Transcription
When we see all the acronyms that aviators use when talking about airspeed, it can get confusing fast. There's EAS, TAS, calibrated, and Mach, and it doesn't end there. Every one of these is relevant in a specific situation. We'll go over them all, and by the end of this video, you'll know what each one means. So let's dive into air speeds for aviators.
When you're driving a car, you have a very accurate and reliable way to measure how fast you're moving. A sensor in the car just measures how quickly the tires are spinning and translates that into a speed. As long as the tires are in contact with the ground and have traction, it's pretty accurate. But when we're flying in an aircraft, by definition, we don't have anything in contact with the ground. So how do we measure airspeed? The answer is with pressure.
Aircraft usually come equipped with two sensors for this task. One of them is called a static port, which measures the pressure of the air near the aircraft. This is the same sensor we use for figuring out our altitude. We get our airspeed by adding in another sensor called a pitot tube. Like the name implies, it's a tube which collects air as the aircraft moves forward. This ram air will be at a higher pressure than what the static port is measuring. As the aircraft picks up speed, the pressure in the pitot tube will rise. It's the difference between the pressure in the ram air and the pressure in the static port that's used to determine air speed. Together, these two sensors are referred to as the pitostatic system.
The direct reading we get from the system is known as indicated airspeed, or IAS. While IAS is useful in a lot of circumstances, there are a few errors that might crop up that need to be corrected. One of these errors is known as position error. What this refers to is how the position of the aircraft can affect the airflow into and around the pitostatic system, which could change their readings. For example, when the pitot tube is pointed directly into the oncoming airflow, it'll show a higher pressure than if the tube was deflected at a high angle. The static port is subject to position errors too. If it's placed somewhere where the airflow changes at high angles, then this will affect the pressure registers, which will change the indicated airspeed. So if your pressure is off, your airspeed is going to be off too.
Aircraft manufacturers correct for this by doing tests on their aircraft during development to measure how far off indicated airspeed is. Engineers can then calibrate the airspeed indicator for any of these errors. This adjusted speed is known as calibrated air speed, or CAS. Calibrated air speed is important in several situations, like landing and takeoff, when a high angle of attack or flap setting could affect the flow of air into the pitostatic system.
Now we know that as we climb in altitude, the ambient air pressure decreases. Since our airspeed instruments rely on pressure, climbing high up into the air is going to cause some errors if we don't compensate for this. Another way to look at this is that lower density air will not apply as much pressure to the system, so the air pressure indicator will show lower speed than what the aircraft is actually doing. What we get when we correct for these errors is called true air speed, or TAS. There's no direct way to measure true airspeed, so it has to be calculated. A pilot can use one of these E6B calculators to do it by hand, but nowadays there are air data computers built into the aircraft that do it automatically. There's also a rule of thumb that will get you close to true airspeed: take the calibrated air speed and then add two percent for every 1000 feet of altitude. So with an airspeed of 200 shown and an altitude of 10,000 feet, the true airspeed would be about 240.
Now there's one other airspeed error we need to go over, and it's known as compressibility. When we talk about compressibility, what we mean is that as air is forced very rapidly into a confined space like a pitot tube, it has a tendency to pile up like cars in a traffic jam. This will alter the measured pressure and introduce a new error into our air speed. Pilots don't need to worry about this unless they happen to be flying fast aircraft, since it takes a lot of speed before this error gets big enough to be noticeable. There are several math formulas out there used to calculate the error, and the result is known as equivalent airspeed, or EAS. This is typically done for the pilot by a built-in computer, but as a rule of thumb, when flying below 200 knots and 10,000 feet, then there's no significant difference between equivalent and calibrated airspeed.
One thing to keep in mind here is that depending on who you ask, true airspeed may or may not include EAS, but there is one airspeed that always includes it, and that's the Mach number. Mach number is sometimes spelled out as IMN for indicated Mach number, and unlike the other airspeeds we've covered, this one is not given in knots; it's given as a ratio of the speed of sound. So if an aircraft is moving at Mach 0.5, it's going at half the speed of sound, and Mach 1 would be exactly at the speed of sound. More importantly, this number is about the local speed of sound, which means how fast sound travels at the conditions present around the aircraft. This local speed of sound changes with temperature, which varies with altitude.
When we look at how the speed of sound changes with altitude, it looks like it decreases with air density, and that makes sense because sound travels faster through dense materials like water and steel but more slowly through air. But as we pass from the troposphere into the stratosphere, we see that the speed of sound actually matches the shape of the temperature line. For most aircraft, the speed of sound will only ever decrease with altitude. That's because they operate down in this region up through the lower portion of the stratosphere. Only specialized aircraft like the SR-71 and spacecraft will ever move through the region where the speed of sound starts to pick up speed again.
So why exactly is Mach number useful to pilots? One reason is because it takes compressibility into account. Compressibility is only an issue under certain speed and altitude conditions, so a pilot could either memorize a chart with all those conditions or just remember that compressibility happens at around Mach 0.3. Since Mach number accounts for both speed and altitude in the calculation, then compressibility is already taken care of for us, so it's a lot easier to remember this one number. The other major reason for using a Mach number is to stay within the structural limits of an aircraft. At around Mach 1, shock waves form around the aircraft that put extra stress on the airframe. If an aircraft isn't built for supersonic flight, then this is a speed limit that needs to be obeyed. Since the speed of sound changes with altitude, it's a lot easier for a pilot to go by this one number than an entire chart. The same goes for aircraft that have an even lower structural limit, whether that limit is Mach 0.3 or 0.7. This is a number that lets you know where that cutoff is, whether you're flying at 200 feet or 20,000.
Now there's one last speed we need to cover, and it's our speed over the ground. So why do we need to cover ground speed when we have all these other speeds already? Shouldn't they tell us how fast we're moving over the ground? Not necessarily. There are two factors that will affect how much ground we cover. One is the angle we're flying at. If our aircraft is flying straight up at 300 knots, it's not moving towards a point on the ground at 300 knots. The other factor is wind. If a plane is flying at 90 knots into a 20-knot headwind, then it's not really moving towards its destination at 90 knots. That 20 knots of wind will register inside the pitot-static system exactly the same as if it came from 20 knots of forward movement. So in this case, the plane's speed over the ground is only 70 knots, and if we bump that wind up to 90 knots, it wouldn't make any progress at all over the ground, but the airspeed indicator would still show 90 knots. The same thing happens with the tailwind, only in the opposite direction. A tailwind would increase the ground speed.
To get a better idea of how these air speeds look, we should jump into the cockpit and see them at work. For this demo, we'll be using our virtual F-16, since it has a set of displays that lets us see several air speeds at once. Here we can see our calibrated airspeed on this tape to the left; just below it is our indicated Mach number. All the way over to the right on this smaller display, we can see our ground speed. Right now we're cruising along at 350 knots calibrated, and we can see that's pretty close to our ground speed and that we're around Mach 0.5. Let's take a look at this same flight, but we're going to add in a significant headwind. [Music] With that headwind, things look the same, right? 350 calibrated and Mach 0.5, but look at our ground speed; it's a lot slower than what we had with no wind. And that's because the air flowing over our F-16 is moving at a relative speed of 350 knots, but our progress over the surface is a little slower than that. It's something we'll have to keep in mind when flying with strong winds.
Now let's take a look at all this again, but we're going to try it at 30,000 feet. Again, we're 350 calibrated, but look at what's happened to our Mach number and ground speed. All of a sudden we're covering ground a lot faster. Air is less dense up here, so we'll move faster and burn less fuel than we would at sea level. The same is also true for a true airspeed, even though we don't have a display for that; it should be pretty close to our ground speed as long as we fly straight and level and have no wind. You'll also notice that our Mach number is a lot higher too. Part of the reason is our higher true airspeed, but there's a second factor at play. At 30,000 feet, the speed of sound is actually lower by about 72 knots. So as we climb, we can expect to get higher Mach numbers.
In this video, we went over several ways of describing airspeed. There's indicated airspeed, which is just the raw reading from our pitostatic system, and it's subject to several errors. Position error is covered by calibrated airspeed; compressibility error is solved by equivalent airspeed; true airspeed takes into account the lower density of air at higher altitudes; Mach is a ratio of an aircraft's speed relative to the local speed of sound; and lastly, we have ground speed, which lets us know how fast we're moving towards our destination, even if there's wind involved. I hope I did an adequate job of explaining this topic, and thank you for watching. [Music] [Music]