[nomind7881]

     So i am trying to build a hoverbike, using helium to provide the lift for the vehicle in two 'lift tanks' made of aluminium.  I know that a balloon with a 20 ft. diameter can lift 263 lbs.  v = (4/3)pi*r^3 = (4/3)pi*10^3 = approx. 4186 cubic feet. So I would need two tanks each with a volume of 2093 cubic feet to lift 263 lbs; about the size of four Buick skylarks...

      Needless to say that won't work for a hoverbike; the ideal volume for a single tank i figure is about 8.18 cubic feet, which is 256th the volume of the above tanks.  I'm assuming that by decreasing the density of the helium by the same factor that the same lift can be achieved.  Density of helium(@STP)=0.1786 g/liter.  0.1786/256 =  0.0007 g/liter.  This is about as far as i get on the problem.  How much do I need to heat the helium to get this density? I'm assuming I can keep it at normal pressure.  I'm hoping the temperature is below the melting point of aluminium...

[Borek]

Aluminium tanks will be way too heavy.

Density of gas is its molar mass divided by its molar volume. Use ideal gas law for the latter.

Your assumption that decreasing the density of helium 256 times will increase the lift 256 times is wrong. Lift is mass of displaced air minus mass of displacing object - so it won't be more that the first, not matter how hot and light the helium will be.

[billnotgatez]

The molecular mass of air is estimated to be slightly below 29 g/mol based on summing of the constituents. Since buoyancy is determined by the difference between air and the lifting gas that would be the theoretical maximum that could be floated. Since you have to take into account the container and items to be lifted, the maximum you could lift would be much less no matter how much you heated the lifting gas.