I want to demonstrate the principles of hot air balloons to some 9 year-old kids.
I remembered a story about someone using dry cleaning bags with the bottom held open by 2 lightweight sticks in a X configuration. They put a lighted candle at the center of the X as the heat source, and when the air inside the bag was warm enough, the bag floated up into the sky with the lit candle glowing. People thought they were UFOs.
Sounds like it shoud work, but I tried it and it didn’t work for me. I even left out the sticks to reduce weight and just held the bag open over a candle. I expected the bag to at least inflate to some extent like the real balloons do, but got nothing at all.
I also tried blowing hot air in with a hair dryer. The bag rose, but it was just because of the force of the air stream, not the hot air effect.
Has anyone else ever done this? I’d like to find out what I might be doing wrong, or if the original story was just a hoax.
Here are some instructions. They specify that you need to narrow the bottom opening (in a manner that will weight the bottom down), empty the bag of air, and fill it up while on the ground.
The Carnegie Science Center (http://www.carnegiesciencecenter.org/) in Pittsburgh has a display with a hot air ballon. You push a button to turn the heat source on (a heat gun IIRC) and the ballon rises. Cool display.
The garment bags with candles inside are fire hazards, and really shouldn’t be used.
At the Museum of Science here in Bostion, they demonstrated hot air balloons by filling mylar balloons with large openings with air from a hair dryer “gun”. It worked pretty well. Maybe you could ghet mylar balloons of the right size and shape, ansd maybe cut off the bottom to get a wide neck.
I have made hot air balloons with my kiddos. I used tissue paper, gluing the panels together with glue sticks. It’s not a quick and easy project and my kids were older than 9 year olds. Finally, the heat source is crucial, as you mention. We used fire in a charcoal starter chimney, covered with door screen material so it wouldn’t set the thing on fire, and we used balled up pieces of paper as the fuel. It worked a couple of times, and once, we set the balloons on fire anyway. But what the heck, we had fun. However, the heat from the fire did cause the balloons to rise as they burnt up. We also used a hair dryer a different time and we held the balloon down until we were sure it was filled with hot air, and then we turned the blower off and released the balloon and it did lift up - indicating that the lift came from the heat, not from the force of the dryer. You can also use a glue gun if you want more heat, but it takes longer. Man, I miss teaching. xo, C.
Would something along the lines of a tea candle attached to a paper bag rise? For some reason I seem to recall that being a device used in a comic, but I’m not sure if it would work in RL. I know the wax would be kind of heavy in relation to the contraption, though.
I’ll let you know from experience that the softer grocery bags will melt together under the application of a hairdryer, leaving you feeling sad with a bunch of melted plastic and tape.
If you then try again with the tea candle/paper constructions, I’d say something at least a meter tall, though you have to be extra super careful with the candle, as a meter tall sphere of paper will go up in seconds. :eek:
That last part was enough even at ~12yo to end my ambition to launch a hot-air balloon!
Oh, yeah, and if you do make your own balloons, and they’re fairly large, you have to build in a little hook so you can hold them up over your heat source. We built in a little hang tag at the top and we used one of those old sticks that we found around the school that they used to use to lift up the windows back in the day, that had a little hook on the end. It was perfect for holding up the balloon over the heat source while we pumped heat in. I’d also suggest doing it early in the morning when there’s minimum wind.
My sister, years ago for a elemetary School Science fair project did a demonstration of Hot air ballons and the principle they worked on. The thing I remember about her display is she took two paper grocery bags (plastic was not an option then) attached them to an old broom handle one on each end with a paper clip pierce trough the bottom of the bag in the center (piece of doweling would work just as well) and balanced the broom handle in the middle with a piece of string. She then placed a living room lamp with I think a 60 watt bulb under one of the bags and that side lifted up, the other side sank down, turn off the light and the whole rig would balance out again.
The way we did it years ago (and we were too smoked to think about the fire hazard) was to use a dry-cleaner’s bag with the hanger hole taped shut. We made an X out of hibachi skewers, and we taped that to the open end of the bag. We taped together a package of birthday candles, glued onto a little piece of cardboard taped to the middle of the X. That way you get many wicks, and not much heavy wax. We heldhe bag by the peak and lit the candles. We used a couple of cigarette lighters to help get the thing inflated. Once it began to rise, we just watched.
We launched five or six of these before we got bored. They all flew farther than we could see. If we burned down anybody’s house, we didn’t hear about it.
It’s not necessary to make your own hot air balloon from scratch. I’ve bought for a nephew a simple kit (for about $25) allowing one to make a hot air balloon. The instructions say that you can fill it using a couple of hair dryers. My sister and brother-in-law report that it was a lot of fun. I’ve also given him a huge balloon (about 8 feet by 50 feet) that one fills up by running down a hill while holding it. One then ties off the ends. The balloon is black, so if you do this on a sunny but cool day, the heat of the sun causes the balloon and then the air inside to heat up. The balloon expands so it rises into the air. This costs about $20. This was also fun, I’m told. I got these balloons from Edmund Scientifics, but I presume they are available elsewhere.