Author Topic: Lighter Than Air Travel  (Read 1724 times)

Offline Tragic Story

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Lighter Than Air Travel
« on: February 14, 2014, 12:14:55 PM »
So since my flight was cancelled today, I was thinking why do we not use LTA aircraft anymore?  With the proper harnessing, it seems ideal to take off/land in any inclement weather since it canít stall so it could just be floated up or cranked down.  Of course, the downside to this is they are slow.  That is where my favorite engineers come in:


How can we get a LTA aircraft to Mach 1? 

Well, that may be pushing it but how fast could one go, assuming you had Elon Musk funding the endeavor?
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Offline scikopas

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Re: Lighter Than Air Travel
« Reply #1 on: February 20, 2014, 04:56:53 PM »
Mach 1 with LTA!? that would be super awesome!  I'd hate to think about the aerodynamic drag though, you need >1m^3 of volume to lift 1kg, and thats even using Hydrogen, not Helium.

The main problems for LTA popularity (as I see it) are:
     a) people need to get over that Hydrogen is dangerous and "oh the humanity." do they think jet fuel, nuclear submarines, and gasoline are safe?
     b) mega giant massive hangars to house these things are hard to maintain.
     c) why do people need to go so fast? take it easy, chill, read a book, have a beer.
PhD student in Materials Science at Arizona State University currently working on high-temperature superconductors and quantum computers or something.
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Offline adrive7

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Re: Lighter Than Air Travel
« Reply #2 on: March 19, 2014, 06:53:01 PM »
The answer is "In Thrust We Trust". Basically, if you want to go fast, you need Thrust. If you want to go really fast, you need a METRIC CRAP TON OF THRUST. Let's do some math:

The drag equation for "low speed" Aerodynamics: D = .5(Air Density)(Velocity)^2(Coefficient of Drag)(Cross Sectional Area)

Airships operate fairly close to the ground, so let's say ~1,000m.

Air Density @ 1,000m = 1.112 kg/m^3 (Standard Day)

Coefficient of Drag (Cd) on a long Ellipse (Airship Shape): ~.2

Cross Sectional Area = .5*(20m)^2*pi = 628.3 m^2 (The Hindenburg was ~40m across)

Things get weird when you start going fast, so lets stay "low speed" for now and say velocity  = Mach .3, which at sea level = 102.9 m/s. Still pretty freakin quick for an airship.

So: .5(1.112 kg/m^3)(102.9m/s)^2(.2)(628.3m^2) = 739,780 Newtons of drag, (166,309.16 pounds force)

To get the power required to move that, we multiply by velocity once again (Power = v^3 relationship!)

739,780 kg-m/s^2*102.9m/s = 76,123,363.8 Watts (102,083.112 HP)

That's a lot of horsepower/thrust required, but it's not outside the realm of possibility. The GE90, two of which hang under a Boeing 777, can make up to 115,000 pounds thrust, but they weigh quite a lot (~8,000 kg).

For reference, the Hindenburg had 4 1,200 HP engines.

Now, once you start talking about going even faster, this starts to happen to your drag coefficient:


Trouble. Drag skyrockets as you approach mach 1. (This chart isn't for an airship, obviously, so probably not that exact chart, but similar) The drag and power required would similarly skyrocket.



Keep in mind, all that force has to be reacted by the airship structure, which means lots more weight. Weight that has to be lifted by gas.

I'm an LTA fan, I'd love to see airships come back as a thing. But I'm ok with slow airships. Make the trip an event. See the country. Treat it like a cruise, rather than transport. Etc etc.

Joe
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Offline adrive7

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Re: Lighter Than Air Travel
« Reply #3 on: March 19, 2014, 06:56:27 PM »
Also, back when airships were a thing, NACA wrote a report on such things: http://naca.central.cranfield.ac.uk/reports/1926/naca-tn-248.pdf
Joe
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