# Is there a way to convert CFM (rate of air moved) to kilograms of lift generated?

Is there a way to convert CFM as in a rate of volume of air moved to kilograms of lift generated?

• Hmm, maybe in particular cases as air flow doesn't always indicate lift (e.g., air flowing in vents). Could you provide a bit more context here (i.e., the specific case you have)? – Kyle Kanos Dec 30 '14 at 3:11
• No. You also need the velocity at which the air is flowing. Lift is a force and measured in Newton. – CuriousOne Dec 30 '14 at 6:19
• Force is momentum per unit time, so it's not just the mass of air moved, it is mass times the change in velocity. – Mike Dunlavey Dec 30 '14 at 15:57

Let's change it from 1CFM to $1m^3/s$. Further I'll assume that your system is accelerating air from rest to the speed necessary to move through a particular size vent. We can calculate the change in momentum and therefore the average force. I'll assume $1.2kg/m^3$ as a density for air.
For case 1, assume the vent is $1m^2$ in cross section. $$v = \frac{\text{flow}}{\text{area}}$$ $$v = \frac{1m^3/s}{1m^2}$$ $$v = 1m/s$$ $$F = \frac{dp}{t} = \frac{m \text{ } dv}{t}$$ $$F = \frac{1.2kg \text{ } m/s}{1s}$$ $$F = 1.2N$$
Do the same calculation for a $1cm^2$ cross section and you get $F = 12000N$
• You seem to be actually using $F=\Delta p/\Delta t$, not $F=dp/dt$ (though you ignore the differential aspect of $dt$ and incorrectly write $t$ in the denominator). – Kyle Kanos Dec 30 '14 at 14:11