My Daughter has her 4th grade science fair coming up, and she decided she wanted to make a remote controlled blimp. (I started a chain reaction with the snap circuits kits last christmas!)
SOO we have been doing research and plan to use the guts from an cheap RC plane, build the envelope from mylar sheets and probably attach a small camera to relay to a 'base station.'
All this is well and good, but I have aspirations to also make a ballast to control altitude. Now I figure we can use a standard latex balloon and inflate / deflate it inside of the mylar envelope.
The thing I am struggling with is how do I remotely control the air pressure in the ballast? Do you think a small electric motor with a fan attached would be sufficient to inflate the balloon enough to decrease altitude?
Perhaps latex is the wrong material, maybe we'll fashion the ballast out of the same mylar film as the envelope (may be easier to inflate than latex)
I was hoping others have tackled something similar and could offer suggestions on how to build the ballast.
I have a roll of aluminized plastic waiting to be turned into some sort of airship. As I've learned about the different ways blimps are controlled, I don't recall anyone attempting to use air as ballast.
It just doesn't seem like a good idea to me. I think the energy and hardware required to pressurize air would be much too heavy to make it practical.
I think the usual way of controlling altitude is to have the blimp slightly heavier than air and then use the motors to control the altitude.
If you wanted to control the altitude by the changing the buoyancy of the blimp, then I think the most practical way to do so would be use water as ballast and release the water to increase the buoyancy. To decrease the buoyancy I don't see any other alternative than releasing some of the lifting gas.
One option would be to carry a small amount of pressurized gas, like a CO2 cylinder used in bb guns or something, and use a pair of small solenoid valves to let gas into or out of the ballonet. This has two advantages, the weight would be less than having a compressor and the CO2 is slightly heavier than air.
Delta_G:
One option would be to carry a small amount of pressurized gas, like a CO2 cylinder used in bb guns or something, and use a pair of small solenoid valves to let gas into or out of the ballonet. This has two advantages, the weight would be less than having a compressor and the CO2 is slightly heavier than air.
I initially thought the question was about pressurizing a ballonet but after rereading the post I realized it the question was about ballast.
I agree a CO2 cartridge would be better than attempting to compress air but I don't think a CO2 cartridge offers any advantage with regards to ballast.
CO2 at atmospheric pressure is indeed denser than air but compressed CO2 is much denser than CO2 at atmospheric pressure. Releasing CO2 from its compressed state to fill a ballonet will increase the overall volume of the blimp while not changing its weight. This increased volume will displace more air and result in an increase in buoyancy rather than a decrease in buoyancy.
I think the a system using CO2 cartridges could be useful if used to hold the shape of the blimp with a ballonet but I think water would be a much better ballast material.
The ballonet are used to hold the blimps shape. They are not used as ballast.
The point of the ballonet is that the volume of the blimp doesn't change. There should be a rigid structure there. Otherwise we aren't talking about a blimp but a balloon and those are indeed handled by dropping sandbags and letting air out (think hot air balloon).
The point of the ballonets is that when you fill them they take up some space inside the blimp thereby compressing your lifting gas and making it denser and creating less lift.
But I think you're right that it can't be done with the CO2 cartridge because you have the weight of the gas with you all the time. If the volume of the blimp doesn't change, and the total weight doesn't change (because the compressed gas in the cartridge weighs the same after it is expanded gas in the ballonet) then the lift doesn't change either.
So you've gotta do it with outside air and use a compressor.
You're right about the ballonet being used to control lift. I was wrong thinking they were just to hold the shape of the blimp.
And if the blimp's volume didn't change when releasing the CO2 then again, you're correct it wouldn't change the buoyancy of the blimp. If the blimp were sagging from low pressure inside the envelope and the CO2 was used to increase the overall volume then it would also increase the lift.
I'm pretty sure I haven't seen any remote control blimps which use ballonets. I'd be very interested in see some examples.
The "blimp" article (in the Principles section) gives the impression the ballonets are primarily used to hold the shape of the blimp.
It looks like full size blimps use a similar strategy for controlling altitude as the model blimps I've seen.
Modern blimps are launched somewhat heavier than air (overweight), in contrast to historic blimps. The missing lift is provided by lifting the nose and using engine power, or by angling the engine thrust. Some types also use steerable propellers or ducted fans. Operating in a state heavier than air avoids the need to dump ballast at lift-off and also avoids the need to lose costly helium lifting gas on landing
I kind of hope I'm wrong, but I just don't see it being practical to control the altitude of a model blimp by regulating the pressure of the lifting gas.
Without doing the math, my usual approach to these sort of pondering is to always consider edge cases (extreme scenarios).
In this example it would be a completely deflated ballonet on one extreme and completely inflated ballonet on the other.
If we assume a constant volume of the blimp, the inflation of ballonet will pressurize the lifting gas. Deflated ballonet will offer the entire blimp volume for lifting gas, while completely inflated ballonet will force the lifting gas into a tiny pressurized pocket offering zero lift, since the entire volume of blimp is now replaced with CO2. Of course, the structural integrity of a blimp is nowhere near strong enough to withstand such pressures, but the concept of decreased buoyancy with inflation of ballonet remains.
If the outer shell of the blimp can withstand mild increase in pressure, then this approach should work, but only for a limited number of inflation/deflation cycles, since the CO2 will be vented to atmosphere when the ballonet is deflated, the CO2 cartridge will lose mass, and the blimp will be more buoyant with each cycle.
Edit,
One morning shower later, it is clear that it will not work after all.
With CO2 cartridge total mass/volume stays the same. Air would need to be pumped from the outside.
Yikes! I wasnt expecting so many good replies! Thank you all for the insight.
So our airship is going to be fairly large 4' long, 2' in diameter at its largest point. (Have to buy a larger helium tank!)
Constructed of primarily mylar. My thought was with this size that perhaps we wouldnt need to fully inflate the envelope to achieve sufficient lift. Inside would be our ballast, probably also constructed out of mylar and perhaps we could use a small fan that would only push air into the ballast while it was running. The idea being that if the envelope isnt fully pressurized a smaller fan could provide enough force to push air into the ballast, at least to some extent.
Either way we could always use the 'sandbag' approach and use paper clips or something to achieve our desired buoyancy.
wjdavis5:
So our airship is going to be fairly large 4' long, 2' in diameter at its largest point. (Have to buy a larger helium tank!)
A 4' by 2' envelope is much bigger than a party balloon but you're still going to have to be very careful about weight.
You get about 1.112g of lift per liter of helium in your blimp. If you live above sea level you'll get less lift.
If your blimp is a cylinder then you should get 395.7g (13.96oz) of lift. The envelope, flight control hardware, battery and camera all need to weigh less than this.
The lift values I gave are likely a bit high. If your blimp has rounded ends it likely has less volume than a cylinder the same length.
You will probably need to trim your blimp for neutral buoyancy. A blimp this size might be able control altitude by angling the trust of the propellers but the mechanism to do so needs to be very light weight.
Hi folks, dont quote me on this but heres my thoughts.
Go up lighter than air go down heavier than air.
Usualy you have balast to hold you down and release it to go up then release the gas to go down.
Keep doing this till you run out of gas and sink or run out of balast and drift off into the blue (neither is good) but
a docco I saw recently turned the idea around and worked on the 'more gas in the bag, more lift! less gas less lift!'.
What they are doing (which should work if you can fit the pump) is pumping the helium from the lift bag back into a pressure bottle compressing the Helium and making it heavier than air to decend, then releasing it back into the lift bag to ascend.
You would have to play around a bit but a simple plastic bottle should do(Use caution here) to store the gas in as you will not be pumping all the gas out, just enough to decend slowly.
Daz1712:
What they are doing (which should work if you can fit the pump) is pumping the helium from the lift bag back into a pressure bottle compressing the Helium and making it heavier than air to decend, then releasing it back into the lift bag to ascend. You would have to play around a bit but a simple plastic bottle should do(Use caution here) to store the gas in as you will not be pumping all the gas out, just enough to decend slowly.
That sounds like an excellent option. Carbonated beverage bottles are good for at least 90 PSI. Use the blood pressure monitor hardware to move lift gas into (pump) and out of (release valve) a 1 or 2 liter soda bottle.
I actually like the pressure vessel idea more than the ballonets but the pressure vessel will leave a blimp limp. A blimp needs the internal pressure to hold it's shape.
I like the blood pressure cuff pump idea but the pump I saw inside the blood pressure device I hacked didn't seem very blimp friendly (it was heavy).
Using a pump to control lift is a cool idea (I've wondered about this many times with my vaporware dirigible), but I don't think it's practical with a 4' by 2' blimp.
The 395.7g (13.96oz) of lift figure I quoted earlier, has to include the lifting gas envelope, remote camera, control electronics, motors and batteries.
Maybe it's possible to use the pump and pressure vessel if the propulsion motors are abandoned or made very small. I personally think a blimp which is able to control its own lift is more interesting than one able to fly around a room. If the lift could be controlled this way, I think it may be a first for small scale blimps. At least I've never seen any small scale lighter than air vehicles which can control lift this way.
If wjdavis5 can provide information about the envelope material (weight per area), details about the shape of the blimp (so volume can be computed), the altitude where the blimp will be used and the weight of the control gear, a better estimate of payload capacity could be computed.
The lift figure mentioned earlier assumes the blimp is a cylinder and the blimp will be flown at sea level. The actual lift figure is likely to be less than the value above.
Glad to see people light up when something new comes along but before going overboard we should remember this is a 4th grade project.
So continuing along a line mentioned before I would go with netural boyancy and two motors with seperate speed controls to stear left and right and mount them to a horizontal boom (operated by a servo) that can be rotated to point them up and down to provide up or down thrust. (Sounds like a simple Arduino project with comms).
This way your Heliums lift cancels out the hardware weight and the motors provide the Up, Down, Forward, Reverse , Left and Right to change direction.
You can either make the boom rotate all the way around to point the motors nearly 360 deg or reverse the engines which would allow you to make one forward and one reverse to turn the air ship almost on the spot.