Set Default Entry Types for elements in a Structure Section
I'm not sure if you can set a default entry type within Craft.

But a reasonably workaround would be to go to Settings > Sections, then click 'Edit entry types' for the section you need to apply this to. Next, drag the entry type you would like to be used by default to the top of the entry-type list.

That way, when a new entry is added to the Section, the default entry type will be whichever type is the top of the list.

Hope this helps, but as I say there may be some other answers that offer a cleaner approach.