Help, hot air baloon Q.

[P.I. Stazzer-Newt]

How big is an "ordinary" hot air baloon?

There was a bizarre accident in Durham this weekend - when an inflatable sculpture 50mx50mx5m broke its moorings and took off - killing two and injuring many.

Quick calc says 12,500 cubic metres - thats a 14 metre radius sphere or 92 feet in diameter - is that a big baloon or a small baloon?

Reuters (http://today.reuters.com/news/newsArticle.aspx?type=oddlyEnoughNews&storyID=2006-07-24T185848Z_01_L24889699_RTRUKOC_0_US-BRITAIN-INFLATABLE.xml)

[S/V Laura Ellen]

Quick calc says 12,500 cubic metres - thats a 14 metre radius sphere or 92 feet in diameter - is that a big baloon or a small baloon?


Copied from http://www.playday.com/activity_geo_Prov.asp?Activity=Ballooning&Geo=ON
Hot Air Balloons come in all shapes and sizes, ranging from a 21,000 cubic ft. – one person hot air balloon, to a 400,000 cubic ft. –22 passenger hot air balloon.

[P.I. Stazzer-Newt]

12500 cubic meters ~> 430,000 ft3.
So the sculpture was the equivalent of a REALLY BIG balloon

[Paul Pless]

well that's bizzare, I guess you never know

http://www.dreamspace-agis.com/images/plang.jpg

[Katherine]

Bet the lawyers are already lining up.

[Bruce Hooke]

There are lots of spaces in the sculpture so the total volume would be less than simply the product of the dimensions. Still, it is certainly in the range of a very large balloon.

Very sad that people lost their lives.

[Meerkat]

Saw video of this captured on a cell phone (so it wasn't all that clear) on news.bbc.co.uk yesterday.

[Todd Bradshaw]

The balloons that I used to fly were "average" sized and were about 55' diameter and 77,000 cubic feet (AX7 class) in volume. People capacity varies greatly with ambient temperature as there is a limit to how hot you want to heat the nylon fabric. In hot, summer weather you might only have enough lift for the pilot and one passenger before reaching "red-line" temp (250-275 degrees F). In cooler weather you could often carry four people without a problem. In winter, once the thing was upright and off the ground, you could damn near fly all the way across town on just the heat from the pilot light. Most commercial "paid-rider" balloons are around 70' in diameter and 150,000 cubic feet and hold 6-8 people. Anything larger than that starts to be a pain in the rear to handle without a huge ground crew.

Hot air produces 17 lbs. lift per 1000 cu. ft. at sea level and at about 70 degrees ambient temp. Helium produces 70 lbs. lift per 1000 cu. ft. in the same conditions. I didn't see the story, but are you sure it was hot-air? It would seem very inefficient as a means of "powering" a sculpture, especially when you would need three or four blasts from the burners every minute just to keep it inflated and upright.

...or was this just a big cold-air inflatable that got loose at ground level and squished somebody?

[Meerkat]

I think the sculpture was a light weight lattice of sealed balloons. They looked big enough to have been old weather (sonode) balloons.

[Todd Bradshaw]

I still haven't found the story. but it might be one of these arcitectural inflatable structures which visitors can actually enter and walk around in. If one got loose while you were inside and started to roll, I can see how you might get injured.

http://www.architects-of-air.com/main.html

http://www.bbc.co.uk/shropshire/content/images/2005/08/02/luminarium_gallery_16_470x300.jpg

[Meerkat]

http://newsimg.bbc.co.uk/media/images/41923000/jpg/_41923934_dreamspace203main_pa.jpg
Here:
http://news.bbc.co.uk/1/hi/england/wear/5208712.stm
http://news.bbc.co.uk/1/hi/england/wear/5208898.stm
http://news.bbc.co.uk/1/hi/england/london/5210138.stm

Googling "Dreamworks Sculpture" may also provide additional links.

[Todd Bradshaw]

Got it. Well, their "theory" about the structure heating up enough to lift from hot air is total B.S. and physically impossible. Even with the lightest construction and skin materials made it would never even come close to developing enough lift to budge it's own empty weight, let alone lift off. In hot weather, you have to heat the inside of a balloon 100-150 degrees above ambient temp to get it to lift off, even if it's built from ripstop nylon. The folks inside would have fried long before they flew anywhere.

What they will eventually find is that the structure was the victim of what balloonists sometimes call "false lift" (Bernoulli's principle) and there's really nothing false about it. It's real lift, just like an airplane wing creates. Wind passing over the top of a big roundish object (or a group of them connected together in this case) creates lift, sometimes a lot of it. When the object is tethered, rather than drifting freely at the same speed as the wind, you can generate enough lift for the object to leave the ground and fly. Once it's airborne, there are two possibilities, neither of which are very good. If the tether breaks and the thing is set free, as it's drift speed approaches the wind speed, it loses the wind-induced lift and falls to the ground. If the tether holds, the object may stay up as long as the wind speed is constant, but will come down as soon as the wind-induced lift diminishes or in some cases the combination of lift, wind speed, being tethered and centrifical force just slams it into the ground.

When you learn to fly a balloon it's something that they drill into you pretty soundly. You can be sitting in a nice clearing in a park, ready to take off with very little wind on the ground. Eighty feet up though, you may have enough wind coming over the trees and the top of the balloon to generate a lot of lift. You heat up until the balloon gets "light" the crew lets go and you begin to climb. At this point, you obviously have enough lift to ascend, but you can't tell whether it's lift from the hot air or "false lift". Once you clear the trees and the balloon reaches wind speed, the false lift goes away and if you aren't heated up enough for the thing to fly by itself, you're in trouble. It takes a balloon about 15 seconds to react to the burners, so you may not have time to generate the hot-lift you need before you get dropped onto somebody's house at the edge of the park. For this reason, most pilots will take off "hot" or do what's called a "pop-up" when launching from a sheltered spot - heating the balloon up enough before leaving the ground that it really climbs out fast and with considerably more than enough hot lift to get it off the ground. That way, even though the false lift may speed your ascent, once it's gone and you've reached windspeed you still have enough hot-lift to keep you from falling out of the sky.