Playing with Density Altitude due to Cold / Hot air...

[Anatoli-Kagari9]

"From High to Low Watch Out Bellow!!! " applies to both pressure AND temperature, although in most sims only the pressure side is correctly modeled...

 

Today I set up a test to figure out if the weather model in DCS calculates the temperature side of the "from high to low" adagio...

 

Picking a default mission ( tf51 quick start free flight ) I dragged my tf 51 d towards the mountainous regions of the Caucasus and started the situation at 11,000 feet until I was at the right spot ( between some mountain peaks, flying across a valley ).

 

I then set the season to SUMMER, temperature to a hot +35ºC ( ISA + 20 ) and the standard pressure 760 mmHg.

 

Went flying and had to climb for a while to align my altimeter with 11,000 feet, noticing that I was a little above the mountain peaks...

 

Then I set the same flight with WINTER season and ISA -35 ( -20C ), same pressure ( standard 760 mmHg ) and went flying again...

 

The aircraft starts it's flight at the exact same height - HEIGHT - but the altimeter was now showing way above 14000 feet, and I then initiated a descent to get to indicated altitude of 11000 feet, but before I got there I smashed into the mountains...

 

So, indeed density of the colder air masses and the lower geopotential height are modelled in DCS!!!

 

Of course density is also calculated in almost all flight simulators, but for instance in FSX and X-Plane, if you keep trhe surface pressure equal, and just vary the temperature and Season, you will be at the very same barometric ( read in the altimeter ) altitude and height above ground level. Mountains are a good reference point. Very few sims model the "shrink" of the troposphere with the cold / denser Winter air...

 

Non military flight simulators taht also model this effect are Flight Gear and Aerowinx PSX, as well as ELITE, just for the sake of curiosity :)

Edited March 3, 2016 by jcomm
Additional info regarding modelling of this feature ...

[xjiks]

Mountains are a good reference point

 

Yes, whatever weather or pressure, they will always show where the ground is :-)

 

Thanks for sharing your tests. I learned something today

[Anatoli-Kagari9]

You're seeing the difference between absolute altitude and pressure altitude. Density altitude has to be calculated from pressure altitude and outside air temperature. A pitot static steam gauge can't show you DA, but some digital ones with an OAT input can, like the Davtron model 655. But you're right, DA is modeled. Just making sure that we all understand that this is an indirect indication of DA. Good test.

 

Yes, but please note that I always set QNH = 760, so, indeed the "jump" in the altimeter read is due to DA and not only PA.

 

At the denser air mass, both ground and a given level ( geopotential ) can have the same pressure reads ( say, 760 and 690), and the same in a hot less dense air mass will naturally correspond to the a higher absolute altitude ( or height above ground level to simplify...)

 

Unfortunately, and just to name a couple, MSFS, P3D, X-Plane all calculate air density, but the pressure "lapse rate" is unchanged, so, you can be in the hotest of the days, with a given QNH and your altimeter set for it, climb and feel the effects of the less dense air, corresponding sometimes to a much higher altitude, but when your altimeter reads 1500m or feet, that will be you true altitude ( height ) as well, the same you will get on a cold winter situation...

 

In AEROWINX PSX ( the 744 sim ) it's inetersting to test this effect on very cold days if you're flying a VNAV approach ( using a "VNAV glideslope" ).

 

What none of the sims considers ( exception being ELITE, Flight Gear's Advanced weather mode and Aerowinx PSX, I believe also Flightdeck A3xx by Airlinetools ) is the effect of moist in air density. I tried to notice differences between situation with all rest being equal less the supposed T / Td proportion, and the readings are the same in our altimeters, also in DCS ... But heck! it already does a lot more than other sims I've used :-)

Edited March 9, 2016 by jcomm

[Maulkin]

I am getting ready for my PPL exam and thought I would experiment with what I have been studying regarding pressure and density altitude. I created a mission where I have a TF-51 sitting on the tarmac at Bselan, the ambient temperature is set to standard (15 degrees Celsius). I then set the altimeter to standard pressure (29.92) and confirm that the altimeter is indicating the pressure altitude of the airfield: 1772 feet.

 

I then change the mission to set the ambient temperature to 25 degrees celsius, set the altimeter to 29.92 inches and the altimeter displays approx 1710 feet! It went down?

 

The formula I have learned is this:

density altitude = pressure altitude + (100' * [actual temp - {standard temp at sea level - standard temp at altitude}])

 

So this comes down to:

DA = 1772 + (100 * [25 - {15 - ~3.4}])

DA = 1772 + (100 * [25 -11.6])

DA = 1172 + (100 * 13.4)

DA = 1772 + 1340

DA = 3112

 

Shouldn't the altimeter display this? Or does this just mean the aircraft will perform as though it is operating at that altitude but the altimeter will not display it? In that case why does the altimeter go down when I increase the ambient temperature?

 

I am using this as reference: http://www.langleyflyingschool.com/Pages/Flight+Operations--Pressure+and+Density+Altitude.html

[DarkFire]

I am getting ready for my PPL exam and thought I would experiment with what I have been studying regarding pressure and density altitude. I created a mission where I have a TF-51 sitting on the tarmac at Bselan, the ambient temperature is set to standard (15 degrees Celsius). I then set the altimeter to standard pressure (29.92) and confirm that the altimeter is indicating the pressure altitude of the airfield: 1772 feet.

 

I then change the mission to set the ambient temperature to 25 degrees celsius, set the altimeter to 29.92 inches and the altimeter displays approx 1710 feet! It went down?

 

The formula I have learned is this:

density altitude = pressure altitude + (100' * [actual temp - {standard temp at sea level - standard temp at altitude}])

 

So this comes down to:

DA = 1772 + (100 * [25 - {15 - ~3.4}])

DA = 1772 + (100 * [25 -11.6])

DA = 1172 + (100 * 13.4)

DA = 1772 + 1340

DA = 3112

 

Shouldn't the altimeter display this? Or does this just mean the aircraft will perform as though it is operating at that altitude but the altimeter will not display it? In that case why does the altimeter go down when I increase the ambient temperature?

 

I am using this as reference: http://www.langleyflyingschool.com/Pages/Flight+Operations--Pressure+and+Density+Altitude.html

 

The altimeters on all the DCS aircraft under-read as altitude increases, even if you calibrate for QFE on takeoff. The effect gets larger as altitude increases. Take a zoom climb for example, something I do occasionally in my favourite Su-27: at the top of the climb the HuD (and altimeter) are both telling me that I'm just over 27,500 metres altitude, whereas the F2 view says that I'm above 35,000m altitude.

 

I gather that the atmospheric modelling in DCS probably needs a little bit of tweaking.

 

Good luck with the exam!

[Bushmanni]

The problem can also be in the planes instruments. Until we get tools to gather direct pressure and temperature data from DCS atmosphere there's no telling which is wrong or if they both are.