Warning, HUEG post imminent.
Ok, before we get to the UV mapping part, some info for Dekker: Your 3ds model has some nasty geometry errors (duplicate polies and other things I don't even know what happened there) in the areas marked below, but don't worry it's fixed now. I also did some other cleanup, made the cockpit and the "hooks" part of the main model and applied smooth groups... it's still not the most efficient model, but to explain UV mappig it will do nicely.
Now, let's get down to business.
Step 1 - We start with the upper part of the main hull, select the faces that are (close to) horizontal and do a planar mapping along the z-axis (= from top down). Observe the yellow rectangular outline on the model showing the mapping plane and the result on the right side. Note: the model could also be mapped so that it's mirrored along the middle, but I find the resulting seam ugly, since it really stands out when you do some scratches/weathering on the texture. So I usually go the non-mirrored route on the central parts (the left/right flank I usually do mirrored, since you never see those parts simultaneously on screen)
Step 2 - Next come the flanks; as mentioned above, select both the right/left parts and do a planar mapping along the x-axis (straight from the side). So far, so easy.
Step 3 - Now comes the first tricky part, the faces between the top and the flanks. Those faces are roughly at 45° between the x and z axis, so an orthogonal mapping would result in serious pixel-stretching, which will give you eye cancer. So what do we do? We select the faces in question and perform a planar mapping to the average normal of all the selected faces, again watch the yellow outline showing the mapping plane.
Step 4 - Now if we look at the top part and the part we just mapped side by side, we see that they fit more or less together. At this point you always have to make a decision: Do you want that part mapped seamlessly or without any stretched pixels? The answer always depends on what you plan to put on the texture in the end, things like panel lines you can usually line up pretty easily on the texture so that the seam is not visible. On the other hand, heavy weathering and other random patterns are very hard to match manually on the texture.
In this case I went with the seamless option, since the resulting pixel-stretching is very moderate and hard to see.
Step 4 - The bottom is relatively easy, select the faces and do a planar z-axis mapping again.
Step 5 - The rear end is a nice example of the paper model approach. The flat part straight to the rear is mapped planar along the y-axis (straight from behind). The parts around it were all done in the same way: select the faces and do a planar mapping along the average face normal, scale/rotate the result so that it matches to the central part and then weld it onto the central part. The identical parts on the left/right side were stacked on top of each other.
Step 6 - The little face there is also part of the main hull (and not the out pods), so we map it planar to its face normal and scale/weld it to the rear section we just mapped before. Also the corresponding face on the front of the wing were done like that.
Step 7 - Ok, let's see what we have so far; we scale the 4 resulting "islands" correctly in respect to each other, arrange them in somewhat efficient way (for now) and park them somewhere outside the 0/1 UV space so they don't get in the way until we need them again.
To check if the scaling is ok, we apply some checker pattern... seems ok so far.
Next up, we do the cockpit, the pods and the other little do-dads, so stay tuned.
