, granted, it’s a bit hard with the light here to really see what’s going on, but if you look at the post I link to above, you’ll see this is just the CNC machined version of the gear leg tooling.  Since one of these molds is two part, the next step is to join the two parts together:

We usually join them together with some fast setting epoxy and then drive some dowels into them for good measure.  Now at this point you’re probably thinking “well that’s nice and all, but now you’ve gotta spray some sort of primer/sealer onto that thing, and hand finish it.”  But, you would be wrong.  You see, I hate sanding.  Actually, that’s not really true, I like doing body work, but I hate sanding when there’s no point to it.  And now let me show you why there’s no point to it:

This, my friends, is the heart of “The Method.”  The green stuff over the mold is an elastic vacuum bagging material called Stretchelon.  Stretchelon is a high elongation vacuum bagging film.  I hate dealing with bagging tape, and so I’ve routered channels into my work table here, coated them with shelf paper and then used hardware store screen door spline to create the seal.  This method works quite well, but you can just as easily make a tube out of the Stretchelon and put the mold in the tube.  I pull my vacuum for the mold from the bottom of the table.  The mold is sitting on a piece of breather so that the air gets evacuated from all around the mold.  The slight porosity of the foam does wonders to allow the air to travel to the edge of the foam.  Closeup, it’s looks like this:

The Stretchelon pulls down right onto the foam, sealing it for use.  Pretty cool huh?  Yes, there is a bit of a texture to the mold.  If you want a smoother mold, just buy higher density foam.  But what I’ve found is that since the first thing I do when I go to prime a part is to scuff it up, that texture simply vanishes with a few strokes of a sanding block.   The final thing to do is put some mold release on it – the film is polyurethane and so epoxy will stick to it.  I’ve used Frekote in the past, but I like to use another Airtech product called Safelease 20L.   It can be dispensed from one of those hand squeeze spray bottles you get at the hardware store, so I like the fact I’m not using a bunch of aerosol cans.  And it works, so that’s nice.  Anyhow, once you spray your mold down with some release and then wipe off the excess, this mold is pretty much like any other – yes, it’s slightly more ding prone than standard hard tooling.  If you want to make lots of parts (let’s say, more than 10) then I would simply go with a more dense foam, say 25 lbs.  If you get any dings, you can simply patch them with spackle.  When I first starting doing this, I was really concerned about mold wear, but frankly it just hasn’t been a real problem.  Some of my molds have been used 10-12 times now, and for the most part they look just like they did the first time.  So now, it’s just a matter of laying up the laminate, in this case 3 layers of 5.7 oz carbon:

And then, you vacuum bag another layer of Stretchelon (or regular bagging film) over on top of the part:

Your standard peel ply/perf/breather fabrics are on top of the carbon.  The vacuum for the top bag comes from a standard bag tap.   So we’ve got two separate pumps running here – one for the mold, and one for the part.  They’re both around 25-26″ of mercury.  You’ll get a few more inches of mercury with a bag tube made with bagging tape, but the cost and labor savings of this whole screen spline method are nice.

After the part has cured, you simply switch both vacuums off, and remove the part.  The Stretchelon comes off the foam with the part, and as long as you haven’t gone and poked some holes in the first layer (which you’d pretty much have to try to do) the mold is just as pristine as when you started.  Here’s some advantages of The Method:

1. It’s freakishly fast.  On small parts, I’ve done a design, mold fabrication, part fabrication and demold cycle in under 24 hours.

2. It’s very accurate.  With the EXAscan laser scanner, what you’ve got is a fully digital production workflow.  The data goes in on the EXAscan, comes out on a CNC mill, and you do NO hand finishing of the mold.  If you need a tight tolerance, hand finishing can quite easily take you out of that tolerance. Of course the same level of accuracy applies to designs that start off in the computer.

3. Far less sanding.  The first time I pick up a piece of sandpaper with this method is when I go to put on primer.

There are a few drawbacks:

1. Certainly, if you want to make molds out of cheap boat cloth and polyester resin, The Method will cost more.

2. There is SOME limit to what the Stretchelon can do.  Not a lot, but some.  One case in particular – if you have a cylinder that you want to lay up over, the Stretchelon will have a seam somewhere along the length of the mold.  So, occasionally you have to be creative.

So there you have it – this is “The Method.”  I’ve heard rumors of other people doing something like this, but I’m not sure if anyone has taken it to the extremes that I have.  I’m going to be outlining a few of the finer points in a few days, and I’m sure people will have some questions, but that’s really the gist of it.  Happy 1st Birthday to Better Living Through CNC, and thanks for reading.  Enjoy!

Go to Part II

Published with permission from Better Living Through CNC.