PDA

View Full Version : I have an engineering problem for the experten.



na85
01-02-2008, 11:19 PM
This summer I'm going to try to build a remote control Zeppelin.

My problem is this: How do I achieve different states of buoyancy?

I've been toying with various ways of compressing the gas inside the bag to alter its density, but all the solutions I've come up with seem easy to implement on a full-size airship, but not so much for a smaller one.

I'm envisioning something 10 feet long, if that helps.

Any thoughts?

leitmotiv
01-02-2008, 11:22 PM
Here is a site dedicated to airship modeling with an information forum for working model airships---good luck! What a wonderful idea.

http://www.airshipmodeler.com/

luftluuver
01-03-2008, 12:41 AM
Originally posted by na85:
I've been toying with various ways of compressing the gas inside the bag to alter its density, but all the solutions I've come up with seem easy to implement on a full-size airship, but not so much for a smaller one.

A pound of gas is pound of gas whether it is in one cubic foot or 10 cubic feet, iirc.

The-Pizza-Man
01-03-2008, 02:27 AM
Originally posted by luftluuver:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by na85:
I've been toying with various ways of compressing the gas inside the bag to alter its density, but all the solutions I've come up with seem easy to implement on a full-size airship, but not so much for a smaller one.

A pound of gas is pound of gas whether it is in one cubic foot or 10 cubic feet, iirc. </div></BLOCKQUOTE>

and your point is....

But anyway, I'd imagine a good way would be to use water ballast and gas release valve. Drop water to make it go higher, release helium to make it go lower. The biggest problem I can think of that would be that you would end up with a limited number of altitude changes.

Redwulf 32 - Nis
01-03-2008, 02:53 AM
Originally posted by luftluuver:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by na85:
I've been toying with various ways of compressing the gas inside the bag to alter its density, but all the solutions I've come up with seem easy to implement on a full-size airship, but not so much for a smaller one.

A pound of gas is pound of gas whether it is in one cubic foot or 10 cubic feet, iirc. </div></BLOCKQUOTE>

This link (http://en.wikipedia.org/wiki/Buoyancy) might help clarify http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif

K_Freddie
01-03-2008, 06:03 AM
Why don't you use the age old concept of ballooning - Hot Air.
Work out the volume of the balloon and how weight it can lift (your small gas supply and control systems). A few vents for bouyancy control, and lightweight motors for speed, ect..

Light weight miniature machinery is going to be your problem., so I think it's going to be trial and error.

Good luck.
http://forums.ubi.com/groupee_common/emoticons/icon_cool.gif

DmdSeeker
01-03-2008, 07:33 AM
Put a ballon inside yuor ballon, and connect it to a soda syphon CO2 bottle.

You can blow up the inner ballon with a relatively heavy gas (CO2) and compress the lighter gas in the outer ballon as the inner one expands.

That should control your bouyancy.

Jaws2002
01-03-2008, 09:10 AM
Originally posted by DmdSeeker:
Put a ballon inside yuor ballon, and connect it to a soda syphon CO2 bottle.

You can blow up the inner ballon with a relatively heavy gas (CO2) and compress the lighter gas in the outer ballon as the inner one expands.

That should control your bouyancy.

Very smart. The perfect solution.http://forums.ubi.com/images/smilies/25.gif

JG14_Josf
01-03-2008, 10:02 AM
If the size of he balloon can change from small displacement size to a large displacement size, then, more and less air is displaced by the size of the balloon.

If the gas used to change the size of the balloon is lighter than air, then, adding size to the balloon will displace more heavy air with lighter gas. The force of gravity will then push the larger and lighter balloon up.

na85
01-03-2008, 10:45 AM
Originally posted by luftluuver:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by na85:
I've been toying with various ways of compressing the gas inside the bag to alter its density, but all the solutions I've come up with seem easy to implement on a full-size airship, but not so much for a smaller one.

A pound of gas is pound of gas whether it is in one cubic foot or 10 cubic feet, iirc. </div></BLOCKQUOTE>

You're right, but buoyancy is a function of density, not volume.

na85
01-03-2008, 10:48 AM
Originally posted by DmdSeeker:
Put a ballon inside yuor ballon, and connect it to a soda syphon CO2 bottle.

You can blow up the inner ballon with a relatively heavy gas (CO2) and compress the lighter gas in the outer ballon as the inner one expands.

That should control your bouyancy.

I've been thinking along these lines. Still trying to come up with a good way to compress the inner balloon if I want to go up again.

JtD
01-03-2008, 11:38 AM
Shifting masses around inside your blimp as in pumping up balloons won't do the trick. Either change the volume of the entire hull, for instance by increasing the size of the hull while reducing inside pressure (some sort of mechanical expansion) or by increasing the size of the hull while maintaining inside pressure (some sort of heating). If you want to keep changing altitude for endless cycles you need to have some independent power input, for instance solar panels to either warm or expand your Zeppelin.

Or you do it like the real things and life with a limited amount of cycles. Best way here indeed seems to be to carry and drop water. It's not that you need a lot.

You can also mount engines that can be directed upwards and downwards and control climb and dive with them.

My 2 cents, good luck. I'd be interested to see your result. http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif

M_Gunz
01-03-2008, 12:16 PM
Originally posted by na85:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by DmdSeeker:
Put a ballon inside yuor ballon, and connect it to a soda syphon CO2 bottle.

You can blow up the inner ballon with a relatively heavy gas (CO2) and compress the lighter gas in the outer ballon as the inner one expands.

That should control your bouyancy.

I've been thinking along these lines. Still trying to come up with a good way to compress the inner balloon if I want to go up again. </div></BLOCKQUOTE>

You could include a controlled vent in that case or you can carry ballast to drop as they
really did.

OTOH you could put a plastic piston and cylinder inside with a mechanism to lengthen or
shorten the whole, a motor-turned screwjack wouldn't need to be very heavy. That cylinder
would need a tube leading outside and its function would be to change the density of the
rest of the gas in the shell.

I can hook you up with a man who has designed and built blimps in the US, Germany (post WWII)
and Japan as well as flew them and trained others to do. He might find your project fun.
Let me know and I'll contact him, he's an old buddy of mine since 79.

JtD
01-03-2008, 01:42 PM
The problem with the mechanical hull extension will be a tight enough seal, I suppose.

general_kalle
01-03-2008, 02:34 PM
sounds like a job for Raaid http://forums.ubi.com/images/smilies/16x16_smiley-wink.gif

M_Gunz
01-03-2008, 03:39 PM
Originally posted by JtD:
The problem with the mechanical hull extension will be a tight enough seal, I suppose.

If you mean about what I wrote, the cylinder is fully internal and the hull (zeps are rigid)
stays the same. An o-ring will hold back more pressure difference than you'd need.

All you want is to make the thing neutral buoyancy or maybe slightly heavy. The props drive
the craft up or down. The tricky part is maintaining buoyancy over changes in alt IRL to
keep the containment intact. Getting into an updraft that takes the bag up faster than it can
vent off is of real and deadly concern to blimp drivers. For sure to zep drivers as well.

Vent details were shown on a somewhat recent show about the Hindenburg. Na85, if you want
to make a model zeppelin then that one would maybe be a best candidate, or would a WWI bomb
dropper suit better?

Viper2005_
01-03-2008, 05:55 PM
Blimps make more sense than Zeps at small scales because the lift scales as l^3 whilst the structural mass scales as l^2, meaning that you've got less margin to play with and are therefore less able to afford the mass of a rigid aeroshell which offers a pretty limited aerodynamic benefit anyway in this context (low speed, low Reynolds number).

I would suggest that at this small scale, at low altitudes and over the short flight durations implied by a hobby project, the best approach would be to carry a quantity of water ballast.

Then you can dump ballast to go up and dump helium to go down.

This allows you to keep the whole system simple and save weight. Of course, this is wasteful of helium, and would eventually limit duration, so you should try to use aerodynamic lift and vectored thrust in preference for small altitude changes.

This trade may be numerically optimised if you feel that way inclined, and you could then use a simple set of computerised control laws to decide which altitude control mechanism to use as a function of the magnitude and duration of manoeuvre demand and available battery voltage (which may be used to infer battery charge).

***It is advisable to insert a fail-safe helium dump valve in your design so that it won't float away into the sunset (posing a hazard to aerial navigation) in the event of transmitter or receiver failure.***

Spinnetti
01-03-2008, 06:01 PM
I didn't read everything, but y'all are making this too difficult. M_Gunz is on the right track.

A Zeppelin works just like a submarine, not like a balloon.

Make it neutrally buoyant, then use the "dive planes" (the fins and motor) to adjust attitude. So basically it just floats there, and you fly it with the control surfaces!

Its relatively easy if your structure is light enough (thats where you will struggle).

You also don't seal the whole structure, but rather have balloons inside the structure to hold the gas (you would of course use helium, not hydrogen right? http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif

Finally, the flight envelope is so small, you shouldn't have to worry about buoyancy changes over the narrow altitude and range you will be dealing with.

Just give it a try before you sweat the little stuff. You will have to really get it light though - I'm not sure it can be both scale and light enough. Its easy if you use mylar and a dorky looking 'zep - I've seen lots of those.

na85
01-03-2008, 07:22 PM
Originally posted by JtD:
Shifting masses around inside your blimp as in pumping up balloons won't do the trick. Either change the volume of the entire hull, for instance by increasing the size of the hull while reducing inside pressure (some sort of mechanical expansion) or by increasing the size of the hull while maintaining inside pressure (some sort of heating). If you want to keep changing altitude for endless cycles you need to have some independent power input, for instance solar panels to either warm or expand your Zeppelin.

Or you do it like the real things and life with a limited amount of cycles. Best way here indeed seems to be to carry and drop water. It's not that you need a lot.

You can also mount engines that can be directed upwards and downwards and control climb and dive with them.

My 2 cents, good luck. I'd be interested to see your result. http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif

I'll certainly post pics when I start construction

na85
01-03-2008, 07:23 PM
Originally posted by M_Gunz:

OTOH you could put a plastic piston and cylinder inside with a mechanism to lengthen or
shorten the whole, a motor-turned screwjack wouldn't need to be very heavy. That cylinder
would need a tube leading outside and its function would be to change the density of the
rest of the gas in the shell.

Ahhhh, that was the missing piece of the puzzle. A tube to the outside air!

Nice, ty.

na85
01-03-2008, 07:27 PM
Originally posted by M_Gunz:
The tricky part is maintaining buoyancy over changes in alt IRL to
keep the containment intact. Getting into an updraft that takes the bag up faster than it can
vent off is of real and deadly concern to blimp drivers. For sure to zep drivers as well.


I wasn't planning on flying it very high. I can't imagine that 100 feet will affect the containment very much, will it?



Vent details were shown on a somewhat recent show about the Hindenburg. Na85, if you want
to make a model zeppelin then that one would maybe be a best candidate, or would a WWI bomb
dropper suit better?

I don't really know. Certainly it would be awesome to drop bombs. Water balloons? http://forums.ubi.com/images/smilies/icon_twisted.gif

na85
01-03-2008, 07:53 PM
Originally posted by Spinnetti:
You will have to really get it light though - I'm not sure it can be both scale and light enough. Its easy if you use mylar and a dorky looking 'zep - I've seen lots of those.

What's mylar?

I was planning to use something like aluminum or fibreglass rods to make a skeleton and cover it with fabric.

If weight allows, I think the ultimate would be to include a wireless webcam so I can get the bird's eye view. But that's a long ways ahead.

JtD
01-03-2008, 10:48 PM
Originally posted by M_Gunz:

If you mean about what I wrote, the cylinder is fully internal and the hull (zeps are rigid)
stays the same. An o-ring will hold back more pressure difference than you'd need...

I was also referring to you, but it was a general statement. Air density is around 1.2kg/m³ at sea level, so in order to achieve something with volume changes you'd need some serious volume to work with. Based on the size of the ship every liter will give you about 0.005 m/s² vertical acceleration, meaning if it goes down with a foot per minute (hardly notable) and you add a liter of volume it will need 10 seconds to stop that movement. Someone sneezing will have a bigger impact.

I'm not sure a simple o-ring is good enough for a seal, but then maybe the seal doesn't contribute as much to the overall Helium loss as I thought yesterday. It's tiny compared to the hull surface.

Movable engines are probably the simplest and most effective solution.

M_Gunz
01-04-2008, 06:15 AM
Originally posted by na85:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by M_Gunz:
The tricky part is maintaining buoyancy over changes in alt IRL to
keep the containment intact. Getting into an updraft that takes the bag up faster than it can
vent off is of real and deadly concern to blimp drivers. For sure to zep drivers as well.


I wasn't planning on flying it very high. I can't imagine that 100 feet will affect the containment very much, will it? </div></BLOCKQUOTE>

No. Not a bit though you'd better not go playing on windy days! If it suddenly gets colder
out the you might need to vent gas quickly so watch for day-evening-night effects, a small
change can put the rig into a steady climb.


<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">
Vent details were shown on a somewhat recent show about the Hindenburg. Na85, if you want
to make a model zeppelin then that one would maybe be a best candidate, or would a WWI bomb
dropper suit better?

I don't really know. Certainly it would be awesome to drop bombs. Water balloons? http://forums.ubi.com/images/smilies/icon_twisted.gif </div></BLOCKQUOTE>

The size of the thing grows vastly with every extra pound it must carry. Perhaps a blimp
would make more sense just because you can deflate it for storage. Did you know that Navy
blimps carried depth charges and 50 cals?

Of course the prime US blimps to model would be the Akron, the Macon or the Savannah.
Then you'd get to try and model the Sparrowhawks as well.

M_Gunz
01-04-2008, 06:55 AM
Originally posted by JtD:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by M_Gunz:

If you mean about what I wrote, the cylinder is fully internal and the hull (zeps are rigid)
stays the same. An o-ring will hold back more pressure difference than you'd need...

I was also referring to you, but it was a general statement. Air density is around 1.2kg/m³ at sea level, so in order to achieve something with volume changes you'd need some serious volume to work with. </div></BLOCKQUOTE>

You seem to miss the part about AFTER YOU HAVE ACHIEVED NEUTRAL BUOYANCY IT DOESN'T TAKE A
LOT TO RAISE OR LOWER THE BALLOON MERELY THROUGH BUOYANCY. Not that it's critical anyway.


Based on the size of the ship every liter will give you about 0.005 m/s² vertical acceleration, meaning if it goes down with a foot per minute (hardly notable) and you add a liter of volume it will need 10 seconds to stop that movement. Someone sneezing will have a bigger impact.

I'm not sure a simple o-ring is good enough for a seal, but then maybe the seal doesn't contribute as much to the overall Helium loss as I thought yesterday. It's tiny compared to the hull surface.

Movable engines are probably the simplest and most effective solution.

Move over Don Rickles, we have a real pro here............

I was talking about a cylinder inside a rigid container, with a tube to outside air, that
would change the density of the gas outside the cylinder but inside the envelope itself.
It's a mechanism that was used on a small scale to control buoyancy in model submarines
back in the 60's that I know of.

If the craft descends at 1 foot per minute then how is that acceleration btw?

As described to me in detail by a man with over 9000 hours piloting airships your speed
and buoyancy both matter. Even when the craft is slightly heavier than air you have the
huge surface, the fins and your forward speed to fly with. Put the nose even a little bit
up and you have plenty of LIFT.

He did make a passable blimp for MSFS back in the 90's as long as you kept the thing moving
and another guy made a Tillamook. I do have the files as well. I've flown it on his PC
with him instructing me and telling me all what wasn't there. MSFS does not let you make
LTA models so movement is how the thing flew.

It's not like I invented any of that. Go find out how the real things worked.

You want to vector the thrust on a blimp then have fun placing the engines so it doesn't
just spin. It's possible, at least one RC toy did it but it didn't much resemble the real.
You don't need to do that. The shape of the thing, the fins and the thrust will carry even
a slightly heavy airship up just fine. No real airship is in great danger while sinking at
1 foot per minute.

Why helium for a model? It's expensive. As long as you don't make sparks or you just keep
the hydrogen pure there is no real danger with hydrogen. I doubt that Na85 will go with
aluminized skin and have large static buildups from prolonged high speed through air to
recreate the Hindenburg disaster so don't bother throwing that herring in.

H311 you could put a bag inside the envelope and inflate it from outside with a very small
pump. Do it right even a fishtank pump in combination with 1 controlled valve would work.

He could even just use a gas valve, water tank and ballast valve to control buoyancy.

M_Gunz
01-04-2008, 07:06 AM
Originally posted by na85:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Spinnetti:
You will have to really get it light though - I'm not sure it can be both scale and light enough. Its easy if you use mylar and a dorky looking 'zep - I've seen lots of those.

What's mylar?

I was planning to use something like aluminum or fibreglass rods to make a skeleton and cover it with fabric.

If weight allows, I think the ultimate would be to include a wireless webcam so I can get the bird's eye view. But that's a long ways ahead. </div></BLOCKQUOTE>

You can quickly wind up with a 10+ foot long model.
Good news is you don't really need to make it float unless you want to hover stationary.

You want me to hook you up with a guy that spent decades engineering and flying the real
articles? He won't be around forever though, he's a WWII veteran.

M_Gunz
01-04-2008, 07:15 AM
Originally posted by Spinnetti:
I didn't read everything, but y'all are making this too difficult. M_Gunz is on the right track.

A Zeppelin works just like a submarine, not like a balloon.

Make it neutrally buoyant, then use the "dive planes" (the fins and motor) to adjust attitude. So basically it just floats there, and you fly it with the control surfaces!

Its relatively easy if your structure is light enough (thats where you will struggle).

You also don't seal the whole structure, but rather have balloons inside the structure to hold the gas (you would of course use helium, not hydrogen right? http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif

Not to do just what I suggested. The LTA gas would have to be in a single fixed volume, or
at least as much as the expanding-contracting volume inside that would be working on.

Yep a real zep has multiple balloons inside for many reasons including being able to trim
the craft, blimps have the same.


Finally, the flight envelope is so small, you shouldn't have to worry about buoyancy changes over the narrow altitude and range you will be dealing with.

Just give it a try before you sweat the little stuff. You will have to really get it light though - I'm not sure it can be both scale and light enough. Its easy if you use mylar and a dorky looking 'zep - I've seen lots of those.

JtD
01-04-2008, 09:48 AM
Originally posted by M_Gunz:

You seem to miss the part about AFTER YOU HAVE ACHIEVED NEUTRAL BUOYANCY IT DOESN'T TAKE A
LOT TO RAISE OR LOWER THE BALLOON MERELY THROUGH BUOYANCY. Not that it's critical anyway.

Dude, you had caps lock enabled.


I was talking about a cylinder inside a rigid container, with a tube to outside air, that would change the density of the gas outside the cylinder but inside the envelope itself.

Oh, were you? I thought you wre talking about a cylinder inside a rigid container, with a tube to outside air, that would change the density of the gas outside the cylinder but inside the envelope itself.


It's a mechanism that was used on a small scale to control buoyancy in model submarines
back in the 60's that I know of.

Yes, for submarines it is a very good idea. Because water has a 1000 times higher density than air.


If the craft descends at 1 foot per minute then how is that acceleration btw?

I don't get you. Speed is no acceleration. Considering you use caps lock you should know that.


blablabla ...
unless you want to hover stationary

Ah!

-----
To cut it short, your proposed device is impractical. If you dedicate the same weight to carrying water and a helium exit valve, you will have a higher benefit at a lower cost. If you dedicate same weight same costs into engines or engine controls, you'll be more effective.

Can you do me a favour and leave me alone? I don't need your "help" and I am allergic to capitals. Thanks.

M_Gunz
01-04-2008, 10:42 AM
Can you do me a favour and leave me alone?

Can you do likewise?


I don't need your "help" and I am allergic to capitals.

Obviously they stop you from being able to read. I only made a suggestion to Na85 and....

JtD
01-04-2008, 11:01 AM
I didn't know that you quote me if you make suggestions to na85. The world's really full of misunderstandings. To avoid them in the future. could you please not quote me if you address others? Thanks.

na85
01-04-2008, 01:51 PM
Originally posted by M_Gunz:


Why helium for a model? It's expensive. As long as you don't make sparks or you just keep
the hydrogen pure there is no real danger with hydrogen. I doubt that Na85 will go with
aluminized skin and have large static buildups from prolonged high speed through air to
recreate the Hindenburg disaster so don't bother throwing that herring in.

Even if I did get sparks.... let's face it, it would be really cool.

DuxCorvan
01-04-2008, 04:20 PM
Here waiting for your gasbag. http://forums.ubi.com/images/smilies/59.gif
http://www.ueec.es/Espanol/ColeccionUEE/Cuadros/Laescopetadecaza1929.jpg

M_Gunz
01-04-2008, 09:23 PM
Originally posted by na85:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by M_Gunz:


Why helium for a model? It's expensive. As long as you don't make sparks or you just keep
the hydrogen pure there is no real danger with hydrogen. I doubt that Na85 will go with
aluminized skin and have large static buildups from prolonged high speed through air to
recreate the Hindenburg disaster so don't bother throwing that herring in.

Even if I did get sparks.... let's face it, it would be really cool. </div></BLOCKQUOTE>

Only if you get enough mixed H and O and then it would also be expensive. Lot of ways to
avoid that. Best might be to keep all power parts outside the bag in the cabin below.
You can generate your own hydrogen from water or using metal and acid, zinc works fine
and very quickly with hydrochloric, slower but still doable with vinegar if you cut it up
fine or use powdered metal but be careful with metal powder, it really generates fast with
possible spraying of acid if not contained and added properly! Just another engineering
problem, that. If you're not up to the necessary handling then don't mess with it. If
you are then its really cheap.

Back in Jr. High we sent up balloons filled with natural gas, IIRC, from the school bunsen
burner outlets. It's the same gas as used in stoves. It's not as good as H or He but it
works too, you just need a bigger balloon to carry instruments aloft.