# How to find force from volumetric flow rate and pressure

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1. Oct 14, 2016

### bl965

1. The problem statement, all variables and given/known data
Hi. This is a problem related to designing a RC hover craft. We are given a datasheet for a fan used as thrust.
http://imgur.com/6liZSid Sorry image post no working, url: http://imgur.com/a/6XLwL
We have to approximate the equation to a straight line through the curve. Then relate it to Fthrust = F0+ γ*v. We have to find γ and what F0 represents.

2. Relevant equations
Q = velocity * Area
F = mass * acceleration

3. The attempt at a solution
I found that the slope is -0.285 [m3/s/mmH20]. To find F0, I think the graph has to be turned on its side: pressure is the independent variable and volumetric flow is the dependent. If pressure is zero, I can find F0 which is the y intercept. But how to relate force to volumetric flow or velocity.

Last edited: Oct 14, 2016
2. Oct 14, 2016

### Staff: Mentor

If P is the pressure and A is the area that the pressure acts on, what is the force?

3. Oct 14, 2016

### bl965

If pressure is 0, volumetric flow is high, and small pressure * area is small force. As pressure increases, there will be more force with less flow. Makes sense. From start the fan is operating at atmosphere. I get high results when I search for one atmosphere in mm of water. Am I on the right track?

4. Oct 14, 2016

### Staff: Mentor

I have no idea. Can you please provide an exact problem statement?

5. Oct 14, 2016

### bl965

(b) We can relate this linear relationship to Fthrust = F0-γv [Note: γ is a constant representing the slope of the F(v) line]. For each fan, based on your approximation in (a), what are the values for F0 and for γ?

6. Oct 14, 2016

### haruspex

No, it looks the right way around already, with pressure on the vertical axis. Seems to me you flipped it around in order to get:
What do you think relates volumetric flow through an aperture to the linear flow rate through it?
As Chester asks, what relates pressure to force?