An aeronautical propulsion system includes a fan having a plurality of fan blades rotatable about a central axis and defining a fan pressure ratio, FPR. The propulsion system also includes an electric motor mechanically coupled to the fan for driving the fan, the electric motor including a number of poles, npoles, and defining a maximum power, P. The relationship between the fan pressure ratio, FPR, of the fan, the number of poles, npoles, of the electric motor, and the maximum power, P, of the electric motor is defined by an equation: n pole p = C 1 · e - 9.062 · FPR + C 2 · e - 1.2604 · FPR ; wherein C1 is a constant having a value between 22,000 and 52,000, wherein C2 is a constant having a value between 4.0 and 9.8, and wherein e is Euler's number.