Density and moisture ?
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Density and moisture ?
What am I missing ? Given that the mass of a cubic metre of water is so much more than the same volume of air ; How is it the case that moist air is less dense than dry (or less moist) air ? is it a function of ambient temperature ? seems counter-intuitive to me but then so does Bernoulli's theorem . I'm sure that I'm demonstrating my ignorance but hey 
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There is a difference between water vapor and liquid water. Same goes for air, liquid air (compressed into liquid for industrial use) has a density of 870 kg/cubic metre.
Liquid water is indeed much more dense that air as a gas, but water vapor is a gas (not steam tiny beams of water), and is less dense than air.
Saturated water vapor at 15 degrees Celsius is roughly 13 gm/cubic metre.
Liquid water is indeed much more dense that air as a gas, but water vapor is a gas (not steam tiny beams of water), and is less dense than air.
Saturated water vapor at 15 degrees Celsius is roughly 13 gm/cubic metre.
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Atomic weight of two hydrogen atoms = 2
Atomic weight of one oxygen atom = 16
Atomic weight of H2O (water vapor) = 18
Atomic weight of an oxygen molecule = 32
Atomic weight of a nitrogen molecule = 28
Equivalent atomic weight of air
(80% N2, 20% O2) = 28.8
18 is much less than 28.8
QED
edit: Mea culpa, kindly substitute "Mass" for "weight" in above text.
Atomic weight of one oxygen atom = 16
Atomic weight of H2O (water vapor) = 18
Atomic weight of an oxygen molecule = 32
Atomic weight of a nitrogen molecule = 28
Equivalent atomic weight of air
(80% N2, 20% O2) = 28.8
18 is much less than 28.8
QED
edit: Mea culpa, kindly substitute "Mass" for "weight" in above text.
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whats an OP ?
sounds like you have the answer ! could you share it with me ? I'm still confused with air and water supposedly acting in physics and aerodynamics as "liquids" but with different viscosities and other characteristics.(temp ?!!) Hence in former times the term "airscrew" > Does Bernoulli apply for ships' propellers ie an area of lower pressure behind the blades "sucks" the ship forward , compared to me driving an Archimedes screw through a piece of wood by dint of elbow grease ? Go on , give me an idiots guide to it all 
Liquids are liquids.
Gases are gases.
Both are fluids.
Bernoulli applies to all fluids.
Definitions:
Fluids cannot sustain a shear force. Solids can.
Gases are compressible. Liquids are not.
Gases are gases.
Both are fluids.
Bernoulli applies to all fluids.
Definitions:
Fluids cannot sustain a shear force. Solids can.
Gases are compressible. Liquids are not.
Last edited by boguing; 9th Aug 2011 at 18:37.
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freewheeler:
Stop confusing yourself with liquid water; A little study of gas laws is necessary.
This is a problem in molecular mass of gas mixtures. Dry air is a gas mixture with a molecular mass of 28.8 (or maybe 28.9). Water vapor (NOT liquid water, and not ice) has a molecular mass of 18.
Thus, the more water vapor you mix in the air, its molecular mass decreases from 28.8. In other words, its density decreases.
And with a less dense working fluid, engines don't develop as much output, and wings don't lift as well.
Stop confusing yourself with liquid water; A little study of gas laws is necessary.
This is a problem in molecular mass of gas mixtures. Dry air is a gas mixture with a molecular mass of 28.8 (or maybe 28.9). Water vapor (NOT liquid water, and not ice) has a molecular mass of 18.
Thus, the more water vapor you mix in the air, its molecular mass decreases from 28.8. In other words, its density decreases.
And with a less dense working fluid, engines don't develop as much output, and wings don't lift as well.
Sorry freewheeler! "OP" is meant as an abbreviation for "Original Poster", meaning in this case YOU, the person who started this thread.
As for the matter at hand, Barit1 has already explained the principle more clearly and succinctly than I could hope to do. Just know that high temperature with a high dewpoint means that the air density is even less than accounted for by considering the effect of temperature alone. High, hot and humid is one condition where published takeoff performance data might best be used with a little extra conservatism.
Best,
westhawk
As for the matter at hand, Barit1 has already explained the principle more clearly and succinctly than I could hope to do. Just know that high temperature with a high dewpoint means that the air density is even less than accounted for by considering the effect of temperature alone. High, hot and humid is one condition where published takeoff performance data might best be used with a little extra conservatism.
Best,
westhawk
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cheers all
food for thought then and yet more proof that what appears to be intuitively the case is not necessarily so. Thanx for taking the time, straight up .
Special thanks to barit1 and boguing for the really first-class answers provided.
I've been in this business for 40 years and never 100% understood the "reasons why". Both of you explained things succinctly in a manner that is far better than any textbook I have ever read.
Michael
I've been in this business for 40 years and never 100% understood the "reasons why". Both of you explained things succinctly in a manner that is far better than any textbook I have ever read.
Michael
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Now that we have that settled, there's another humidity-related factor to consider: Condensation when static pressure drops (for example, at Vr on a very humid day). A cloud forms over the wing - you see it a few dozen times an hour at ATL or MIA or other steamy burgs.
That same cloud forms inside the inlet of a jet or fan engine, and the condensation has a side-effect: The heat of condensation increases the air temperature a few degrees. The engine behaves just as you'd expect - a bit lower output because of reduced density.
To simplify the aircraft performance, this is already accounted for in your AFM performance charts. Thus, if you take off in a very dry desert condition, your actual performance will be a bit better than book, since there's no condensation to worry about.
That same cloud forms inside the inlet of a jet or fan engine, and the condensation has a side-effect: The heat of condensation increases the air temperature a few degrees. The engine behaves just as you'd expect - a bit lower output because of reduced density.
To simplify the aircraft performance, this is already accounted for in your AFM performance charts. Thus, if you take off in a very dry desert condition, your actual performance will be a bit better than book, since there's no condensation to worry about.
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any fool can ask......
very grateful for your replies ! as an amoeba in aviation's food chain its great to hear from you all. I guess I have this thing with water as also interesting is water injection into piston engines (apparently as far back as WW2 ) and I'm amazed at the water ingestion abilities of jet engines . By the way ,I'm working for a certain manufacturer in the field of composites ; there is a big debate going on about production methods and processes anyone out there involved ? Again, just a low form of life but one with an evolving brain 
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as an aside ...
When I was very young just pre-PPL .. I flew in a very overloaded FR172 from a popular holiday island in the Med back to the UK with two owner pilots who should have known better. I sat nessled next to an oversized liferaft, while the baggage compartment was stuffed with suitcases ... oh and a limestone barbacue someone took a fancy to.
The Riems Rocket always looked rather nose heavy with that big 6 cylinder engine ahead of the nosewheel, but on this day adoped a far more nose light attitude .. naturally no loadsheet was done .. big engine .. it always flys was the order of the day ... and so it did, to this day I dare not guess how far out of aft limits.
Curious thing was noted though, every time the aircraft entered cloud a high frequency buffet rang through the airframe, loud and unmistakable, and at the time nobody could figure it out. Then, many years later came my CPL exams and the matter of dencity vs water content was dicussed.
After a bit of reflection, it occurred to me that the buffet was probably the airflow seperating from the tailplane .... hmmm nice.
The Riems Rocket always looked rather nose heavy with that big 6 cylinder engine ahead of the nosewheel, but on this day adoped a far more nose light attitude .. naturally no loadsheet was done .. big engine .. it always flys was the order of the day ... and so it did, to this day I dare not guess how far out of aft limits.
Curious thing was noted though, every time the aircraft entered cloud a high frequency buffet rang through the airframe, loud and unmistakable, and at the time nobody could figure it out. Then, many years later came my CPL exams and the matter of dencity vs water content was dicussed.
After a bit of reflection, it occurred to me that the buffet was probably the airflow seperating from the tailplane .... hmmm nice.