Creating graphic designs is one of my favorite leisure pleasures. For this I prefer Corel Draw as a most suitable tool. Sometimes I’m just playing around, exploring different functions and options. I’ve found out that certain tools are throughout practical for model designing applications. This has lead me to a new idea recently. To draw a plan for our next super stunter we sometimes want to interpolate a given airfoil to generate transitional ribs of different shape across the span of the wing. This process is called “lofting”. I do know that there are special model designing programs, and those CAD programs are perfect for this task. But they are quite expensive, and offer much more functions than I’ll ever need (and being able to use !).
So acquiring one of these software bundles doesn’t appear reasonable to me. Also none of these software versions includes one of our typical control line aerobatics airfoils. Apart from our well known NACA 0018 there’s nothing we can use. Yes, some of these programs can modify airfoils. But again - I don’t want to spend a fortune for just a few basic functions. Many people use Corel Draw and this software can do a lot for us. So it may make sense to explain what I did.                
   
               
  It’s the “Blend” tool which got my attention; the symbol is shown in graph 3. All we need is two shapes: one for the beginning and one for the end. After setting the number of steps, this tool draws the desired shapes in a precise linear transition between both original shapes. Of course these “shapes” can be airfoils, too. There’s only one big problem: this fantastic blend tool doesn’t
work with bitmap files. I’ve tried it, but it didn’t work. This function needs vector files! ( or maybe other types which I’m not quite familiar with ). So this means we have to get a vector file of our beloved airfoil first! We can use our own personal super airfoil or one of the better known shapes, like Nobler, Stiletto, Cardinal, Trivial Pursuit, Snaggletooth, etc.etc. I have scanned a desired airfoil , colored in a light colour, and opened this file in Corel. Now the hard part begins.

shown here are the required functions: 4= Bezier curve, 5= Shape tool, 6= line into curve tool

Corel has several tools for drawing lines. What we need is the “Bezier” tool (graph 4). I like to use a line colour with high contrast to the colour of the scanned airfoil, so the new line is well visible. I started with a click at the rear end of the airfoil outline. The next click is put at the point of highest thickness ( of course you can choose another spot ). In doing so we’ve crated a “node”.
While holding the mouse button down, the mouse is dragged down to make a curve out of the straight line. You’ll easily find out how to move the mouse to achieve the correct curve. Then release the button (8). At the same time a little “handle” appears which may help to control the shape of the curve. Repeat this process to draw the next part of the airfoil outline with another node( 8b). I used a point just short of the leading edge. The last click goes to the most forward point of the airfoil at the center line. Usually the finished outline is not perfect. We need the “form” tool (5) to modify it. The mouse pointer now looks like(10)      
         
           
  I select one part ( double click on desired spot ) and choose the tool “line into curve” (6). Now this curve is editable and gets two “handles”. With these handles the curve can be pulled to any desired shape (9).
It’s quite reasonable to draw only one half ( usually upper half )of the outline. The bottom half is easily drawn by duplicating ( Strg+D ), flip the curve vertically ( mirrored, 11), and then moved to the correct position. Both halves are selected and grouped into one unit.    
    There’s also another method which can be used. We simply apply straight lines by clicking at several points along the airfoil outline, thereby making several nodes. Later we use the “form” tool to pull the line to the desired spot. This method needs a big number of nodes between which we can alter the line. This method is easier, but it’s more work and not so elegant.
It needs some time to get used to the Bezier tool and to master the Form function. When we are satisfied with the outline, the whole drawing is saved - and this is important - as a GIF file . GIF files allow transparent areas, and this is very helpful in the following process. Which actually is our main goal.
The “Blend “ function is the tool we need for the “Lofting” process, it’s quick and easy. We just import our airfoil file, put it at the top of the screen, duplicate it, put it at the bottom of the screen, scale it to desired size, and select both. The Blend tool offers several options, but I used the most simple one only: choose the number of steps, click on “Apply” - and all is done (13). Depending on the number of steps, the result may look like (14) . I’ve included this only to show how this tool works. This page can be printed out. If one particular section ( or all of them ) are needed separately, the required number of sheets can be printed and the respectiv airfoils are cut out.
 
      Now in most cases we don’t need the airfoil outline. What we need is the actual rib shape. Well - no problem. Instead of the airfoil outline we draw the rib shape by the same method as described above. In this case we cannot simply scale down the second ( smaller ) shape from the original shape, because we would also scale down the spar cutouts. So a separate drawing is necessary for the blend (15).
So far I’ve only described the method for making a linear lofting of the rib shape, which is required for construction of a trapeze shaped wing planform. Many will not go to this effort since they produce the ribs with the sandwich method; means, clamping the required number of wood sheets between two rib patterns and shaping this whole stack. So for them all of what is written above may appear superfluous. However - as soon as the wing shape deviates from the simple trapeze, things become more complicated. This is where special software would be required. And this is where I discovered Corel Draw as a practical substitute. Let’s suppose we want to design a “somewhat” elliptical wing planform for a flapped wing. Here is what I did.  
         
  At first a vertical line is drawn. This represents the trailing edge = hinge line . Then we need the leading edge line. This may follow our fanciful inspiration or any given outline. Again this is done by using the Bezier tool. I have drawn the line, and with the second “click and draw” I’ve shaped the curve of the leading edge (16). Again the root rib is placed, duplicated, positioned as tip rib (17) , and the Blend tool applied. We still have only a straight taper (18), and all the rib outlines are still grouped together in the Blend group. We must unlock this group by Arrange/ Unlock. Only now can we select each rib separately . After doing this, we can click on the side handle and drag the airfoil till it touches the leading edge line (19). We do this with all rib shapes. In the end we have all ribs aligned according to intended wing outline (20).
Now I can already hear the cry of several shocked experts who fear the end of Western culture with their hairs standing on end.
 
   
I fully realize that this is not exactly a very scientific method. Of course I know that the thickness distribution is not in accordance with the elliptical planform.
By stretching the rib shape as shown in (20) airfoil shape is changed and thickness is reduced; thus thickness of the whole rib group is reduced linear. Looks like disaster, but I’m really not afraid of that. As long as even experts think of shoe sole contours being fully adequate for drawing airfoil outlines, I don’t feel hesitant to apply such a simple method as described here.
If for some reason the outline of one special rib shape is needed ( like a plywood stiffener ), we can easily select this particular object , remove it from the group (19), and print out separately.
Now - having already explored this funny Blend tool, I can see lots of applications of the lofting process which can make our design task easier and quicker. Think about all those shapes we might want to create. Just imagine those cases where we have to draw a line of cross sections; for example fuselage formers (21).
                                 
Draw the first and the last shape and then - just blend! Actually Corel offers many more options within this function which can perform some sophisticated tasks. It’s only my limited skills which keep me from using these. If you can control them, you’ll find an astonishing choice of valuable tools for us in Corel Draw which go far beyond what was shown in this story.
                                           
             
PS. Please apologize - the text given above may cause some confusion. My friend Jeff Reeves has discovered a problem. So I’ll add an update here.
When looking for the “blend” tool you can find it in the tool bar shown on the left side of the Corel screen. However it depends on the Corel version you are using whether it will work that way. Instead I recommend to click on “Effects” in the menue bar at the top of the screen. A fly-out menue will show the function “blend”; clicking on this will show the docker which will allow you to type in the parameters ( as shown in the graphic on the right ). Steps value is what we're most interested in. Alas the text of the graphic is in German here. Sorry if I have confused you.
More additions: when trying to draw the ribs for an elliptical wing planform, I just “pulled” the respective rib shape to the required length (as shown in graph 20 ). Thereby - of course - reducing the thickness of the airfoil. If anybody wants to create a true elliptical lofting of the rib shape, he has to click on a “corner handle” of the respective airfoil and then pull it to the desired size. Unfortunately the result of this is that the main spars will have to be “bent” in two plains, horizontal and vertical. Judging from my own building skills, I wouldn’t like to recommend this curved exercise to anybody.
Also I forgot to point out that we cannot simply reduce the airfoil shape for the tip rib, because that will also reduce trailing edge thickness. Before we use the Blend tool as shown in graph 14, we’ll have to modify the outline of the “small” rib. This is done by first “rotating” the upper airfoil line. With the line marked, the “anchor point” is moved exactly to the most forward end of the line. Now the line can easily be rotated around this center marker, thereby lifting the end of the line as much as necessary, until we have arrived at the correct trailing edge thickness ( half of it). After that we continue as shown in graph 11: Strg + D / vertical mirror, to get the complete airfoil.
( everytime I re-read my articles I detect some errors requiring correction !)