The power of a bit of air!

So, I finally made the time to show the sheer power available due to a small change in air pressure. I wanted to show how much force is created over the majority of a Formula 1 car floor but in a way that could be done without going at Formula 1 speeds. F1 cars generate tons (yes tons) of downforce at high speed which is why they are so fast around the corners. For a while now I've been looking to demonstrate this at a lecture and my visit to Keating Supercars and the University of Bolton presented the perfect opportunity. I had some fun demonstrating this!

I had been at Keating Supercars the day before and surprised one of my protege's, Dr Kathryn Richards, at a talk she was giving along with Josefine Lissner at Bolton on behalf of "Dare to be Different".

Dr Kathrine Richards, Prof Willem Toet and Josefine Lissner at Bolton Uni - Sat 12th May

The theory. Using an equation derived (and simplified a bit) from the famous Bernoulli equation.... we can calculate the force on an object using the average pressure coefficient (or a drag / downforce coefficient) and the reference area (frontal area for example).

1/2 Air Density * Velocity * Velocity * Coefficient of Pressure * Area = Force in Newtons In metric, converting from a coefficient to a force is pretty easy. Let's take air density at sea level because that's easy to find 1.225 kg per cubic metre. Velocity has to be in metres per second so let's go for 80 (that's about 288 kph or nearly 180 mph) - that's nowhere near the top speed of a Formula 1 car. For coefficient, let's go for 4 - most F1 floors will have peak suction that's of this order of magnitude. For area let's use 1 square metre - the floor of the F1 car is much bigger but I've gone for a high coefficient - and this gives us an impression of the force created. The answer is in Newtons (divide by 9.80665 for Kg).

If we multiply this out we get a force of: 0.5 * 1.225 * 80 * 80 * 4 * 1 = 15,580 Newtons or 1598 kg = about 1.6 tons - on one square metre of area!

Wheel without tyre used for the demonstration.

But I wanted to do this demonstration without travelling at the speed of a formula 1 car and I wanted to do this without spending lots of money. Thanks to the Sauber F1 team I was able to borrow an old (slightly damaged) F1 wheel. Inside rim of the wheel has a diameter of about 12.7 inches (13 inches less material thickness) - that's about 322mm diameter (0.322 metres). Area = Pi * Radius * Radius = 3.142 * 0.161 *0.161 = 0.088 square metres. Not big at all compared to the one metre squared but what could you do with that?

So if we could create the same level of force as our F1 car suction would create we'd have a force of 1598 kg/m2 * 0.088 m2 = 141 kg. That's a bit less than twice my weight. And to create the pressure reduction? Every household has one (hopefully) - a vacuum cleaner. It turns out that (using a different mechanism than a Formula 1 car but, none the less using the laws of physics) a vacuum cleaner should generate about the same suction pressure. To be clear I need to point out that a vacuum cleaner does not create a (full) vacuum. A strong one is capable of creating about 20% less pressure than the pressure around it. At sea level that will mean a force of about 2000 kg per square metre. Of course, real vacuum cleaners used daily and with a few leaks will produce less suction. I borrowed one from the university and was fearful it would not be strong enough. For my first my test, everything was very tentative.

Wheel - prepared for the demonstration

I taped up the spokes of the wheel (using fabric reinforced tape) and taped in an adaptor to allow the vacuum cleaner's hose to be connected. Taped up the valve hole. Put the wheel face down on the floor, turned on the vacuum cleaner and carefully tried to lift the wheel. I could not - so that gave me hope. I need not have worried. Please - if you're tempted to try this "at home", be VERY careful - all sorts of things can go wrong! We turned the experiment upside down (on a strong, low ceiling!). I found that I could lift myself up on the wheel - held up only by the difference in pressure outside the wheel to that inside the wheel. We photoshopped the helmet off my head as it didn't look as casual as the look we wanted.....- OK, OK, I wasn't really wearing one - but I should have been! Air is thin and easy to move aside (when going slowly) but powerful when you know how to harness that power. Be careful lifting an object against a vacuum - it may suddenly fly up (using your strength) when the vacuum is "released" and could hit you. Be even more careful if you try to lift yourself using vacuum - the test surface needs to be structurally strong - most household and industrial ceilings will be pulled apart if you try this with a person hanging on them - then you'll have heavy things landing on you - and have some serious repair work to do!

I'm pleased that the power of the air held me up there - glad the theory worked in practice - and that I can still do a pull up! Photo Dr Anthony Keating, Keating Supercars.

This experiment is a low cost version of a famous one done way back in the year 1654. Now that's going back a "bit"!! Otto von Guericke invented a vacuum pump and wanted to demonstrate it's power. His vacuum pump was able to get much closer to a full vacuum and he used more surface area but the impact was huge. Have a look here if you're interested. https://en.wikipedia.org/wiki/Magdeburg_hemispheres

PS - if you're been awake you may well have noticed the implied effect of having 1 atmosphere of pressure one side of a surface and a full vacuum on the other - about 10 tons per square metre. Autoclaves around the world have used more positive pressure (typically 6 times that) to make carbon fibre parts (press the layers of fibre together and into the mould). Amazing what you can do with "just" air. So, the question is - what useful tasks can YOU do with a bit of air pressure. Remember, be careful!

Prof. Willem Toet. More posts from me here.