When all the lift is created by the frontwing and the rearwing is there mostly for traction/stability why are most of the rearwings curved down or up in the tips?
Its logical in the frontwing to reduce wing tip wortices, but for the traction rear wing the pressure should be 50/50% up/down so a straigt foil should create less drag, so why?
Well, almost, as I think there could be a slight downward lift on some, in order to get a neutral trim at different speeds, or the opposite for beginner foils, but that is an extremely small contribution, close to and sometimes zero I think.
That is another issue, not important here...
So you are right, the stabilizer could just be flat/straight for minimum drag.
I also believe many, actually most foil stabilizer wings ARE flat, right ?
But curving the stabilizer can be used instead of having a rudder (yaw stability).
Some also make the stabilizer in a "V" to avoid having a rudder.
This way is also used in planes, to avoid drag and mechanical nuissance
Although, it is a bit of a religous item also, like many design features unfortunately still is
Apart from drag and yaw stabilizing effects, some also want to avoid having the stabilizer in the direct wake turbulence of the main wing at the same time, so having a different curve or V than the main wing, means the stabilizer is never fully in the turbulence shade, but only part of it