I'll second what Erich has written. As an aviation artist myself (published, and with original oils and pencil sketches sold, and hanging, worldwide,), I ALWAYS seek a story for each of my paintings. From there, I produce a (very) rough sketch, in order to give me an idea of how my visualisation of the scene will translate onto canvas; this 'rough' is used to discuss, with the client, such things as angles, lighting, movement etc. The next step is a detailed drawing, called 'the working drawing', which is how the finished picture should look, within about 95%. This is a fully-toned pencil sketch, normally to A3 size and, in it's own right, is a piece of individual artwork. Again, this is used to finalise details with the client, and to gain final approval, before putting brush to canvas.
Although I will consult photographs in order to ascertain fine details, placement of certain features and so on, they are only used as reference. If I don't have an actual aircraft to use as my model, which, of course, is rarely possible, especially for airborne subjects, then I use a model, normally to 1/32nd scale, which I can pose as required, in order to get the correct lighting angles, perspective etc.
It may well be that the original sketch for the working drawing is made using this model. Sometimes, but by no means always, this sketch, or at least the outline of the main subject, might be enlarged, in order that a trace of the OUTLINE can be made, for transfer onto the canvas. This is only the outline, and serves as the basis for the composition and perspective of the picture. The rest is down to me - drawing-in the basic curves, shapes etc, before painting the background, and then the subject(s).
Although I might be inspired by a photograph, or another painting or print, I never copy these, although I know of at least two occassions when artists have copied MY work.
Working this way, I know, and the VIEWER knows, that the work is totally original, done by hand, without any 'artificial' aids.