Airspeed indicator indicate faster than GS

[Guest]

When I was practicing landing on a Supercarrier, when I calculated the ground speed by Lua script and displayed it by trigger.action.outText, it was slower than the airspeed indicator.
My calculation results are almost the same as the speed displayed in F10 View, so I don't think it's wrong.

Please check the attached miz file.

F-14A_CS_test.miz

[Tom Kazansky]

Isn't that the reason why planes land against the wind to reduce GS?

[Swordsman422]

There are four speed measurements in aircraft; indicated airspeed, true airspeed, calibrated airspeed, and groundspeed. In a real aircraft, it's not often that your indicated airspeed is going to match your groundspeed. IAS is the simplest to measure but the least accurate and there are several factors that affect it, including wind speed and direction, altitude, and air density. IAS into the wind is going to be faster than GS, where IAS with the wind will be slower. True airspeed is the aircraft's speed relative to the air it's flying through.  Groundspeed true airspeed corrected for wind. I'd be more concerned of a bug if the IAS and GS matched all the time in DCS.

 

 

Edited July 31, 2021 by Swordsman422

[razo+r]

Isnt the speed in the F10 map also TAS and not GS? That would make an additional 9.something Kts difference according to the wind in your picture

[Home Fries]

TAS is what is shown in F10, and you can expect differences in TAS and IAS/CAS the higher altitude you go and the more your weather deviates from standard temperature and pressure.

 

Of the four major types of airspeed, Indicated and Calibrated Airspeed (IAS/CAS) are almost equivalent.  CAS is IAS with adjustments for pitot-static errors, and is generally within a knot or two of IAS.  IAS/CAS is simply a measure of airflow over the wing, and is useful to the pilot to stay within the performance envelope.

 

True Airspeed (TAS) is simply the speed of the aircraft going through the mass of air regardless of wind.  TAS is useful for navigation in order to determine the winds (or to determine track and groundspeed if you know the winds and are not using an INS/GPS).

 

Groundspeed (GS) is the speed of the aircraft over the ground.  It is quite simply TAS+/- the wind component of the aircraft heading.  In no wind, GS = TAS.  In most INS/GPS systems, GS is determined by the navigation system, then compared with a TAS computed by a flight computer to determine the winds.

 

So looking at the picture:

 

We have the aircraft on a ~043 heading, which is the same as the 10kt wind vector creating a 10kt headwind (rounding up the 9.5kt wind component).  IAS shows 250, and GS shows 245.  With a 10kt headwind, TAS should be 255.  With the aircraft near sea level (can't see the altimeter, but the aircraft is below the ILS needles and therefore below glideslope), we can assume that IAS is within a knot or 2 of CAS and CAS is roughly equivalent to TAS.  This indicates a possible three knot discrepancy between TAS and IAS/CAS, which isn't that out of whack, especially considering the lag in pitot-static dependent gauges like speed, baro altimeter, and VSI.

Edited July 31, 2021 by Home Fries

[Guest]

Oh, I'm sorry, that's right.
The mistake was in my understanding...
Thank you for pointing out.

[draconus]

9 hours ago, Fuujin said:

When I was practicing landing on a Supercarrier, when I calculated the ground speed by Lua script and displayed it by trigger.action.outText...

Why? GS is displayed on the HSD.

[Guest]

 

1 hour ago, draconus said:

Why? GS is displayed on the HSD.

Really?
Where is it?

F-14A_CS_test_2.miz

[Wingmate]

 

3 hours ago, Fuujin said:

 

Really?
Where is it

Right above the red line