The files are short because they continually repeat.
Bearing in mind the props make the same noise each revolution as they spin, and the engine will also make the same noise as it runs, the short sound file with the repeat loop produces a consistent sound which only varies with speed, which will change the pitch and speed of the sounds.
I experimented with the sounds a bit myself some time ago, trying to make more individual sounds for various ships according to their size ( hull resonance/engine size/number of props etc.)
Somebody gave me the info for the name of the modding program, and a research link recording the different sounds certain ships made.
Unfortunately, because it was some time ago i can't remember the name of the program i downloaded to do it with, and having done a couple of re-installs since then i lost the mod files and reverted back to stock.
Others will undoubtedly be of more help.
Good luck with it
I'll dig the info out by going through my old posts if you want.