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Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
It has been touched upon before, but it needs to be further emphasized: note - I use energy loosely to refer to work/power done on the system, with the assumption that you understand power is the time derivative of work, and that energy is what allows you to do work. The power, from a 'strictly physics--all energy must be conserved' standpoint is 0. The only time you need energy is to change the height or velocity of your vehicle. You must generate a force equal/opposite to that of gravity, so for your 1kg vehicle you need 9.8N of force. The key thing to remember is that all of the physics equations define work as force times distance, so if if your vehicle is stationary it, by definition, cannot be using any energy.
However, from a practical standpoint, generating that 9.8N of force will take some amount of energy! Impractical implementations aside (you never said that you can't just set another earth sized mass on the other side of it to generate your force, but it would be an unstable equilibrium anyway so lets not worry about such cases), it is going to take some energy. At this point you consider your 'force generator' with more scrutiny, and calculate its 'thrust efficiency' in terms of watts in to force out. It is my understanding that ducted fans are the most efficient at ~1 N/W for very high performance fans, with typical RC gear putting out closer to 0.2N/W
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
BigBad wrote ...
Nah. It doesn't take any energy at all.
For example if you have a huge flat plate of graphite (weighing 1kg) resting on a huge flat plate of neodymium magnet!
yes, but your body did the work against gravity to get the graphite plate above the magnet. the magent is a stationary force "generator" which is cool but useless for an aircraft or spacecraft .
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
Yes, raising something up against gravity does take energy, in strict accordance with conservation of energy!
But so what? The levitation itself takes no energy. The answer to your original question is 0 joules/second/kg is the minimum you need, and this isn't a limit case, it's routinely achieved.
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
The limit would be an infinite large propulsion unit with finite mass pushing on an infinite amount of air. That should give you levitation with no energy use.
It is unclear what you are really trying to caculate. If you want a real world value you don't calculate, you look up an existing fan unit and get your values from there, then you buy one and double check the numbers.
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i need another opinion on energy of flight...
