Airspeed Indicator & Type of Airspeed Overview – Video Transcript
Today, we are going to learn a little bit about how the airspeed indicator works, and the nuances involved. To get us started, what I want to do, is take a look inside of the instrument and see if we can understand how the airspeed indicator displays our appropriate airspeed.
Behind the face of the instrument, what we see here is this diaphragm inside, and what we notice is that this pitot pressure is what is going inside of this diaphragm. Then, the static pressure goes into the case around that diaphragm. So, what is the significance of this? Well, as you may recall from one of our previous videos, the pitot pressure is a combination of the static air plus our speed, or sometimes they call that dynamic pressure. So, what we are trying to do is, we do not want a factor of change in altitude which would actually change the pressure of the pitot air to affect what is indicated on the airspeed indicator. So, in essence all we are doing is just comparing the pitot pressure to the static pressure. Another way to say that is, we’re comparing the static and the velocity of pressure to just the static pressure, so in essence what happens then is these basically negate or cancel each other out and that way what’s indicated on the airspeed indicator, by the expansion and contraction of this diaphragm, is simply a result of the velocity change and not a result of any static pressure changes.
Let us go ahead and look at how we read the airspeed indicator. On a standard single-engine airplane, we usually see a configuration kind of like of this, where we’ll see what we call a white arc on the airspeed indicator, we’ll have a green arc on the airspeed indicator, we have a yellow arc on the airspeed indicator, and we usually will find a red radial line. These different colors indicate different ranges. This white arc indicates our flaps operating range, this green arc identifies a normal operating range, this yellow arc represents a sort of caution or smooth air only range, and then the red line indicates our maximum speed that we can go in this aircraft. These lines, or these colors, have a starting point and an ending point; and these starting points and ending points are indicated by a particular V speed, as we call it. They are usually speeds that we often have committed to memory, but we can also read them from the airspeed indicator.
Let us go over a few that we do find on the airspeed indicator. The first one is the beginning of the white arc, known as VSO, which is our stall speed in the landing configuration. The end of the wide arc down here is VFE, which is our maximum flaps extended speed. The green arc starts from VS, sometimes it is VS1, and that is our stall speed in the clean configuration typically in most aircraft. The top of the green arc is VNO, which is our maximum structural cruising speed. Maximum structural cruising speed indicates when we could go outside of our load factor tolerance relatively easily so one of the big elements to VNO is if we’re going to fly into this yellow arc, we need to be in smooth air only and we predominantly are going to just be flying straight and level, not a lot of aggressive maneuvers. Last speed is the end of this yellow arc. There is a red radial line VNE. VNE is our never exceed speed, which means we are never able to go faster than this speed. If we do, we run the risk of structural damage occurring to the aircraft and obviously we don’t want the airplane to have any problems while we’re flying, so it’s a good warning for us and know never to exceed that speed.
Next, what I want to do is talk a little bit about the different types of air speeds. The first one is relatively simple. What we had just finished talking about when we read our air speed on the airspeed indicator is known as our indicated air speed. Just like what we have talked about before, as we look at the face of the air speed indicator, whatever your speed is indicated that is what we get. Moving from that we have what is known as calibrated airspeed. Now with calibrated airspeed, we are applying a correction value to the indicated airspeed based on installation or instrument errors. The most common example of where we will see something like that is if you imagine as we’re flying along in normal straight and level flight, that pitot source is pointed almost exactly into the relative wind and therefore, we have the most accurate amount of ram air pressure. If, however, we slow the airplane down, what happens if we maintain altitude and slowdown is, we end up flying at relatively higher pitch attitude. With that relatively higher pitch attitude, that means that our pitot source is not directly in line with the relative wind, so there is a little bit of a miss on the exact ram air pressure that it is sensing. An easy way for us to prove that is in most smaller aircraft. If I look at the correction table that we can find in the POH (Pilot’s Operating Handbook), what I usually identify is that the greatest calibration error for most small aircraft is generally at slower speeds and at the normal cruising speeds there’s very little calibration error that exists. So, it is just a simple example of what it means by this calibration error.
Next, I want to talk about what is known as true airspeed. To do that, I have put a diagram here that represents the ground, or the earth and I have a pitot mast from a Piper Aircraft – like what we fly here at AeroGuard – flying at a relatively lower altitude. Then I have another one flying at a relatively higher altitude and what you see here are these little dots and those little dots are representing air molecules. Why I have them displayed like this is because of a change in air density. At this lower altitude, the air molecules are denser which means they are closer together and for all intents and purposes, what we are trying to determine is velocity pressure of this air. That is how we get our speed. So, if for example, I have 10 air molecules here and let us say if all 10 of those air molecules are able to go into the pitot in one second, that is equivalent to let’s say 100 knots indicated air speed. Well imagine then that we climb to a higher altitude, or now where this airplane that is at a relatively higher altitude. What happens as we climb in altitude? The answer is the air molecules will begin to be less dense. So, will they become less dense if they become less dense? It means that they are more spread apart. So now these 10 air molecules are more spread apart which means they take up more volume. So now technically this airplane would need to fly a greater distance in the same amount of time in order for us to also have 100 knots indicated airspeed. What that means then, is this, we have actually traveled a greater distance at this higher altitude than we have at this relatively lower altitude. So if we assume that this was basically at sea level, or was the standards, and we would say that this was equivocal to 100 knots true airspeed this covering this distance or this volume of air in that amount of time and then that would mean then that if we went through this volume of air in the same amount of time we would be at a relatively faster speed. So maybe this is something like, I do not know, 150 knots true airspeed. The point I’m trying to make here, is this, our true airspeed is dependent upon the density of the air so for all intents and purposes the true airspeed is our calibrated airspeed that’s been adjusted for this change in air density whether that’s pressure or temperature.
Finally, we have what is known as ground speed. So, the ground speed is simply the true airspeed corrected for the wind. So, in this example, if I had a true airspeed link in this case of 150 knots that is fine and if there was no wind, then that distance through the air is equivalent to the distance over the ground. If, however, this air mass, this volume of air, was moving let us say that direction or facing us, so a headwind, and let’s say that wind was 10 knots, we know then that we’re moving at 150 knots through the air. But relative to the ground, it would be the difference between the two. So, we would then see our ground speed to be 140 knots in this example. Vice Versa, if the winds came from behind us, we call that a tailwind, then we would add that wind to our true airspeed. I hope that helps illuminate a little bit of the different types of air speeds and how we can go about calculating them.