[IW] Zeppelins mean Alternate Timelines. So, how are they made feasible?

[Ulzgoroth]

Originally Posted by Fred Brackin

I couldn't actually read the document due to useless formatting but the fuel efficiency claims are almost certainly false on a point A to Point B basis.

The LTAs might use less fuel on a per hour basis or something like that but once you multiply gallons per hour by hours necessary for the trip that advantage is going away. There really isn't any way around the drag penalty.

It's a NASA technical memorandum with a bunch of references. So while it's an appeal to authority (barring reading the references, anyway), I think it's got a bit of that. I couldn't find a reference to higher fuel economy in my quick skim, though.

It also seemed to claim that airships were potentially as survivable as airplanes in bad weather.

[Fred Brackin]

Originally Posted by Ulzgoroth

It also seemed to claim that airships were potentially as survivable as airplanes in bad weather.

Relatively unlikely. The problem is a matter of surface area exposed to aerial turbulence v. the structural strength of the airframe (LTA or HTA) with a fudge factor for size since larger size allows for greater divergence of the forces affecting the vehicle i.e. the wind at the nose isn't pushing in the exact same direction as the wind at the tail. R-r-r-r-r-r-i-i-i-i-p.

This effect is fairly obvious in pictures of the US Navy airship Shenandoah which broke in half and went down over land in a storm.

http://www.history.navy.mil/photos/e...5/zr1-loss.htm

This problem is reduced but not eliminated for non-rigid airships. Those also are helped by their relatively small size on the LTA scale. Their survivability might be close to that of light aircraft.

[malloyd]

Originally Posted by Fred Brackin

The LTAs might use less fuel on a per hour basis or something like that but once you multiply gallons per hour by hours necessary for the trip that advantage is going away. There really isn't any way around the drag penalty.

Sure there is, and its the one airships use. Drag is proportional to both wetted surface and velocity squared (and power is hence proportional to area and velocity cubed and energy consumption per unit distance is proportional to area x velocity ^2). Airships move more slowly than airplanes.

[Fred Brackin]

Originally Posted by malloyd

Sure there is, and its the one airships use. Drag is proportional to both wetted surface and velocity squared (and power is hence proportional to area and velocity cubed and energy consumption per unit distance is proportional to area x velocity ^2). Airships move more slowly than airplanes.

But that isn't a choice. With the same engines and same payload as an airplane a dirigible has to move more slowly because of the drag.

When you equip those engines with, say, 8 hours of fuel and when your 8 hours are up the faster airplane has traveled farther on the same amount of fuel. The difference was all in the energy the LTA spent overcoming drag.

Reduce the engines on the LTA so the fuel lasts 16 hours and you'll travel more slowly yet. There probably is some point where the LTA can travel the same distance on the same amount of fuel but then you find that your LTA is inefficient on the basis of payload hauled per unit of time.

That's important for getting the most bang for your buck too. Your vehicle has to be _much_ cheaper (like a river barge) to be really efficient at hauling cargo slowly. I don't think you ever get to the point where LTAs compete with river barges.

[vicky_molokh]

Originally Posted by Fred Brackin

That's important for getting the most bang for your buck too. Your vehicle has to be _much_ cheaper (like a river barge) to be really efficient at hauling cargo slowly. I don't think you ever get to the point where LTAs compete with river barges.

Hmm. Divergent-tech contragravity-charged gas?

[malloyd]

Originally Posted by Fred Brackin

Reduce the engines on the LTA so the fuel lasts 16 hours and you'll travel more slowly yet. There probably is some point where the LTA can travel the same distance on the same amount of fuel but then you find that your LTA is inefficient on the basis of payload hauled per unit of time

It's pretty easily figured from those relationships. Suppose area is 100 times as great, you travel 1/10 as quickly and drag is the same. Energy use for the trip is force x distance, so for the same distance you use the same amount of fuel. It takes 10 times as long, so you need 1/10 the power (and engines)

That's important for getting the most bang for your buck too. Your vehicle has to be _much_ cheaper (like a river barge) to be really efficient at hauling cargo slowly. I don't think you ever get to the point where LTAs compete with river barges.

Actually, you probably do if you are just considering drag energy costs - drag and displacement lift both being proportional to fluid density and volume scaling faster than area it looks to me like the dirigible actually wins in terms of energy per unit transported by virtue of being bigger than the barge if they move at the same speed. You end up losing on the anciliary costs that haven't anything to do with the fundamental differences in motive system - barges are made of cheaper stuff, compensating for leak rate by pumping water is cheaper than replacing hydrogen etc. - and the unfortunate tendency of air to move relative to your destinations faster and less predictably than water tends to.

[Green-Neck]

Is there an energy saving in the dirigible's engines not having to provide lift?

[David Johnston2]

Originally Posted by Green-Neck

Is there an energy saving in the dirigible's engines not having to provide lift?

The decreased drag from not having wings is more than compensated for by the size of the bag you are lugging. Basically we need the other times to not discover Bernoulli's principle.

[malloyd]

Originally Posted by Green-Neck

Is there an energy saving in the dirigible's engines not having to provide lift?

Some. For finite length wings aerodynamic lift generates induced drag - raising the drag for the airplane above what it would be simply for its area. But it's not enormously big compared to other drag terms.

[Fred Brackin]

Originally Posted by Green-Neck

Is there an energy saving in the dirigible's engines not having to provide lift?

Only when you're hovering or at least travelling below the airplane's stall speed.

Wings provide lift in exchange for drag. Gasbags provide lift regardless of speed but inflict drag when attempting to move forward.

When trying to travel from point A to point B both wings and gasbags are trading lift for drag and the wing does it more efficiently at any speed above stall speed.