IAS TAS and Ground Speed
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IAS TAS and Ground Speed
(Virtual/PC pilot)
I've noticed that as you ascend to your cruise altitude of 360, your 'speed' drops in the primary display. Additionally, I notice that max speed (Vne?) reduces as well. My reading leads me to believe that the speed reduction on the primary display reduces because the air pressure on the pitot decreases, or less air is being forced in. But why does the Vne reduce as you climb? Is it a calculation of Vne based on altitude?
Then I see we have Ground Speed. So that would be using GPS over the ground, I'm guessing. Is that what ATC would see, when the aircraft reports speed?
Then we have IAS... So indicated airspeed would match what we see on the primary display, I think.
And TAS would be the real speed the aircraft is moving through the air, regardless of pitch (i.e. If we were climbing vertically, speed would not be 0 - it would be relative to the airflow).
Is TAS based on the altitude (air pressure).
If all of these are correct - why wouldn't the primary display just display TAS, which seems to be the most important speed for the pilot? With Ground Speed being the most important to ATC. IAS seems to be the least useful.
I've noticed that as you ascend to your cruise altitude of 360, your 'speed' drops in the primary display. Additionally, I notice that max speed (Vne?) reduces as well. My reading leads me to believe that the speed reduction on the primary display reduces because the air pressure on the pitot decreases, or less air is being forced in. But why does the Vne reduce as you climb? Is it a calculation of Vne based on altitude?
Then I see we have Ground Speed. So that would be using GPS over the ground, I'm guessing. Is that what ATC would see, when the aircraft reports speed?
Then we have IAS... So indicated airspeed would match what we see on the primary display, I think.
And TAS would be the real speed the aircraft is moving through the air, regardless of pitch (i.e. If we were climbing vertically, speed would not be 0 - it would be relative to the airflow).
Is TAS based on the altitude (air pressure).
If all of these are correct - why wouldn't the primary display just display TAS, which seems to be the most important speed for the pilot? With Ground Speed being the most important to ATC. IAS seems to be the least useful.
This is a bit simplified and not strictly accurate but as a rough explanation. Aerodynamicists will have conniptions but here goes...
Generally for flying the aircraft, IAS (as measured by the ASI) is primary for the pilot. It measures how the aircraft 'feels' the air.
As air density decreases, the aerodynamic force provided by the movement of the air over the craft decreases. IAS will reflect what the air "feels like" to the aircraft. So for the same IAS but different altitudes, the aircraft seems to fly the same way (meanwhile the TAS will change).
Vne is a bit different.
Vne depends not just on the 'striking force' the air imparts on the aircraft (if it did Vne would remain at a constant IAS regardless of altitude).
It also depends on the actual speed of the aircraft through the air (even at lower densities).
eg aerodynamic "flutter" is more like a resonance or vibration rather than the result of the direct force of air striking the airframe. It is more related to actual airspeed and so as we fly higher our IAS may remain below the lower altitude Vne but if our TAS is high enough to cause flutter we may run into dramas. So the IAS for Vne will change as we go higher.
Like I say this is very simplified and not 100% accurate (I am not an aeronautical engineer) but hope it gives a rough idea of why the ASI is used so prominently.
NB there is also CAS (calibrated Airspeed), EAS, (equivalent airspeed) as well!
.
Generally for flying the aircraft, IAS (as measured by the ASI) is primary for the pilot. It measures how the aircraft 'feels' the air.
As air density decreases, the aerodynamic force provided by the movement of the air over the craft decreases. IAS will reflect what the air "feels like" to the aircraft. So for the same IAS but different altitudes, the aircraft seems to fly the same way (meanwhile the TAS will change).
Vne is a bit different.
Vne depends not just on the 'striking force' the air imparts on the aircraft (if it did Vne would remain at a constant IAS regardless of altitude).
It also depends on the actual speed of the aircraft through the air (even at lower densities).
eg aerodynamic "flutter" is more like a resonance or vibration rather than the result of the direct force of air striking the airframe. It is more related to actual airspeed and so as we fly higher our IAS may remain below the lower altitude Vne but if our TAS is high enough to cause flutter we may run into dramas. So the IAS for Vne will change as we go higher.
Like I say this is very simplified and not 100% accurate (I am not an aeronautical engineer) but hope it gives a rough idea of why the ASI is used so prominently.
NB there is also CAS (calibrated Airspeed), EAS, (equivalent airspeed) as well!
.
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Wow... OK, so that opened my eyes (and mouth) a bit wider! Thanks for that.
Then I went and googled V Speeds, and came across wikipidias link:
https://en.wikipedia.org/wiki/V_speeds.
I didn't count, but estimated around 60!
One thing, unrelated, but if the speed is between V1 and "V2min"
That seems to spell disaster....
Then I went and googled V Speeds, and came across wikipidias link:
https://en.wikipedia.org/wiki/V_speeds.
I didn't count, but estimated around 60!
One thing, unrelated, but if the speed is between V1 and "V2min"
That seems to spell disaster....
As jonkster has said IAS gives a measure of how the aircraft will behave aerodynamically, even at altitude, even though it is underreading relative to the True Airspeed. At altitude your Vne is most likely going to be determined by Mach effects (e.g shock wave formation) so on a all singing dancing PFD you’ll quite possibly see the high speed Limit varying relative to indicated airspeed.
It could, so that is one of the reasons why we do a performance calculation before every take off - to ensure that the combination of runway, thrust and aircraft loading is such that there is no “gap” between the various controllability speeds...i.e. you are not going to be in a situation where you are too fast to stop but not going fast enough to be able to control the aircraft. If you can’t satisfy that criteria you need to look at changing one of the variables (runway, thrust, loading).
It could, so that is one of the reasons why we do a performance calculation before every take off - to ensure that the combination of runway, thrust and aircraft loading is such that there is no “gap” between the various controllability speeds...i.e. you are not going to be in a situation where you are too fast to stop but not going fast enough to be able to control the aircraft. If you can’t satisfy that criteria you need to look at changing one of the variables (runway, thrust, loading).
Last edited by wiggy; 1st Aug 2018 at 07:18.
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If you have an engine failure at V1 you will still accelerate to Vr and by the time rotation is finished even on one engine you should be at V2.
So you should never be below V2 during flight.
Regarding your other question i would suggest reading some wikipedia:
https://en.wikipedia.org/wiki/Coffin..._(aerodynamics)
So you should never be below V2 during flight.
Regarding your other question i would suggest reading some wikipedia:
https://en.wikipedia.org/wiki/Coffin..._(aerodynamics)
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(Virtual/PC pilot)
I've noticed that as you ascend to your cruise altitude of 360, your 'speed' drops in the primary display. Additionally, I notice that max speed (Vne?) reduces as well. My reading leads me to believe that the speed reduction on the primary display reduces because the air pressure on the pitot decreases, or less air is being forced in. But why does the Vne reduce as you climb? Is it a calculation of Vne based on altitude?
I've noticed that as you ascend to your cruise altitude of 360, your 'speed' drops in the primary display. Additionally, I notice that max speed (Vne?) reduces as well. My reading leads me to believe that the speed reduction on the primary display reduces because the air pressure on the pitot decreases, or less air is being forced in. But why does the Vne reduce as you climb? Is it a calculation of Vne based on altitude?
Then I see we have Ground Speed. So that would be using GPS over the ground, I'm guessing. Is that what ATC would see, when the aircraft reports speed?
Then we have IAS... So indicated airspeed would match what we see on the primary display, I think.
And TAS would be the real speed the aircraft is moving through the air, regardless of pitch (i.e. If we were climbing vertically, speed would not be 0 - it would be relative to the airflow).
Is TAS based on the altitude (air pressure).
Is TAS based on the altitude (air pressure).
If all of these are correct - why wouldn't the primary display just display TAS, which seems to be the most important speed for the pilot? With Ground Speed being the most important to ATC. IAS seems to be the least useful.
Hope that explains it all.
Basically, an aircraft flies because of INDICATED air speed (IAS) which is the actual effect of the air on the wing similar to what you would feel if you were able to stick your hand out the window. If the best airspeed is 280 KIAS, then it will to a degree be the same at all altitudes.
But, the air is a lot thinner at altitude, so to achieve that 280 KIAS you will have to go FASTER through the air mass as you climb. At FL400, it would be about twice as fast so, that gives TAS, or true airspeed. For a constant IAS, the TAS will increase as you climb, and for a constant TAS, the IAS would be reducing.
The aircraft only cares about IAS...that's what makes it fly. TAS is what gets it from point A to B once corrected for the wind, and that gives GS or groundspeed.
Overriding all of that is mach number which is speed as a percentage of the speed of sound. It gives one set of maximum speeds and is related to temperature, and reduces as you climb. It's TAS. At sea level it will be around 680 knots, reducing to roughly 560 at FL400. So, that 560 kts of mach 1 is roughly 280 KIAS, which is not much faster than best IAS. Drag rises enormously around mach one, so it is limiting for a number of reasons. Above about .85, you'll slow to below the best IAS to keep away from it. In a climb, at about FL300 the climb IAS will hit roughly mach .8 and from that point you climb at a mach number.
An IAS, at a given weight, will give a fixed 1G angle of attack. An airframe has a 'best' angle of attack, so we use IAS to indirectly give us that best AoA.
But wait, there's more.................
as an aircraft burns fuel, and the weight reduces, it needs a lower angle of attack to produce the required lift. To maintain that best angle, the speed is reduced which is why cruise speed is constantly being reduced as a flight progresses.
And more........................
The engines have an RPM at which they function best too, sometimes it's 90% or even closer to full power. Reducing the speed means the engines may not be at that best power setting any more, so after a while you'll be better off climbing. The 'best' altitude would be the one at which the IAS giving that best angle of attack simultaneously requires the engines to be set at their best RPM.
The upshot of this is that you end up with a series of climbs and decelerations during any given flight.
But, the air is a lot thinner at altitude, so to achieve that 280 KIAS you will have to go FASTER through the air mass as you climb. At FL400, it would be about twice as fast so, that gives TAS, or true airspeed. For a constant IAS, the TAS will increase as you climb, and for a constant TAS, the IAS would be reducing.
The aircraft only cares about IAS...that's what makes it fly. TAS is what gets it from point A to B once corrected for the wind, and that gives GS or groundspeed.
Overriding all of that is mach number which is speed as a percentage of the speed of sound. It gives one set of maximum speeds and is related to temperature, and reduces as you climb. It's TAS. At sea level it will be around 680 knots, reducing to roughly 560 at FL400. So, that 560 kts of mach 1 is roughly 280 KIAS, which is not much faster than best IAS. Drag rises enormously around mach one, so it is limiting for a number of reasons. Above about .85, you'll slow to below the best IAS to keep away from it. In a climb, at about FL300 the climb IAS will hit roughly mach .8 and from that point you climb at a mach number.
An IAS, at a given weight, will give a fixed 1G angle of attack. An airframe has a 'best' angle of attack, so we use IAS to indirectly give us that best AoA.
But wait, there's more.................
as an aircraft burns fuel, and the weight reduces, it needs a lower angle of attack to produce the required lift. To maintain that best angle, the speed is reduced which is why cruise speed is constantly being reduced as a flight progresses.
And more........................
The engines have an RPM at which they function best too, sometimes it's 90% or even closer to full power. Reducing the speed means the engines may not be at that best power setting any more, so after a while you'll be better off climbing. The 'best' altitude would be the one at which the IAS giving that best angle of attack simultaneously requires the engines to be set at their best RPM.
The upshot of this is that you end up with a series of climbs and decelerations during any given flight.
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'Atlas Shrugged', that was an incredibly helpful bit of text you wrote there. It all clicked nicely! And then it went crazy! I can now understand why it's happening. Thanks for the clear understanding.
I can now understand why it's happening. Thanks for the clear understanding.
