Most of the books/manuals i have referred to just stay vague about it..
I'm no aerodynamics expert, but I'd imagine the reason the books are vague is because there are a number of factors involved, some of which will result in an increase in Vmc and some of which will result in a decrease. All of these factors are very marginal, so whether the total effect will be an increase or decrease will vary, I'd guess, from type to type, and maybe depending on the amount of flap selected too.
We've already discussed the increased rolling effect due to slipstream. Also, the CofG might change marginally when flaps are lowered. The CofP will certainly change, which I would think would affect Vmc by altering the effectiveness of the rudder. The flaps themselves may have a slight stabilising or de-stabilising effect too. I doubt that any of these would make any appreciable difference, but the number of factors involved would, I would think, make it very hard to say - in general terms, at least - exactly what difference the flaps would make.
FFF
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