Expert: Paul Soderman - 3/17/2008

Question
I am 10 and home-schooled. I want to make a fan pack (like a jet pack)only with a fan to lift me. I want to run it on compressed air.  I have been researching to find out how many revolutions per second it takes to lift 200 pounds off the ground but it has been hard for my Mom and I to find an answer.  The fan would need to be 6 inches by 6 inches.  If I have 40 pounds of compressed air spinning the fan, how many revolutions per second will it make the fan go?  Thank you, Andrew (and Mom)

Answer
Andrew
I can't answer that without knowing something about your fan and how the air makes the fan rotate.  You probably need to find a fan and see what it's performance characteristics are.  You can Google on lift fan to get some ideas.  One problem - you can't get more thrust out of the fan than you can get out of an optimum nozzle exhausting your compressed air.  And at 40 psi, 200 pounds of thrust requires a pretty big nozzle - one that might exhaust your supply pretty fast.  (Try to calculate that.) Another problem, if you did get 200 pound of thrust, how can you control your attitude so you don't tip over and slam into the wall or something ?  The jet pack is actually a non-trivial problem that NASA and others have struggled with in the past.  No one has built a safe one that I am aware of.  I hate to deter your efforts, but you might be better off starting with some model lifting device that didn't have your body in the system.  Check out   http://jlnlabs.online.fr/gfsuav/index.htm   for an example of a Coanda lifting device that is kind of like a lift fan.

Good luck.
Paul