You do - a sphere three feet across holds just over 14 cubic feet. Then you need to lift the weight of the balloon itself, so at least 16 cubic feet would be sensible. If you have more lifting capacity than you need, you can always add ballast to adjust the blimp's flight (angle as well as buoyancy). If you don't have enough lifting capacity, it's a lot harder to shave weight off your load.

Volume of a cylinder is Pi x radius² x length.

A cylinder 2 feet wide, five feet long, will have a volume of Pi x 1 x 5 = 15.7 cubic feet.

I'll let you work things out from there - working in feet instead of metres is making me grit my teeth, and I had a filling 5 hours ago.

Then wouldn't it have been easier to just work it out in meters and then convert ? :-)

Yes, what I was say to Kiteman was that if he had difficulty doing the math in base 12 (inches in a foot), then he could use meters and then just convert the answers in the end, which is what you were looking for anyway.

Two feet is the diameter, making the radius one foot. Thus, r²=1.

Sheesh, Kiteman! Feet aren't that bad! It's furlongs and fathoms and gills and leagues and chains and rods and links and drams and hogsheads and minims and pecks and bushels and barrels and hundredweights and board-feet that are confusing.

Let's not forget English "stones" LOL

actually weight in stone isn't hard, it's 2.2lbs to a kilo, 14lbs to a stone. thereby to change from metric to imperial you take (weight in kilos)*2.2 / 14

Um, yeah, I have that memorized now (until I go to another page *sigh* ) the difficulty is not that it is hard, but it's unfamiliarity

Yeah lbs does the same to me, also american money can befuddle me now and again, we have 20p pieces no 25p pieces...

Is your 20p like 1/5th of a (what) pound ? 25 cents is 1/4 of a dollar (100 pennies in a dollar), but I see how that could be disheartening anyway.

just seems odd when adding up...

I understand. I know I had difficulties trying to figure the price of things in Canada, when the currency was the same, but the values were different *sigh*

Ah the simple complications of life...

That's Imperial...All of mine are official measurements that are part of the US system.

Vernors-No problem. Math with a purpose is actually fun. Arbitrary equations in a classroom that mean nothing aren't as much fun.

Well, except for the equation e^i*pi=-1. That's a cool enough equation that it gets to be fun.

as can fooling about with: c²=e/m
;-)