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stickvsjoke
4th Mar 2019, 12:41
I know that humidity makes the air less dense and therefore degrades aircraft and jet engine performance.

I was was wondering if Airbus flysmart or Boeing OPT performance has a factor for humidity at all?
It can’t work out humidity from the data entered as the due point isn’t asked for?

ScepticalOptomist
5th Mar 2019, 00:13
Humidity would only be a factor in isolation wouldn’t it?
If you look at two samples of air at the same temperature and pressure - the more humid air would be less dense as you said.

In our actual environment though, more humid air would be accounted for by the temperature and /or pressure increasing, which is taken into consideration.

stickvsjoke
5th Mar 2019, 05:27
Humidity would only be a factor in isolation wouldn’t it?
If you look at two samples of air at the same temperature and pressure - the more humid air would be less dense as you said.

In our actual environment though, more humid air would be accounted for by the temperature and /or pressure increasing, which is taken into consideration.



Thank you for your reply. If you run the figures for two airports that theoretically were exactly the same in terms of pressure and temperature but one was nearing 100% humidity then you would have the same figures for both but the performance would be worse at the humid airport. I assume they work on the basis that the conservative nature of the performance will take account of the humidity.
You said it would be compensated with a higher temperature? But that would also impact the performance negatively along with the humidity. If there was a higher pressure then this would improve performance but the figures flysmart pop out still haven’t taken into account a really humid atmosphere such as SIN?

megan
6th Mar 2019, 05:16
You may find this informative.

https://commons.erau.edu/cgi/viewcontent.cgi?article=1124&context=ijaaa

john_tullamarine
6th Mar 2019, 06:18
If the concern raised is that the design rules ignore (ie don't consider) humidity then, perhaps, a read of FAR 25.101 might be useful.

That the rules haven't changed in a long time suggests that the residual effects are, in the overall scheme of things, not overly critical ?

That is to say, the AFM performance includes a consideration of humidity. I guess that there is naught to prevent running a subsidiary calculation for a more accurate correction, providing that the application does not result in non-conservative corrected data when compared to the basic AFM information.

See https://www.ecfr.gov/cgi-bin/text-idx?SID=a4e9b5c6319b2803576d41fbf55f6478&mc=true&node=se14.1.25_1101&rgn=div8

Smythe
6th Mar 2019, 15:23
In the calculations, you will find that temperature has the greatest affect on barometric pressure. At 3 decimal place differences, humidity effects are negligible.

B2N2
6th Mar 2019, 17:06
What gets interesting to me at least is the effects on the operation of piston engines, turbo charged and normally aspirated and the effects on the operations of jet engines.

tdracer
6th Mar 2019, 19:04
High specific humidity does adversely affect engine thrust - due to the lower density of the air. This has a much larger effect at N1 than at EPR (obviously signficant only at higher temps, since you need high temps to have high specific humidity).
For N1 rated engines, this is accounted for in the power setting charts - basically added performance margin at high temps to account for high humidity.

Smythe
7th Mar 2019, 00:57
High specific humidity does adversely affect engine thrust - due to the lower density of the air. This has a much larger effect at N1 than at EPR (obviously signficant only at higher temps, since you need high temps to have high specific humidity).

Ummm,, what?!?! You need High temps to have high specific humidity??

The standard is BASED on 80% to 34% rel humidity. Looking at the calculations, the variable in the calc for humidity is multiplied by .0621, and the humidity ranges between .0008 and .007. Therefore .0621 * .0008 to .007 is meaningless as an additive to the performance calculation. That is why is is negligible, and not added to any performance software, even at any altitude.

Temperature governs, plain and simple by a vast margin.

megan
7th Mar 2019, 01:27
A picture is worth a thousand words they say.


https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/687x386/moist_air_density_temperature_relative_humidity_e4b7867e3385 57b5ccc6f79960a844803ae71046.png

Calculator here.

https://wahiduddin.net/calc/calc_da_rh.htm

Effect on piston engine power calculator.

https://wahiduddin.net/calc/calc_hp.htm

tdracer
7th Mar 2019, 06:38
Ummm,, what?!?! You need High temps to have high specific humidity??

The standard is BASED on 80% to 34% rel humidity. Looking at the calculations, the variable in the calc for humidity is multiplied by .0621, and the humidity ranges between .0008 and .007. Therefore .0621 * .0008 to .007 is meaningless as an additive to the performance calculation. That is why is is negligible, and not added to any performance software, even at any altitude.

Temperature governs, plain and simple by a vast margin.

YES!!! You need high temperatures to have high specific humidity. Basic physics. Look at the chart Megan posted - specific humidity is all that matters since it reduces the air density - and isn't accounted for by temperature. At ~90+% relative humidity and ~100 deg F, you've reduced the air density relative to dry air by ~1% - which is ~1% thrust at N1 (as noted, effect on EPR is significantly less).
You don't believe 1% thrust is meaningful?

PS Thanks megan

DessertRat
7th Mar 2019, 08:28
OMG it’s like an Airbus training captain bukkake session.

Don’t fall into the Airbus trap of overthinking EVERY detail. Put the numbers in; get the data out; it will fly.

stickvsjoke
7th Mar 2019, 09:14
OMG it’s like an Airbus training captain bukkake session.

Don’t fall into the Airbus trap of overthinking EVERY detail. Put the numbers in; get the data out; it will fly.

I fly the 747 so I wouldn’t suggest it’s an “Airbus trap”?
I was asking out of curiosity. Thank you to those that have posted. I have a rule of thumb from the Embry riddle document and now aware it’s all taken account of in the certification documents.

Smythe
7th Mar 2019, 11:27
Umm..look at the graph in aircraft operating temperature ranges. How often are 90% humidity and 100 degrees encountered? Arent most operations temperature limited at that range?
All that for a max of 1% deviation in thrust? What is the deviation in thrust for a dirty turbine?

Between 20 and 40 degrees, there is virtually no difference in density relating to humidity.

Run BCOP, tell me again how humidity affects performance.

Please keep this chart handy, for the next time you fly at 80 degrees.

GordonR_Cape
8th Mar 2019, 03:58
A picture is worth a thousand words they say.


https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/687x386/moist_air_density_temperature_relative_humidity_e4b7867e3385 57b5ccc6f79960a844803ae71046.png



Just a reminder of the obvious: This diagram extends up to the boiling point of water, which is not representative of the real atmosphere. In practice the dew point never exceeds 30C/86F, due to condensation and precipitation processes.

The maximum water vapour content of the atmosphere at sea level is 4%, which puts an upper imit on the correction factors in the density formula.

Anecdotal reports of temperatures of 37C/100F and 100% humidity should be disregarded.

tdracer
8th Mar 2019, 04:10
Umm..look at the graph in aircraft operating temperature ranges. How often are 90% humidity and 100 degrees encountered? Arent most operations temperature limited at that range?
All that for a max of 1% deviation in thrust? What is the deviation in thrust for a dirty turbine?

Between 20 and 40 degrees, there is virtually no difference in density relating to humidity.

Run BCOP, tell me again how humidity affects performance.

Please keep this chart handy, for the next time you fly at 80 degrees.

Arent most operations temperature limited at that range?

Thanks for making my point. At high temps, you are temperature limited - now take away more thrust because of very high humidity and see what happens...
If you're operating from someplace like Singapore or Jakarta, 100 deg. F and 90% humidity is close to the norm. Safety is not determined by 'well, that won't happen very often so we don't need to worry about it' :mad:
A dirty turbine (or compressor) makes nearly zero difference in thrust at N1 - EGT yes, fuel burn yes, but not thrust.
That's why it's necessary for the engine companies to build conservatism into the power setting charts to account for the thrust uncertainty - and the higher the temperature the more uncertainty there is - in large part due to humidity.
People have died due to 1% less thrust.... That's why the regulations require better than that.
Or do you routinely set thrust 1% low on a hot day and not worry about it? :confused:

tdracer
8th Mar 2019, 04:19
The maximum water vapour content of the atmosphere at sea level is 4%, which puts an upper imit on the correction factors in the density formula.

Anecdotal reports of temperatures of 37C/100F and 100% humidity should be disregarded.

4% specific humidity is ~1.5% density compared to dry air - that's 1.5% thrust at N1. You may not consider 1.5% thrust significant, but those of us who worry about engine performance sure do.
And there nothing anecdotal about 37C and 90+% humidity if you're operating near the equator.

GordonR_Cape
8th Mar 2019, 04:38
4% specific humidity is ~1.5% density compared to dry air - that's 1.5% thrust at N1. You may not consider 1.5% thrust significant, but those of us who worry about engine performance sure do.
And there nothing anecdotal about 37C and 90+% humidity if you're operating near the equator.

1. I didn't say its insignificant, I suggested its an upper limit, already included in the calculations.
2. The highest recorded dew point anywhere in the world was 35C/95F in Saudi Arabia, and 33C/91F in Florida. The norm is definitely closer to 30C in the tropics.
3. 37C and 90%RH is false (dew point 35C/95F). I challenge you to show me a Metar showing a dew point above 30C.

Reference: https://en.wikipedia.org/wiki/Dew_point#/media/File:Dewpoint-RH.svg

Edit: An explanation of why this 'myth' arises: https://www.washingtonpost.com/news/capital-weather-gang/wp/2013/07/08/weather-weenies-prefer-dew-point-over-relative-humidity-and-you-should-too/ (https://www.washingtonpost.com/news/capital-weather-gang/wp/2013/07/08/weather-weenies-prefer-dew-point-over-relative-humidity-and-you-should-too/?utm_term=.e5daf5d58fa3)

tdracer
8th Mar 2019, 05:47
1. I didn't say its insignificant, I suggested its an upper limit, already included in the calculations.


Which if you go back and read my original post, is exactly what I stated:
"For N1 rated engines, this is accounted for in the power setting charts - basically added performance margin at high temps to account for high humidity."
In other words, humidity is not insignificant, it's accounted for with conservatism.
Takeoff on a hot, dry day, and you get extra thrust (with an N1 rated engine).

Smythe
8th Mar 2019, 14:47
Must really mess you up when you take a dirty turbine loss of 15%

tdracer
8th Mar 2019, 17:09
Must really mess you up when you take a dirty turbine loss of 15%

Which affects thrust at N1 how exactly?

megan
9th Mar 2019, 00:46
Effect of humidity on gas turbine performance.

https://www.ijser.org/researchpaper/Effect-of-Compressor-Inlet-Temperature-Relative-Humidity-on-Gas.pdf

I challenge you to show me a Metar showing a dew point above 30C. The world's highest reported dew point is 35 degrees, recorded on July 8, 2003, at Dhahran, Saudi Arabia The highest in the US was 31.1 at Moorhead, Minn July 19, 2011

https://www.wunderground.com/blog/weatherhistorian/record-dew-point-temperatures.html

GordonR_Cape
9th Mar 2019, 07:47
Effect of humidity on gas turbine performance.

https://www.ijser.org/researchpaper/Effect-of-Compressor-Inlet-Temperature-Relative-Humidity-on-Gas.pdf

. The world's highest reported dew point is 35 degrees, recorded on July 8, 2003, at Dhahran, Saudi Arabia The highest in the US was 31.1 at Moorhead, Minn July 19, 2011

https://www.wunderground.com/blog/weatherhistorian/record-dew-point-temperatures.html

Those record values were in my earlier post! Other than freak conditions, dew points above 30C do not routinely occur in outdoor air. And certainly not in the tropics, which was what I was pointing out to @tdracer.

The NWS office in Grand Forks, North Dakota issued this statement explaining how heavy rainfall the morning of the event had saturated the soil and left ponding water near the instrument site. Furthermore, the AWOS is surrounded by sugar beets and soy beans “two of the most prodigious transpiring plants”.

Have you looked at any Metars from Singapore, Bali, Jakarta, and other airports close to the equator? That was my challenge...

https://en.allmetsat.com/metar-taf/singapore.php?icao=WSSS
https://en.allmetsat.com/metar-taf/indonesia.php?icao=WADD
https://en.allmetsat.com/metar-taf/indonesia.php?icao=WIHH
METAR: WSSS 090630Z 14010KT 080V150 9999 FEW020TCU SCT300 32/24 Q1010 NOSIG
METAR: WADD 090630Z 26010KT 9999 FEW017 31/24 Q1010 NOSIG
METAR: WIHH 090600Z 01008KT 6000 SCT020 33/22 Q1010 NOSIG


Tropical dew points are mid-20s C (70s F), slightly higher during El-Nino/La Nina conditions. Moisture in the air is a dynamic between evaporation and precipitation, and dew points do not exceed sea-surface temperatures (except as noted above). See: https://en.wikipedia.org/wiki/Sea_surface_temperature#/media/File:SST_20131220_blended_Global.png

There are exceptional areas outside the tropics, notably the Read Sea, and also the Persian Gulf. See: https://en.wikipedia.org/wiki/Bandar_Abbas#Climate

In the summer, Bandar Abbas sees some of the highest average dew points of any city in the world, averaging 27 °C (81 °F) and frequently exceeding(*) 30 °C (86 °F).

* 88F/31C maximum dew point on several days in July