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M_Gunz
08-06-2010, 09:58 AM
AFAIK speed of sound depends on air temperature, not density. Is this the whole truth?

So is sea level IAS always the same as TAS? Only when the temperature is standard.
Are all IL2 maps standard? No, there may be one that is at some time of day and Crimea summer is *close* at noon.

IRL morning, noon, 3PM, evening and night tend to differ which holds with flying too.

M_Gunz
08-06-2010, 09:58 AM
AFAIK speed of sound depends on air temperature, not density. Is this the whole truth?

So is sea level IAS always the same as TAS? Only when the temperature is standard.
Are all IL2 maps standard? No, there may be one that is at some time of day and Crimea summer is *close* at noon.

IRL morning, noon, 3PM, evening and night tend to differ which holds with flying too.

DrHerb
08-06-2010, 10:46 AM
This might help.

http://www.sengpielaudio.com/c...lator-speedsound.htm (http://www.sengpielaudio.com/calculator-speedsound.htm)

runyan99
08-06-2010, 11:37 AM
IRL, TAS includes a correction for both temperature and pressure altitude. Your question prompted me to look it up.

JtD
08-06-2010, 12:12 PM
IAS needs to be corrected for position error and density to become TAS.

Speed of sound depends on temperature, not density. For many purposes, that's the whole truth. If you are a physicist, it isn't.

No il-2 map is standard, Crimea used to be the one closest or even at standard conditions, but no longer is. In 4.09 Smolensk is closest to standard conditions.

K_Freddie
08-06-2010, 04:03 PM
Take two extemes..
Speed of sound in space = not very fast or far.
Speed of sound in water = a lot faster than in air.
The deeper you go (in water) the greater the pressure, causing sound to bend upwards. A diffraction/speed differential at depth/higher pressure/density.

http://forums.ubi.com/groupee_common/emoticons/icon_cool.gif

BillSwagger
08-06-2010, 06:13 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by JtD:
IAS needs to be corrected for position error and density to become TAS. </div></BLOCKQUOTE>

My understanding is that Il2's IAS is actually what would be EAS in RL.
I'm talking about the speed bar readings, but the instruments might vary, but from what i can tell they look the same.

ElAurens
08-06-2010, 07:55 PM
BlitzPig Bury (aka BBury) has done a mod map that has true "standard day" conditions.

It is included in HSFX.

M_Gunz
08-06-2010, 11:18 PM
Actually IAS/TAS not about Mach at all while it is about density which also changes with temperature. It is generally easier to take off at night when the air is cooler and more dense, not to mention usually less turbulent.

The thing is that during sunlight hours the air often changes, how long is standard good for? Try takeoff on a winter map compared to same map summer. Seems easier and shorter run to me.

JtD
08-07-2010, 12:00 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by BillSwagger:

My understanding is that Il2's IAS is actually what would be EAS in RL.
I'm talking about the speed bar readings, but the instruments might vary, but from what i can tell they look the same. </div></BLOCKQUOTE>

Yes, that's right. Some gauges might show a wrong number, but that would be a bug (say they show km/h instead of the indicated knots). Generally gauges show the EAS in game.

WTE_Galway
08-07-2010, 05:42 AM
Temperature (and to a lesser extent humidity) are mainly significant in GA when calculating Density Altitude.

Density Altitude is used in turn to calculate minimum takeoff distance and maximum takeoff weight.

This is very significant in places like New Guinea where on a hot humid summer day at midday in the mountains you may be at a density altitude that makes your average Cessna completely unable to take off with 4 passengers and a full load of fuel (actually happened to friends of mine luckily they only hit a fence at the end of the runway and not trees).

In game terms, if temperature altitude was implemented it would mean things like in summer from Pacific high altitude runways your bomb and fuel load would be restricted.

Bremspropeller
08-07-2010, 06:12 AM
It would be nice to see that modelled in future combat-sims - as well as (carburator/ airframe) icing and a much better deal of atmospheric (weather) simulation.

But I can already see the whiners asking why their B-25 can't haul the full cargo off a 8,000ft MSL gravel-airstrip at 30°C.

M_Gunz
08-07-2010, 10:14 AM
The hard part may be getting them to realize the temperature is 30 C and for some to get from there to 86 F. The thing about those takeoffs is they may be possible by supper time. I am sure that British night bombers able to carry more bombsand especailly so in winter than summer, there is enough difference just from day to night. This must affect IAS.

KG26_Alpha
08-08-2010, 01:28 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by M_Gunz:
The hard part may be getting them to realize the temperature is 30 C and for some to get from there to 86 F. The thing about those takeoffs is they may be possible by supper time. I am sure that British night bombers able to carry more bombsand especailly so in winter than summer, there is enough difference just from day to night. This must affect IAS. </div></BLOCKQUOTE>

No as there's a thing called operational limitation, its a standard to allow aircraft to operate safely within its performance range.
If it took 10,000lbs payload it wouldn't matter if it was day, night, winter or summer as its within its safe operational performance range.

robtek1957
08-08-2010, 04:59 AM
But.... the performance range depends on those outside factors.
As colder it gets as more payload is possible!
The operational limitations are not fixed!!!
at least not in war, in peace, shure.

Bremspropeller
08-08-2010, 05:47 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">No as there's a thing called operational limitation, its a standard to allow aircraft to operate safely within its performance range.
If it took 10,000lbs payload it wouldn't matter if it was day, night, winter or summer as its within its safe operational performance range. </div></BLOCKQUOTE>

Getting off the ground is only one fraction of performance-caculations.
Reaching the target with enough fuel to return safely is another one http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif
Winds aloft temperature at altitude can pretty much spoil your plan.

An as such, there is no standardized "operational safe payload" as it would be pretty much useless when getting close to max range.

M_Gunz
08-08-2010, 09:11 AM
I'd been given to understand that the Navy and AF have people just to determine loads for planes.
But yeah, just because you can get off the ground with it doesn't mean it's what you should take.

Still, IAS/TAS at sea level does vary with local temperature. Perhaps for those long trips it is better on a hot day as you'd get more true speed for whatever drag you have?

Bremspropeller
08-08-2010, 09:40 AM
But you're also having less lift and less performance due to the lesser compression by the engine.

The negative effect is thus greater than the positive one.

M_Gunz
08-08-2010, 10:34 AM
Is that why the highest top speed of the P-51 is so high up?

Bremspropeller
08-08-2010, 12:15 PM
It's not a matter of temperature over altitude, it's a matter how mutch the actual temperature is off the "standard" temperature for the altitude.

Colder than normal = good

JtD
08-08-2010, 02:06 PM
But M_Gunz point would be that having 20k feet conditions at sea level would improve some aspects of aircraft performance.

And I'd agree that range and speed can be higher in a low-density-at-sea-level environment than they'd be in standard conditions.

Bremspropeller
08-08-2010, 02:33 PM
Got an example?

M_Gunz
08-08-2010, 03:47 PM
In lower density air up to FTH the speed of the P-51 is higher than in denser air near sea level.. and you need an example?

Bremspropeller
08-08-2010, 03:59 PM
I want an example on warmer than standard air performance-increases.

BillSwagger
08-08-2010, 10:07 PM
Here's a question:
One day,
50 degrees and 30" on the barometer

vs

Another day,
90 degrees and 30" on the barometer

Wouldn't they present similar flight conditions at sea level?

Bill

M_Gunz
08-08-2010, 10:56 PM
Since the barometer is measuring air density, right?

WTE_Galway
08-08-2010, 11:54 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by M_Gunz:
Since the barometer is measuring air density, right? </div></BLOCKQUOTE>

No ... it measures pressure not density.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by BillSwagger:
Here's a question:
One day,
50 degrees and 30" on the barometer

vs

Another day,
90 degrees and 30" on the barometer

Wouldn't they present similar flight conditions at sea level?

Bill </div></BLOCKQUOTE>

Absolutely not. The atmospheric pressure may be the same but the air is far less dense in the 90F situation.

I will try and avoid equations here as maths seems to many people in this forum to equate to "must be wrong" !!

1. As a rule of thumb, Density Altitude is 120 feet greater than pressure altitude for each 1 °C that the temperature exceeds ISA standard temperature for that level.

(or 120 feet lower for every 1 °C below standard ISA standard temperature)

2. In your example the ISA standard temperature is the value for sea level, 15 °C .

3. Your first example is 10 °C which is 5 °C BELOW standard giving a very approximate density altitude of 600 feet BELOW sea level. Your second example is 32 °C which is 17 °C above standard giving an also very approximate density altitude of 2040 feet ASL.

All up we have a difference in density altitude of around 2600 feet between the 50 F case and the 90 F case using our rule of thumb.

4. Avoiding equations ... 2500 to 3000 feet difference means the air is about 10% less dense than sea level. Assuming a normally aspirated engine that is about 10% less horsepower. We will also get roughly 5% less thrust from the prop because its less effective when air is less dense and about 5% less lift. Also the ratio of TAS (true air speed) to CAS (calibrated air speed) is about 5% higher meaning a higher actual takeoff GROUND SPEED.

(If you want the equations for these numbers look here http://www.auf.asn.au/groundschool/umodule3.html, the example "Armidale NSW" is actually my local airport where I did flight training)

5. These 5 and 10% differences all add up. In your example the takeoff roll will be substantially longer and on a short field the difference could seriously ruin your day if you did not reduce weight to compensate.

*****

http://www.dmjwilliams.co.uk/gbsep_performance.htm

Go to the takeoff distance calculator for a Cessna 172 half way down the page and change NOTHING except temperature. Keep takeoff weight, atmospheric pressure and humidity identical. Choose the same type of runway, change nothing except temperature.

You should find the 172 at the default weight etc on a 10 °C (50 °F) day has a takeoff distance of 883 meters versus a takeoff distance for the 172 at 32 °C (90 °F) in otherwise identical conditions of 1041 meters.

It should be reasonably obvious a 160 meter longer takeoff roll is not " similar flight conditions" and in fact might get you killed on a short field http://forums.ubi.com/groupee_common/emoticons/icon_biggrin.gif

JtD
08-09-2010, 08:26 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Bremspropeller:
Got an example? </div></BLOCKQUOTE>

Less Ar, more N2.

---

Good post, Galway!

Bremspropeller
08-09-2010, 09:21 AM
Lower EGT margin.

M_Gunz
08-09-2010, 09:25 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Bremspropeller:
I want an example on warmer than standard air performance-increases. </div></BLOCKQUOTE>

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">
1. As a rule of thumb, Density Altitude is 120 feet greater than pressure altitude for each 1 °C that the temperature exceeds ISA standard temperature for that level.

(or 120 feet lower for every 1 °C below standard ISA standard temperature)
</div></BLOCKQUOTE>

Speed of WWII fighter X at sea level compared to speed of same plane at 500m tends to be?
A Faster
B Slower
C Same

Bremspropeller
08-09-2010, 09:30 AM

Speed for MSL @ standard temperature (= 15°C) compared to non-standard conditions is what is important.

Daiichidoku
08-09-2010, 09:37 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Bremspropeller:

Speed for MSL @ standard temperature (= 15°C) compared to non-standard conditions is what is important. </div></BLOCKQUOTE>

OT, but isnt that a NACA standard, and many european countries didn't apply such a standard when assessing ac performance in the 30s/40s?

JtD
08-09-2010, 10:05 AM
There were a couple of standard atmospheres developed. The 15 °C now is international standard. From what I remember, the international standard is close to the US standard, but not identical in all points.

JtD
08-09-2010, 10:06 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Bremspropeller:
Lower EGT margin. </div></BLOCKQUOTE>

?

Sentences?
Full?
Can you use?
Words?
Ditto?

Bremspropeller
08-09-2010, 10:20 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Less Ar, more N2. </div></BLOCKQUOTE>

How about speaking in full sentences in the frst place? http://forums.ubi.com/images/smilies/16x16_smiley-wink.gif

JtD
08-09-2010, 12:03 PM
Three word question, four word answer. You got a good deal already.

Less Argon, more Nitrogen. Or Water, or anything with a smaller molar mass not inhibiting combustion. Gives the air a lower density, with no power loss occuring. Results in a higher speed.

M_Gunz
08-09-2010, 12:36 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Bremspropeller:

Speed for MSL @ standard temperature (= 15°C) compared to non-standard conditions is what is important. </div></BLOCKQUOTE>

And we have the density altitude formula telling us that for every 1 C above standard is the same as + 120 ft height in standard. So tell me that top speed for WWII fighters is higher at sea level or 500m above sea level. I think this holds true for most GA planes today as well. Or tell please what planes have higher sea level top speed than at any height above, and don't forget the Wright Flyer if you need at least one.

Part of what I've been trying to point out is the rarity of standard conditions and how temperature change during the day does affect IAS/TAS. If I have a plane that can reach 120 kts IAS at sea level up to 3000 ft then guess where it will reach the highest TAS? If the local temperature raises sea level density from standard to 1000 ft above standard then my TAS will be higher for the same IAS even at sea level. My lift will be by my IAS and my takeoff run will be longer but I should go faster.

BillSwagger
08-09-2010, 01:26 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by M_Gunz:
And we have the density altitude formula telling us that for every 1 C above standard is the same as + 120 ft height in standard. So tell me that top speed for WWII fighters is higher at sea level or 500m above sea level. I think this holds true for most GA planes today as well. Or tell please what planes have higher sea level top speed than at any height above, and don't forget the Wright Flyer if you need at least one.
</div></BLOCKQUOTE>

After glancing at some tests, i can tell you that certain models/configurations of the F4U had a faster speed at sea level than they did just a few hundred meters above. At least test results reflect this, and it might have something to do with using heavier mixtures on take off.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">WTE_Galway

http://www.dmjwilliams.co.uk/gbsep_performance.htm </div></BLOCKQUOTE>

Thanks for explaining this. I guess what had me confused was that the barometer measures the weight of the atmosphere and not necessarily the density of the air.

JtD
08-09-2010, 02:07 PM
F4U characteristics are probably down to the low full throttle altitudes in neutral blower. If you raise the manifold pressure high enough, full throttle alt would easily be below sea level. The engine therefore loses power quickly.

Bremspropeller
08-09-2010, 02:17 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">And we have the density altitude formula telling us that for every 1 C above standard is the same as + 120 ft height in standard. So tell me that top speed for WWII fighters is higher at sea level or 500m above sea level </div></BLOCKQUOTE>

What you're obviously not understanding is that the same compression-ratios can not be acheved by warmer than standard air, thus leading to a decrease in thrust.

WTE_Galway
08-09-2010, 05:42 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by M_Gunz:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Bremspropeller: