From .dat to foam. Make your airfoils

At fabLAB Asturias, we've been experimenting with airfoil designs for our scratch builds. Our first designs used simple airfoils like the Armin Wing from ExperimentalAirlines or the ones from the FliteTest crew.  Then we wanted to try how different airfoils changed the way a foamy flies, but we also wanted to stay with the simple techniques of the foamboard and the laser cutter. So we started to search for a compromise solution, and designed a procedure to take a true airfoil and make a foamboard one that is close to the real thing.

For this we use InkScape, a free and opensource vector drawing program. It's a great tool and we use it for several things at the Fab Lab. You can get it at http://www.inkscape.org. But if you're used to another vector drawing program such as Adobe Illustrator or Corel Draw, you can adapt this techniques to it.

First we start with a real airfoil. You can get airfoils from the AirFoil Tools database at http://airfoiltools.com. For this example I'm going to use the s7055 airfoil, a nice airfoil that glides nicely at low speeds. So we go to the airfoil tools database, search for the s7055, and then click on the "send to airfoil plotter" link at the lower right of the airfoil picture.

Then we click on the "SVG image as text file" link to download the SVG file (a vectorial format) of the airfoil. And then let's open InkScape.

Go to File->Open and select the s7055-il.svg file you've just downloaded. First we're going to delete everything we don't need. Select everything dragging with the mouse and hit CTRL+SHIFT+G to ungroup everything (perhaps you'll have to hit that several times until you see the selection boxes around every line. Then delete everything (selecting with the mouse and pressing DEL), but the airfoil and the blue chord line (I leave the chord line as a reference for the neutral angle of attack).

You shoud have something like this:

Now we can make our airfoil the size we want. Select the units you like to work with, be sure that the lock is closed, and enter the new width of the airfoil. I'm going with 190mm.

Then, as we can't make that nice and smooth curves in foamboard without a lot of work, we are going to make an approximation to the real thing using several flat bends. Select the red airfoil and copy it with CTRL+C, CTRL-V. Then select the Bezier tool and start drawing straight lines above the airfoil, trying to follow the shape. Just click at the starting point, then move the mouse and click again at the ending point of that segment, then you can continue adding segments, until you reach the starting point. In smooth places you can make long lines, but in steep curves you'll need to make shorter ones. The result after removing the original red airfoil will be something like this:

Now we are going to mark the transitions between segments with perpendicular lines. I usually make a copy of the current step and work on the new one. The lines don't have to be perfectly perpendicular, but try to do your best:

After this, it's time to draw the real shape of the foamboard. First we have to take into account the thickness of the board, here in Europe we usually use 5mm foamboard, so we are going to draw a 5mm diameter circle with the circle tool (draw a circle and then resize it to the thickness of your material, 5mm in my case), and copy and paste it inside the airfoil above the perpendicular lines like this:

The two last ones aren't above any perpendicular line, but they'll help us for the next step, drawing the inside of the foamboard. Select the Bezier tool again and connect the points where the circles meet the perpendicular lines in the inner side of the airfoil. The leading and trailing edges of the wing are a bit tricky. For the leading edge you'll need to remove foam for making that acute bend, so we are going to observe just the thickness in that point (touching the red circle). For the trailing edge, we are going to draw the foam board as it is, square, then we can make a bevel cut if we want when we are building the wing. The result:

And after cleaning, the section of our wing will be something like this:

And using the same technique you can draw a rib that will go inside of the wing to define it's shape. You'll need several of them depending on the length of your wing. Don't go all the way to the front or to the back with the ribs:

But before building anything, we need to take this design to a flat foamboard, so we are going to measure each segment. For this, on the previous step (before cleaning) select the Bezier tool again, and starting on the INNER face of the trailing edge, click and then move the mouse to the end of that segment. If you look at the lower bar of the inkscape window, you'll see the angle and lenght of the segment between the previous point and the position of the mouse. If the lenght is in pixels, go to File->Document properties, and change the Default units to the one you like best.

So now write down that value and continue writing down all the lengths of the inner segments.

Now we are going to take that measurements to a flat line. First we need to learn how to use guides in inkscape. You can insert a guide line in inkscape just clicking on one of the rulers and dragging to where you want the guide. You can make vertical or horizontal guides from the vertical or horizontal rulers. So we are going to make a vertical and a horizontal guide in another place on our document and starting from the intersection of the lines (you can snap to the intersection), we are going to draw a rectangle line with the rectangle tool. Draw a thin rectangle of an arbitrary length and then resize it to the lenght of your first segment. After that drag another guide to the end of the rectangle:

Now repeat for the other segments, drawing the new rectangles just above the previous ones using the new intersection between guides and resizing them to the length of the next segment of your list. If doesn't matter if the rectangles are not all the same width. After this, you'll have all you need to design a straight wing:

Using the guides you can draw a rectangle from the bottom guide to the top one, and the width you want depending on the lenght of the wing you are designing,  and then draw red lines (for the usual 50% score cuts) for the the rest of the guides. After removing the guides(hovering the mouse above them until they go red and clicking DEL), you'll have your base wing:

If you cut this and several ribs you can build it right now. But you can make a better design adding a spar, refining the outer shape, adding a small undercamber, deciding what amount of dihedral you want, etc. You can get the heigth of the spar from the rib you designed before. Remember you'll need to add some cuts to the spar to fit the ribs in. After designing everything, you'll get something like this:

That's half the wing we are using on our trainer design (that we'll publish soon), 1 metre wingspan and 19mm chord.

If you want to make a wing that's not straight, like tapered, swept-back, etc, you can start from the first line of rectangles at the root of the wing, copy them, and resize accordingly. For example if you want a tapered wing that's 19cm chord at it's root and 10cm chord at the tip, that's about 53% smaller at the tip. So for example for a 50cm half-wing, copy the rectangles you drawed before, move them 50cm apart, select "%" for the size and resize them to 53%. Then align them taking into account where do you want the taper. For example, tapered at the front and straight at the back:

You can use guides to align everything. Now join the leading edge marks:

As the wing will need to be folded at that line, you'll need to rotate the rectangles abobe that line so they are at the correct angle. You need to measure the angle of the leading edge from the vertical. You can use the bezier tool, start a line just at the beggining of the leading edge line and look at the angle at the bottom status bar of Inkscape. In this design we get something like -10.64º. So 90º minus 10.64º is 79.36º. Now with the circle tool, we are going to draw a big arc that begins at 0º and ends at 79.36º, it's just a matter of drawing a circle and defining it's start and end angles:

Now move and rotate the arc so one of its sides ends aligned with the leading edge, and its center at the beginning of it:

Now rotate and move the rectangles above the leading edge, so they end parallel to the other side of the arc:

Now move the arc to the other side of the wing and repeat the process of rotating and moving the rectangles, then, join the all the marks and draw the contour as we did for the straight wing. After cleaning the guides and small rectangles, we have the base of our tapered wing:

Remember that the rib at the tip of the wing will need to be resized to the 53% from the rib at the root (and if you add more ribs at the center you'll have to resize them accordingly), and the height of the spar will also need to be smaller at the tip of the wing.

With these techniques we've been able to test different airfoils for the same plane, our FabTrainer, and confirmed that different airfoils make huge differences for the same wing area being the rest of the plane also the same.

Now go and make some wings!!!