Thermal
airships are a hybrid between conventional
non-rigid airships and
hot air balloons. Conventional modern airships use
helium as a
lifting gas and rely on the
pressure difference for lift.
A thermal airship is an airship that generates its lift via a
temperature differential between the gas inside its
envelope and the
ambient air.
Currently most thermal airships use hot air, as used in a hot air balloon, as their lifting gas. However, an airship that uses steam would also qualify as a thermal airship. This is being investigated by a British team called 'The Flying Kettle'.
Thermal airships are a fairly recent innovation. The first recorded flight of such a craft was in 1973 by Don Cameron. This first ship took three years to develop and has been the father of many beautiful craft since.
Most thermal airships are non-rigid. Some are pressurized. In some cases, the pressurized air is taken from a duct located behind the propeller. In other cases, the pressurized air comes from a separate fan. This is a very rare method, but it is effective.
In 2006, the Sky yacht was born. This is an exciting new type of thermal airship that is semi rigid. Aluminum ribs are used to keep a tear-drop shape and they are help in place by a tension cord running down the center of the envelope. The resulting effect is an umbrella-like craft that folds away when not in use. Perfect for the flyer who does not have massive amounts of money to spend on a hanger, but wants to fly a cheap airship. The structure also permits the mounting of a steerable engine/propeller on the tail of the aircraft. The tail-mounted enables vectored thrust steering allowing a much tighter turns which is a problem for many airships currently.
Thermal airships are much less expensive than helium ones. This is a great advantage for the pilot who has little cash, but wishes to tour the skies. Sadly, thermal airships cannot reach fast speeds or turn very well when going slowly. If the wind is above 5 knots, they can be very difficult to handle on the ground. Most of these disadvantages have been over come by the Sky yacht.
For those interested, they are still in the experimental stage but will be available to buy for about $100-200,000 (depending on requirements).
For more information, go here http://www.personalblimp.com/.
and here http://flyingkettle.com/