Updated 10 Aug 2003

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A Wing in Rhino part 2

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11.  Now we have an accurate curve of our wing section, we must scale and rotate it into its correct location within our 3D model space. Usually a background bitmap image of a drawing will be used to do this, as we set up in our first Rhino tutorial. Looking at the Right view, we are lucky as the drawing of the Martin Baker MB5 we are using shows locations for both the wing root and tip sections, so we can simply maneouver our pair of curves into place. Mirror the curves about a vertical axis to get the aerofoil facing forwards correctly. Then drag it so the trailing edge lines up with the bitmap image, then scale the curves from that point so that the length matches as well. You should end up with something like this, with the aerofoil shown in yellow.

12.  Immediately we notice a couple of points. Firstly, while the length is OK, our section is not as ‘fat’ as the drawn image, and secondly it is rotated a couple of degrees clockwise relative to the drawing. These are not errors, but show very well some more aspects of aerodynamic design.  When an aerofoil is defined, the points along each curve have a specific relationship to each other, but the curve can still be ‘scaled’ in the y axis. The degree of ‘fatness’ is given usually as a percentage of the length measured at the sections thickest point, so a 12.5% section would be 8 times longer than its maximum thickness. So, to fully define a wing section you need to know the co-ordinates of the master section, and its thickness ratio. On our model we can simply do a 1D scale operation, increasing the depth of the section without changing its length.

13.  The section is still not located correctly as we have to rotate it a little anti-clockwise to line up with the drawing. The section co-ordinates are arranged as if the line from leading to trailing edge is exactly horizontal. However this might not be the optimum location on our aircraft. Both lift and drag change greatly depending on air speed and angle of attack, so the wing will usually be tipped up slightly relative to the aircraft centre line. This will give minimum drag at high speeds, vital in this case, for a high performance fighter. Rotate the section about its trailing edge so it lines up with the drawing. It will look like this, if all has gone well.

14.  When we looked up the aerofoil data for the MB5 we found out that the same section is used at the root and tip; it is not unusual for them to be different, so do check this when modelling other types. However, in this case we can simply cut and paste our root section, locate it over the drawing like this...

.... but this time we find the section is thinner, and rotated the other way! Again, the drawing is correct, and you can easily reduce the thickness and rotate to match the drawing. Why is this, however?

15.  The change in relative thickness will allow the root of the wing to be deeper, and therefore stronger, at the expense of a little more drag. Also the extra depth gives more room for undercarriage, radiators, guns etc. The thinner outer section reduces drag and weight. The apparent twist in the wing has several subtle effects, the most important of which is to delay the point at which the wing tip stalls at high angles of attack. If the aircraft is pitching up and approaching a stall, the root of the wing will start to stall first, and the pilot will probably notice some buffeting. The outer part of the wing, being at a slightly shallower angle, will continue to lift allowing the pilot to keep control.

16.  We now have accurate root and tip sections for our wing, but they are currently on the Right view construction plane, in this case on the aircraft centre line. We will have to move them outboard to the correct locations in 3D space. On to Part 3