Types of Airspeed, Explained

so in a car the only speed that we really need to concern ourselves with is the speed of the car over the ground if we're in an airplane we're moving that airplane through an air mass usually an air mass that's moving at varying densities and pressures and the speed at which that air mass passes over the airplane can have different implications in terms of aerodynamics or structural implications so we need to be able to talk about air speed in terms of more than just the speed of the airplane over the ground we need to be able to talk about air speed in terms of the air mass that it's moving through or in terms of the instrument that's measuring it or in terms of the speed of sound so we have several different types of air speeds that we need to be familiar with all right let's start out with the simplest one first ground speed ground speed is just the speed of the airplane moving over the ground or the speed of the airplane in relation to the ground it has nothing to do with altitude or temperature or density or pressure or any sort of atmospheric conditions it's just the speed of the airplane moving over the ground so if we want to calculate how long it will take us to get somewhere right using our distance equals speed times time we need to use ground speed so true airspeed is just the speed of the airplane as it moves through the air or the speed of the airplane in relation to the air mass that it's flying through so in still air true airspeed is the same as ground speed because the ground is not moving either so there's no difference in saying the speed between the air mass and the airplane and the speed between the ground and the airplane if neither are moving now if we have a headwind the true airspeed is going to be greater than ground speed because the relative speed the speed that the air moves past the airplane is going to be higher than the speed that the airplane moves over the ground because the because the relative air mass is moving in the opposite direction as the airplane so it passes the airplane faster than the air passes the ground if we have a tailwind true airspeed is going to be less than ground speed because the air that's the relative speed between the air mass and the airplane is going to be less than the relative speed between the ground in the airplane because the air mass is moving in the same direction as the airplane so the relative speed between these two are going is going to be less now another way of thinking about this is is just to say that the headwind sort of slows down the airplane so ground speed is going to be less than two air speed in a headwind and the tail wind speeds up the airplane so ground speed is going to be higher than true air speed in a tailwind so indicated airspeed is just the measured speed of the airplane as it moves through the air it's the speed of the airplane in relation to the air mass that it's flying through according to the pitot tube so the pitot tube measures air speed using the pressure of the air and as altitude increases the pressure of the air decreases so at a constant true air speed the airplane is still going to be moving past these air molecules at the same speed it's just that they're going to be there fewer of them to enter the pitot tube so the pitot tube reading the indicated airspeed is just going to be less than your than the true airspeed just because it's just a limitation of the instrument but tahrir speed is the same because we're moving past these air molecules at the same speed or garlis of altitude in this example it's just that at a higher altitude there are fewer alma air molecules to enter the pitot tube so indicated airspeed is less than true airspeed so we could say that as altitude increases indicated air speed will decrease at a given true airspeed okay so let's look what that might look like again in still air so if we're holding a constant true airspeed of 200 knots so that is to say that we're moving past all of these air molecules at 200 knots as we increase our altitude and the pressure of that air decreases so we increase our altitude there are going to be fewer air molecules to enter the pitot tube so that pitot tube reading will eventually will eventually go down or it's lower than true airspeed just because of the density of the air but we're still moving at a constant career speed we're still moving past those air molecules at the same speed now let's look at it the other way around because we're going to be using indicated airspeed in our climb because that's what we see on our airspeed indicator so if we want to maintain a constant indicated airspeed throughout our climb that means that we need to be putting enough air molecules in that pitot tube to hold that same indicated airspeed even as the pressure of the air decreases with altitude so as we increase altitude we're gonna be we're gonna have to move past those air molecules even faster to keep that same indicated airspeed for to keep that indicated airspeed from decreasing because of the decreasing density so if we hold a constant indicated airspeed in our climb true airspeed will increase okay so calibrated airspeed is just indicated airspeed or airspeed measured by the pitot tube corrected for what we'll call instrumentation or position errors so anytime air flows around a body it's going to change the pressure of the air the speed of the air in different ways so ideally the pitot tube would be measuring the free flow of the air uninterrupted by the airfoil or skin friction or angle of attack or flap setting or anything like that but unfortunately it's not possible to completely avoid the interference of those things so calibrated airspeed just basically corrects for any error caused by the changing of the airflow from other parts of the airplane so this is usually no more than a couple knots and it's most prevalent at slower speeds low altitudes like when we have the flaps out things like that but it's not that important to where you're going to notice a difference it's just important to know the definition of this and that this this these errors can exist so as airplanes fly at higher altitudes and higher air speeds the airflow and their airspeed the speed of sound and when airflow approaches the speed of sound it behaves a little differently compressibility changes and things like that things that are important to keep the airplane in a safe manoeuvre within safe maneuvering margins so for this reason at high altitudes and high air speeds we use Mach number to measure the aircraft's airspeed so Mach number is just the ratio of our true airspeed to the speed of sound so if we're at Mach 1 then that means that our true airspeed is at the speed of sound if we're a Mach 7 zero that means our tree airspeed is 70% of the speed of sound so it's just a way of saying how close we are to the speed of sound so the speed of sound is a function of temperature so as temperature decreases speed of sound also decreases now as altitude increases air temperature decreases so therefore as altitude increases speed of sound decreases so speed of sound decreases with altitude so if our Mach number is how close they are to the speed of sound and as we get higher speed of sound gets lower then we're going to get closer to the speed of sound if we hold a constant true airspeed so at a constant true airspeed Mach number increases so let's look at this the other way what if we hold a constant Mach number in a climb if we hold a constant Mach number that means we're holding a constant percentage of the speed of sound which is decreasing with altitude so for example for holding Mach seven zero as we climb that means we're holding 70 percent of the speed of sound which is decreasing we're holding 70 percent of a decreasing number so seventy if we if we our true airspeed is 70 percent of a decreasing number that means as we climb at a constant Mach our true airspeed is going to decrease so at a constant Mach in a climb true airspeed decreases

Channel: Vector Pilot Prep

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Keywords: Aviation, Flying, flight training, pilot training, student pilot, flight lessons, pilot lessons, commercial pilot, flight instruction, Flying lessons, true airspeed, indicated airspeed, groundspeed, mach, mach number, calibrated airspeed, types of airspeed, airspeed, ground school

Id: tAGgax4SmkQ

Length: 8min 47sec (527 seconds)

Published: Mon Dec 10 2018