Heavy aerial lift quadcopter?

Hi, I am in a local competition where they are asking for a quadcopter that can lift a box of lead. The box weigh's 25kg. Yes, you read that right 25 kg. The rules state that it must be a quadcopter and it must fit in a meter by meter by meter cube. This limits the prop size to 500mm diameter unless the height of the blades is different so they overlap. I would prefer not to do this. each rotor must lift 7000 grams (payload + weight of quad). I have been using this thrust calculator : http://personal.osi.hu/fuzesisz/strc_eng/ and according to it a 500mm (20") at 4.5" pitch (I just put that as standard) will need to spin at 6000 RPM to produce 7.12 kg of thrust. It also says it will need an 800 Watt motor. I plan on using these motors http://www.hobbyking.com/hobbyking/store/__24433__NTM_Rotor_Drive_450_Series_1700KV_930W.html. They are 930 watts and 1700kv. they run on a 6s lipo. A 6s lipo is 22.2 volts. 22.2 X 1700 = 37,740. 37,740 RPM is the theoretical speed of the motor. I only need 6000 RPM. 37,740 / 6000 = about 6. so the motor has an 8 tooth gear on it and the prop has a 42 tooth gear on it. This makes a roughly 6:1 ratio to slow down and power up the rotor. The motor is 30A constant current, 42A burst. The ESC will be this one http://www.hobbyking.com/hobbyking/store/__25366__Turnigy_Multistar_45_Amp_Multi_rotor_Brushless_ESC_2_6S.html and the battery will be this two of these: http://www.hobbyking.com/hobbyking/store/__9176__Turnigy_5000mAh_6S_20C_Lipo_Pack.html.          When I started writing this I had a question that I answered myself but now I just want to know if you see any faults with this. PS, the winner of the competition is the one who can hover the longest. I also want to do it for as cheap as possible

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vtxstar4 years ago
Take care to calculate and consider the rating of the propeller max rpm.
One brand has this page for ratings of their products http://www.apcprop.com/v/html/rpm_limits.html

monty324 (author)  vtxstar4 years ago
Thanks. I hadn't thought about max RPM's but I should be fine with anything but multirotor or slow flyer blades. The blades will run at near limits but they cost about £2.50 so its negligible.
monty324 (author)  monty3244 years ago
That being said I don't actually know what negligible means precisely so i might have just made myself seem like a massive idiot.
rickharris4 years ago

Have a look, solve your problem.
monty324 (author)  rickharris4 years ago
Thanks for the answer and my god you replied fast

Sorry the original URL was wrong. (USA spelling)
monty324 (author)  rickharris4 years ago
yes, when I first followed the like it went somewhere terrible but I tried it again and I got the good one.
Nothing to do ;-) Raining today.

Flight lifted a building block (cinder block) although with a fixed wind but they also have designs for multi rotors. and LOTS of good ideas and suggestions.