Tuesday, March 25, 2008

A Question of Temperature - Warm air and Superheat

Duncan from the Blimpship website and blog asked an interesting question in the forum, he asks:
I've been reading more and more about the operations of airships recently and can't decide whether I'm missing something or there is some contradictory information out there.

Some info speaks of the rise in outside (air) temperature and how it degrades the performance of an airship, whilst other reports tell of how sun warming the envelope (and I assume heating the helium inside) provides increased lift.

Can anyone clear this up for me? What effect does temperature have on the capability of an airship?
Since I don't want to leave Duncan in the dark and nobody else has responded yet. I will try to bring a little bit of clarity. If I make any mistakes, please correct me.
So there is one major thing that makes airships fly and that is static lift, generated through the helium in the envelope. If an airship is flown light it has positive buoyancy, if it is flown heavy one speaks of negative buoyancy. Usually airships are flown slightly heavy so that they always sink back down to earth in case of an engine failure, the last bit of lift is usually aerodynamic lift generated through the forward movement of the airship. Since the helium in the envelope has a specific density, the air surrounding the airship either has a slightly higher or lower density or exactly the same if the airship is in equilibrium.
Now coming back to Duncans question. So now we have to go back to our old physics classes and think about temperature and density. If we look at the density of air we see that the warmer the air the lower the density. This leads so far that you can heat up the air and start flying, for example in a hotair-airship. So hot air rises because it's less dense, that means for our airship if it is flying from cold into warm air it looses lift because the density difference smaller. That is the one side of the medal which says warm air is bad for airships.
On the other side we have a term called Superheat, this describes the process of an airship heating up and becoming lighter because in addition to it's usual lift through the helium we now have warm helium that just like air has a lower density and produces more lift. Superheat happens when the sun warms up the helium in the envelope, which happens faster than the warming of the surrounding air. And suddenly the airship becomes a lot lighter.
Those two effects have a strong impact on airships and their performance and have to be taken in account when constructing and flying an airship. While I was researching for this blog entry i found a few articles on the web that are worth a read.
Especially a 12 Page Article written by David Samuels called "The Light Stuff — Learning to fly the Goodyear blimp" which was published 2003 in the New Yorker. But also the most recent entry in the Goodyear Blog mentions superheat. And last but not least we found an article on the Website of 21st Century Airships that mentions superheat. I hope that this answeres the question of Temperature and maybe we can pick up the discussion in the Forum. I am sure there is more to talk about. What other environmental parameters have to be taken into account? Rain, Snow, Wind ? Let's discuss in the Forum. If you enjoyed reading the article please consider posting it to Digg, Stumble Upon and Del.icio.us

2 comments:

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