I developed this technique because X-Plane still uses just one bitmap texture file to render all engines nacelles and I begun having trouble with this scheme once I started building different parts of my planes by modeling different nacelle shapes.

The Convair XFY-1 Pogo was the first model where I used this technique, taking advantage that it has two engines coupled on his nose, I used one nacelle to shape the propellers spinner and the second nacelle to shape the engines intakes (that is part of the real plane's fuselage).

I did not desire the intakes all black, and further in the plane's development I also did add Mr. Pilot "Included" that was shaped as part of nacelle #2, and by no way I would like a carbonized pilot inside the cockpit.

X-Plane always stretches the texture map to cover all the rendered part, so the idea is to divide the bmp in areas that will fit each nacelle. It is easier to do when you need just two different textures. For more than this you will have to experiment many combinations and test your luck also.

The easy approach:

1. Build your engines nacelles as usual but leaving two consecutive stations closed (that is, all nodes aligned as a single point) in each nacelle. For one nacelle you leave the two initial stations forming a line and for the other nacelle you leave the two ending stations;
   
   If your engine already start/end at a single closed point (as usual) it only means that you will need to spend one more station to set another closed point far away, hence building a hair line.
   
   
2. For nacelle #1 you make those two reserved stations a line with the size of nacelle #2 contour* and for nacelle #2 you make those two reserved stations a line with the size of nacelle #1 contour (continue reading the next block to understand the meaning of this "contour");
   
   

(*) I mean by nacelle contour a line that would conform to the external half-skin of the nacelle (traced in the above image with a blue line).

As we have no ways to know the exactly algorithm that X-Plane uses to unfold the texture bitmap, I suggest you to take the absolute value from the distance of one station to the next (the yellow values sampled above) and sum them all to obtain a first guess of the size you need for this line; it may need some tweaking depending on the shape of your nacelles (for the above example the real value would be approximate 47.0 feet, and you can see below, how it was applied to both nacelles);

3. Generate the starting texture maps from Plane-Maker and paint in the same "plane nace.bmp" file the two different engines arts, one at each half of the texture map, as shown below;
   

You can see the beautiful result in this A340 technique demonstrator. I have to advise you to take care with the size and placement of those stretched lines because they can touch the ground during takeoffs and landing procedures, perhaps causing your plane to crash down.

As many things on life go first by the difficult approach, and just further in time you infer the easy way, it was just what happened with me while implementing this technique for the first time in the above cited Pogo.

But this difficult approach can be of use whenever you need more than two nacelle textures or if the stretched line method could not be used.

The difficult approach:

Use the same method of the easy approach but without adding that stretched line for every nacelle. Do it for the ones you can (or want to), or even do not do at all. As X-Plane will apply the same texture on every nacelle and one nacelle can be different shaped from others, you can find spaces inside the texture map that is visible for one nacelle and hidden for another one.

Let's illustrate it with the example of my Pogo model: The nacelle #2, the one that has the pilot and the engines intakes, has that stretched line at its foremost part. But if you notice on nacelle #1 behind the propeller spinner, where the painting changes from black to gray tones, you will find the pilot uniform texture repeated (but it does not bother me once it will be at a hidden spot inside the body hull).

This following image is the full texture map file used to render both nacelles and the pilot body on my Pogo model:

Almost everything here done by me: Marcelo M. Marques - codename 31 M.M.M
mmarques@frontier.com.br