Project Hybrid airship

[boogabooga]

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?

 

[markp]

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.

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

 

[boogabooga]

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:

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.

 

[markp]

The length of the airship is close to that of Airlander 10 which has a envelope volume of 38000 m3.

 

[boogabooga]

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.

 

[markp]

That volume might be for a box that encompasses the mesh.

I am thinking that the airship's envelope mesh volume could be scaled to the required volume calculated from information given in the scenario file.

Controlling the neutral buoyancy altitude by the pilot would be made perhaps by adding or dropping the ballast. The rigid airship sounds like a good idea for now. I haven't found any good sources yet to compare performance capabilities of rigid and non rigid airships.

 

[Urwumpe]

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.

There are two methods to calculate volume and PMI AFAIR, the monte-carlo method is slower but more accurate.

---------- Post added at 11:09 ---------- Previous post was at 11:06 ----------

Controlling the neutral buoyancy altitude by the pilot would be made perhaps by adding or dropping the ballast. The rigid airship sounds like a good idea for now. I haven't found any good sources yet to compare performance capabilities of rigid and non rigid airships.

I am already researching the old NACA documents for good information about rigids to create a more or less generic "rigidairship.dll". There had been quite many experiments in the 1920s about how to improve their performance.

---------- Post added at 19:17 ---------- Previous post was at 11:09 ----------

Something completely different:

---------- Post added at 22:58 ---------- Previous post was at 19:17 ----------

No surprise the R.101 had so much trouble during its flight. Its bouyancy was eaten up by over 64% deadweight, according to this report about the different structural designs of airships:

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930094544.pdf

And 47.9% of this deadweight was framing (For comparison: Its just 39% for the Akron, with just 51.8% deadweight to lift). Not surprising, if you look at the technology used.

 

[boogabooga]

 

[Urwumpe]

The NACA report to the first half of this video:

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930091641.pdf

 

[boogabooga]

Hello Mark,

Are you able to use Excel?

Ignoring ballonets for now, I've worked out the math(s) for an airship. This version assumes a constant envelope volume with the lifting gas at atmospheric surface pressure and temperature regardless of the gauge pressure (think non-rigid airship-or a gas bag- made out of a super-strong material).

It demonstrates for instance how to account for the lifting gas mass. It also demonstrates how to calculate either the payload mass or the ballast mass required to achieve a desired neutral buoyant altitude. I think that this technique would make more physical sense than keeping the mass constant and varying the envelope volume. We might change our minds about the "effective" envelope volume once ballonets get implemented, but I think that in general ballonets are there to keep the pressure from rupturing the envelope.

I don't have time now to go through the equations, but I can help you later. I might be able to take over the math modeling in the code, if you wish.

I'll work on a more sophisticated version of the workbook later with ballonets, more atmospheres, etc.

*gauge pressure is the pressure difference between the inside and outside of the envelope.


P.S., if anyone could help me provide atmospheric constants for other planets, that would be great.

 

[markp]

Thanks for the calculations, that's great. I had a look.

That would make things more convenient. I guess the pilot could query an onboard 'flight computer', consisting of your calculations, to determine how much ballast to add or what payload weight could be carried. The calculations could also be used to automatically inform the pilot of the new neutral buoyancy altitude if ballast is dropped or when the payload is released from the airship.

 

[Urwumpe]

Thanks for the calculations, that's great. I had a look.

That would make things more convenient. I guess the pilot could query an onboard 'flight computer', consisting of your calculations, to determine how much ballast to add or what payload weight could be carried. The calculations could also be used to automatically inform the pilot of the new neutral buoyancy altitude if ballast is dropped or when the payload is released from the airship.

Luckily Orbiter is not giving you many surprises there. In real airships, this is a lot tougher since the atmosphere can be very dynamic.

Also, for a hybrid airship, you should usually be above neutral buoyancy altitude, since you make use of dynamic lift.

 

[boogabooga]

That's the weird thing,

If you run the numbers in my workbook for Airlander 10, 38000 cubic meters of Helium at 20000 kg solid mass is not heavier than air, not even close. :blink:

 

[Urwumpe]

One cubic meter of Helium results in 10.9 N lifting force at sea level. So, you should get 42222 kg maximum weight.

Makes sense. In reality, the ballonets should be fully filled with air on the ground, so you have much less than 38000 m³ helium volume then. And since the ballonets are not shrinking to zero, this explains why the maximum payload of the airlander is less than the theoretical maximum.

 

[markp]

boogabooga if you could add a function or routine into the code to calculate the ceiling that can be queried from the main program. That would be fairly easy to implement I think.

Are you planning to factor in the engine thrust and aerodynamic lift into the calculation of a 'ceiling' as well?

The code is a bit of a mess at the momemt but I can spend a small amount of effort to make it more readable, without breaking anything, then post the updated version within the next few days.

I think it is ok to have a lighter-than-air hybrid airship, at least for now, although hybrid airships are supposed to be heavier-than-air.

I would certainly like it to have a lighter-than-air capability to enable it to do what I'd like it to do, i.e. delivery of heavy oversized objects like buildings. It also needs the ability to take on ballast. This is to maintain its neutral buoyancy altitude after delivery of the payload.

 

[Urwumpe]

What I wonder - would it be possible to produce a H2/He mixture that can't ignite?

Of course, the gas in the cells would need to be circulated, because otherwise, the H2 would concentrate on the ceiling of the cell. But just being a bit crazy there: Maybe this could increase payload capacity of a Helium airship significant. If the mix would have just 2% more lift force than helium, it would already be a huge difference in payload after subtracting the deadweight.

 

[kuddel]

Someone thought of this earlier
http://www.madsci.org/posts/archives/2003-07/1057952816.Ph.r.html

8.7% (Hydrogen) is stated there as a "save" mixture

 

[Urwumpe]

Someone thought of this earlier
http://www.madsci.org/posts/archives/2003-07/1057952816.Ph.r.html

8.7% (Hydrogen) is stated there as a "save" mixture

OK, even a bit less than I thought. Nevermind. :lol:

 

[markp]

Been playing around with boogabooga's excellent spreadsheet that calculates the lift capability of a non-rigid airship. To fly on Mars I think the airship envelope would most likely have to be at least 3 times its current length or about 30 times the volume of Airlander 10 to fly on Mars.

I can post my guesstimates and sources for the hybrid airship mass budget if anyone's interested.

During the course of tidying up the code I added the ability to configure the airship envelope size plus aerodynamic surfaces in the scenario file. The gondola and engines remain the original size and are not scaled. A few more days until I'll be ready to post a new version with the tidied up and commented code.

 

[boogabooga]

I don't have time this week to look at this but I'll try more next week.