Plotting Wing Ribs.
Note:- Since publishing this site I have come across Dr. Martin Hepperles web site where you will find a superb free program (ConCord) that will more or less do all the work detailed below in a flash. It has the facility to accept a wide variety of co-ordinate inputs and regurgitate it in another format. For example it will take as input a .dat file and produce a .scr file in a flash. It can also scale to any dimension and has a facility to stretch in the X and Y axis. Ideal for Geodetic ribs. (stretch in the X axis only) Always bear in mind that web sites may be withdrawn at any time, and knowing how to do things 'manually' can always be used as a backstop. It is for this reason that the information given below, although largely redundant, has been retained. Log on to Martins site and scroll down to Software. The ConCord program is the second one down.
Aero Modellers who wish to produce their own wings, using the built up rib and spar structure or a foam core, are faced with the problem of plotting the chosen aerofoil at the correct form scaled to the required chord. Published coordinates produce a rib of unity length (1) and must be scaled as required. This form is usually in the shape of two alloy templates for simple straight or tapered wings, or a set of individual templates for an elliptical wing where every rib is a different chord. Further complications arise with Geodetic or Lattice rib structures where the rib is 'stretched' in the X axis (length) whilst still retaining the correct form (depth) in the Y axis. Even if you wish to produce the ribs on a CNC Router, full size drawings or correctly scaled coordinates are required as the software cuts to this data. 'Step 4' besides producing a CNC milling/routing machine also market 'Wing Designer' software. If you need something more sophisticated try a commercial Foam Wing supplier or the Compu-Foil 3D software. See links page under Construction & Design. There are a few suitable free programs on the web, try Michael Selig's site as a start at http://www.ae.uiuc.edu/m-selig
The information given below will enable modellers to produce very accurate, but simple plots, of virtually any known aerofoil using the extensive coordinate databases available on the web. See links page Even without any co-ordinates a simple drawing taken from a plan or magazine can be scanned to produce a "raster" image. See use of scanned raster images in AutoCad below.
At first sight the usual set of aerofoil coordinates appear very confusing, converting these to actual ribs, even more so. There are a number of formats used to present aerofoil plots, the two most common types give coordinates that run from the Leading Edge to the Trailing Edge for both upper and lower surfaces, and data that run from the Trailing Edge to the Leading Edge and then back to the Trailing Edge. If you intend to do your plots manually both types require further mathematical work to 'scale' the basic coordinates into an actual rib of the required chord, whilst at the same time retaining the essential aerodynamic shape. There are a number of books on the subject 'Model Aircraft Aerodynamics' by Martin Simons is one of the better ones, though by no means the only one.
Some of the more recent Computer Generated Aerofoils have 140 plot points, 69 upper, 69 lower with point 1 at the trailing edge and point 70 at the leading edge. Converting these to the required chord and then plotting manually takes hours but will give results as near perfect as possible. See also scanned images.
If you click on one of the sample aerofoils below you can download a very simple Excel Spreadsheet that will do all the required calculations for you in an instant. All you need to input is the chord required (input into the RED cell) and the spreadsheet will calculate the upper and lower coordinates for any chosen chord in an instant. One of these sample spreadsheets can be used as a 'template' by inserting fresh coordinate data as required.
You still need to input these figures into your favoured CAD drawing package (AutoCad etc.) which will then plot out the rib form full size for the chord selected. Units are not involved if using AutoCad until you do an actual plot, it is only at the plotting stage where you select mm or inches. The finished plot is essentially a collection of straight lines joining the coordinate points (in theory they should be very slightly curved) but in practice this is irrelevant. No one cuts balsa to a thou, not even with a CNC machine. ConCord plots to correct dimensions. Using a scanned raster image by-passes most of this work.
Symetrical Aerofoils
There are very few symetrical aerofoils around but one of these is the NACA 0010, apparently used on the Avro Vulcan Bomber. With the help of an Excel Spreadsheet this basic shape can be altered to give a thicker or thinner symetrical sections and even to alter the shape top and bottom whilst still retaining the basic co-ordinate base. Using the given spreadsheet input the length at C4, top thickness % at C5 and lower % at C6. By inputing 100% top & bottom you get the basic shape. Once you have the X & Y co-ordinates required save these results as values. (Cut/Paste Special/Values) and then save as a .csv file to a convenient location in Word etc. Alter to the basic Script format and save as a .scr file. AutoCad will then plot this shape. See plotting .scr files below and some examples.
Plotting Geodetic Wing Ribs.
Modellers wishing to build a Geodetic wing will need to modify the standard aerofoil by stretching the wing rib X axis by an amount dependent on the angle of the rib to the fuselage centre line, whilst keeping the rib depth Y axis the same at any given chord point. The columns on the extreme right of the spreadsheet do this calculation for you. First input the normal chord at C4 (input into the RED cell) then input the angle of the rib to the fuselage centre line at K4 (input into the RED cell) You will notice the depth coordinates do not alter, but the rib length coordinates are stretched as a function of the rib angle. Plot as above.
Note:- You will need a spreadsheet (Excel) and suitable CAD software (AutoCad) installed on your PC. Failing this you can always resort to good old graph paper.
AutoCad Users.
(See also Martin Hepperle's program.)
Raster Images
AutoCad has a very useful facility that can be used to produce very accurate wing ribs, or for that matter any drawing you can scan into your PC. This is the ability to input "Raster" images. A raster image is usually in the format of a .bmp, tif or jpeg file. If you have a drawing of an airfoil you wish to use, (any size will do as long as it is accurate) scan this and save the image into a convenient file location. Open up AutoCad and go to the Insert Menu then to Insert Raster Image. You will be asked for a rotation and scale. Leave Rotation but scale up by x25 as the image usually comes out so small it is microscopic. The raster image can now be seen on your AutoCad screen. It may still be very small so the next step is to produce it as a unity length drawing. ie 1 unit long. Measure the length of the raster image and scale as required. Make sure that the image is not at a slight angle as it is only scanned in. Rotate the whole image as required. Lets assume that the raster image is 0.078842 long. Scale up to 1 by multiplying by 12.68359504 This will give you a rib 1 long (exactly) Super accuracy is required as you will scale this drawing up perhaps 500 or 600 times so even a slight error will be magnified. Now open up a new drawing layer, this is most important, and copy the raster image onto the new layer. This is easily done by enlarging the raster image so that the outline is at least 1mm wide using the Zoom function. With the Line function and the mouse draw round the image with very short lines, the more the better. With care you can spot inside the 1mm outline. When you have gone round the full outline you can delete the raster image. You are left with an AutoCad drawing of the airfoil 1 unit long on the new layer. By measuring the models wing chord at the rib station required, you can determine the length of each rib. Say we require a rib 476mm long. Just scale the unity rib by 476 and print. Hey presto you have a template for a rib 476mm long. Repeat as required. This gives perfect results for odd wings such as Ellipticals etc. Even cheap printers produce drawings to very accurate limits. Much closer than we need. Spar slots must be put in later as required as they will scale up along with the rib. You can draw a simple single line to note the spar position if required.
Reproduction of Scanned Model Aircraft Drawings
You can use exactly the same procedure above to produce very accurate "copies" of virtually any drawing that you can scan into your PC. I use it to produce AutoCAD Model Aircraft Drawings from the A4 size plans in Aeromodeller etc. Once you have a raster image on your screen, scale as required to give the span you require for the finished model. Do not forget to open up a new layer to do your "tracing" otherwise you will get into a real mess. Takes time, but the results are as perfect as you can get. With care you can get to within the line width of the original. This method is also very useful for converting 1/72 scale outline drawings up to a full size for an RC Scale model. Where complex curves are involved this is possibly the only way to get a 100% accurate result based on the original. If the original is too large for your scanner it can be scanned in sections and joined up later. Difficult but not impossible.
Printing your AutoCAD Model Aircraft Drawings
Once your full size AutoCAD drawing is completed you can take this to your copy shop on one of those little USB Memory Sticks, who will print as required. I have had trouble in the past as errors seem to creep in. I now save the AutoCAD files as a .pdf file (Portable Data Format) and take this to the print shop. Some print shops can do this for you and for some reason this method gives perfect results every time.
Plotting AutoCAD Script Files or .scr
AutoCad also has a facility to plot from a script file (***.scr) You will find below some sample script files. If you download these they will run on AutoCad and plot an aerofoil in seconds. In AutoCad go to Tools, Run Script... and select the correct .scr file. See below how to produce a .scr file.
The **.scr file will plot a 'unity' Aerofoil, ie: it has a length of 1. - 1 inch, 1 foot, 1 mm etc. Use the 'Plot to Scale' facility to produce a rib of the required size on your normal printer or plotter. Set the plotter to inches or mm and scale as required. 10 to 1 will produce a rib 10 "or 10mm long etc. The 'stretch' facility in AutoCad can be used to produce Geodetic ribs. The results are virtually perfect and accurate to within +/- 0.5mm. This one unity plot should be saved, as it will produce any rib size you may require in future. You will of course need a separate plot for each aerofoil design (shape). Once you have a selection of .scr files in your 'database' it is interesting to compare various contours. Leave AutoCad open, and run a 2nd .scr file. This will overprint to the same scale showing in great detail the differences between say an Eppler 211 and a 212. (not a lot)
If you wish to use an AutoCad file on a CNC machine you will need to plot the rib full size in the correct units. This is best done by plotting with a .scr file that has been scaled using the .xls file, the data extracted, and converted into a .scr file. Very hard to explain but quite easy once you try it.
Producing a script (.scr) file.
Select your rib profile from the Aerofoils database and download the coordinates into Microsoft Word as a text file. (.txt) This needs to be modified manually to the same format as the sample files. Delete any textual information, Name etc. and reformat to the samples. You can save some time by using the Edit, Replace facility. The file must start with Osnap none followed by Pline on the second line then two columns of numbers (comma separated) with a leading zero. 0.12345,0.23456 etc. NOT .12345,.23456 No spaces or tabs allowed. See the sample files below.
NOTE. Microsoft does not recognize .scr files as 'script' files. (it sees them as screensaver files) They are therefore saved on the web as .txt (text) files. Download as a text file and simply change the extension to .scr. Use this file to run the script application in AutoCad. AutoCad will plot the aerofoil, usually very small in the lower left hand corner. 'Zoom to Extents', add the name etc. & save as required for use next time.
An alternative method is provided by the superb ConCord Program by Martin Hepperle on his "Aerodynamics for Model Aircraft" web site. This little gem enables you to convert virtually any coordinate input to almost any output, even into an AutoCad .scr format. It has a 'stretch facility for Geodetic Ribs, besides this program there are loads of other excellent Model Aircraft information. Click on Software for a selection of free downloads.
The .scr file is more or less a 'comma separated file' and can be used by Excel as the 'External data Source' Delete the Osnap none and Pline before using.
Choosing an Aerofoil
There is a very good site http://gliders.dk/choosing_airfoils.htm with some excellent advice regarding Aerofoil selection. Mainly for the Glider Guiders.
Aerofoil Excel Files.
See note above regarding .scr / .txt files
MH 32 Electric Pylon Racing and F3B by Martin Hepperle see gliders.dk above for a full write up on this and many other popular foils.
Script file MH 32 Note that the coordinates on the www.nasg.com site give a slight error at the Leading Edge. These coordinates here are Martin Hepperle's own and are correct.
RG-15a This is a slightly modified RG-15 by Ralph Girsberger. Currently very popular with German Sailplane modellers.(see article in 'Aufwind 5' Sept/Oct 2005)
Eppler 211 A more recent Eppler design.
Eppler 387 Semi aerobatic slope soarers. Do NOT remove the undercamber.
NACA 0010 Fully symetrical as used on the Vulcan Bomber
Script file for Selig 3021 - 095 - 84
Clark Y For 30% flaps Standard aerofoil used in many model aircraft designs.
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Text © Colin Usher 2009 Illustrations © Colin Usher 2009
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