Good points Multiflyer, thanks. I agree with everything, but one thing. We were both correct, it seems!

Or better, I would like to ask you guys this thing, about which I would like to learn more. Multiflyer, in the first part of your post, you say that high thrust line doesn't solve the "spiraling problem" , because of the difference between lever arm in the distribution: I totally agree. I said infact that a weighted sum must be computed (area * lever arm). You said also that a subfin would do the trick, ok...

But, the question that I have in mind is:the spiraling slipstream is centered on thrust line (which I think) or on the stab line?

If it is centered on the thrust line, which seems to me natural, then if you look at Oxalys tail, you'll see that it is almost symmetrical, with a fin and a sub fin (here with sub fin I mean the tail part under the TRHUST LINE)! This is what I thought at the beginning with my question. In other words, ok that we have to look at the tail since it has a greater lever, but with high thrust line the tail can be splitted in two almost equals surfaces! Do you understand what I mean?

Ok... this seems to be reasonable to me, but we have to take into consideration the stab also, since it can disturb the spiraling slipstream and perhaps it can hide the subfin under it (here I mean under the STAB) ? If this happen, then the stab should be placed higher in order to yeald a more symm tail, but that perhaps can hurt other flight capabilities?

Thanks all, good discussion guys.