An attached texture is a necessity for importing into real flight. It can be a solid color and must be square divisible by 256.
So 256x256...512x512 and so on. with 4096x4096 being the largest for most models.
It calls it a kex because the import process converts your fbx to a .kex and it errors out because of the missing texture.
Proper naming is also expected. ~CS_ is a prefix to consider a part individual and breakable. Unnamed parts will be considered a part of it's parent changing the parent's shape in the editor.
The exception to the prefix rule is "Fuselage" It is the root frame therefore is automatically considered individual and it is the parent of all other parts.
So with a multi rotor you will have a Fuselage parent to the number of arms each of which will be named ~CS_Tailboom1 through the total number of arms present. Those will be parent to their own corresponding ~CS_EnginePivot and finally as children of the EnginePivot you have ~CS_Spinner and ~CS_Engine each of which will have the same number suffix as the tailboom they belong to.
the part of your model that will visually rotate will be your ~CS_Spinner objects. E.g. motor bells and prop nuts
~CS_Engine is a piece of geometry that will not render in the game and it's pivot serves as the position to show the prop. The prop is a part added by the sim and not modeled within a new model.
It is possible to model your own propellers but they are imported into RealFlight as a separate file to be used on any aircraft you choose.
Pivots for the model must be correct to Real Flights expectations with the model facing to positive Y axis and Z up. Pivots can have some rotation on any axis as in the case of ailerons on a dihedral wing or even swept back.
You will need 3ds max or Blender to allow for precise pivot alignment. Fusion360 will not work for this purpose.
Heirarchy or Parenting Is a process of linking objects in a specific rank telling RealFlight how you want the model and it's parts to follow one another within the sim.
Again you will need 3ds Max or Blender to properly set this property of your model.
Exporting as FBX has a few settings to be successful.
Smoothing Groups
Units - Automatic
FBX version 2014/2015
Uncheck
TurboSmooth
Split per-vertex Normals
Because of these settings I am not sure if Blender exports 100% correctly. Someone familiar with Blender could comment to that.
Textures:
Diffuse texture or main color map can be named whatever you wish and must be saved as a .TGA up to 32bit works well and RLE compression also works. Example: QuadAwesomeCS.tga
The next two texture maps are not required but improve the models visual impact
All maps must be multiples of 256 and square but each map can have different resolutions
Normal Map must have the same name as the diffuse texture with _n suffix Example: QuadAwesomeCS_n.tga
Specular Map also must have the same name as the diffuse texture with _s suffix Example: QuadAwesomCS_s.tga
These textures must be in the same folder as the .FBX file for import into RealFlight