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Friday, 26 July 2013

Aero geekery – how the weather makes you go quicker (or slower…)

  Time trialling can be such a ‘drag’….
  …in fact, aerodynamic drag absorbs 80% of the effort that you put into the pedals.

  So we all have pointy shaped tubes on our bikes, disc wheels, pointy hats and crippling aero positions on the tri-bars to help slice through the air.

  I seem to have been proclaimed club ‘geek’ following a recent Facebook conversation with a fellow tester about air density and CdA (drag) so I thought I’d do a blog post about it (plus I’ve got the day off and I’m bored…)

  I should explain that my interest in all this comes from spending many years flying for a living and teaching people to fly (or trying to!) but the irony is that the perfect atmospheric conditions for TT’ing represent a pilot’s worst nightmare for taking off from a short airstrip with a heavily fuelled aircraft and a hardcore ‘salad dodger’ in the passenger seat! I shall explain….

  In aviation & meteorology, there’s a thing called the International Standard Atmosphere. This is a standardised ‘average’ set of conditions against which cars, aircraft, TT helmets etc are tested. The ISA is set at:

  An atmospheric pressure of 1013mbs.
  15 deg Celsius
  Dry air (ie 0% relative humidity)

  You just have to follow the weather for a while to realize that all of those parameters vary hugely in the real world but as a ‘standard’ it works pretty well.

  We’ve grown used to our atmosphere around us and so we don’t really give a second thought to how dense and treacley it actually is.
  If you were to put a cubic meter of ISA air on the scales, it would weigh a whopping 1.22 kgs.  Put another way, an area the size of your thumb has 14lbs of air weighing down on it from the atmosphere above.  So whenever we’re walking, cycling, driving etc, we’re having to force our way through surprisingly dense surroundings – we’re trudging along the bottom of a deep ocean of air that extends 75 miles above us.

  So, the density of air under ISA conditions is 1.22 kgs/m3. The weather in the UK varies massively though:

  •  Atmospheric pressure can swing between 970 and 1040mbs here (the highest ever recorded pressure was 1088mbs in Siberia!! 890mbs has been recorded in the centre of hurricanes.)
  •   Temperatures can vary between -15 and +35 celsius
  •   Humidity can vary between 0 and 100%

  If you calculate the density of air using the most extreme conditions above, you get a range from 1.07 kgs/m3 up to 1.40 kgs/m3.  The difference in power needed to ride at 25mph through those two extremes of density would be huge.  If you turned up at a flat dual carriageway course in zero wind at an atmospheric density of 1.07 kgs/m3 you would most probably set the PB of your life! Riding in 1.4kgs/m3 would probably feel like riding with flat tyres!
  Most summer TT's here in the UK probably occur with air densities of around 1.2 kgs/m3 but when you consider that air resistance is eating up 80% of the watts you put into the pedals, just a 5-10% reduction in air density will help you chop chunks of time off. It's a tangible difference.

Air pressure

  In aircraft of course air density changes massively due to changes in height. The higher you go, the less atmosphere above to squash down on you so the pressure drops – 1mb per 30 feet in fact.  So at 5000 feet, air density is already 15% less than at sea level.  This is why so many world records are set at altitude.

  Lower pressure = lower air density

  This is also the reason that airliners fly so high. At 35,000 feet, you may be winging your way to Benidorm at 550mph but if you were to stick your head out of the window, it’d only actually feel like 300mph (although I wouldn’t recommend it…)
  That’s because at this height the air pressure (and hence density) is a fraction of that at sea level and so the airframe is encountering air molecules less frequently.


  Temperature has an effect upon pressure following Charles’s Law.  Heat a gas up and it expands. Once expanded, the same amount of molecules are now occupying a larger space and the density therefore reduces.  This process which is harnessed for hot air balloon trips etc is also the main driving force of climate and weather due to the sun’s uneven heating of the earth’s surface.

  So higher temps = lower air density

  Relative humidity

  A lot of student pilots struggle to get their heads around this and I suppose it is a little counter-intuitive:

  Higher humidity = lower air density

  No, no, it’s true. Bear with me….

  Water IS denser than air but we’re talking about water VAPOUR here, not liquid water. Bit of geeky science coming, sorry!:

The two most abundant elements in the atmosphere are Oxygen and Nitrogen. Oxygen has a 16 atomic units mass while Nitrogen has a 14 atomic units mass. Since both these elements are diatomic in the atmosphere (O2 and N2), the atomic mass of diatomic Oxygen is 32 and the diatomic mass of Nitrogen is 28.

  Water vapour (H2O) is composed of one Oxygen atom and two Hydrogen atoms. Hydrogen is the lightest element at 1 atomic unit while Oxygen is 16 atomic units. Thus the water vapour atom has an atomic mass of 1 + 1 + 16 = 18 atomic units. At 18 atomic units, water vapour is lighter than diatomic Oxygen (32 units) and diatomic Nitrogen (28 units). Thus at a constant temperature, the more water vapour that displaces the other gases, the less dense that air will become.

  •   So low atmospheric pressure, high temperature and high humidity make for the perfect time-trialing conditions.

  Unfortunately, the fine, dry weather that we hope for when racing is generally accompanied by high pressure (an anti-cyclone). You could go racing in the eye of a hurricane at 890mbs but we don’t get those round here too often! However, there is a weather phenomenon called a ‘col’ where calm, warm, humid conditions can occur in an area of reduced atmospheric pressure.

  So I’d imagine that those mythical ‘float days’ occur on warm, wet days slap bang in the middle of a col.  I’ve yet to check back on the conditions for the day but this warm, wet, calm superfast day on the E2 last year seems to fit the profile well.

  As I mentioned at the start, the above conditions would make most pilots a bit twitchy. The wings and propeller would have less air to claw on but, to make things worse, the engine would be sucking in low density air and reducing the compression ratio and hence full power available. Plus the low density might make the carburetor run excessively rich too. AND, high humidity increases the risk of carb icing… etc, etc, etc…..
  All these ‘marginal losses’ (Sorry Dave B) can add up quite quickly, seriously impairing the performance of an aircraft.  The take-off distance is normally quoted in the operator’s manual at maximum take-off weight but is given under ISA conditions and usually includes the distance required to clear a 50’ high obstacle at the end of the strip (That pesky tree, house, set of power cables etc).  So you can imagine the potential problems of trying to take to the skies in air much less dense than that found in International Standard Atmosphere conditions.

  However, there’s normally a card or slide rule type calculator handy somewhere (hopefully) that allows you to ‘factor’ in stuff like air density, runway surface etc.

  So, add 10% for the high temp, 10% because you're taking off from an airfield at a high elevation… oh, and it rained earlier and the grass is a bit wet so add another 10%...  add a bit more for the wife and kids… an extra 50 yards for luck. Yep, 200 yards to spare. Excellent.
  Full power and fingers crossed…..

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