Introduction to airships

 

This is a follow up from the article on types of flight. Here, we’ll be taking a further look at airships. 

I’m sure most of you know what an airship is already. You might be familiar with advertising blimps. You may have seen old photos. Perhaps, you’ve encountered the Hindenburg disaster in your physics lesson about electricity (that’s how I encountered it).

These are pretty interesting, actually, because they are almost ridiculously efficient in terms of energy requirement. That’s mainly why it looks like they will make a comeback. To be precise, it’s hybrid airships making a comeback, but they are a topic for another time. The main focus here is your traditional airship.

First things first, we’ll look at a diagram of an airship.

The big balloon on top (blue) is the envelope. That is a large balloon-like structure filled with a gas lighter than air. This is what keeps the airship airborne.

The gas inside the envelope is called the lifting gas. As it is lighter than air and because there is such a lot of it relative to the rest of the structure, it makes the overall structure less dense than air. This means it experiences an upthrust that is higher than its weight, making in float. This happens until the upthrust and weight balance. As air gets less dense with altitude, you will reach an altitude where the upthrust that the airship experiences is the same as its weight, and that will be where your airship floats.

The gas itself is usually Helium on modern airships. It is very light and extremely safe, but our supply of helium is limited. As a result, it’s expensive.

Hydrogen has been used historically. The problem with hydrogen is that it is flammable when mixed with air, so if the envelope leaks and something sets the mixture on fire, the whole airship can burn up. However, pure hydrogen is not flammable, so if you have a spark inside a container of pure hydrogen, nothing with happen. Preventing leaks completely on a moving ship, especially one with a mostly flexible, thin exterior can be very difficult if not impossible. That is why it’s not used in great quantities anymore.

Another option is hot air. You can use a burner to heat air inside the envelope. Air expands when heated, which reduces its density, essentially making it lighter than the air outside the envelope. Air is not a limited resource, and here, the lifting gas itself is not flammable. The problem here is that you will need a fuel supply. You can burn a fuel like kerosene, or you can use electric heaters inside the balloon.

With this method, the fuel weight can become significant. This can be true for batteries or solar panels as well. Besides, if you’re burning your fuel, you’ll be losing fuel weight throughout the journey, making the balloon lighter throughout the journey. You will have to compensate for that when flying. Besides, the fuel itself can be flammable, or in the case of batteries, explosive.

All said, however, it’s a happy medium, as long as all the risks are managed.

For a writing project, you can always make up a lifting gas (provided you’re writing fantasy), along with its properties, availability, price, and all that. Another possibility is using a vacuum. It will lift the airship alright, but the envelope will have to be very strong, and altitude control will be a bit of a problem. I will probably discuss this in detail later. (Look, I’m a flying obsessed mechatronics engineer, what else do you expect?)

Going back to the diagram, the small section below the envelope (green) is your gondola. That’s the section for passengers, crew, storage, and fuel. The gondola can be quite roomy for the number of passengers it can carry (due to the need to maintain a low weight). It also means the gondola needs some form of communication between its various areas. It’s probably a bit like being on a ship.

You may have noticed the engine (yellow) on the diagram. That is what provides power for forward motion. Usually, they would be fitted with a propeller, as few other types of propulsion make sense for the speed an airship flies at. The actual engine type can be pretty much anything you’d see on a propeller-driven plane. I’ll elaborate on this later. The engines are usually fitted on the sides of the gondola to allow differential thrust for steering.

The fuel for the engines can be pretty much anything. You could use an electric motor, powered by a battery or even solar power. You can use a conventional fuel like diesel or petrol or even biogas.  

The airship in the diagram has a vertical stabilizer and rudder, as well as horizontal control surfaces. These will be covered in more detail in later as I would go off on a 20,000 word tangent if I tried to cover it here.

The main disadvantages of an airship, compared to a conventional airplane, are,

•      Lower speeds, as the envelope’s enormous cross-section produces a lot of drag (I’ll explain it in more detail, I promise. I am aware it’s a lot of things to explain later).
•      Lower service ceiling in general. This is dependent on the difference in density between the lifting gas and the surrounding air, how much lifting gas there is, and the weight of the airship overall. As a result, it’s capped by the possible size of the envelope.
•      Properties of the lifting gas can make it a fire hazard.
•      Huge, slow moving target if anyone wants to take a pot shot at it. This can be especially dangerous if the lifting gas is flammable.
•    Susceptible to weather, as it is large and slow moving.

Advantages are:

•      Lower fuel requirement, as there is no energy needed to provide lift (under most circumstances).
•     Can stay aloft forever (there will be some gas leakage, but well, for a very long time). Is great for sightseeing and observation as a result.
•     Not so easily brought down by enemy fire, unless it catches fire. A bullet hole will produce a hole that is so small that it can take hours for the envelope to deflate completely, giving enough time for a safe landing.
•      If used for passenger transport, the sheer amount of space on it compared to an airliner. You wouldn’t be cramped into one seat for eight hours, instead you can walk around, sleep, eat, and enjoy the view for two days. It’s up to you whether it’s actually an advantage, honestly.

That’s it for a brief introduction. I promise I will follow up on everything I promised to follow up, but that’s all for now. 

The painting in the video is available here. 

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 See you next time!