Lighter Than Air Concepts

Contents

1. Introduction

2. Hot Air Balloons

3. Lighter-than-air crafts
3.1. Rigid Airships
3.2 Semi-rigid Airships
3.3 Non-rigid Airships

4. Hot Air Airships

5. Aeroscraft

6. New developments

7. The pros and cons

8. Summary

9. Sources

1. Introduction

When thinking about lighter-than-air concepts many people have the fascinating pictures of large Zeppelin airships and the Hindenburg disaster of 1937 in mind.

At first view, making use of buoyancy seems to be an old-fashioned approach to human flight that does not comprise significant technical challenges. But even today, there is a small fleet of technically relatively simple and small-sized airships operating over the world - mainly for the purpose of advertisement and commercial leisure flights. The most famous and oldest lighter-than-air concept is the traditional hot air balloon - available for passenger rides and fun. On the other side, a growing global interest and quite ambitious activity in developing large high-tech airships for specific promising applications (e.g. cargo lifting) can be observed in the field of LTA-crafts.

But what exactly is an lighter-than-air craft? An aircraft is anything that flies. A lighter- than-air craft is any vehicle that flies because it is lighter than air, like balloons, airships and aerostats¹. What makes it lighter than air is the fact that it uses a lifting gas (e.g. helium, hydrogen, hot air) in order to be lighter than the surrounding air and by this making use of buoyancy. Any gas or gas mixture that has a lower density than air is "lighter than air" and therefore suitable as a lifting gas. Under standard conditions (0°C, 1bar) air has a density of 1.275 kg/m³. To see the difference of lifting power, there is the rule of thumb that 1m³ of hydrogen lifts 1.1 kilogram, 1m³ of helium lifts 1 kg and 1m3 of hot air lifts only 300 grams.²

2. Hot Air Balloons

Hot air balloons are the oldest successful human-carrying flight technology, dating back to the Montgolfier brothers' invention in France in 1783. The first flight carrying humans was made on November 21, 1783, in Paris and stayed in the air for a grand total of 20 minutes.³

A hot air balloon consists of three parts: the basket, the burner and the envelope. Latter is the huge bag which is capable of containing hot air. Suspended beneath is the (wicker-) basket, also called gondola or capsule, which carries a source of heat capable of producing a sufficient temperature gradient between the air inside the envelope and the surrounding air mass to give enough lift to keep the balloon and its passengers aloft. Air that has been heated to about 70°C has a lower density than atmospheric air. Its density is approximately 1.02 kg/m³ and therefore significantly higher than that of hydrogen or helium resulting in less lift. As the balloon derives lift from the hot air which is blown into the hull of the balloon and makes it fly. It is then following the direction of the wind (horizontal movement) and usually has a mechanism to control the vertical movement. A great disadvantage is that heat has to be added to the air constantly because otherwise it cools down and consequently loses lift as density increases with cooling.

Unlike gas balloons, the envelope does not have to be sealed at the bottom since the rising hot air only exerts pressure on the upper hemisphere of the balloon to provide lift.⁴In today's sports balloons the envelope is generally made from nylon fabric and the mouth of the balloon, which is located close to the burner flame, is made from fire resistant material such as Nomex.⁵

A large-scale technical application of hot air for the later described airships is impossible, because high heating costs and the low amount of lift makes it uneconomical.

That is why hot air balloons are used primarily for commercial leisure flights, recreation and as a sightseeing attraction for tourists nowadays. The hot air balloon baskets may vary in size and carry anything between 3 up to 20 passengers.⁶Due to the huge envelope of the balloon, it is often used as advertising space and some of them are even used for sport (records) as well: The record for the longest duration of a hot air balloon flight was set up by the adventurer Steve Fossett, who traveled 20,602 miles (32,963 km) around the world in 14 day, 19 hour and 51 minutes in Summer 2002.⁷

Constitutive parts of a hot-air balloon:

illustration not visible in this excerpt

3. Lighter-than-air crafts

In contrast to a balloon, an airship is a powered lighter-than-air craft, which is propelled forward by an engine including some means of controlling the direction, usually with rudders. Because of this advantage these airships are also called "dirigible", which is derived from the french word "diriger" meaning steerable.

We distinguish between three main types of airships: rigid airships, semi-rigid and non- rigid airships, also known as blimps. All three types have four principal parts: a cigar­shaped bag, or balloon, that is filled with a lighter-than-air gas, a gondola that is slung beneath the balloon and holds the crew and passengers and one or more engines that drive propellers as well as horizontal and vertical rudders to steer the airship.

3.1. Rigid Airships

As their name implies, rigid airships are rigidly constructed and keep their shape with an inner latticework of metal supporting a bag containing pockets of lifting gas. The⁸

most famous rigid airships were the Zeppelins and the USS Akron and Macon. The era of the Zeppelins began with the flight of the first Luftschiff Zeppelin D-LZ-1 over Germany's Lake Constance in July 1900 and ended with the disaster of the D-LZ-129 (The Hindenburg) in May 1937.⁹The most famous of the Zeppelin airships was the D- LZ-127, which was launched for the first time in 1928 and was designed for carrying 20 passengers and a crew of 40. This airship set a host of records, including the longest non-stop flight from Germany to Lakehurst, New Jersey, a 6200 mile trip, in 111 hours and 44 minutes. In August 1929, it made the 20,500 mile around the world trip in a total of 12 days in the air.¹⁰Until the Hindenburg accident in 1937 about 405,000 passengers travelled across the Atlantic by a Zeppelin.¹¹During that time, airships like the Zeppelin had an advantage over the airplane on intercontinental flights, but that advantage had vanished by the end of World War II. Airships like these were also used during World War I, but by mid-1917 the airship could no longer survive in a combat situation where the threat was airplanes. During World War II they were still used for anti-submarine patrol flights over the pacific.

illustration not visible in this excerpt

Airship D-LZ 127 "Graf Zeppelin"¹²

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¹Derived from the greek words “aer“ and “statos“, which means standing or staying in the air.

²http://www.ebaNoon.org/history/history-of-baNooning.html

³http://www.peacesoftware.de/einigewerte/luft.html

⁴http://science.howstuffworks.com/hot-air-balloon1.htm

⁵http://blog.adventureballoons.co.uk/category/the-balloon-envelope

⁶http://www.eballoon.org/balloon/commercial-leisure.html

⁷http://www.stevefossett.com/html/press_archives/balloon_04july02.html

⁸http://www.eballoon.org/balloon/how-it-works.html

⁹http://www.century-of-flight.net/Aviation%20history/up%20to%20WW%201/Dirigibles2.htm

¹⁰http://www.altfrankfurt.com/Spezial/Zeppelin/GrafZeppelin

¹¹http://www.nytimes.com/slideshow/2008/07/06/business/0706-BLIMP_5.html

¹²http://www.altfrankfurt.com/Spezial/Zeppelin/GrafZeppelin/pic/GrafZeppelin3.jpg