Confusion in magnetic neutral plane of DC machine
The actual air gap is smaller this so the field is almost perpendicular to the conductor movement around most of the rotor however at the top and bottom of this rotor there is a area where the direction of rotation is parallel to the magnetic field.

At the top and bottom of this machine the rotor coils are moving horizontally. This means that  they are not cutting the flux of the magnetic field so no EMF is induced. The MNA is the plane that cuts both of these points.This means that the MNA runs vertically through this machine.