This fan will fly up at the height of a few meters, like a real helicopter, if you twist it in your hands.

Military transport aeroplanes or passenger planes operating on short distances are powered by similar propellers. While jet planes fly at high altitudes of 10-11 km where the air is thinner and resistance is smaller, turbo-prop planes have to fly at lower altitudes, e.g. 5 km, where the air is dense enough for the propellers.

Big wind turbines provide electricity. In Denmark more than 10% of electricity is produced by wind turbines. Contrary to popular belief, wind turbines are quite efficient. Their efficiency of transforming wind energy into electricity is around 15%. It seems that even a small turbine can provide enough electricity for a number of households. There are a lot of wind farms in mountainous regions of Spain and on plains of Germany.

People have been using wind energy ever since the beginning of history. Can you imagine Holland without windmills? Modern windmills change the landscape, they are still expensive, but they generate a considerable amount of power.

Assuming that the density of air is ρ=1,3 g/dm₃ = 1,3 kg/m₃, and the velocity of wind is v = 10 m/s (quite strong, but 'acceptable') the mass of the air blowing within 1 second over the base of a cylinder of a radius of R = 10 m is equal to M = πR²vρ=4000 kg, and its kinetic energy amounts to ca. 200 kJ. Supposing the efficiency of transforming the energy of the air flow into power is 15% (due to the fact that after passing the windmill the air still flows with some velocity) we can establish that a single windmill generates a power of 30 kW. An average household has a contract with electricity board which guarantees the maximum power consumption of 3 kW, but even if a washing machine, and a refrigerator and TV set are on, power consumption amounts to ca. 2 kW.