Steal this airfoil

The last few weekends at the shop have been pretty boring and unphotogenic. And I’ve also been forgetting to bring my camera with me. But all I’d have to show for it is a bunch of gray dust where I’m sanding the wing molds to clean up some tooling coat oopsies and plug defects, and the 3-ton hydraulic tension rig I’m using to do pull-testing on some rock climbing gear. I’ll get some photos of that stuff the next time I’m up there.

Anyhow, what I have this time is another Development Basics unit: How to steal an airfoil.

The genesis for this Update is an IM session I had with a friend a few days ago, where he wished he had the coordinate set for the wing of his glider. That puzzled me for a few minutes.

I believe in the motto of Make Magazine: If you can’t open it, you don’t own it. Well, here’s how to own your glider’s wing section.

Or, at least, here’s one relatively easy and convenient way of doing it. It is by far and away not the only way, and depending on the skills, tools, and materials at your disposal might not be the best way. This is just one of several ways I can think of, and one of two ways I have employed in the HP-24 development project.

I call this method triple-trace because it involves tracing the same path using three different offsets.

First off, here are the conditions: you need to have full access to the wing in question, and you need to be able to stick stuff to it with gaffers tape or similar. If you’re working with a borrowed wing or surface (as I did when I last used this technique), you’ll want to take every precaution not to damage it.

Here’s what you need to get started:

  • Wing or surface with the airfoil to be copied
  • Drawing or drafting program that lets you import a scanned picture
  • Access to a large-format scanning service (your local blueprint firm will be happy to help you with this)
  • Fine-tip ballpoint pen with cylindrical point (inserts for retractable pens are good for this)
  • Perfectly round metal or plastic disk (2″ OD or so) with hole in center that matches the OD of the pen
  • Gaffers tape (duct tape may work for this)
  • Electrical tape (optional)
  • Cardboard or foam-core matte board, at least 6″ wider than the wing chord and 6″ taller than the wing depth
  • Several 2″ square pieces of foam, matte board, or plywood.

Here’s the plan:

Start by assessing where on the surface you want to obtain the profile. For most wings, you’ll probably want to get at least two profiles; one near the root and one near the tip. Avoid sections so close to the root and tip that they include portions of the side-of-body fillet or the actual wingtip. The shapes in those areas are usually done on-the-fly, and are often dictated more by aesthetics than by airfoil considerations.


Once you’ve chosen where you want to take the profile, apply a single wrap of gaffers tape to that area. Take care to get a nice smooth surface with the tape, without wrinkles, creases, or bubbles.


Next, take your piece of cardboard and cut a hole in it that approximates the airfoil. What you’re looking for is to approximate the airfoil to +1″/-0″ tolerance. You can get pretty close by measuring the chord and eyeballing the depth and going from there.


Next, slide the carboard down the surface until it is over the taped area. What you really want is for it to be straight and square, and about 1/4″ from one edge of the tape.

Secure the cardboard over the taped area by gluing it to the tape with a bunch of 2″ square (or triangular) chocks cut from plywood or whatever is handy. I like to use low-temp hot-melt glue for stuff like this.


Only apply the chocks to one side of the cardboard, so the other side presents a clean surface to trace the airfoil onto.


Insert the pen into the center hole of the disk, press the disk against the cardboard.


Roll the disk along the taped portion of the surface so the pen traces the path of the center of the disk onto the cardboard. Work slowly and carefully so that you get as clean a trace as you can manage. This is the first of the three traces.


Stop tracing after you have gone all the way around the airfoil. Then, trace the disk onto an unused area of the cardboard as a scaling reference.


Remove the cardboard from the wing or surface. Restore the wing surface to pristine condition by removing the gaffers tape and cleaning off any tape residue.


Take the cardboard down to your local blueprint firm and have them scan it into a .jpg or .tif file. Then import the scanning into your drawing program.


Use your line segment tool to create a curve path that traces over the traced line on the scanned cardboard. This is the second trace.


After going all the way around the path, do whatever you have to to make the trace as clean as you can. Use lots of endpoints, and zoom in real close so that the tracing is as accurate as possible. When I do this in CorelDraw, I can pretty easily keep errors down below 0.001″.


If your drawing program has an “offset path” function, use it to create a new path inside of the second tracing that is offset by the radius of the disk plus the thickness of the tape.

Otherwise, select the curve you created in your drawing program, and set its outline width so that it is the same as the diameter of the disk you used to trace the airfoil onto the cardboard, plus twice the thickness of the tape.


Use your line segment tool to create a curve path that traces the inside edge of the thick line that represents the second tracing. Again, take care to work closely and accurately. This curve is the third trace.


Delete or hide the thick line, and what you have left is a pretty close approximation of the original airfoil.


Now you can use your drawing program to do an X/Y plot of the airfoil coordinates, resize or rescale the profile, or anything like that. Those might be good topics for the next time we’ve got lots of IBS (itty-bitty stuff) going on at the shop but not much VP (visible progress).


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