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Old 06-07-2018, 03:12 PM   #31
boogabooga
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Actually, I don't understand fully what is meant by "venting" the lifting gas, even in the old days of constant volume rigid airships. A pure venting would decrease the internal density and increase the buoyant lift, right?

So it must mean replacing some of the lifting gas with air, but I'm not sure of the details.

Edit:

On further reflection I see that the old airships had separate internal gas bags that were not constant volume at all. So I assume that air would take up the space between the external and internal skin as the gas bag deflated?

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Originally Posted by markp View Post
 I am thinking of it as a non-rigid airship.
This means that the internal gauge pressure must be kept slightly positive and probably about constant to maintain the airship's structure. Venting the lifting gas (intentionally) is probably out of the question.

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 Well for Mars I am assuming the engines are ducted fans like on Earth but spinning at ten times the speed like for the NASA Mars helicopter. I haven't looked into it in any depth though.
I get the ducted fans.

I mean, on Earth it would have some kind of air-breathing combustion engines. On Mars...nuclear? Solar? Fuel Cells? You might need to have a configuration option to switch propulsion systems.

Last edited by boogabooga; 06-07-2018 at 03:34 PM.
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Old 06-07-2018, 04:00 PM   #32
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Quote:
Originally Posted by boogabooga View Post
 Actually, I don't understand fully what is meant by "venting" the lifting gas, even in the old days of constant volume rigid airships. A pure venting would decrease the internal density and increase the buoyant lift, right?
Yes. Remember: The gas cells inside the rigid hull can grow and shrink. The smaller the gas cells become, the more air is of course inside the hull. And we are talking about really small changes in volume there to have big effects.

When the pressure difference between inside the cell and outside the cell becomes too large by heating the gas or climbing, it has to be vented.

Also, if you want to land, its is one bad choice to vent gas - more skilled captains preferred to land during the evenings preserving the gas by letting it cool at altitude and the drop into a layer of warm air close to the surface, making the Zeppelin "heavy" relative to the air.

Generally, you often needed to vent during long distance trips because the fuel consumption made the Zeppelin lighter, which increased the tendency to climb. One solution to reduce this tendency was for example heating the gas before take-off. Or using large radiators to cool the engine exhaust and produce ballast water that way.
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Old 06-07-2018, 04:40 PM   #33
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How much overpressure could the gas bags support?

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  Or using large radiators to cool the engine exhaust and produce ballast water that way.
That was used in the Akron class. Helium was very expensive in the 1930s and venting was out of the question.
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Old 06-07-2018, 05:32 PM   #34
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Originally Posted by boogabooga View Post
 How much overpressure could the gas bags support?

I still try to find it out, did not find it yet. The USS Macon for example had a maximum pressure altitude of 900 meters. But I know that the German Zeppelins reached higher without problems. One military Zeppelin had a "static altitude" of 2000 meters, but had to vent 1% of its hydrogen for every 80 meters climbed.

Looks like German Zeppelins had rather been using a maximum volume of the gas cells as reference, rather than the pressure inside the gas cell. Especially the pre-129 gas cells had been really fragile, they had been made from one special skin of a cows stomach, because it is very gas tight and light, and multiple pieces can easily be glued together to one large bag by using salt water.




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Originally Posted by boogabooga View Post
 That was used in the Akron class. Helium was very expensive in the 1930s and venting was out of the question.

The Los Angeles also used it, since she was also filled with Helium after arriving in the USA.

---------- Post added at 19:32 ---------- Previous post was at 18:50 ----------



Of course, they did not mix hydrogen and air inside the gas cells. 95% hydrogen meant that the gas cell was inflated with just 95% of its maximum capacity with pure hydrogen.

Thus, not accounting for tension by gravity on the gas cells, the pressure inside the gas cell is equal to the outside pressure until reaching maximum volume. Once the pressure starts to increase significant, the gas cell is getting overinflated. So, practically, it might be as little as just a few Pa over outside pressure to start venting.

Last edited by Urwumpe; 06-07-2018 at 05:10 PM.
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Old 06-07-2018, 06:19 PM   #35
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Quote:
Originally Posted by Urwumpe View Post
 Especially the pre-129 gas cells had been really fragile, they had been made from one special skin of a cows stomach, because it is very gas tight and light, and multiple pieces can easily be glued together to one large bag by using salt water.
Goldbeater's skin. Because it was a polymer before the plastic industry was a thing. The number of bovines that must have had to be slaughtered to make one of those things is incredible, though I suppose that was going to happen anyway.

Airships. They are like giant air sausages. In more ways than one.
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Old 06-07-2018, 06:43 PM   #36
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Originally Posted by boogabooga View Post
 Goldbeater's skin. Because it was a polymer before the plastic industry was a thing. The number of bovines that must have had to be slaughtered to make one of those things is incredible, though I suppose that was going to happen anyway.

700000 for one WW1 military zeppelin.

Seven Hundred Thousand.

---------- Post added at 20:43 ---------- Previous post was at 20:28 ----------

Found the usual overpressure of a typical gas cell made of goldbeaters skin in a NACA report (NACA-TM-172) about the usage of this skin: 30 mm of water (294 Pa)

Last edited by Urwumpe; 06-07-2018 at 06:31 PM. Reason: Memory was off by one order of magnitude. Sorry.
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Old 06-07-2018, 11:53 PM   #37
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Airship Engineering (101)

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Old 06-10-2018, 08:21 PM   #38
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Uploaded version 5 to post #1

In the next version I hope to add a simple representation of the ballonets. For trimming the pitch this is already implemented via the ability to move the centre of mass in the model. I just need to revise the messages and perhaps limit the range of pitching. I also plan to add some fine control over the buoyancy within some realistic limits. I have kept the venting capability for the moment as I find it useful for testing.

This version has improved fan animations, a dust effect (not quite fully working yet), reverse thrust, larger propellant tanks and a revised manual.
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Old 06-11-2018, 09:04 AM   #39
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Big improvement.

I miss the ability to vary the neutral buoyancy altitude directly in the simulation.

Perhaps the pilot could increase or decrease the payload and/or starting ballast mass when landed and get a prediction of the buoyancy altitude?

Edit:
It could be that the ceiling is being changed, but not making it to the HUD.

---------- Post added Jun 11th, 2018 at 05:04 AM ---------- Previous post was Jun 10th, 2018 at 05:25 PM ----------

Version 5 bug reports:

Some HUD messages don't disappear, such as those relating to to changing the swivel speed

Some important parameters are not saved to the scn file. Especially GAS. Speaking of which, why is the default GAS 90 now?

Last edited by boogabooga; 06-10-2018 at 09:55 PM.
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Old 06-11-2018, 10:45 AM   #40
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Quote:
It could be that the ceiling is being changed, but not making it to the HUD.
The current ceiling value can be viewed by pressing c after pressing 1.

Quote:
Some HUD messages don't disappear, such as those relating to to changing the swivel speed
Thanks I'll look into it

Quote:
Some important parameters are not saved to the scn file. Especially GAS. Speaking of which, why is the default GAS 90 now?
GAS 90 is to make the airship is heavier than air so it can fly as a hybrid airship. The other 10 percent is taken up by some imaginary air filled ballonets.

This then makes the use of a neutral buoyancy CEILING somewhat useless. Not sure what course of action to take at the moment.

One possibility would be to add a physical model of the airship which can be used to predict the airship's performance, e.g. maximum altitude, given a set of parameters, e.g. ballonet pressure? Then the predicted results could be presented to the pilot who can then adjust parameters, such as the amount of air in the ballonets, until the predicted performance matches the desired performance. Not entirely sure how it's done in real life so will have to check that a bit.
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Old 06-11-2018, 11:44 AM   #41
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Quote:
Originally Posted by markp View Post
 The current ceiling value can be viewed by pressing c after pressing 1.



Thanks I'll look into it



GAS 90 is to make the airship is heavier than air so it can fly as a hybrid airship. The other 10 percent is taken up by some imaginary air filled ballonets.

This then makes the use of a neutral buoyancy CEILING somewhat useless. Not sure what course of action to take at the moment.

One possibility would be to add a physical model of the airship which can be used to predict the airship's performance, e.g. maximum altitude, given a set of parameters, e.g. ballonet pressure? Then the predicted results could be presented to the pilot who can then adjust parameters, such as the amount of air in the ballonets, until the predicted performance matches the desired performance. Not entirely sure how it's done in real life so will have to check that a bit.
Yeah, we need to have a discussion about airship physics.

I'll say now that I greatly prefer the CEILING system to the GAS system. It's helpful to the pilot to know the neutral buoyancy altitude, and to be able to set it. Perhaps that's how to implement ballonets.

What is the current model that you are using for airship physics, could you please describe it?
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Old 06-11-2018, 09:03 PM   #42
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Quote:
I'll say now that I greatly prefer the CEILING system to the GAS system. It's helpful to the pilot to know the neutral buoyancy altitude, and to be able to set it. Perhaps that's how to implement ballonets.
Yes that could work.

Quote:
What is the current model that you are using for airship physics, could you please describe it?
ok ...

Buoyant lift:

Firstly the atmospheric density is obtained from the desired ceiling altitude (neutral buoyancy altitude) set in the scenario file or via the HUD. The volume of the airship's envelope is then calculated as follows:

airship_volume=airship_mass/density_of_air_at_ceiling_altitude

where airship_mass=structural_mass+propellant_mass (+payload_mass if declared in the scenario file)

Structural_mass is around 20000 kg which might be similar to Airlander 10. Lifting gas has no mass in the model at the moment.

Then the buoyancy force acting on the airship during the simulation is calculated as:

buoyancy_force=airship_volume * density_air * acceleration_due_to_gravity

The buoyant force is added to the airship with AddForce() and acts at 5 m above the central point of the airship. Centre of mass movement is simulated by moving the point where buoyant force acts along the z-axis.

Aerodynamic lift and drag:

Aerodynamic lift and drag are modelled with Orbiter's aerofoil model with profiles that result in a maximum L/D of 4 at an angle of attack of 10 degrees.

Engine thrust:

The engine power is set as horsepower in the scenario and is converted into thrust as thrust=40*horsepower

Last edited by markp; 06-11-2018 at 09:06 PM.
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Old 06-11-2018, 09:48 PM   #43
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I'm going to have to take some time later to go through this (i.e. let's talk now but do not change the code yet), but this stands out immediately:

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Originally Posted by markp View Post
 The volume of the airship's envelope is then calculated as follows:
Let's ignore the ballonets for a bit.

For a non-rigid airship, my first approximation would be to treat the envelope volume as constant. What is the volume of the mesh itself?

The way that this would really work would be to fill the envelope to a slight overpressure with lifting gas. This is required for it to maintain structural rigidity. As the airship rises, the gauge pressure inside would tend to increase. I don't think that would change the volume much- just put more stress on the skin. Perhaps there is an emergency release, or perhaps this is where ballonets come it; have them inflated on the ground and let them out to keep constant gauge pressure. I'll have to read about this.

You could use what you have with a variable volume if you just say that it is a rigid airships and the volume represents an internal gas bag.

You have more variables floating around in the payload mass and the ballast. Decreasing either or both of those would increase the ceiling. Those should get worked into the ceiling altitude calculation.

Last edited by boogabooga; 06-12-2018 at 05:49 PM.
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Old 06-12-2018, 06:58 PM   #44
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The length of the airship is close to that of Airlander 10 which has a envelope volume of 38000 m3.
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Old 06-13-2018, 01:40 AM   #45
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Shipedit.exe is saying around 60,200 m^3 for the entire mesh, but admittedly I don't know how to use the program well.
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