I am an aerospace engineer and I use CAD daily. I use a program called Pro/E (http://www.ptc.com). This is $30,000.00 software/license. But, you can get student versions for $20.00 and they have all the functionality. I think they are limited in that you can not output certain types of files like iges, dxf, step, neutral etc. You can print drawings/plans. It has a fairly steep learning curve as well... not anything difficult, but there is so much functionality! It is regarded by many as being superior to Catia. There are some lower dollar CAD packages like solid edge, solid works, and Cadkey. For typical model plane type projects, they would be OK. I am building the plane in the attached photo... It is a 72" YAK 55. I am going to have the fuselage master CNC cut. So in this instance I am leveraging the ability of Pro/E to directly import the part file into PTC's CNC software with no "Translation" into a different file type. I have seen several AutoCad lite advertised, but I can not comment on these.