When doing calculations in engineering, many of the variables that we deal with are not constants. For example, if we were to calibrate a digital altimeter to collect accurate altitude position, we need the atmospheric pressure at sea level. It would be nice if pressure stay the same, then we could just hard code that number in. Unfortunately, that is not the case. Pressure changes constantly due to a number of factors.
Because of the continuously changing nature of some of the variable we deal with, this present us with a challenge in engineering. How are we supposed to design the aircraft if conditions are keep on changing?
Well, that’s when the Standard Atmosphere come into play. In simplest terms, the Standard Atmosphere is pretty much just a model that engineers and scientists came up with that generally describe what the average conditions are like.
The atmosphere on Earth is mainly composed of 78% Nitrogen, 21% Oxygen, and 1% of other gasses. Since most of our atmosphere are perfect gases, they follow the perfect gas law, hence described by the relation: P = ρRT. It is also worth mentioning that the heat capacity ratio (ɣ) is equal to 1.4 in most cases in our atmosphere.
Standard Day at Sea Level (SI UNIT)
Pressure (p) = 1.013E5 N / m^2
Density (ρ) = 1.225 kg / m^3
Temperature (T) = 288 K or 15 °C
Gas Constant (R) = 287 J / kg-K
Standard Day at Sea Level (ENGLISH ENGINEERING UNIT)
Pressure (p) = 2116 lbs / ft^2
Density (ρ) = 0.002377 slugs / ft^3
Temperature (T) = 519 °R or 26 °F
Gas Constant (R) = 1716 ft-lb / slug-°R
The above are data for sea level. And yup, plane usually don’t fly at sea level. They fly at a range of different altitude. And guess what? The air properties are different at different height. Luckily, there’re tables on the back of the book.