Steady winds aloft affect IAS/CAS and EAS

[Stickler]

First of all, this is a general DCS bug. The F-16 was merely used as an example; I can also reproduce it with the F-4E.

Observe the below screenshots taken from the exact same .miz upon entering the game and selecting active pause before clicking "Fly".

In both cases, the aircraft is set to fly at 500 KTAS at 20000 ft MSL, standard day, no turbulence.

 

Screenshot 1: No wind

 

Screenshot 2: 50 kts headwind

 

Note how the headwind increases CAS (HUD) and IAS (F2 bar), remembering that "IAS" provided in the F2 bar is actually EAS (Equivalent Airspeed).

This is physically incorrect. Steady winds aloft do not affect IAS or CAS in real life.

Conversely, winds an aircraft experiences while on the ground DO affect IAS/CAS. This effect is (at least qualitatively) modelled correctly in-game.

Note also that the usage of active pause is not the cause for the issue; after resuming the game the error persists.

It must be stated that this is not merely an aesthetic issue but affects the aerodynamic behaviour of aircraft. For example, when setting maximum tailwinds (206 kts at 1600 ft) at a reasonable TAS (depending on the aircraft, say 300), the aircraft will stall out of the sky due to insufficient IAS/CAS.

I've been playing DCS for about 15 years and this is the first time I've come across this bug. Not sure if I've just never noticed it or if it was introduced recently.

Edited Friday at 08:40 AM by Stickler

[razo+r]

Just to confirm, how did you enter the 500KTAS? In the mission editor you enter GS, so that could perhaps explain why. Did you crosscheck with the TAS in the F2 bar or with switching what the aircraft shows for a speed?

[Stickler]

Thanks, I believe that was in fact the error in my thought process. I set 500 in the ME. What threw me off is that the "TS" value shown in the F2 view was "500" in both scenarios, whereas the actual true airspeed (which then translates to KIAS etc.) when entering the game was 500+50 = 550 KTAS with headwind (KTAS increased "automatically" to maintain 500 KGS) and 500+0=500 KTAS without any wind.

I find the whole thing terribly confusing. Witness the below screenshots of the exact same situation in the game and in Tacview (note the dive angle of approximately 45°, this is with 50 kts headwind):

 

Summary:

TS (F2 view): 499 kts

TAS (F-4E): 535 kts

TAS (Tacview): 443 kts

GS (F-4E): 319 kts

GS (Tacview): 284 kts

KCAS (F-4E): 390 kts

KCAS (Tacview): 315 kts

IAS (F2 view, not seen but imported directly by Tacview): 363 kts

IAS (Tacview): 363 kts

 

If I were to try, I could probably explain half these discrepancies. I believe the original bug report has resolved itself though.

Edited Friday at 09:39 AM by Stickler

[Stickler]

For everyone else's benefit, I think I figured it out.

• IAS: No discrepancy between game and Tacview since IAS is directly imported. The only problem is that IAS is misnamed and should actually be EAS.
• TAS: For human players, "TS" in-game is equal to "TAS" in Tacview with the caveat that "TS" is an instantaneous value whereas "TAS" is averaged over the last second. Conversely, if winds are present, "TAS" in Tacview/"TS" in the game are unequal to TAS in the cockpit because Tacview's "TAS" and the in-game "TS" are the distance the aircraft has travelled between two 3-dimensional coordinates in DCS's coordinate space divided by the time required to do so, whereas "TAS" in the cockpit is derived from IAS by correcting for pressure and temperature, thereby indirectly obtaining the distance travelled through the air mass in a specific time. In headwind, the aircraft will travel less absolute distance in DCS's coordinate space in the same amount of time than without wind (resulting in a lower calculated "TAS"), whereas the wind has no influence on the distance travelled by the aircraft through the airmass surrounding it. Note that due to this logic, with the same headwind and a 90° dive angle, or with perfect crosswind, cockpit TAS and Tacview TAS/"TS" are virtually the same whereas the difference is at its maximum in straight-and-level flight with perfect head or tailwind. Interestingly, for AI, "TS" is always their actual TAS regardless of wind, but their "TAS" in Tacview will be off in the same manner as for humans.
• GS: GS in-game is equal to GS in Tacview with the caveat that it is an instantaneous value in-game whereas it is averaged over the last second in Tacview, even though they are obtained in a different way: In-cockpit GS is obtained from TAS by factoring in wind (drift) and dive/climb angle, while Tacview's GS is the distance travelled between two 2-dimensional (X/Y) coordinates in DCS's coordinate space.
• CAS: Tacview's "CAS" will only match cockpit IAS/CAS on a standard day and with the caveat that for DCS, Tacview calculates CAS from TAS. If TAS is incorrect due to winds, so will be CAS.

I have no idea why the Tacview values I posted above were so grossly off. I wasn't able to reproduce this issue.

Edited Friday at 07:30 PM by Stickler