Erratic altitude and mach indications

In the attached mission starting conditions (Alt 12000m, speed 1050km/h, default weather), every other DCS aircraft shows 11800 alt, 0.97 Mach on the instruments

MiG-21 gauges indicate 12700 alt, 1.04 Mach.

The altimeter has almost 1km error, and the mach indicator suggest we're going supersonic while we don't.

zSpeedtestMig.miz

Was this test conducted just changing the planes in the mission?

It's correct that altimeter and airspeed indicator do not read accurately in certain conditions around Mach. There is a chart of the discontinuity zone of the real airspeed indicator.

Barometric altimeter won't read correctly in general even without Mach effect because of temperature effect and choice of pressure calibration. But there is a effect of compression near Mach as well and another chart about it.

Basically there are a lot of charts of all sorts of errors and corrections but it's in Russian and decoding them is labor intensive. It's hard to decrypt all of the subscripts that are in use.

There is a special option to disable these effects so you can compare with them on and off.

Are the effects very accurate? I don't know. But it is strange that altimeter would unwind due to static pressure effect but vertical speed gauge (which is also static) and autopilot are strangely unaffected. Probably VSI and AP should follow the baro alt in the same dip.

Was this test conducted just changing the planes in the mission?

Yes.

Barometric altimeter won't read correctly in general even without Mach effect because of temperature effect and choice of pressure calibration. But there is a effect of compression near Mach as well and another chart about it.

 

Basically there are a lot of charts of all sorts of errors and corrections but it's in Russian and decoding them is labor intensive. It's hard to decrypt all of the subscripts that are in use.

 

There is a special option to disable these effects so you can compare with them on and off.

I don't see an option to turn these effects off anywhere in the sim.

Not counting the transition around mach number, the altimeter should show the same value in the same temperature/pressure setting as in other planes, that's the whole point of having an altimeter. At 12 km the altimeter in the MiG-21 shows 13 km at any speed I tested from 500 km/h to 1500 km/h, a little less around Mach 1.

The only error chart I found in the real manual for the altimeter is for IAS vs. Altitude, and the error is in the opposite direction to what is modelled in the DCS MiG-21. (should show about 400 meters less that real altitude at high IAS values).

The altimeter is bugged, the small scale is not correctly calibrated in the sim. There is no such methodical error on the real Mig-21bis, which would cause the altitude be different by 1000 meters. I use the fix devised by Frederf here:

https://forums.eagle.ru/showthread.php?t=212575&highlight=Altimeter

Options>Special>MiG-21bis>Disable Pitot Shock Errors

I guess that only applies to airspeed (Mach, TAS) since the rest is static stuff.

Yeah, best way to see the airspeed-altitude modeled error is to set autopilot level and go supersonic from M0.8. You'll see the altimeter do a canned unwind despite being actually level.

Yeah there is a bug where the "hour hand" doesn't progress whole number of 1km ticks every time the "minute hand" of the altimeter sweeps through 1000m. It's an animation issue. The long hand will sweep increasing 10 times but the short hand won't increase to the 10km mark. Obviously the real instrument is geared in such a way that the 10:1 hand motion is enforced.

But I will say that if you set for example QNH such that it shows airfield elevation perfectly at takeoff that doesn't mean it reads exactly true altitude at any other height, dramatically so at extreme temperatures. The other altimeters in other modules have this effect as well.

Options>Special>MiG-21bis>Disable Pitot Shock Errors

I guess that only applies to airspeed (Mach, TAS) since the rest is static stuff.

Ah, yes, that option fixes the errors around Mach 1, but doesn't change altimeter erratic indication at any other speed. It's still 1 km too high.

But I will say that if you set for example QNH such that it shows airfield elevation perfectly at takeoff that doesn't mean it reads exactly true altitude at any other height, dramatically so at extreme temperatures. The other altimeters in other modules have this effect as well.

Of course it doesn't show "true" altitude, but it should show roughly the same altitude as the altimeters in other modules in the same conditions, minus small error specific to the unit. 1 KM error at typical cruise altitude is not a small error.

Yeah, best way to see the airspeed-altitude modeled error is to set autopilot level and go supersonic from M0.8. You'll see the altimeter do a canned unwind despite being actually level.

Such behavior makes no sense and only shows another bug in the autopilot system.