A thermal airship is an

airship An airship or dirigible balloon is a type of aerostat or lighter-than-air aircraft that can navigate through the air under its own power. Aerostats gain their lift from a lifting gas that is less dense than the surrounding air. In early ...

that generates

buoyancy Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the ...

by heating air in a large chamber or

envelope An envelope is a common packaging item, usually made of thin, flat material. It is designed to contain a flat object, such as a letter or card. Traditional envelopes are made from sheets of paper cut to one of three shapes: a rhombus, a ...

. The lower density of interior hot air compared to cool ambient air causes an upward force on the envelope. This is very similar to a hot air balloon, with the notable exception that an airship has a powered means of propulsion, whilst a hot air balloon relies on winds for navigation. An airship that uses

steam Steam is a substance containing water in the gas phase, and sometimes also an aerosol of liquid water droplets, or air. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporizatio ...

would also qualify as a thermal airship. Other types of airships use a gas that is lighter than air at ambient temperature, such as

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic ta ...

, as a

lifting gas A lifting gas or lighter-than-air gas is a gas that has a density lower than normal atmospheric gases and rises above them as a result. It is required for aerostats to create buoyancy, particularly in lighter-than-air aircraft, which include free ...

. Some airship designs that use a lighter-than-air lifting gas heat a portion of the gas, which is usually maintained in enclosed cells to gain additional lift. Heating the lifting gas causes expansion of the gas in order to further lower the density of the lifting gas, which results in greater lift.

Advantages and disadvantages

Thermal airships have the advantage of being much less expensive than

-based airships. They are also routinely deflated after each flight and can be readily packed for storage and/or transport, making them

blimps A blimp, or non-rigid airship, is an airship (dirigible) without an internal structural framework or a keel. Unlike semi-rigid and rigid airships (e.g. Zeppelins), blimps rely on the pressure of the lifting gas (usually helium, rather than hyd ...

rather than rigid airships. Hot air craft produce much less uplift per unit volume than

- or

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...

-filled craft (about 30% depending on air conditions). This necessitates lighter construction, with fewer controls and hence more difficulty in maneuvering. This leads to: *lower airspeeds *difficulty in handling on the ground if the ground wind is above 5 knots *difficulty in steering, particularly at low air speeds *lack of elevator (pitch) control, causing the airship to pitch up or down in response to changes in the throttle setting (a motion called 'porpoising') In recent years, the steering of these ships has improved somewhat. The most successful approach has been to use higher pressure in the tail fin structures than in the rest of the envelope, or to use an internal structure (see below).

History

The first public flight of a hot air airship was made by Don Cameron (UK) in a Cameron D-96 at the Icicle Meet in January 1973. The aircraft took 3 years to develop.

Envelope structures

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. In 2006, a new type of envelope employing a tensile membrane structure was developed by Skyacht Aircraft. This design uses an unpressurized envelope and an internal structure that uses ribs made of

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...

to keep the envelope in shape. When not in use, the structure folds up in a manner similar to an umbrella. The structure also permits the mounting of a steerable engine/propeller on the tail of the aircraft. The tail-mounted propeller provides for

vectored thrust Thrust vectoring, also known as thrust vector control (TVC), is the ability of an aircraft, rocket, or other vehicle to manipulate the direction of the thrust from its engine(s) or motor(s) to control the attitude or angular velocity of the v ...

steering, allowing tight turns.Youtube video of Skyacht
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Operation

Like hot air balloons, thermal airships are first inflated partially with cold (ambient temperature) air. Once the envelopes are sufficiently full, a

propane Propane () is a three-carbon alkane with the molecular formula . It is a gas at standard temperature and pressure, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as ...

burner is ignited and the inflation is completed using heated air.

References

{{Reflist Airship configurations Steam vehicles Tensile membrane structures

Advantages and disadvantages

History

Envelope structures

Operation

See also

References