Create LOD levels

We now have reduced our propeller plane to only 1 mesh with 12 materials and 58,383 triangles and we're ready to generate some LOD levels for it.
Select the object in the hierarchy again and click Tools->Simple LOD in the menu. The Simple LOD panel will open again.

Make sure it says "GameObject: Propeller plane".

Set the sliders for the compression levels to the values you want to use. There is no best practise for this, I usually start with values of 0.3, 0.6 and 1.2, then see what happens and adjust them to better values for a second run. At the bottom of the window there is an additional slider for removing small parts like bolts, or shoelaces. The default value for this is 1, but when you see that small parts are remove too early or too late, you can change this.
I found that switching the checkbox "Recalculate normals" off for low compression values and on for high compression values gives the best results. But this depends on the model.

When everything is set you can click the button "Create LOD levels". And this time we will receive a warning beforehand, because this operation may take a while. On my 2013 Macbook Pro it takes about 2 minutes to complete.

When it is ready, you get a message that tells you where the assets for the LOD meshes were saved.

When you look in your Console, you can see how much was saved.

LODSwitcher

When you look in the object inspector, you will find a new component. The LOD Switcher. It is set with the 4 meshes for LOD 0, 1, 2 and 3 and it has 3 float properties that tell it when to switch between the LOD levels.

The first float of 0.6 tells the LODSwitcher to switch between LOD 0 and LOD 1 when the size of the object relative to the size of the screen is less than 0.6. The second value of 0.3 means: switch between LOD1 and 2 at a relative screen size of 0.3 etc.

Draw the slider to switch between the LOD levels in editor mode. When you activate the game view, you can switch on the toggle "Move camera with LOD" to move the camera to the exact distance at which the LOD level gets activated.

If you are not satisfied with the end result, you can change the compression level for the specific LOD level and re-run the operation.

Removing small parts

When you look at LOD3 of the propeller plane, you will see that several smaller parts have been removed entirely, like the landing gear. With the slider under advanced settings in the SimpleLOD editor panel, you can control the maximum size of the parts to be removed.
But before you slide it all the way back: Keep in mind that the LOD levels are only used when the object is viewed from a distance and small parts become so small that the user will not notice or see them anymore.