How much lift thrust would a 252 CFM 120mm fan provide?

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Discussion Overview

The discussion revolves around calculating the lift thrust provided by a 120mm fan capable of pushing 252 CFM, in the context of building a miniature RC plane. Participants explore the relationship between airflow, thrust, and the weight of the aircraft, while considering various factors affecting lift production.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Leon seeks to determine how much weight a 120mm fan can lift, given its airflow capacity of 252 CFM.
  • Some participants calculate thrust based on airflow and fan dimensions, but express uncertainty about the accuracy of their calculations.
  • There is discussion about the relationship between thrust and weight, with some arguing that thrust must exceed weight for the aircraft to ascend.
  • Concerns are raised about the suitability of the fan for lift production versus maximum airflow, with distinctions made between airflow and thrust generation.
  • Participants debate the importance of factors such as cross-sectional area, angle of attack, and blade design in determining lift capacity.
  • One participant provides a calculation for thrust based on mass flow rate and air velocity, suggesting a maximum hover weight of approximately 0.6886 lbs with two fans.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the calculations or the suitability of the fan for lift production. Multiple competing views on the relationship between airflow and thrust remain, and uncertainties about the calculations persist.

Contextual Notes

Some calculations rely on assumptions about air density and fan performance, which may not be universally applicable. The discussion includes various interpretations of thrust and lift, indicating potential misunderstandings of the underlying physics.

  • #61
russ_watters said:
No. It comes from Bernoulli's equation and Bernoulli's equation is actually about kinetic energy. Note the similarity to the kinetic energy equation: 1/2 M V^2.

Thank you russ, this has explained it.

Also, thank you for confirming the 0.75N value. (I'm just glad laser and myself got there eventually)
 
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  • #62
russ_watters said:
No. It comes from Bernoulli's equation and Bernoulli's equation is actually about kinetic energy. Note the similarity to the kinetic energy equation: 1/2 M V^2.

well that's good, it means there was no dependence on knowledge of some funky aspect of fans; it came down to properly using Bernoulli's equation.

i used to be afraid of being wrong, but the only way to improve and master things is by trying and learning from mistakes. thanks for the correction/information.
 
  • #63
last night i was too lazy to look at the NASA page (general thrust equation) i had previously linked to see why it didn't including the 1/2. it does include but not in a "thrust for idiots" type way. yet i looked at their page on propeller thrust and that one is totally a (propeller) "thrust for idiots" page, which really helped me understand. i trusted russ_watters answer, but i wanted an extended version. i figure there is a few others who may find it useful too. here is the link:

http://wright.nasa.gov/airplane/propth.html

btw, it does say that you use the blade length to calculate the area, so the fan in this topic will have a higher thrust than calculated. looking at typical ducted fans, the thrust will be 25% greater when taking the revised area into consideration. minus the 5% losses, it'll be about 20% greater thrust than calculated, so 0.9N per fan.
 
  • #65
Lazer57 said:
Keep it under 3 pounds if you want it to lift off the ground with two of those.

Yeah, I know. I can't imagine everything else in the aircraft weighing more than 3 pounds with those fans.
 
  • #66
What power source are you going to use? Weight considerations from batteries etc?
 

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