Sunday, March 29, 2009

Air Density

As a private pilot I had to learn the basics of aerodynamics – Bernoulli’s principle being one of them where a liquid speeds up in a constricted zone to create areas of high pressure and low pressure, this same principle is what creates aerodynamic lift on an airplane’s specially curved wing.

Another major item we have to pay attention to is Air Density. Air Density affects aircraft performance. An aircraft uses up more runway to take off or land at altitude or on hot days at sea level where the air is less dense and where there is a decrease in aerodynamic resistance.

A combination of high altitude and high temperature could spell disaster for an uninformed pilot. The combination of high attitude and high temperature produces a situation that drastically reduces the aerodynamic performance of an airplane. But on the other hand this is a good thing for cyclists since cyclists will go faster under these conditions.

Air Density is the mass per unit volume of Earth’s atmosphere. Atmospheric pressure (Barometric Pressure) is the amount of downward force exerted by the weight of the air above us and is measured in inches of mercury using a Barometer.

Air density decreases with an increase in altitude, it also decreases with increasing temperature or humidity. Warm air is less dense than cold air because there are fewer air molecules in a given volume of warm air than in the same volume of cooler air. Why am I mentioning Air Density? It is because Air Density is a factor in aerodynamic drag.

As you go up in elevation, the Air Density lessens and the resistance to movement is reduced. It therefore takes less energy to overcome drag at altitude than at sea level. However at altitude oxygen pressure becomes limited so aerobic performance will suffer unless you become acclimated. (In a carburated aircraft engine there is a mixuture control knob to adjust the fuel to air mixture to compensate for this.) On the other hand in cycling events which are short and anaerobic such as 200m match sprint, kilometer and 500m time trials, the times at elevation will be faster than at sea level due to the reduction in Air Density.

Kissena track is an outdoor bumpy track at sea level, so one cannot expect super fast times there. Expect times to be considerably slower during opening weekend in late April when the temperatures are still cool and the air more dense than during the state championships in August when the temperatures are hot and the air less dense.

One might think the times are slower on opening weekend because the racers haven’t been training, but many of the racers who will be at Kissena Track have already started racing road races starting the last week of February. So they are coming to opening weekend with a high level of fitness, but it will be the increased air density of the cool air which will result in slower times for the timed events.

To summarize - a cyclist will perform better in hot weather and high altitudes in timed events since there is less aerodynamic drag. An aircraft will not perform well in hot weather and high altitudes since there are not enough air molecules and a decrease in aerodynamic resistance to help create lift within given operating parameters.

Hope you are not thoroughly confused by my explanation, but next time you check the weather, make a note of the Barometric Pressure.


Below is some more information from Analytical Cycling

The air resistance acting on a rider is directly related to the density of the air, the greater the density, the greater the force.

Under standard conditions, the air's density at altitude is less than at sea level. The following table shows typical values:

Elevation......... Air Density............. Units
Sea Level........ 1.226
1500 m ...........1.056....................... kg/m3
3000 m.......... 0.905....................... kg/m3

Although the above values are typical, on any given day the air's density may be much different from values given in the above table.

Was it a bad day, or was it Air Density? As an example, Denver, Colorado, at an elevation of 1500 meters with temperature of 24°C and barometric pressure of 29.01 inches of Hg could have an air density of 0.960 kg/m3 (lower than standard pressure). At the same time Wilmington, DE, at sea level with a temperature of 12°C and a barometric pressure of 30.29 inches of Hg could have an air density of 1.253 kg/m3 (higher than standard). Due to differences in Air Density it would require 28% more power to ride the same pursuit time in Wilmington as in Denver, even though 15% is more or less the nominal difference.

Air density depends on temperature, barometric pressure and altitude and to some extent on water vapor (humidity). Air density is calculated here as a function of temperature, barometric pressure, and altitude, neglecting the effect of water vapor which is small.

Current and historical daily temperature and barometric pressure for most places in the world can be found at Weather Underground, Inc..

An explanation of the relationship between Air Density and barometric pressure can be found at the USA Today Weather Page


40x14 said...

Interesting topic Michael.

I'm curious why it's 28% more power computationally (to ride the same speed) but only 15% more nominally? Both seem like a huge difference.

I guess this mean that Kissena trackies may be faster than Colorado trackies even though we're slower on paper? ;-)

Kissena Track said...

Don't have the answer to that one, those were numbers in the Analytical Cycling article written by Tom Compton. Could be a good research topic.

Anonymous said...

Elevation is most likely the cause for the normal 15% difference.