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M_Gunz
04-27-2007, 04:07 PM
I am looking at Density Altitude for effect on performance and I think maybe I could use some
help because what I am looking at....

Difference between 15C at sea level Standard Atmosphere and 25C at sea level day is as if on
the warmer day sea level performance is equal to 1000 FEET higher on the Standard day.

1000 FEET difference for 10 deg C is not a lot of difference on performance correction.

Hope I am clear about this, I see climb at sea level on a day with 25C at sea level as being
same as climb at 1000 feet on a day with 15C at sea level. And I am thinking that cannot be
a whole lot of difference.

What am I doing wrong please?

Lurch1962
04-27-2007, 04:23 PM
Look here...

http://wahiduddin.net/calc/calc_da_m.htm (http://wahiduddin.net/calc/calc_da.htm)

(Options can be found in the web site's left panel.)

For simplicity, you can use altimeter settings applicable to standard seal level pressure; 29.92 in. Hg., or 1013.2 Hectopascals (= millibars).

The dew point is the temperature at which water vapour would spontaneously condense out of the air. When the dew point equals the air temp, the relative humidity is 100%. (The dew point can never be greater than the air temp.)

The dew point is used in the calculation because water vapour content must be considered. Why? Because the H2O molecule is lighter that either O2 or N2, the two main constituents of the atmosphere. In a nutshell, more humidity means lower density because more water molecules per unit volume means less oxygen and nitrogen molecules.

--Lurch--

M_Gunz
04-27-2007, 04:40 PM
In most places the difference is small. By the time you get 1000m up it is very small.

AKA_TAGERT
04-27-2007, 09:32 PM
bingo!

The difference between the current definition of the 'standard day' and the 'standard day' of the 40s is nill for altitudes below 40kft

During the mid 20s all country had it pretty well mapped out.. it was not until the 50s with rockets that they even started poking around above 40kft. Thus during the 50s and 60s they simply extended the standard atmosphere data from the 20s to include altitudes above 40kft!

Long story short, those small differences are not going to explane away a 25% better ROC.

AirWireFox
04-28-2007, 04:35 AM
It is vital to know your Density Altitude for testing, which is why we use the Crimea map.
Pick a plane on the map that has a Outside Air Temp (OAT) gauge and you'll see at Sea Level (SL) the air temp is 15deg C.
As the SL OAT is 15 deg at 0 ft then this map is at International standard Atmosphere (ISA) conditions.
Every 1000ft you go up in the ISA the OAT drops 1.98 deg C
Air pressure at 0ft in the ISA is 1013 millibars.Every 1000 ft you go up the ISA pressure drops 27 Mb.
All test data is corrected to ISA conditions or a historical equivalent (practically the same). Data charts usaully show a set of results for ISA and ISA+10deg and ISA+20deg etc etc.

So for example you are taking off from Madrid in the high summer OAT 40 deg amd airport elevation of 2000ft your density altitude is about 7000 ft
(source Airtour CRP-5 circ slide rule). http://www.pooleys.com/prod_detail.cfm?product_id=567
This would be a big hit on performance.

Simillarly if you were taking off in Aberdeen with a deep depression weather system over northern UK. You might have a QNH (altimeter setting) of say 965 mB. This is a 48 mb difference on ISA which at 27mb per 1000ft is a pressure atlitude of 1700ft which can be a big hit on performance.

So to know what you can get out or your machine at say 6000 ft you need to know the OAT at 6000ft and the air pressure at 6000ft.Then you can use a computer to find the density altude.
Then go into your chart at the density alt amd read off your Speed or ROC.

Humidity effects on air density are have not been included in any end user performance calcs (test pilot stuff maybe) that I know of.

M_Gunz
04-28-2007, 05:51 AM
Originally posted by AKA_TAGERT:
Long story short, those small differences are not going to explane away a 25% better ROC.

I look at the density alt chart at Wiki and I don't think that for planes like 109K and La-7
that correction from historic will account for 5% difference but that's just my take based on
the climb charts at low alts not having a whole lot of difference over even 1000m.

But I could be wrong and would like if Oleg were to tell me something about how since it was
his reply to you that got me onto this tack. I tried to email him direct but his address is
no longer the same.

AKA_TAGERT
04-28-2007, 09:53 AM
Originally posted by AirWireFox:
As the SL OAT is 15 deg at 0 ft then this map is at International standard Atmosphere (ISA) conditions.
Exactly!

And the one's that don't 'typically' will state it right on the graph that the data has not been converted to std atm.

I have looked at..

<pre class="ip-ubbcode-code-pre">
NACA Report No. 147 dated 1923
NACA Report No. 218 dated 1925
NACA Report No. 1235 dated 1952</pre>
And the difference in those values, if any, is very small. Granted I did not verify all of the thousands of values, but looking at the T/To, P/Po, p/po at altitudes below 40kft (i.e. where most of the WWII testing was limited to) the difference, if any are nill!

For example, in NACA Report No. 218 they provide the following formula for converting TEMP. Which notes that this "formula not only fulfilled the requirements of simplicity and reasonable accuracy but <span class="ev_code_yellow">also had the advantage of being extensively used in Europe</span>"

<pre class="ip-ubbcode-code-pre">T = To - 0.0065*Z</pre>
Where
T is temperature in ?C at altitude Z in meters

Punch in 304.8m (1,000ft) and you get

<pre class="ip-ubbcode-code-pre">T = To - 1.9812?C</pre>
Which agrees with what you said about the 'modern' standard, i.e. "Every 1000ft you go up in the ISA the OAT drops 1.98 deg C".

Since that time, the only thing the new standards (read updated) have done is extend the 'standard day' data beyond the 40kft mark and provide 'standard day' data for Latitudes other than 40? (New York, Paris, Berlin, etc)

Therefore the 'standard day' data is and was the same, thus the only area left for any errors is in the conversion process itself that makes use of this data. I have recently ordered a few books that may shed some light on each countries conversion process, but I can not belive that they would be so different as to account for a 25% error in climb rates.

M_Gunz
04-28-2007, 02:49 PM
Biggest possible gap I've seen is what base temperature was used as standard.
Modern standard at sea level is 15C.
IF (and this is a biiig IF) some country worked with 25C then I can see that the sea level
performance at 25C would be about the same as a 15C day and 1000 ft, again unless I have
that wrong. 25C is a warm day. Another +10 C gets performance at 2000 ft, i.e. not much
difference but you'd notice either in a small GA prop plane.

AKA_TAGERT
04-28-2007, 03:35 PM
I have seen nothing to even hint at some countries not being able to measure the temp at sea level to such an extent as to miss it by 10?.

As I pointed out, by the early 20s everyone was on the same sheet of music with regards to the temp at sea level being 15?.

What is it that you have seen that makes you think there is a 'potential gap' that is off by 10? (25?-15?)?

M_Gunz
04-28-2007, 05:03 PM
Only looking at the math, didn't catch that -everyone- used the 15C standard. The only way
I so far see a possible gap of even 1% within SL to 1km alt is by that difference. I do NOT
say that anyone did and I did ask if there is some other factor I missed.

Could I decide that you must be 42 since I know someone who is 21 and only half nuts?
And what does it then mean that I am 50?

It's all in the math, right? http://forums.ubi.com/images/smilies/16x16_smiley-wink.gif

AKA_TAGERT
04-28-2007, 05:28 PM
Ah ok!

I was just wondering where you got that 25? value from.. As in I though you were saying you saw some data that looked like they used that instead of 15?.

So, it appears that we agree?

That by the 1940s the differences, if any, in the definition of a 'standard day' used by one country vs another was small and would not account for a 25% better ROC as 'some' were trying to sugest! http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif

M_Gunz
04-28-2007, 11:20 PM
Originally posted by AKA_TAGERT:
Ah ok!

I was just wondering where you got that 25? value from.. As in I though you were saying you saw some data that looked like they used that instead of 15?.

So, it appears that we agree?

That by the 1940s the differences, if any, in the definition of a 'standard day' used by one country vs another was small and would not account for a 25% better ROC as 'some' were trying to sugest! http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif

From the density alt graph it appears to me that a base temperature of 40C wouldn't account for it!