Planes, Trains and Automobiles Answers

by Deb on September 3, 2011


Someone did very well yesterday, unfortunately they’re anonymous but congratulations dj302. If you want to join in, close your eyes and click back.

1.     The tread on tyres works by giving water somewhere to flow. Imagine you put a big flat tray onto a puddle. It would slip and skate around because not all the water gets out and some is trapped under there. The same thing happens with a bald tyre, causing a skid. Tread patterns are designed to give the water a place to collect and flow out of the way, which means the rest of the tyre can touch the road surface.

The grip of a tyre is a function of surface area – the more area, the more friction and the better the grip. Even though a tyre with tread has less overall surface area because it loses some to the channels, by getting the water out of the way more rubber is effective and will give you the best grip.

2.     That big fan on top of the helicopter isn’t just providing lift, it’s spinning really fast. Because it is connected to the helicopter which is hanging in midair, it would make the helicopter spin really fast too. The tail rotor pointing sideways counteracts the spin to keep the helicopter going straight.

3.     Possibly unfair here – I said ‘avgas,’ not realising that was a specific type of aviation fuel when I meant general. Diesel, petrol and aviation fuel all come from oil. Oil and gas are the massively compressed and cooked remains of plankton and other sea creatures that settled to the bottom of ancient oceans and were buried. They are made up of various hydrocarbons, chemicals made from bonded hydrogen and, well, carbon. These have a lovely naming convention that is like a little code that tells you how many carbons they have in them. So methane (1), ethane (2), propane (3), butane (4), pentane (5) and so on.

Basically, the longer and bigger the chains are, the heavier they are. Makes sense. The early ones are very light and they are gases at room temperature and pressure. As they get bigger they are liquids. That all changes when you heat them because they boil and evaporate, and the lighter ones boil at a lower temperature. If you control the temperature then catch and cool the gases you can separate the different hydrocarbons.

So the short answer – diesel, petrol and aviation fuel have different proportions of the heavier hydrocarbons in them, so they come out at different times of the distillation process and they burn differently. Petrol is one of the lighter ones (although it’s a complicated mixture) and you’ve probably heard of ‘high octane’ fuels. It vapourises easily and the fuel/air mix will explode with a spark, which is what happens in your engine. Aviation fuels are on the heavier end of the light fuels, and diesel is one of the middle distillates, meaning it has longer chains of hydrocarbons.

4.     Regenerative braking is a very clever process, almost like getting something for nothing. It doesn’t really break any laws, but the price you have to ‘pay’ is something you want to get rid of anyway. In normal brakes on a car or bike you are stopped by friction on the wheels and the movement energy you had is lost as heat (and sometimes sound!). In regenerative braking you take away the movement energy by turning it into something useful.

A little while ago I wrote about electricity and mentioned that movement, electricity and magnets always go together – if you have two of them you can produce the third. This is what electric trains and cars do with regenerative braking, their electric motor can switch into a generator when they are stopping. Generally they put electricity into the motor where it moves through magnets and causes movement that runs the wheels. But during braking it uses the movement coming from the wheels and runs it through the magnets to produce electricity that recharges the battery. With nothing powering them the wheels lose energy quickly and stop.

Getting rid of movement and producing more electricity to run on, that’s definitely clever.

5.     dj302 in yesterday’s comments quite rightly identified that the smoke in the picture is from friction, because the plane is going faster than the wheels are spinning and they are sliding instead. The only bit I would quibble with is saying that rolling would be better. The best description I’ve heard of landing a plane was along the lines of

‘You need to go from just faster than the speed at which you’re airborne to just under, and then stop before you run out of runway.’

This all has to be done extremely quickly, which means you need to dump a lot of velocity and therefore energy.

In any normal situation moving things on roads, wheels are much better than trying to drag it precisely because rolling wheels generate less friction and therefore use less energy. But a landing plane is trying to convert as much movement energy as it can into anything else, so in this one special case rolling wheels are bad. Sliding wheels turning movement into lots of heat and noise will stop the plane faster.

And because I always finish with a video, here’s one from They Might Be Giants ‘Here Comes Science’ CD.

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{ 1 comment… read it below or add one }

Coran September 3, 2011 at 10:36 am

Actually, I think the smoke is a bad thing. As you point out it means the tyres are skidding as they spin up to match the speed of the aircraft. In the process some of the aircraft’s kinetic energy is converted into heat, noise and tyre ablation. I would say that a negligible amount of kinetic energy is transferred from the body of the plane to the spinning wheels. If these wheels didn’t have brakes, then yes, the conversion of kinetic energy into heat through friction with the tarmac would cease when the wheels got up to speed.

But these wheels have brakes. The amount of force that the aircraft can place on the tarmac to counter its kinetic energy is considerably greater when the tyres are in contact with the tarmac and not skidding and ablating than when they are (static vs dynamic friction). The brakes, then, are the bits that skid and ablate, but now we have a brake disc skidding against a brake pad – two objects engineered to be able to apply a *lot* of friction to each other and convert that kinetic energy into heat energy. Far more energy can be expended by the sliding brake discs than can be expended by the sliding tyres.

This is why we have anti-lock brakes and why engineers have looked for ways to start spinning aircraft wheels before they hit the tarmac, because you can’t slow the aircraft down until the wheels grip – all that smoke is just so much burnt rubber.


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