The printing of the templates was described in a previous post. The templates are glued (Pritstick works OK) to some thick cardboard ( about twice as thick as that from a cereal box). The card template is carefully cut out with a modelling knife. You will need three. Base, middle and tip. The edge of the template is covered in a strip of adhesive backed copper foil tape about 5mm wide. (Type in “Copper foil tape” into Google.) This produces a more solid and smooth edge to the template. They will only be used a couple of times so they don’t have to be over engineered.

Using the hot wire cutter, cut out the 4 wing blocks (inner and middle for each wing) from the foam. Measure and draw a line all the way round each block about 2cm from the base. Using dressmakers pins (ones with plastic heads) pin the template to the block, lining up the line in the centre of the template with the line on the block. The templates will overlap the block by the amount equivalent to the leading and trailing edge. Be careful that you get the templates the right way around. Yes! I’ve cut wings with the trailing edge at the front! I stick the inner and middle panels together using UHU-POR glue (Copydex will also work). When they have set I sand the leading edge to produce a gradual sweep. I saw part way through the leading edge quadrant to make bending easier. I glue on the leading edge with epoxy and use masking tape to hold it in place. The trailing edge is glued on with epoxy and the ailerons are held in temporarily with masking tape. A block of foam is cut to the approximate size and glued with UHO-POR to form the tip. It is then carved and sanded to shape. The back edge of the wing and aileron are then carved and sanded to shape. Try to make this edge as thin as possible without losing strength.

First decide on the airfoil section you want to use. Because I mostly design and build PSS aircraft I make use of articles posted at http://www.pssaonline.co.uk/

For this exercise I will choose the Selig airfoil S3021.

Go to http://www.ae.uiuc.edu/m-selig/index.html follow links to a file containing the coordinates for the S3021 section name S3021.dat. Clicking on this file name automatically downloads the coordinates into a text file on my computer. Open a new Excel spreadsheet. Choose “Data” from toolbar, then “Import External Data”, find your file s3021.dat. In “Text Import Wizard” select Fixed Width option and defaults for next screens. s3021.dat is now converted into a two column spreadsheet in column A and column B. Delete any rows that contain description (ie the top row). For safety, save the spreadsheet as s3021.xls but do not close it.

From the plans of your model determine the width of the wing at your required chord location. Let us say this is 250mm. Enter 250 into location F1 of your spreadsheet. Select cell C1 and enter the formula =(A1*$F$1), select D1 and enter formula =(B1*$F$1). C1 and D1 should now show the result of this calculation. Select C1 and copy (under Edit option), then select C2 and the rest of this column by dragging the mouse. With this column selected click paste (under Edit option). The whole of column C should contain the result of multiplying the column A value by F1. Repeat the process for column D.

Now to plot the airfoil section.

Select columns C and D (both columns highlighted). From menu select “Insert” and then “Chart”. The wizard should appear. Select XY(Scatter) chart type and chart sub-type “data points connected by smooth lines without markers”. Hit Next and you’ll see a picture of the foil section. It’s no good, the X and Y axis need transposing. Click on the “Series” tab and on the lines where you see C change to D and change D to C. Hit Next and the section is drawn up the page. Hit Next and you are asked if you want to save as new sheet. Choose this option. Hit Finish. Great, but will it fly. I doubt it.

Now the tidying and the trial and error bit.

Make sure you are on the chart page. Select “File” and  “Page Setup”. Set page to Portrait (not Landscape). Make top and bottom margins as small as you can get away with (0.3 worked for me) and make Header and Footer zero. Right click in the Plot area and choose “Format Plot Area”. In the “Border” box choose automatic and in the “Area” box choose None. Click OK.

Right click the Y-Axis (the vertical one in the middle) and choose “Format Axis”. Pick “Scale” tab, un-click Auto minimum and set the value to zero. Click OK.

Now the trial and error bit.

Unfortunately I don’t know if it is possible to define an absolute measured grid on the page in Excel so I use the “Print and Pull” method. Click the Plot area to display the sizing handles. Pull in the right hand side until the airfoil looks about the right shape and then print it. Check the actual measurements with reference to columns C and D. Pull and squeeze and print again. Save it when you get it right.

To produce a section with a different chord, simply change the value in F1 and repeat.

The paper section will be glued onto the template material to be used in the Hot wire wing cutter.

Fig a

These are my basic materials. Trailing edge balsa stock, Quadrant dowel, Brown parcel paper and Extruded Polystyrene foam.

I had been made redundant when I embarked on the building of the Spitfire and I was looking for the cheapest materials I could find. The trailing edge stock was used for ailerons (and trailing edges) as I felt this would be stronger than using foam. The quadrant I used for the leading edge of the wing. It needs a bit of work to create the correct profile but it’s pretty strong and I find that I can do away with any further wing reinforcing. I know it’s a bit weighty but luckily this weight is in front of the CoG. Brown paper is the lighter weigh version used for wrapping parcels, obtained from the Post Office or those “everything for a pound” shops. The lighter grade is preferred because it’s easier to form across curved surfaces. It has a “grain” like wood. It is easy to tear with the stripes but much stronger in the other direction. For this reason when covering wings the stripes go from root to tip and for the fuselage from front to back.

Fig b

Fig b is the specification sheet for Extruded Polystyrene. It has several trade names, “Styrofoam”, “Floormate”, “Wallmate” and also called “blue foam” (except mine was pink). I used the lighter grade of 30kg/cubic metre. A board of 50mm thick by 600mm wide by 2438mm (about 8 foot) long cost about £8.00 (2006) from Encon Insulation, Gelderd Road, Leeds, LS12 6DP. Take a wood saw with you, and you can cut it in half in the car park, so it will fit in the car. This foam is slightly heavier than Expanded Polystyrene (white foam) but is stronger and much easier to shape by sanding. Beware! Wear a mask when sanding. I got a headache the first time I sanded it and I now always wear a mask. The main disadvantage with pink foam is that it “dings” easily. If it gets a knock the mark will be permanent. The brown paper covering will help in to alleviate this to some extent.

Fig c

This is what happens when you try to build an Me109 using white foam. Attempting to reinforce the inside of the cockpit with brown paper didn’t work. I have used two plywood doublers inside the fuselage over the wing roots in the Spitfire. When I asked an old hand on the slope how I could stop my planes from breaking over the wing root, his advice was “don’t crash them”.

Fig a

Fig a shows the plans I used for scratch building my 1/10th scale Spitfire. They are 1/72nd scale and can be obtained from http://www.aviation-news.co.uk/aircraftplansservice.html I do not scale up the whole drawing but make rough sketches with the new 1/10th scale dimensions written on. It is useful to have a calculator handy so that the dimensions taken off the drawing can be quickly multiplied by 7.2.

I made more accurate 1/10th scale drawings of the Fin and Rudder, and Tailplane profiles as these were used to “pin-through” the outline onto balsa.

I photographed the cross-section drawings in the bottom right hand corner with my digital camera and after manipulating them with “Word” was able to expand them to 1/10th scale. After expanding these small drawings by 7.2 their outline becomes a little indistinct but they are good enough to make a reasonable template for the hot wire cutting. “Word” seemed to give me the best option for dragging and expanding the image but I printed, measured and re-dragged a couple of times before I got it right.

The templates for cutting the wing sections were produced from coordinates downloaded from the web, calculated and graphed in “Excel” and stretched, dragged and printed in “Word”. More detail later.

Fig a

Note:- Click on image to get a more detailed picture. Double-click to zoom.

Fig a shows the cutting bow with the wire stretched between the lower ends of the 2 vertical supports.

To perform hot wire cutting you need  an electric current high enough to heat the wire. The transformer from a mains powered high output current car battery charger is ideal. They can output a current of 4 or 5 Amps at a low voltage of about 15 volts AC. I salvaged my transformer from an old charger inherited from my late father-in-law. Some method of varying the output current will also be required.

Mains voltage is lethal. Do not attempt to wire up a mains transformer unless you know what you are doing. There is a real “Danger of Death”.

The output current is fed into the hot wire cutter shown in Fig a.

The cutter is made from 3 pieces of wood or dowel. The 2 vertical pieces are attached at their centres to the end of the horizontal piece. They are free to pivot at the attachment. I screwed 2 small screws into the bottom ends of the verticals and soldered the feed wires, one to each screw. A length of constantin resistance wire was then solder to these 2 screws. This can be obtained from Maplins http://www.maplin.co.uk/module.aspx?moduleno=2196#features. A loop of nylon string is threaded through holes drilled into the top of the 2 vertical supports. A length of dowel is inserted into the middle of the loops of string and the string wound up and tightened. This action in turn will put tension on the resistance wire. It should be tight enough to make a clear ping when plucked. Alternatively, wearing eye protection, tighten until the wire snaps. Then you will know how tight is too tight. Replace the wire and don’t tighten it quite as much. To keep the tension slide the winding dowel through the string so that the lower end rests on the side of the horizontal bar. Tape the electric feed wires to the frame, out of the way.

Spitfire banded to top of camera tripod. Bedroom wall as background. Will airbrush out tripod and paint some clouds in the background when I can get the hang of Photoshop.