Our goal is to understand the differences between each airspeed.
Typically, when planning a cross-country flight, it is our goal to find ground speed, and sometimes indicated airspeed. What’s wrong with IAS? Why do we need to find True Airspeed all the time? When we read our airspeed from our airspeed indicator, as the name would suggest, the number we see, we call “Indicated Airspeed”.
What’s wrong with this “Indicated” airspeed?
To answer this question, we must understand how our airspeed is measured.
So, How is our airspeed measured? Lets explore this below.
Below is a image of the actual components involved in measuring our “ram” air to move a needle to tell us the number of the rate of nautical-miles-per-hour (“Knots”) we are flying at
This image can be hard to understand when trying to understand it by itself. Let us return to this image after we familiarize ourselves with an analogy.
Speed is Relative
This is an analogy to help explain and visualize airspeed indicators:
Suppose we are sitting on a boat. Many of us have been on boats before. Lets imagine we are floating along on a boat down a river.
Here we see, We are standing on our boat traveling NORTH (up). As we float, imagine we want to know how far to the next dock. Basic physics says:
How long it takes = How far we travel / How fast we go
*Our subject today is not actually calculating time, we Are only concerned with the theory*
So, from the above scenario, we see how on our boat, Like on our plane, we might want to know our speed. So, let’s continue. We have established, in our boat (or our airplane), we do have a need to know our speed.
Imagine, sitting in our boat, you put your hand into the water. Can you feel the water rush past your hand? You would be able to if you were actually in a boat! Based on how hard the water is pushing against your hand (since we are on a motor boat, and the engine of our boat is propelling us), you estimate we are traveling 25 Knots.
Lets call this our “Water-speed” — that is, our speed through the water. This is similar to “airspeed” in our airplane.
In the boat, you now look to your left and right. You see the shore, trees, buildings. Just by looking at how slowly we’re passing the buildings and trees, you estimate we are traveling 10 knots.
Wait a minute! The water is pushing against our boat at a speed of 25 knots, but we’re only passing the buildings and trees at a speed of 10 knots! Are we really traveling at two different speeds at the same time?
This is why we said earlier, 25 knots is our “water speed”. Our speed through the water. Our “ground speed”, our speed relative to the land, is only 10 knots. So, our speed is really relative. We can compare our speed to the water or the ground.
Our ground speed would be important in planning how long it would take to reach a destination along the shore, where all destinations are, after all.
Our “water speed” would be important to know how much stress the hull of our boat is sustaining. If we go too fast, our boat might get damaged from the fast moving water.
In an airplane, our Airspeed is just like our “water speed” if we go too fast in our airplane, and exceed Vne, the force of the air against the thin metal of our aircraft will damage it.
Also, in a discussion of going too slow, our speed relative to the air would be important, since the more (or less) air flowing over our wing can be a good indicator of how much lift we are (or are not) making. If we don’t have enough air flowing over our wing (going to slow), then we wont make enough lift, and will aerodynamic stall. Further consideration on stalls will need to be a different discussion
Ok. So Ground speed is our speed when we compare our movement to the ground, and airspeed is our speed when we compare our movement to the air passing around our airplane.
Lets return to our boat scenario again. Sitting on the boat, with our hand in the water is quite uncomfortable. And after a long day of sailing, our arm will hurt. Instead of sitting with our arm extended over the edge of the boat, lets build a sensor to tell us how fast the water is flowing around us.
Imagine we build a tube, and have a “waterspeed sensor” at the end of it. When we put the tube in the water facing forward, it will tell us our speed. This is an analogy for our pitot tube.
Calibrated Airspeed
When our “waterspeed measuring tube” is pointed straight into the oncoming water, the speed is read correctly. But if the water hits the tube at an angle, less water enters the tube, and therefore waterspeed measurement decreases. This is a position error. Imagine you have a soft drink straw. When you hold it in front of your mouth and blow, all the air you blow will flow through the straw. If you now tilt the straw upwards (keeping the open end close to your mouth) and blow, a lot of the air will pass around the straw and not go through, since the opening is at an angle.
Back to our airplane. Our airplane has a tubular “speed measuring device”, we call it the pitot tube. The pitot tube suffers the same errors as our newly made “waterspeed measuring tube” from our boat.
When the airflow hits our pitot tube at an angle, less air enters the pitot tube since the opening is smaller to the relative wind. What this means to us: When our relative wind is not coming straight into our pitot tube, our indicated airspeed will decrease, even though our actual speed hasn’t changed (only the amount of air entering our measuring device has)!
When will we fly with our Airflow not aligned with our pitot tube? In times of high angle of attack. For example, when we fly in slow-flight. Our nose — and pitot tube — is quite high, but our actual flight path is perfectly straight, not climbing! So, our flight path is straight (and relative wind), but our pitot tube is pointing up. This will result in an even slower indicated airspeed, since the misalignment will, again, cause less air to flow into our pitot tube.
To Calibrate our airspeed during these times of low speed and high angle of attack (both of which often happen at the same time), we need to look in the POH. Airspeed Calibration Table, found in the performance section, will show us: at a given indicated airspeed, our airspeed really will be “this”. We call this airspeed, after it has been calibrated for this position error, “Calibrated airspeed”.
If there is any confusion so-far about calibrated airspeed, seek further assistance before proceeding.
Equivalent Airspeed
Only two things need to be said about Equivalent Airspeed.
1.) Equivalent Airspeed is Calibrated airspeed that has been corrected for air compressability.
2.) At our scale of flying (Slow and low, that is), Equivalent airspeed is the same as our Calibrated airspeed. Since air doesn’t compress much when it flows slow and is very dense. We do not need to calculate equivalent airspeed. We do, however want to know that it does exist, and at higher levels of training (jets) it will be applicable.
True Airspeed
Calibrated airspeed is a great first step into making our speed more precise. However, if we were to fly a constant airspeed as we climb, we would find ourselves flying faster and faster. Why does this happen?
When air is less dense, it will exert less force on our pitot tube, therefore, our tube will think we flying slower!
Refer to the diagram. Both blue pitot tubes are traveling at the same true airspeed. In this picture, we see how many air molecules will enter the pitot tube in a given second. Notice, the high altitude pitot tube will “catch” fewer than the lower altitude pitot tube (imagine Pac-Man eating the dots along the path). We know both airplanes are actually flying the same TRUE airspeed, but the high altitude airplanes’ airspeed indicator will show a lower airspeed (since there are less molecules entering the pitot tube).
Now, if instead of maintaining the same TAS, our two airplanes (high altitude and low altitude, from the diagram) maintain constant INDICATED airspeed, the high altitude airplane will actually be traveling faster. Since the air molecules are fewer up at high altitude, the airplane will need to go faster in order for the same pressure to be felt in the pitot tube to sense that same indicated airspeed.
* If our equivalent airspeed were different from our CAS, we would use that, but as previously mentioned, calculating EAS at our low and slow flight environment is irrelevant.
# We use CALIBRATED airspeed to obtain true airspeed, this focus on the indicator is simply for illustration porpoises.
Groundspeed
Now that we know how fast we are moving through the air mass, we can now figure out our speed over the ground.
Recall our discussion of our boat. If our waterspeed tells us we’re going 80kts, but the water is flowing against us, at 20 kts, then the speed at which we will pass the buildings, trees — the ground– will be 80kts minus 20kts. our groundspeed is 60kts.
The air moves, similarly to an ocean. Instead of having a “current” our air masses blow in directions, and we call this wind.
When our wind comes us from ahead and blows against us, we call it a headwind. When the wind blows in the same direction we’re flying, we call it a tailwind. Headwinds make us fly slower over the ground, it subtracts our speed, and headwind adds to it.
Very rarely will our wind come perfectly from ahead or behind us, so in addition to ground speed differences, our wind will affect our track over the ground. much like a boat trying to cross a flowing river, it would get pushed with the water downstream. But, thats a discussion for another time, today we are only focused on the speed changes.
We could use some advanced math to calculate our groundspeed (its just a matter of vectors), but math by hand is neither easy nor fun, so we’ll use our trusty E6B instead.
Groundspeed is ultimately our speed over the ground. Given the fact that we’d like to land on an airport on the ground the rate at which we travel over the ground is important, so we will certainly use it in our nav-log planning.
HOW DO WE Calculate?
To calculate any speed, we can refer to the “speed triangle”
To get to groundspeed:
- Starting with IAS, we need to convert to CAS using POH
- With CAS, we need to use an E6B and weather information to get TAS
- With TAS, we need to use an E6B and wind data to get our groundspeed
Notice the order. It is a pyramid. Each step must come after the other. also notice the images on the left. IAS comes straight from the Airspeed indicator, as depicted on the pyramid. Then, to find CAS, we need to use our airspeed calibration table in our POH, so to remind us, we can see a table from our POH at the “CAS” level of our pyramid. In our TAS section, we see a rotary E6B to remind us this is how we find TAS. Lastly, a winds aloft table reminds us that to get to groundspeed, we need to use winds aloft data (and also an E6B).
Sometimes, though, we already know our TAS (like when we read it from our cruise performance in our POH), and would like to know what our groundspeed is. All we have to do is make that final step to GS since we already have TAS. All we need in this case is wind data and our E6B.
Sometimes, we need to find what indicated airspeed is required to maintain a desired true airspeed or groundspeed. To do this, we simply walk our way down the steps of the pyramid.
If we want to find what indicated airspeed is required to maintain a groundspeed, we first need to convert GS to TAS. To this, one could very easily use their rotary E6B. Then with our TAS, we convert to CAS, again, we find “what cas would we need to maintain TAS”. A lesson in E6B usage may be needed to maintain proficiency in these calculations. Then, with our CAS, we would refer to our POH and get our IAS needed.
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