Density and moisture ?

[freewheeler]

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 :cool:

[Radix]

............

[westhawk]

In a liquid state, water obviously has a higher specific density. What if it was in a gaseous state though?

[FLEXPWR]

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.

[westhawk]

You ruined the fun of watching the OP figure it out one step at a time!

But thanks are in order for wanting to help.

[Microburst2002]

When you can see water, it is liquid (includes steam and clouds)
when it is invisible, then it is vapour

[barit1]

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.

[freewheeler]

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 :confused:

[boguing]

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.

[barit1]

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.

[Hydromet]

ie an area of lower pressure behind the blades "sucks" the ship forward?
The zone of (comparatively) lower pressure is in front of the blades.

[westhawk]

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

[Green Guard]

to confuse yourself more if you wish, please consider ALSO
a dark air vs AIR FULL of Light...:cool:

[fantom]

Following on, does this mean my aircraft will fly higher in a cloud?

[freewheeler]

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 .

[V1... Ooops]

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

[barit1]

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 (http://www.weatherquestions.com/What_is_condensation.htm) 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.

[boguing]

Why thank you V1, I've edited it to add the compressibility bit that I felt should be in there.

[freewheeler]

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 :D

[Teddy Robinson]

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.

[boguing]

This could run and run.

Elements and compounds can have some astonishing properties.

Water has some unique properties - one that is commonly known is that expands as it freezes.

Aviation-wise, it's not only water. Carbon fibre does too.

Merely fascinating until you realise that the A380 has an Aluminium spar in it's Carbon horizontal stabiliser. Don't know how it's dealt with, but I really really hope that they are not bonded together (originally).

My Father was an Aeronautical engineer (back when they were looking at ceramic blades), and I have a friend who he would love to have met - does composite stuff for RR. If I were to tell you too much about intelligent metals, black cars would be parking outside my house!

What are you working on?

[boguing]

Oh, and, you're aware of Glycol injection into Merlins etc?

Better than water because the 'latent heat of evaporation' is more effective. Or in simple terms, takes more heat out with it.

I did Naval Architecture, so a bit closer to Aerodynamics than a Mech Eng. So while people still argue about Bernoulli vs. plank theory there is a funny little thing that my Dad told me about.

I did just write a paragraph explaining it, and saying that we never came up with a practical use for it. But then realised that there might well be, so forgive me for the tease!

[boguing]

My sport has been yacht racing. Other than the heading of the entire boat, we constantly adjust AoA, camber and the longitudinal position of max camber. And sometimes change sails to suit wind speeds. (And twist the sails to allow for what we call wind shear and you might call ground effect).

That can occasionally be bizarre. Most of the time the closer to the water, the slower the wind. Which is lucky because it would be very complicated to twist the sail the other way. But on two occasions (both Autumnal with T inversions) the higher breeze was slower. Very odd, and felt like I was hallucinating. Top of the sails working nicely, bottoms just flapping.

There are always beer fuelled debates about wind density, temp/humidity, and its' effect on forward speed (l/d). Problem is that we are all racing in the same stuff, and with so many other variables it's very hard to quantify - so we don't because getting to the bar first is more important.

But I will agree that a damp day seems to help turbulence/not help laminar flow.

[Bradda G]

I used the following day-to-day observation to help with the density concept during my PPL training:

1. When boiling water, the vapor rises. Therefore, water vapor (aka moisture) must be lighter than dry air.

Just my $0.02

[barit1]

Funny - I once heard an explanation of operation of a humidifier. Seems that you really don't need a fan to distribute the water vapor. It mixes naturally - Brownian motion (http://en.wikipedia.org/wiki/Brownian_motion) - or, if you will, "under its own steam". :8 :}

[westhawk]

1. When boiling water, the vapor rises. Therefore, water vapor (aka moisture) must be lighter than dry air.

So does the "dryer" air above the flame if you remove the pot. But humid air does so to a greater degree. Warm moist air rising within cooler dryer air is the essence of convective storm formation.

[freewheeler]

thanx for taking the time. I'm involved with C series and L-85 at the production level huge investment going on here in Belfast with dry fibre RTI technology and I have been out to Mexico to try and help with porosity problems with the "Lear" 85 fully-comp fuselage. Have to be a bit careful with confidentiality and all that. The Belfast factory is huge and there have been no end of visiting dignitaries . Some people from Sukhoi came and were caught on the way out with bits of post-cure resin ! At the minute its a grossly labour intensive process but test panels have resisted huge stress loads. Lightning strike , water ingress over time and accident damage are some of the many considerations causing sleepless nights here. Hold on.......someones at the door.......