Let me piss you off a bit! 😋
The total load on the water pump will be the sum of the load due to the moment of inertia and the load due to aerodynamic drag.
Let's assume some example values to perform a simple calculation:
• Metal Fan (2 Blades):
• Mass per blade: mass =0.5 kg
• Radius: r=0.5 m
• Plastic Fan (6 Blades):
• Mass per blade: m=0.1 kg
• Radius: r=0.5 m
Let's Calculate Moment of Inertia:
For the metal fan: I (metal fan) = 2×(0.5×0.52)=0.25 kg⋅m^2
For the plastic fan: I (plastic fan) =6×(0.1×0.52)=0.15 kg⋅m^2
Let's Calculate Aerodynamic Drag:
Assume:
• ρ=1.225 kg/m^3
• v=10 m/s
• Cd=1.2
• A (metal fan) =2×0.05 m^2 = 0.1 m^2
• A (plastic fan) =6×0.05 m^2 = 0.3 m^2
For the metal fan:
F (d,metal fan) = 12×1.225×102×1.2×0.1=7.35 N
τ (metal fan) =7.35×0.5=3.675 N
For the plastic fan:
F (d,plastic fan) = 12×1.225×102×1.2×0.3=22.05 N
τ (plastic fan) = 22.05×0.5=11.025 N
Now let us calculate the total load:
For the metal fan:
Total Load =I (metal fan) × ω2 + τ (metal fan)
where ω is the angular velocity in rad/s.
For the plastic fan:
Total Load = I (plastic fan) × ω2 + τ (plastic fan)
The plastic fan, despite being lighter, may impose a higher load on the water pump due to significantly higher aerodynamic drag caused by having more blades.
The exact numbers will depend on the precise values of mass, radius, speed........😋
This is just to justify your point on the load imposed on the water pump.
FYI: This load is insignificant, the type of bearing using in any water pump can withstand it.
For maximum efficiency in terms of air thrust (cooling) which is the most important point to consider here, it is the plastic fan that wins the battle.