Hey! Here is the input of X-Plane creator Austin Meyer. Thanks!
Blown flaps are different than what this airplane does.
Austin: "Blown flaps put air in (or take it out out) of the boundary layer to reduce airflow separation and get higher coefficients of lift from wings. The F-4 Phantom has them, and bonus points: they make a screeching noise when activated so flybys are better with them activated. (Fun-fact: Flyby score is directly proportional to the number of pants wet by spectators as a result).
But this is not what you want here at all.
Instead you want to have the wings get plenty of propwash from your props to work at lo speed, and luckily X-Plane handles this quite well, but un-luckily I don't really get GOOD at it until 11.30, which is not in public beta yet but will be soon.
So in X-Plane before 11.30, it was just momentum theory to find propwash and put that over anything in line with the prop.
Now, in 11.30, it is much better: Air speeds up ahead of the prop, hits 60% of it's wash speed at the prop, and goes to terminal wash speed well behind the prop, all with assymptotic curves just as in reality. So, if the wing is behind the props, it will get washed by the props, and in 11.30 it will even be washed according to HOW FAR behind the props the wing is!
But here is the thing: X-Plane does NOT understand props that are just ABOVE or just BELOW the wing, trying to wash only ONE SIDE of it: The wings are either in propwash, or they aren't, and the wings speed is changed accordingly, same change to both the lower and upper surfaces!!!!
So your props below the wing will MISS the ing and have NO impact in X-Plane: They need to be in front of the wing to do anything! Just look at the Spruce Goose."