So, I have a small ducted fan which has a listed "thrust" of 75 g at a set voltage. I'm going to assume the "g" is short for "grams". Why would this be measured in grams instead of newtons and how can this be calculated to yield the vertical acceleration provided by the fan? Is this meant to be the thrust-to-weight ratio and if so then shouldn't this value not have any units assigned?

Now, if I attached this ducted fan to a small ground RC vehicle and rotated it 90 degrees so that the direction of thrust was confined to the x-y horizontal plane. Assuming that the drag of the car and the friction from the wheels was negligible, how do you calculate the acceleration provided by the ducted fan from the 75 grams?


1 Answer 1


The $g$ is apparently being used to mean gram-force, a non-standard unit of force equivalent to 9.80665 mN. According to the Wikipedia article, it was historically common for rocket thrust to be measured in kilogram-force, and kilogram-force is still used occasionally for expressing thrust today. The information for the fan should really say that it produces a thrust of $75gf$, for clarity.

Your $75gf$ fan thus nominally has a thrust of about $0.735$ Newtons, from which you can of course calculate the acceleration by using $F=m a$.

  • $\begingroup$ But if it isn't used in a vertical position, doesn't that mean that finding acceleration is different for the RC car example I provided? Although gravity is acting on the vehicle, it's direction of motion (and acceleration) is independent of that force (assuming friction and drag are negligible) right? A better way to convey what I'm trying to explain is, lets pretend that it was a propulsion engine attached to a vehicle in deep space (with no external gravitational factors), assuming you knew the total mass, you couldn't work out the acceleration of the craft with that 75gf value could you? $\endgroup$
    – user155876
    Sep 17, 2014 at 6:15
  • $\begingroup$ The fan should provide approximately the same amount of force regardless of what orientation the fan is in. Yes, if you know a rocket's mass and the thrust of the rocket's engine as measured in gram-force, you can calculate the rocket's acceleration. Gram-force is just a unit of force, not a mass or a weight or anything. You can just pretend that the fan is listed as providing 0.735 Newtons of force. $\endgroup$
    – Red Act
    Sep 17, 2014 at 6:41
  • $\begingroup$ Okay, thank you for that, I had a bit of a fundamental misunderstanding regarding how the gram.force value can be used. $\endgroup$
    – user155876
    Sep 17, 2014 at 6:57

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