Unfortunately, because they lack fixed wings to help provide lift, they are also expensive to operate.

ECONOMIST: Monitor

The simplest option is to scale down the existing standard of a fuselage with two fixed wings and a tail.

ECONOMIST: A personal eye in the sky

Rather than use fixed turbines to generate power, Makani has been experimenting with "flying wings" adorned with several smaller turbines that act as propellers as the craft takes off.

BBC: Google acquires kite-power generator