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

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The discussion revolves around calculating the lift thrust provided by a 120mm fan capable of 252 CFM for a miniature RC plane weighing approximately 15 pounds. Participants clarify that thrust must exceed the aircraft's weight for it to ascend, and they explore the relationship between airflow, fan size, and lift production. Calculations indicate that the fan's thrust is insufficient for the desired weight, emphasizing the need for a fan designed specifically for lift rather than airflow. The importance of factors like cross-sectional area, velocity, and blade design in generating effective thrust is highlighted. Ultimately, the consensus suggests that a more suitable fan type is necessary for the project to achieve flight.
  • #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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