Introduction: CD and Balloon Hovercrafts Engineering Challenge
This is part of a series of Instructables intended for teachers about educating students in the classroom around making and tinkering. For more about the details of this project, check out this video on YouTube.
CD balloon hovercrafts are a tried and true instructional tool in the classroom for teaching the concepts of friction, air flow, momentum and a host of other physics concepts. There are a lot of other Instructables out there with different methods, like this great one from The Children's Museum of Houston. Use whatever method or materials suit your needs best.
These devices float rather well on any smooth surface, and depending on what size balloon you use and how much air you allow to escape, can work for several minutes at a time. They are quick to make for almost any age. We've added a design challenge component to the facilitation of this activity to both make the device work better and challenge the participants to add their own ideas to the activity. Building a Better Hovercraft is a great way to explain it. Traditional ones get some drag when the balloon deflates enough to tip over - how can we creatively solve this issue to make the hovercrafts travel further and faster? Read on to find out!
Step 1: Materials and Tools Needed
We do ours with film canisters and CDs, both items that used to be readily available and either free or cheap. Alas, as the progress of technology marches on, both are becoming less prevalent in the world. You just can't pick up AOL CDs out of the street for this activity anymore! Fortunately, you can still find all of the necessary materials for this activity online and locally.
- Film canister (a thread spool, cut piece of PVC pipe, sports drink nozzle, etc. also work well - most anything with a hole in it that will fit over the hole in the middle of the CD works fine). Most photo developing shops will give you their old ones for free, so check there first before buying online! I get mine in bulk on Amazon.
- CD - any old used CD or DVD will work. You can also pick up packs of blank CDs at most electronics superstores. I grab mine on Amazon for large classes.
- Balloon. I recommend a larger one (9" or even 12") as you can get a lot more flight out of them, and they are easier to stretch over the nozzle than the smaller varieties.
- Hot glue gun and hot glue.
- Large tack or small nail. I love these novelty size push pins for kids, as they have a large area to grip for safety and are not super sharp.
- Pipe cleaners.
- Craft sticks.
- Paper and scissors. Markers are good for decorating, too.
- Tape - I love masking tape for this activity.
- Optional: For large groups of kids, I get these balloon pumps from Target or party store and put a piece of vinyl tubing over the tip so that it fits the hole in the CDs. It makes it MUCH easier to inflate the balloons over and over again without removing them or going breathless trying to blow them up through the bottom of the CD. Most party supply stores will also stock balloon pumps. I also designed a 3D printable piece made out of flexible filament that can fit over the tip of the pump to lock the CD in place - the file is attached below.
Step 2: Attach Film Canister to CD
First, you'll need to build the base of your hovercraft. Take the lid off the film canister - you can save it for other projects or chuck it. Put a little glue on the open end of the film canister and stick it to the cd, trying to center it over the center hole in the CD. At this point, there isn't a good seal at the bottom of the film canister, so some air could escape out the sides. Add a good bead of hot glue around the rim of the film canister to completely seal it up. Allow the glue to cool completely before going on to the next step.
NOTE: Don't get hot glue on the bottom of the CD, or else it will hang up when you're launching! If you get a bit and are already most of the way through this process, simply glue another clean CD to the bottom of your existing CD. The added weight will slow it a bit, but not appreciably.
Step 3: Poke Holes in Film Canister
Now, we need to create a way for the air to escape the balloon and reach the bottom of the CD. Grab a tack, small nail, sharp knife, etc. and make a few slits or holes in the top of the film canister. At this point I remind the kids I'm facilitating with that you can always add more holes but it's hard to take them away (you can tape over them, but that's a pain!). Poke a few holes - see how it works and you can always remove the balloon to add more. Be careful with this step to not stab yourself!
It's good to ask at this point how the number of holes changes the float dynamics of the hovercraft. Will more make it go faster? Float more? How will the increased amount of air flow change the flight time of the hovercraft?
Step 4: Install Balloon and Inflate
Now we're ready to install the balloon. Simply stretch it over the top of the film canister. I find it easiest to stretch out the hole in the stem of the balloon and then slip it over the film canister. Make sure it sits down a little bit on the canister - about 1/2-3/4" or 12-18mm. This will ensure it doesn't fly off. At this point you can blow through the bottom hole of the CD to inflate the balloon, or use a balloon pump with a piece of vinyl tubing (1/2" ID tubing works well) or the attached adapter file printed in flexible filament.
Alternatively, inflate the balloon and twist the top, then slip it over the canister - this is much harder to do, but can sometimes be easier than blowing through the bottom of the CD, especially if you have very few holes in the canister.
Once the balloon is inflated, twist the top around a few times to prevent the air flowing out before you're ready to launch. You can also just pinch the balloon stem, especially if you build a structure in the next step that won't allow for twisting. Give your hovercraft an initial test, and seal any gaps left over from the first step if there are any. Notice how floppy the balloon is, especially if you give it a firm push? The next step will take care of that.
Step 5: Build a Better Hovercraft!
Now it's time to find a creative solution to our dragging balloon problem. You can use anything for this, really, and there's no right answer - I've seen a ton of really ingenious solutions! It can be as simple as a tube of paper with a piece of tape holding it together, to a fully blown pipe cleaner cage that wraps around the fully inflated balloon. I'm particular to the cone of craft sticks with pipe cleaner reinforcement as shown above. Try something, modify it and improve it, and keep going. Then, decorate your creation however you would like.
Step 6: Questions for Facilitation
- How does the number of holes change how the hovercraft flies?
- What happens if you don't seal the bottom of the film canister?
- How can you make your hovercraft float longer? Glide faster?
- What happens when we add the additional weight of the support structure?
- What would happen if we had a bigger disc? Would the balloon we use be able to lift it?
- How are hovercrafts used in real life?
- Where else is the idea of creating a layer of air between things to reduce friction used (i.e. air hockey tables, lifting large loads, manufacturing)?