The goal is to win the American Helicopter Society (AHS) prize of 250k. The regulations (http://www.vtol.org/awards-and-contests/human-powered-helico...) specifically outlaw your suggestion -- although it was a good one ;)

4.1.1 The machine shall be a heavier-than-air machine. The use of lighter-than-air gases shall be prohibited.

Thanks for the link. It appears that you are allowed to drop the ground level from below the helicopter :-)

One thing that is slightly more in the spirit of the contest is to have rotors tht do not provide lift at all and hence have low drag, build up lots of potential energy, and then adjust the angle of attack to get lift. Storing potential energy to get a 30 second launch and subsequent 30 second drop will be 'challenging', though.

I guess that both will be covered by "3.2 For any and all questions regarding acceptance of entries, eligibility of an entrant, pilot, crew or aircraft under the regulations, or any other matter relating to the AHS prize, the decision of the AHS is final."

OK, thanks. So how about I use strong carbon fibre shell with air pumped out. There is no lighter than air gas involved and the whole machine can be (just slightly) heavier-than-air. I bet they would outlaw that too, under the 'catch all' rule mentioned below. Nothing but extremely hard work will do ;)

It'd have to be a very strong shell to actually give you buoyancy above simply not having anything. Carbon fibre is strong in tension, while a material which can support a vacuum must be strong in compression. The weight of any shell is going to be much greater than the buoyancy you'd gain.

Nah… http://pepijndevos.nl/2011/11/09/vacuum-balloon.html