Effect of humidity on Airbus or Boeing performance.

[stickvsjoke]

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]

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]

Originally Posted by ScepticalOptomist

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]

You may find this informative.

https://commons.erau.edu/cgi/viewcon...&context=ijaaa

[john_tullamarine]

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-id..._1101&rgn=div8

[Smythe]

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]

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]

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]

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]

A picture is worth a thousand words they say.




Calculator here.

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

Effect on piston engine power calculator.

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

[tdracer]

Originally Posted by Smythe

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]

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]

Originally Posted by DessertRat

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]

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]

Originally Posted by megan

A picture is worth a thousand words they say.

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]

Originally Posted by Smythe

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'
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?

[tdracer]

Originally Posted by GordonR_Cape

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]

Originally Posted by tdracer

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_po...ewpoint-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/

[tdracer]

Originally Posted by GordonR_Cape

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]

Must really mess you up when you take a dirty turbine loss of 15%