I've gone the carry-on route for ~30K miles so far this year. Batteries, props and all. I use the drone trekker softcase backpack.
Drone Trekker Does an amazing job. I also use some soft silicone motor mount protectors for added protection of the plastic prop attachments. I do think this backpack fits a bit too snugly to not have those protectors. Just my opinion.
I've taken this on-board aircraft on multiple US transcontinental flights, no issues whatsoever with domestic US TSA. Do note that, on smaller regional jets there is no way that it will fit in the overhead. So be aware So be aware of what type of acft you are flying. On the smaller regional jets, I've still managed a carryon- there I will try to book an exit row seat or an aisle seat and plan for the backpack to be a very tight squeeze underseat and to take up most of my legroom.
I did run into an issue once on a small regional flight from Frankfurt, Germany to Luxembourg-- In Europe, you are not allowed to put
anything under your seat in the exit rows. Fortunately, there was an empty seat a few rows back so I tucked my drone backpack under that empty open seat.
Otherwise, I've taken the p4 thru airport security (without any issues) as a carry-on in: Germany, Luxembourg, Croatia, France and the US. Only once was I asked to open my backpack and remove the batteries for inspection (I did not have anything taped, wrapped or covered). Security just wanted to look at the model of battery-- while the agent was inspecting the batteries (I carry two) a supervisor (this was in Croatia) looked over and said, "Drone. Is OK. Yes. Let him thru." and that was that.
No one has ever questioned the props (they are effectively plastic silverware).
I will say, I think the soft-case backpacks also
stand out a bit less than the hardshell. I chose it with this in mind, so that I looked like less of a target for a robbery while traveling abroad. Having the backpack (as opposed to a hard sided camera case to check under the plane) once in-country also makes it a heck of a lot easier to take out with you on hikes and adventures. Have fun!