Aeronautics require accuracy and efficiency. Here are the design software made for aeronautics, that will allow you to create reliable 3D models, such as STL files.
SharkCAD Pro
SharkCAD Pro is a professional modeling software allowing to design for 3D printing, rendering, drawing or animation. With this software you will find a wide range of design tools, for surface, curve, solid and mesh modeling. This program is a good tool for aeronautics and aerospace engineers
This program is not made for the aeronautic industry, but you can add an interesting plug-in. Indeed, there is a special package available and specifically made to design for aircraft. It is called AeroPack and it is developed by DARcorporation. Aircraft designers will be pleased by these amazing modeling tools With this CAD software and its dedicated plug-in AeroPack you will be able to work on professional 3D files with a great accuracy. Also, it allows to make good 2D drawings.
ADS
ADS (Aircraft Design Software) is a design software developed by OAD (Optimal Aircraft Design). This program is perfect for aircraft designers and builders amateurs . If you are a beginner and you are just getting started with building aircraft, this design software will be perfectly adapted to your needs. It is great to work on aerodynamics and aircraft designs and it has a lot of tools to optimise the configuration of your aircraft.
Cart3D
Cart3D is an efficient software powered by NASA, specially made for aerospace and aeronautics. If you are specifically looking for a program to help you with aircraft design, you have to take a look at the tools offered by Cart3D called GoCart. It is made for aircraft aerodynamics analysis and design. It will allow to deal with geometry inputs and mesh preparation. GoCart has perfect visualisation possibilities and includes a lot of post processing options. Thanks to this design program you will be able to work on computational aerodynamics and control your aircraft stability.
OpenVSP
OpenVSP (Vehicle Sketch Pad) is a parametric aircraft geometry tool that will allow you to create 3D models for aircraft. This modeling tool is an open source aircraft design software developed by NASA. It is an intuitive tool, allowing to create models quite quickly. You can also take a look at VSP Hangar, the database of the community, where you can check other 3D models to get some inspiration.
Even if this 3D software is very easy to use, you might want a little help to make your first steps.