Take a look here https://github.com/Microsoft/AirSim/blob/master/AirLib/include/vehicles/multirotor/api/DroneApi.hpp
Please read the comment of the class…
The Api is meant to be executed on a Worker thread to not block your game.
From what it looks: you didnt have to expose RPCclient to blueprints
but you had to work with the DroneAPI which seems have done everything for you.
If this is not the case - then make your own thread and synchronizations.