max lift by blimp calculation

[dual]

guys, I'm trying to compute the maximum weight that my blimp can lift. I have made some calculations but I'm not so sure.

I have a blimp 36inch around and a helium volume of 4ft^3 or 113.267L.
Density of helium = 0.7186 g/L
Density of air at 25 deg Celcius or 77 deg Farenheit = 1.184 g/L

So what I did is this.
Net density = Density of air - Density of helium = 0.4654 g/L
Mass = Net density * volume of helium = 52.7g or 0.116 lbs

Does this mean that the maximum mass that it can carry has to be less than 0.116 lbs? Am i correct in my calculations, I'm not really mechanical so I hope someone can help

[madchimp]

Well I don't know about the calculations but I would think you would need to figure the pressure the helium is at in the envelope of the blimp since a higher pressure means more helium is in there and the overall blimp would have a different density but the kind of pressure you are dealing with that difference might not be enough to worry about.

[Finnik]

Your calculations are alright, you may indeed need to think about pressure as madchimp mentioned, but if your blimp has a more or less solid structure it won't matter that much.

One thing you should consider is that the mass to be carried includes the balloon-like structure that contains the helium, not just the payload.

[hgordon]

Easier approximation is 1oz of lift per cubic foot of helium or 1kg of lift per cubic meter of helium.  Compute your total lift, then subtract the weight of the blimp envelope, gondola, etc, and the whatever is left over is your payload capacity.

[Finnik]

I just now noticed you've made a small error, you wrongly copied the helium density from Wikipedia, it should be 0,1786 g/L. This brings us closer to hgordon's approximation seeing as a difference of 1 g/L = 1 g/dm³ = 1000 g/m^3 = 1 kg/m³.

If you do the math now, you'll have more than double the lift you had previously calculated.

Thank you hgordon for that approximation, otherwise I wouldn't have noticed the density used for helium was wrong.

[dual]

Yes, I just figured that out as well. I can now lift a whole system that's at least a quarter of a pound. and I believe that's about 110g.

I'm looking at micro servos and they're mostly .3oz or 9g. I'll be using 3 of them. so that's 27g
Three sonars that are 4.3g each so that's about 13g.
accelerometers and photoresistors that are about a gram each, so I wanna say another 10g

Now if I can only find a development board that's atleast 30grams, then I'll be able to lift the whole system up.

Do you guys have any recommendations on that?

For the chassis and balloon though, I think I'm gonna use the body of a saucer type RC Blimp from RCTOYS.com
http://www.rctoys.com/rc-products/MACH-3S.html

What do you guys think?

[hgordon]

We use http://www.rcguys.com/66blimp.html in http://www.surveyor.com/YARB.html .  If you order without the motor controller, you can save an additional $100.

[Finnik]

Now if I can only find a development board that's atleast 30grams, then I'll be able to lift the whole system up.

You probably mean under 30 grams 

You should take a look at the MEGAnitty, comes in at a mere 2 grams according to the specs.