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[NOT BUG]Aircraft speed in different units don`t match


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Check airspeeds of aircraft on the HUD and VTB (HDD), note the knots and Mach conversion: 303knots is 0.79 Mach?

303knots*1,852=561km/h=0,45Mach

 

Same goes for my and the plane infront of me, all wrong.

 

 

One more thing to ask here, why is IAS different in F2 and F3 view?

 

I am attaching tacview.

Tacview-20210610-194620-DCS.zip.acmi


Edited by skywalker22
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6 minutes ago, skywalker22 said:

Check airspeeds of aircraft on the HUD and VTB (HDD), note the knots and Mach conversion: 303knots is 0.79 Mach?

Looks correct (assuming DCS's default 20°C (ISA +5°C) and standard air pressure 1013 mBar).

 

303 CAS at 29290ft +5C ISA temp equals 470 TAS, 0.79 Mach

 

5txtJWy.jpg

 

https://aerotoolbox.com/airspeed-conversions/

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Well CAS is basically IAS with some error correction.

 

IAS or CAS are derived from the pressure difference between the static air pressure and the dynamic one of the attacking air - basically the Pitot tube has two openings one facing forward and one in non-moving air and displays that difference as IAS/CAS. That difference depends on the density of the air, so the higher up you are the bigger the difference between true air speed (TAS) and indicated speed will be.

 

That's how it is in real life and knowing CAS/IAS is quite useful because it represents the "aerodynamic situation" of the aircraft.

 

The speed of sound otoh does not change with density (but a little with temperature) so that number is a pretty good indicator of your true speed and is important because it defines the speed region your plane is in (sub-,trans-,supersonic).


Edited by Blackeye
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1 hour ago, skywalker22 said:

Why an airplane shows CAS? Why not TAS, or IAS?

CAS (calibrated air speed) is a better representation of dynamic pressure acting on a plane i.e. a plane flying at 300 CAS, flies pretty much the same at sea level or at 30,000 ft i.e. same roll rate, stall speed, etc.

 

IAS (indicated air speed) is similar to CAS but hasn't been corrected for instrument error due slip stream effects, pressure effects, etc. In older/simpler aircraft there'll be correction charts/tables to help the pilot calculate CAS from ISA.

 

TAS (true air speed) is the speed through the air (i.e. past each air molecule) but doesn't take it's density into account. Without a head/tail wind TAS equal your ground speed, so it's used to calculate route times, time on target, etc.

 

At sea level CAS, IAS and TAS are close to each but at high altitude CAS/IAS is very different from TAS/Ground Speed.

 

1 hour ago, skywalker22 said:

Is that how is suppose to be IRL?

For the M-2000C, yes. As a fighter it's important for the pilot to know it's turn rate, etc. so CAS is displayed in the HUD.

 

For other manoeuvres, mach is a better guide,  so it's also displayed.

 

The INS also measures ground speed, so Ground Speed can be independently be displayed on the PCN.

 

What, where and how a particular air speed is displayed, varies between aircraft, for example the AV-8B's HUD displays CAS in navigation mode but switches to TAS when in Air to Ground mode as it's weapons tables use TAS.

 

Grim Reaper's video

 

... but it's a common FAQ.


Edited by Ramsay
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