Heavy lift LTA

Technical discussions regarding LTA-technology.
HeavyLifter
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Heavy lift LTA

Postby HeavyLifter » Mon Mar 27, 2017 1:50 am

There is a big problem that puts a limit on scaling up an airship The load bearing structure of an airship goes up proportional to the 4th power of the diameter, while the lifting force goes up with the 3rd power of the diameter. For example, consider the cables that hold the gondola on a blimp. Lets suppose you have a certain sized blimp in which the weight of those cables takes up 10% of the lifting force available. Not too bad. Let's say that the blimp aerostat will lift 1000 pounds and the cables weigh 100lb and the skin, gondola and engine weighs 450 lb. SO, you have a payload capacity of 450 pounds (45% of the gross lift). I am just making these numbers up for an illustration. So you have a nice blimp that can carry several people with their luggage.

Now, scale this thing up by a factor of 3. The gross lifting capacity will go up 27 times The skin area goes up by a factor of 9, and maybe you should increase the skin thickness a bit, so that the skin, gondola and motor weight to 1500 pounds. However, take a look at the load bearing cables. They need to support 27 times the load, so their cross-sectional area increases by 27 times, but the length also increases by three times, so their volume and hence weight, goes up by 81 times. Summary: gross lift 27000 pounds, skin and gondola, 1500 pounds, load bearing cables, 8100 pounds. The load bearing cables are now taking up 30% of the weight, and the payload weight is now 17,400 pounds (64% of the gross lift) Wow, we are gaining, so this looks like we can continue to scale up and the sky is the limit.

Try again to increase this by 3 times. The gross lift goes up to 728,000 pounds, the skin, and gondola maybe go up to 40,000 pounds, but the weight of the load bearing cables goes up 81 times to 656,000 pounds. Uh oh. Who wants to make a huge blimp like this where the payload is 32,000 pounds, 4% of the gross lift? Somewhere along the line the load bearing cables' weight exceeds the lift. Well, back off a minute and realize that the Payload is only 32,000 pounds, plus the 40,000 lb, so the cables can be slimmed down, so that their weight will be less, so the payload can actually increase quite a bit, until the load increase and the cable capacity match. However, the sky is not the limit and scaling up runs into an impassible barrier eventually.

A new approach will be needed. I'll stop for now and see if I get any indication of interest in pursuing this line of thought. Questions? Comments? Wondering where I am going with this? I don't want to talk to myself here.

turtleairships
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Re: Heavy lift LTA

Postby turtleairships » Mon Mar 27, 2017 11:24 pm

You may receive some comments about how "new" materials used for cables would reduce their weight; but the overall argument you make is valid.

the true potential of future Lighter-than-Air flight can only be achieved if "blimps" are left behind; back in the 19th Century where they belong......

HeavyLifter
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Re: Heavy lift LTA

Postby HeavyLifter » Wed Mar 29, 2017 12:57 am

Yes, stronger materials and more efficient structural designs push the envelope out further, but the limit will be reached somewhere.

Blimps work well for the purposes they were made for. However, to create a bigger limit on heavy lift, we should look at other designs.

I want to add that the upper limit of a particular design will not be a fine line. There is a margin of safety needed, which blurs the maximum limit depending on the acceptable risk. In design engineering, it is very difficult to define when something will break. What engineering science has achieved is to define the limit where you are sure it will NOT break. In designing airships, it is tempting to reduce that safety factor, and not shoot for long life and being able to handle the worst reasonable cases. The Hindenburg failed because it was not able to take hard turns and in frustration and pride the pilot put it into a hard turn. A cable snapped and ruptured a hydrogen bag. Static electricity lit it off. The Akron apparently suffered fatigue failure.

So, when I am talking about the maximum lift possible, I am talking in the context of a safe design that will go on for decades like the Goodyear blimp.

turtleairships
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Re: Heavy lift LTA

Postby turtleairships » Thu Mar 30, 2017 8:36 pm

@HeavyLifter
Your experience with AeroLift is intriguing; it would appear that we are contemporaries.
I'd enjoy learning more about your ideas towards future airships and
would welcome a private email dialog.

I must offer a disclaimer though.......I do not favor the idea of heavy lift airships as a viable business model; the opportunities seem to be somewhat limited for hauling large/heavy payloads into places without infrastructure; as so many insist.

I always suggest that a (big box whatever) only wants to move ONCE.....whereas, a living passenger will want to fly many times; making carrying passengers the most attractive business.

HeavyLifter
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Re: Heavy lift LTA

Postby HeavyLifter » Thu Mar 30, 2017 9:54 pm

A PM discussion sound good. I have limited time right now, though. Maybe next week.

I am thinking of Heavy Lift to do some things that are not possible otherwise. I agree that regular cargo transportation is probably not a good business model for the LTA concepts I have in mind. The Airlander with its body lift design can have a good use for those applications in the great white north. I wouldn't want to try to compete with it, because as you say, the market for that is quite limited.

More later.

HeavyLifter
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Re: Heavy lift LTA

Postby HeavyLifter » Sun Apr 02, 2017 2:24 am

Basically I am thinking of competing with helicopters, not long range transportation, particularly ecological logging. If you think of all the things helicopters do, you will see where a ship that will lift 10 times as much with much less energy has a niche to fill.

HeavyLifter
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Re: Heavy lift LTA

Postby HeavyLifter » Thu Apr 06, 2017 2:02 am

I guess with over 100 views, I am not talking to myself, although I hope to get more feedback.

The scaling problem not only means that there is a limit to how heavy a given type of airship can be sized to lift, it also means that proof-of-concept demonstrators are not useful, except for stimulating imagination (and maybe crowdfunding dollars destined to disappoint investors). It seems that currently there is a trend to develop a cool concept, make a model, take lots of video, ask for funding to make a bigger version, say a 10 ton model, with the idea that if this 10 ton model works well, then it deserves a bunch of funding so they can make the 200 ton version. No, every size of ship is a different story. If we want to make a 200 ton ship, we have to design a 200 ton ship, check that the engineering analysis of the detailed design is estimated to work well in real conditions with appropriate safety factors. Then, you have to build the big ship directly.

We need to go for the maximum from the beginning. To find that maximum, start with a design for the load lifting ship that is bare bones. This means only load lifting and handling structures in a reasonably aerodynamic shape, but don't get too aerodynamic. Remember that we are not producing a long haul craft, but a heavy lift craft to do lifting of a scale not done by any other craft available. Leave out propulsion devices, fins and tails. To fight the scaling problem, all of those functions should be transferred off of the load carrying ship onto external craft. This will ensure that we are getting the most potential lift. As long as we are including all of these other things we are starting with a handicap that will bring us to the limit sooner. But when we minimize structures under bending load, we start out in the scaling game with a decided advantage.

Once we have an optimized design for the biggest capacity load lifting aerostat possible, then the mobilizing craft can be added in. The first option would be to use this super heavy lifter with ground based handling like a logging balloon. However, to really be effective as an airship, the mobilizing craft should be airships themselves designed to optimize that function. They need to be agile, able to apply forces in any direction, so they need to be roughly neutrally buoyant themselves. They need to have the combined force capacity of the aerostatic lift of the load carrying ship to hold it down when it drops its load, and with that much force capacity, they would be able to hold the load carrying ship against winds, move it horizontally at a reasonable speeds and also to double the lift for short hauls.

This specialized multi-ship concept is the only way to reach this maximum load capacity, whatever it turns out to be, because a single ship has a built-in disadvantage in the scaling game, and therefore cannot compete. I am hoping that all of you will be able to see that and momentum can develop to eventually go down that path. I am open to hear any suggestions or criticisms.

HeavyLifter
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Re: Heavy lift LTA

Postby HeavyLifter » Sun Apr 09, 2017 1:21 am

Another thing I want to point out is that the wind forces on an airship are proportional to the cross-sectional area, which means proportional to the diameter squared, all other things like drag coefficient and wind speed being equal. However, the propulsion force capability would have to be proportional to the gross weight, which normally goes up by the 3rd power of diameter. So, if you make an airship 3 times bigger, you will be looking at 27 time the capacity, but only 9 times the wind loads. This means that in general the bigger the ship is, the better it will handle wind. This is another reason on the positive side to make the big ship directly. A smaller scale "demonstrator" might have a lot of trouble with the wind, but the bigger ship might not.


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