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ATPL Principles of Flight - Class 1: Atmosphere and I.S.A – Transcript
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- Air around us can be described in simple terms of density, pressure, temperature, humidity, and viscosity.
- Density varies first according to the pressure, then the temperature, and then the humidity.
- The international standard atmosphere considers the temperature at sea level to be 15 degrees celsius.
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are you studying for your hpl exams do you need a little bit of help with principles of flight if so you've come to the right place to help with this big old subject hi i'm grant i'm a first officer based here in the uk flying for a major uk airline i've been flying jets since 2016 and i'm here to help you
through principles of flight in an to understand manner giving you some real world examples from my own experiences along the way in this first class we're going to be looking at air and the atmosphere all around us we first have to understand the various components that make up the air before we can understand what happens when a wing passes through the air around us can be described in simple terms of density pressure temperature humidity and viscosity
the first we're going to look at is density so when we're measuring the mass of a solid object it's fairly straightforward because all the molecules here are packed in close together to so to gather them all up and find out the mass is very straightforward they're already all gathered up for us however in the air around us the molecules are all spread out as so so to gather them all up
and find out the mass of every individual molecule would be very difficult to do so in this case we use the measure of density density is the mass of all the molecules per unit volume is given the symbol rho which is a greek letter and it is described as a mass
divided by some volume kilograms from the uk is very common measure of this the molecules in here are also affected by gravity which means there are more molecules lower down towards the surface of the earth the reverse is also true there's fewer higher up
in the atmosphere so we can say as altitude increases our density decreases density is also varied according to the other elements that we're going to cover the biggest to affect these is caused by the pressure so all the molecules in the air are
moving around constantly in a random motion as so and when they collide with any object they impart a force to that object and it is felt equally in all directions static pressure is felt equally in all directions the measurement for this pressure is the force that is imparted by these molecules per area so we've got newtons
per meter squared and one newton per meter squared is the same as one pascal pascal was a scientist who discovered a lot of things about static pressure i see static pressure because the total pressure in the air is consisting of static pressure and also something called dynamic pressure which we'll cover in the next class and it is essential to generating
the forces of lift and drag are affected in the same way as with density there are more lower down so because there are more lower down there's more moving in random directions they're making more collisions they're hitting things more often meaning that the pressure static pressure lower down is higher the inverse is also true higher up there
are fewer molecules moving around colliding less and the pressure is lower so we can say as altitude increases our static pressure decreases the same way as density decreases another thing that reduces with altitude is temperature when we're talking about temperature in principles of flight
we generally deal with something called the kelvin scale kelp scale is very easy because uh one degree of celsius equals one degree kelvin so the increments are the same the only difference is this zero point for celsius the zero is at the freezing point of water for kelvin the zero is at something called absolute zero which is where it's so cold the molecules stop moving around
this temperature here is equivalent to minus 273 degrees celsius so the zero on celsius would actually be 273 kelvin degrees kelvin so to convert from celsius to kelvin you simply add 273.
for example 30 degrees c in kelvin just add that 273 and you come up with 303 degrees kelvin very simple to convert between the two and only though fahrenheit is also still sometimes used by people if you ever have to convert between fahrenheit and celsius the conversion is so the degrees in
fahrenheit equals the degrees in celsius times 1.8 and then you add on 32. so that's the scales we use now what actually is temperature so temperature is a measure of the kinetic energy of these individual molecules what kinetic energy well they vibrate as they're
moving around the air as well as moving in those random directions the hotter the temperature it means the more these are vibrating and when they vibrate they're moving back and forth really fast stopping any other molecules from coming directly close to them aka
hot air expands this is the principle behind a hot air balloon because hot air expands we can also say that it is less dense if it is hotter because there are going to be molecules more spread out and therefore the density will reduce temperature drops steadily as we increase the altitude until reaching a
point called the tropopause which is the entry point into the stratosphere from the troposphere basically it means that the conditions the regular sort of conditions that we're laying out here switch over at this point and the temperature remains constant in the stratosphere now you may have noticed a bit of a contradiction here
we said earlier that density reduces with altitude so density goes down as the altitude goes up we also said that the pressure decreases as the altitude goes up but if the temperature
is going down as well as the altitude increases then that would make the density higher because temperature going up makes the density go down that's hot air expanding so the inverse is true as well temperature decreasing would make the density go up so as we reduce temperature surely
the density should go up so why is the density decreasing with altitude not increasing the answer is simple basically the pressure has more of an influence over the density than the temperature there is a hierarchy in essence so our density varies first according to the pressure then the
temperature and then the humidity humidity as i'm sure you're aware is to do with the moisture levels in the air the more humid the more moisture there is in the air now an individual water molecule actually weighs less than the individual molecules of oxygen or of nitrogen or any of the
other trace gases that there are in normal air that means that when the air is full of these little water molecules which weigh less it displaces the heavier oxygen and nitrogen molecules out of the space which brings the overall mass of the air down so if we plug that into our density equation
rho equals the kilogram mass per unit volume that means if we have more water molecules which are lighter that means the overall mass would reduce which means the overall density would reduce so then the density reduces as our humidity increases
humidity varies vastly depending on if you're next to the sea if depending on where you are if it's recently rained so there's no neat altitude comparison with humidity humidity is very changeable the last factor to look at with air is the viscosity of air viscosity is basically a measure of the stickiness of something
so water is not very viscous but honey would be very viscous so it measures how good or bad a fluid is it pooling along other molecules with it as it goes so if you imagine pouring honey all that honey starts to get pooled over the lid of the jar because all of the individual molecules are being pooled by the ones before it if i apply a force to this first molecule something very viscous would pool these other ones along with
it whereas something very unviscous would you know leave air is considered a fluid because it flows and it is also considered viscous as the molecules are dragged around objects in a predictable fashion which we'll come on to in the next class the elements that we've just looked at
vary vastly all over the globe and from day to day so aviators use an international standard atmosphere which includes all these elements to simplify calculations and it is as follows so we consider the temperature at sea level
to be 15 degrees celsius pressure also at sea level we consider to be 1 0 1 3 hecto pascals hector just being the prefix and pascals are
newtons per meter squared density also at sea level is considered to be 1.225 kilograms per meter cubed temperature lapse rate this is basically how much the temperature reduces as we increase in altitude so that's
considered to be 1.98 degrees celsius every 1000 feet so every 1000 feet we go up the temperature drops by 1.98 degrees celsius this is of course until reaching that tropopause that we talked about earlier which is the point between the troposphere and the stratosphere and that point the tropopause