# Christmas STEM Challenge: Santa’s Aerosleigh

Image credit: Neil Downie

Our STEM Challenge is back briefly for Christmas 2022, hurrah! This weekend, why not join in with the engineering fun by making a Santa aerosleigh.

Santa Claus has a big job on his hands. One calculation reckons that without his team of elves and herds of reindeer Santa would have to travel at half the speed of light to get to everyone. We reckon we can make the job easier with an Aerosleigh, which uses propellers rather than reindeer for propulsion.

But there’s a big problem with sleighs: they work by sliding on ice or snow. And these days we have global warming: good for Rudolf but bad because it’s melting that ice and snow! So if the weather doesn’t give ice and snow, what can Santa Claus do? How can he get presents to all those good children at Christmas? Santa must get through! But we may have the solution: be an Aerosleigh where you take along your own ice, and put it underneath, between the sleigh and a smooth surface.

Friction generally stops things from sliding around. The force of friction stopping two surfaces from sliding is the coefficient of friction x the force pushing the two together. The other thing about friction is that it produces heat. Force x distance = energy, and friction force times distance = heat energy. Now ice has an amazingly low coefficient of friction, which is why we want to use it – and why we don’t need wheels when we’ve got it. How does that happen?

People like Victor Petrenko, the author of a classic book on the science of ice, uncovered the reasons behind its low friction. Ice is melted slightly by frictional heat as something moves across it, forming a thin layer of water which provides continuous lubrication. And because the viscosity of water is so low, even a thin layer is enough to ensure that the frictional force is really low. Even better, as you go faster, ice friction goes down with the square root of speed. So, if you go 4x faster, the friction is 2x smaller.

Now it is difficult to steer on ice and snow. That’s why we have ice skates with blades, not just flat-bottomed shoes for skating, and why skis have sharp edges. However, if you don’t have ice on the ground, but have it on the bottom of the vehicle, you can’t steer at all! That’s why our ice-on-board Aerosleigh must run along a track of some kind. Two things that work are half-round guttering and steel U-channel (as used for protecting cables built into plastered walls.) The guttering works well, but you have to ensure that the propeller won’t hit the sides. The channel works well too and being smaller allows the use of a larger prop.

You’ll need a sleigh body big enough for Santa, an elf or two, and sacks of presents. Balsa wood is good, because it is a good insulator, which means it won’t melt the ice, and it’s strong and light. You’ll need hollows to retain the blocks of ice underneath. The central part of the sleigh could be made of a piece of plastic drainpipe cut longitudinally, which is strong and gives you the opportunity to put the batteries low down and get the centre of gravity low.

A toy motor will be okay, although not one of the lowest-power types. Try a 3-6V motor powered by 4-5 NiMH batteries. The propeller needs to be a low ‘pitch’. The pitch of a propeller is the distance it would go forward for each turn, if you imagine it screwing its way through super soft butter. A pitch of 3 inches (75mm), should do, with a diameter of 4-6” (100-150mm). Check out model plane and drone propellers. With a 3” pitch, a propeller turning at 3000 rpm can make a blast of air up to 150” per second, which is 4m/s, 9mph. Add an on/off switch and you’re done. Testing time!

Lengths of steel track can be overlapped so that the sleigh can pass freely over from the higher to the lower, while guttering can be similarly overlapped or jointed in-line using clip-on joining pieces.How fast does it go? Good acceleration? Good blast of air from the propeller? More or fewer batteries or a change of propeller needed?

Why not carry on and improve the design? You could go bigger, try a twin steel track, or try more power, maybe a ducted fan instead of the propeller. And what about radio control for stop/start and maybe even for reverse thrust braking?

Finally, you’ll need to put some Christmas artistic ideas into the design. Red/white colour scheme? Seating for a few elves, Santa and presents? Headlights and fairy lights? Jingle bells?

If you liked this, you will find lots more fun science stuff in Neil Downie’s books, like ‘The Ultimate Book of Saturday Science’ from Princeton University, and for lots of other things (and a free copy of the ‘Exploding Disk Cannons’ book), visit www.saturdayscience.org. In line with this experiment, Neil’s current work includes developing a new ventilator system to support people with breathing difficulties – get more information on this great project at Exovent.org.

There is a back catalogue of STEM-related challenges from the past two years to choose from if you are looking for more options. The IET also has a host of resources that adults can use to engage children with the world of STEM. Also, check out the IET’s Santa STEM workshop.

Sign up to the E&T News e-mail to get great stories like this delivered to your inbox every day.