TAS / CAS airspeed.
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Hello Forum,
In one of Micro’s excellent TE creation videos, when he is checking the details at various steer points, for time/altitude etc I notice that both the TAS and CAS shown are at the same speed.
When I’m making a TE the TAS and CAS speeds are shown as different. The TAS is always faster. It’s confusing.
Is there a way to align them please.
Thanks,
Bayonet.
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M8… before you write anything more… (you’ll thank me later )
Please check wiki for - TRUE airspeed , vs CALIBRATED airspeed , TAS/CAS.
Cheers
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Hi @white_fang
Thanks for your reply and the warning to keep foot out of mouth
I do understand that TAS and CAS differ.
My query, perhaps unclearly worded, referred to a segment in Micros’s
If you have a moment take a look from time stamp 12:30 where he adjusts various items in steer points.
You’ll see that both TAS and CAS coincide. That’s my question, how do they?
I know that at certain altitudes, perhaps over water, they can be close.
So, is it because the altitude shown is only 100FT (NOE) ??
Have I managed to redeem myself?
Thanks,
Bayonet.
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@Bayonet
If you know the difference between CAS and TAS you know that altitude, and temperature are the point.CAS and TAS are of course similar near the sea level and at constant TAS, the CAS decrease with altitude increasing.
For navigation and timings management, you have to work in TAS (actually, rather in GS).
For performances, you have to think in CAS. -
@Bayonet said in TAS / CAS airspeed.:
Hi @white_fang
Thanks for your reply and the warning to keep foot out of mouth
I do understand that TAS and CAS differ.
My query, perhaps unclearly worded, referred to a segment in Micros’s
If you have a moment take a look from time stamp 12:30 where he adjusts various items in steer points.
You’ll see that both TAS and CAS coincide. That’s my question, how do they?
I know that at certain altitudes, perhaps over water, they can be close.
So, is it because the altitude shown is only 100FT (NOE) ??
Have I managed to redeem myself?
Thanks,
Bayonet.
Yes it is the reason. Test by yourself by changing the altitude and check the results.
If you were not sure about the reason why TAS and CAS were the same or different in that video, it might be useful to check what they mean again.EDIT: Ninja’d by Dee-Jay
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Thank you both.
If I may @Dee-Jay, could you elaborate on using CAS for performance. Do you mean fuel efficiency?
Bayonet.
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@Bayonet - Directly, CAS is the speed of the aircraft with respect to the airmass, and adjusted for position error of pitot tubes and instruments location on the aircraft. The “calibrated” part takes into account the effects of variation in air density (altitude) and temperature to allow the pilot to fly within expectation at all points in the flight envelope - this is why pilots fly to CAS - it can also be thought of as “performance airspeed”, and I’ve seen it referenced as such on more than one occasion.
TAS is the true speed of the aircraft over the ground, without the consideration of the effect of winds - this is why most ballistic and time/speed/distance calculations are performed using TAS and AGL altitudes - it makes the vector math more readily applicable to varied situations/locations.
The two are almost never the same, and should not be expected to be.
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@Bayonet said in TAS / CAS airspeed.:
If I may @Dee-Jay, could you elaborate on using CAS for performance. Do you mean fuel efficiency?
Nope. Airspeed (lift, manoeuvrability).
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@Stevie said in TAS / CAS airspeed.:
TAS is the true speed of the aircraft over the ground
Are you quite certain of this? Maybe you were thinking about GS?
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Right, I see. So, in planning a series of steer points when creating a TE, it’s TAS I use for Nav and timing and not the (normally) slower CAS.
Thanks again,
Bayonet.
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@Bayonet said in TAS / CAS airspeed.:
Thank you both.
If I may @Dee-Jay, could you elaborate on using CAS for performance. Do you mean fuel efficiency?
Bayonet.
Flight performance.
For example at 400 CAS you have the same lift with the same AoA.
But a SL with std. atm and 0 wind 400 CAS = 400 TAS.
At 30k feet the 400CAS = 602 TASThe point of the CAS to see the speed which is related direct to the maneuverability because not even the Mach number is helpful in this area.
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@Bayonet said in TAS / CAS airspeed.:
Right, I see. So, in planning a series of steer points when creating a TE, it’s TAS I use for Nav and timing and not the (normally) slower CAS.
Thanks again,
Bayonet.
Correct, but at the same time, you’ll have to ensure that CAS will remain high enough for the considered plane and mission. (For an F-16 for exemple, don’t go below 250-300KCAS.)
This will gives you an altitude limitation.For COMAOS, it is convenient to have all the flights flying at the same TAS. That helps in deconfliction and time management.
KTAS 360, 420, 480, 540 are even more convenient for math.360KTAS = 6Nm/min
420KTAS = 7Nm/min
480KTAS = 8Nm/min
540KTAS = 9Nm/min
…But to choose the highest TAS you’ll have to know what are the most limited a/c (think loadout and drag factor) … the chosen TAS must be sustainable with a bit of margin without the use of afterburners.
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I think that TAS speed should be GS while you are changing things and times in flight planning tab.
I am saying this because while you are setting everything up in this tab wind and angles of decent or climb are not considered. When you set for example 420TAS time is calculated for straight level flight and zero wind. So 420 is GS.
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@Ironman3 said in TAS / CAS airspeed.:
I think that TAS speed should be GS while you are changing things and times in flight planning tab.
I am saying this because while you are setting everything up in this tab wind and angles of decent or climb are not considered. When you set for example 420TAS time is calculated for straight level flight and zero wind. So 420 is GS.
Yep. But mission planner can’t take the wind in account. Also, wind is usually not taken on account in ATO (because mission can be postponed anytime).
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@dumba - I probably mangled that…because back when I was doing a lot of RL A/G weaponeering we always did calculations based on TAS so that the effect of winds could be easier calculated realtime using INS winds in the area. In fact, in a Harrier you automatically get TAS in the HUD in A/G Master Mode…at least you did back when I was working Harriers in the late 80s.
GS = vector sum of: TAS + windspeed; so there’s also some trig involved.
More correctly, TAS is the absolute speed of the aircraft wrt the airmass but uncorrected for the effects of temperature and density, i.e.; altitude.
Wiki explains it better than I do(!):
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When you are about to plan your route and times you should consider only GS and the job will be done more accurately.
GS says how fast you are moving forward.
For example if you are climbing with 90 degrees your TAS has a value but your GS is 0 which means you are not moving forward.
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@Ironman3
We are speaking military (combat) flight. Not commercial.
You’ll never be able to have an accurate wind component for your computations.
TOT or assigned blocks can change anytime before or during the mission/flight.What you say is not wrong … but that is how it is in military combat aviation (except maybe ferry flight).
In combat, you need as much as combat fuel possible… so it is pointless to calculate an MFQ accurate at 100bls.See what I mean?
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@Dee-Jay @dumba @Ironman3 @LorikEolmin @molnibalage @Stevie @white_fang
Thank you all, this certainly proves the point that BMS is a “study Sim”.
Bayonet.
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@Dee-Jay
I am not talking about fuel calculations.
The thing I want to say is that when you are planning to be at a specific point (RSP maybe) all times till that point should be calculated with GS as the only thing you worry about is not to fight but be there at a specific time in order to fight.With GS you don’t have to calculate wind from the beginning as the aircraft gives you the solution and you just follow the correct GS.
If there is great altitude delta from the one you planned and the performance of the aircraft can not support pre planned timing then there is rolex or slip deviation but this is another part of the planning