True Airspeed Calculator

[Legioneod]

So I've been using this calculator to try and get an accurate conversion from indicated airspeed to true air speed but I'm not sure if I'm getting accurate results. My main problem is getting the correct altimeter setting as I don't really know what this is or how to determine the proper setting. Does Altimeter setting change with altitude and is there a way to find out what it is?

 

I plan on making performance charts for each aircraft but I need to be able to have accurate conversions from IAS to TAS, so far I'm getting pretty close results but I want to be more accurate.

 

https://www.dauntless-soft.com/products/Freebies/TrueAirspeedCalculator/

[unreasonable]

The Altimeter essentially just reads pressure, so the setting here is the pressure at which the altimeter is calibrated to zero: same for the temperature.  Standard settings are 29.92 inches at 15 C for sea level which I think is also on the Kuban Autumn map.  You can check for any map by loading it in the mission editor and looking at the atmosphere settings.You do not have to manually change the Altimeter setting for different altitudes once you are flying - the calculator, like the altimeter, does that for you based on an assumed reduction in pressure with altitude. So you just have to set it for the pressure at your starting altitude. 

 

For BoX testing purpose, just be sure if you are reading heights above SL or above the airfield, make sure that the settings and measurement in the calculator are consistent: all from SL or all from airfield.  I think the HUD altimeter always reads from SL, but you might want to check that.

 

 

[Legioneod]

21 minutes ago, unreasonable said:

The Altimeter essentially just reads pressure, so the setting here is the pressure at which the altimeter is calibrated to zero: same for the temperature.  Standard settings are 29.92 inches at 15 C for sea level which I think is also on the Kuban Autumn map.  You can check for any map by loading it in the mission editor and looking at the atmosphere settings.You do not have to manually change the Altimeter setting for different altitudes once you are flying - the calculator, like the altimeter, does that for you based on an assumed reduction in pressure with altitude. So you just have to set it for the pressure at your starting altitude. 

 

For BoX testing purpose, just be sure if you are reading heights above SL or above the airfield, make sure that the settings and measurement in the calculator are consistent: all from SL or all from airfield.  I think the HUD altimeter always reads from SL, but you might want to check that.

 

 

 

I was reading it from the altimeter gauge, the hud gave me altitude above the ground so it was constantly changing. I was also doing the test on the Stalingrad autumn map so I;m not sure if this affects the altimeter setting or the test results at all.

[unreasonable]

Stalingrad Autumn is also 760 mm Hg = 29.92 inches and 15 degrees, so also standard atmosphere which is what you want.

 

I do not use the HUD so I was not sure:  but you could use the HUD and the Kuban map and fly your tests over the sea: that might be the easiest.

 

The default cockpit altimeter reading on mission starts from the runway is zero, so it is reading altitude above the airfield height.  Press Alt-A and the altimeter will move up to the airfield altitude, so you are now reading height above SL.  In air starts, the altimeter starts at the SL height: press Alt-A and the height goes down (presumably to the  height above the nearest airfield). 

 

You just have to be clear about what your altimeter was reading: you want above SL heights to make the calculator easy to use.

[Legioneod]

20 minutes ago, unreasonable said:

Stalingrad Autumn is also 760 mm Hg = 29.92 inches and 15 degrees, so also standard atmosphere which is what you want.

 

I do not use the HUD so I was not sure:  but you could use the HUD and the Kuban map and fly your tests over the sea: that might be the easiest.

 

The default cockpit altimeter reading on mission starts from the runway is zero, so it is reading altitude above the airfield height.  Press Alt-A and the altimeter will move up to the airfield altitude, so you are now reading height above SL.  In air starts, the altimeter starts at the SL height: press Alt-A and the height goes down (presumably to the  height above the nearest airfield). 

 

You just have to be clear about what your altimeter was reading: you want above SL heights to make the calculator easy to use.

I think by default my altimeter is set to height above sea level, when I spawn in it was around 100ft and not exactly at 0.

 

The calculate is giving me some accurate speeds at low alt but as soon as I get higher it gives me some really fast speeds.

 

For example, at around 2,000ft in the Spitfire Mk IX I'm getting around 335 IAS and the calculator gives me 347 TAS, this number is pretty close to the charts that I've read. At 16,000 ft I get an IAS of around 300 and a TAS of 408, this is much higher than what the speed charts give, at 16k I should only be getting around 390 TAS from what the charts say, the calculator is giving me 408 instead.

 

Not sure what the problem is.

 

Should I be changing the temperature since temps decrease with altitude?

Edited August 13, 2018 by Legioneod

[unreasonable]

If you are  doing air starts, yes you are in ASL, which is presumably why there is no option to airstart at below 250m:  if you do start at 250m you can see that you almost at tree top level.  If you start from the runway you must press Alt-A to get ASL.

 

I checked from the runway at one of the northern most airfields in Stalingrad Autumn map. When I spawn on runway, the cockpit altimeter was showing exactly zero. After pressing Alt A it went up to 200 m. So it starts at zero = airfield altitude on the runway, not ASL. Both Spitfire IX and Fw190 - I assume that this is general for all planes.

 

When I used the same QMB point but air start at 1000 m in the settings tab, my altimeter showed 1,000m when I spawned and down to 800m after pressing Alt A.  The airfield is at 200m,  ie it is starting at 1,000m ASL and changing to 800m above airfield when you Alt-A.  

 

I am sure that the way these calculators work is by assuming a curve for the decrease in pressure and temperature with altitude to calculate density altitude, from which the IAS-TAS conversion is driven, so you do not have to change the temperature or pressure numbers,  you just have to make sure they match the SL values for the map you are using.

 

Having said that, real world tests are never done on a standard day, which only exists in theory, and even if it is close to standard pressure and temperature on the ground there is no guarantee that the real temperature and pressure gradients were exactly as per the standard.  So the particular charts you are looking at may not be the same as the ones the team used: 408/390 = 1.046, so they might feel that they are within 5% of your reference document anyway. 

 

There is also the issue that while our model planes have no PEC real ones do, and there might be some variance in how those errors are corrected in the real tests.

 

I think it is highly unlikely that you will ever get a perfect match.

Edited August 13, 2018 by unreasonable

[Legioneod]

1 hour ago, unreasonable said:

If you are  doing air starts, yes you are in ASL, which is presumably why there is no option to airstart at below 250m:  if you do start at 250m you can see that you almost at tree top level.  If you start from the runway you must press Alt-A to get ASL.

 

I checked from the runway at one of the northern most airfields in Stalingrad Autumn map. When I spawn on runway, the cockpit altimeter was showing exactly zero. After pressing Alt A it went up to 200 m. So it starts at zero = airfield altitude on the runway, not ASL. Both Spitfire IX and Fw190 - I assume that this is general for all planes.

 

When I used the same QMB point but air start at 1000 m in the settings tab, my altimeter showed 1,000m when I spawned and down to 800m after pressing Alt A.  The airfield is at 200m,  ie it is starting at 1,000m ASL and changing to 800m above airfield when you Alt-A.  

 

I am sure that the way these calculators work is by assuming a curve for the decrease in pressure and temperature with altitude to calculate density altitude, from which the IAS-TAS conversion is driven, so you do not have to change the temperature or pressure numbers,  you just have to make sure they match the SL values for the map you are using.

 

Having said that, real world tests are never done on a standard day, which only exists in theory, and even if it is close to standard pressure and temperature on the ground there is no guarantee that the real temperature and pressure gradients were exactly as per the standard.  So the particular charts you are looking at may not be the same as the ones the team used: 408/390 = 1.046, so they might feel that they are within 5% of your reference document anyway. 

 

There is also the issue that while our model planes have no PEC real ones do, and there might be some variance in how those errors are corrected in the real tests.

 

I think it is highly unlikely that you will ever get a perfect match.

 

I guess I'll just make the graphs with the calculator, even if they're not 100% accurate, I'll make sure to do all the aircraft in the same way, that way we can at least get a good comparison between them.