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Author Topic: Air Density  (Read 22422 times)

John Leidle

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Air Density
« on: July 12, 2011, 08:45:13 PM »
  This year in Muncie I witnessed my plane falling in manuvers. Everytime I turned a square corner it stalled in a big way. I took the plane home & flew in Monday all conditions were  the same except the air was in the Pacific Northwest instead of Indiana. The plane was fine  & I tried to stall it & it would not stall.
  It hs been mentioned the air in Muncie requires more nitro for the engines,, can you guys explain this & maybe the differance between humidity & low & high air density.
  John

Offline Air Ministry .

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Re: Air Density
« Reply #1 on: July 12, 2011, 08:55:53 PM »
All the Traffic Fumes give it more grip up youre way ! :##or possibly you were trying to hard there ? n1

Popping a ' hit full up ' turn out of a wingover, but DEFINATELY at say 20 , or 15 ?? Ft. as ' a test ' at both sites might show if its that .
Planes seldom take full deflection in Squares . If you havnt been , and then you do , you could be past the limits . Where whenst thoust
more relaxed onest flying the Sq. Turns. Conscious of it or not . Olde Boye . Z@@ZZZ

The thick clammy coastal air in N.Z. required half the intake in a FP magnum 40 to 2.000 M Alt in the boondocks in Aus .
The Hot Dry air here is discernably less ' adhesion ? ' unless temp is down below 20 Deg, more like 16 , & near sea level .

There must be differant compositions of air, Sea Salt humidity ?? Calm winterish post dusk air is fastest in New Zealand .
And thats good country air at that . LL~

Offline Matt Colan

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Re: Air Density
« Reply #2 on: July 12, 2011, 09:02:33 PM »
  This year in Muncie I witnessed my plane falling in manuvers. Everytime I turned a square corner it stalled in a big way. I took the plane home & flew in Monday all conditions were  the same except the air was in the Pacific Northwest instead of Indiana. The plane was fine  & I tried to stall it & it would not stall.
  It hs been mentioned the air in Muncie requires more nitro for the engines,, can you guys explain this & maybe the differance between humidity & low & high air density.
  John

Hi John,

I noticed the same thing with my ares, no matter how much I softened up the corners, it would still stall on my hourglass, triangles, and squares which resulted in some 2 foot pullouts.  The Air Density was much lower here according to Richard Oliver, than he's ever seen it.

When the air density goes down, the air gets thinner, so it's like the plane is flying at a higher altitude, or it even gained weight.  Also, the engine makes less power so you need less fuel.  That's my way of understanding it.
Matt Colan

Offline Steve Helmick

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Re: Air Density
« Reply #3 on: July 12, 2011, 09:24:26 PM »
A few years ago at Fall Follies (October, Salem Oregon), Bruce Hunt checked the Altitude Density at under minus 1,000 ft. below sea level...and Salem is 214' ASL. It was cold, below freezing in the early AM, and the baro pressure would have been around 31" of mercury. Not possible to stall, I don't think. The engines ran like Jack (the bear) and burned lots of fuel.

Your situation at Muncie was exactly opposite. You needed to add more nitro to make the engine return to normal  power plus some, dial in less elevator to give more flap, and wider handle spacing to make up for the loss in control response. That, or take out 10 ounces of ballast.  ;) Steve
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Offline Garf

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Re: Air Density
« Reply #4 on: July 12, 2011, 09:28:35 PM »
Back in 97, at my first trip to the NATS, I flew a plane I called "old Reliable". It never gave me any problems in Miami. In Muncie, It fell out of the overhead 8's and crashed into the asphalt. Conditions in Muncie are radically different than in Miami, or any other coastal city. Next time I go, I'm bringing some high nitro mix with me.

Offline Garf

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Re: Air Density
« Reply #5 on: July 12, 2011, 09:30:59 PM »
What is the field altitude in Muncie in feet?

Offline Peter Nevai

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Re: Air Density
« Reply #6 on: July 12, 2011, 09:36:53 PM »
As far as your stall tolerance is concerned, the differential in air density is miniscule. At most it is a difference of a couple hundred feet. Unless you are already at the ragged edge of wing loading, it is a non issue. Fuel air mix for the engine is a whole other matter, the tolerances for that are far more critical as a small change in either changes the ratio significantly. Humid air is more dense, hot dry air less so, but not so much as aerodynnamics is concerned. I'm with the first guy, the heat and pressire of the Nats  competition had you trying too hard and impacting your perceptions
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Offline Garf

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Re: Air Density
« Reply #7 on: July 12, 2011, 09:47:27 PM »
I looked up the field altitude of 2 airports in Muncie. Their elevation is between 937' and 991'. Compare that to Miami at about 6'. I remember putting up a flight in Miami at Masters Field right after a rain shower. That Johnson never ran sweeter. Wether it was the drop in temp or the increase in humidity, I don't know.

Offline Howard Rush

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Re: Air Density
« Reply #8 on: July 12, 2011, 10:40:32 PM »
Density at the Nats got down to about 90% of sea level standard day (59 degrees F, 29.92 in. Hg).  Reynolds number was also down.  This is also a factor.  To turn the same corner, you needed a greater lift coefficient than at sea level because of density, but less was available than at sea level because of Reynolds number.
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Offline Howard Rush

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Re: Air Density
« Reply #9 on: July 12, 2011, 10:46:29 PM »
I looked up the field altitude of 2 airports in Muncie. Their elevation is between 937' and 991'. Compare that to Miami at about 6'.

Pressure on a given day and temperature also affect density.  Effective altitude at the Nats was 3000-some feet some of the time.
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Offline Howard Rush

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Re: Air Density
« Reply #10 on: July 12, 2011, 10:49:59 PM »
Humid air is more dense, hot dry air less so, but not so much as aerodynnamics is concerned.

Show me.  Lets see some calculation.  I don't think you can even spell aerodynamics.
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Offline wwwarbird

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Re: Air Density
« Reply #11 on: July 12, 2011, 11:06:23 PM »
As far as your stall tolerance is concerned, the differential in air density is miniscule. At most it is a difference of a couple hundred feet. Unless you are already at the ragged edge of wing loading, it is a non issue. I'm with the first guy, the heat and pressire of the Nats  competition had you trying too hard and impacting your perceptions

  LL~ LL~ LL~ Bull.

  Hot and/or humid conditions can make a huge difference. This is of course unless all of your airplanes simply fly like crap, then you may not notice it.
Narrowly averting disaster since 1964! 

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Offline Mike Scholtes

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Re: Air Density
« Reply #12 on: July 12, 2011, 11:28:59 PM »
Doesn't density decrease as humidity increases? If I recall my long ago pilot training, a parcel of humid air is less dense than a parcel of dry air at equal temp and pressure. If my recollection is right that would account for less lift and less reaction to the same control input at Muncie compared to lower, drier locations. I have only flown Free Flight at the Muncie Nats so have no experience with comparative performance of CL stunters. Here in dry and sea level coastal California humidity is never much of an issue.

Offline Howard Rush

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Re: Air Density
« Reply #13 on: July 13, 2011, 06:47:54 AM »
As I recall, a mole of any gas is the same size at as given temperature and pressure.  Is that right?  Matt should have learned that in chemistry class by now.  Molecular weight of dry air is about 29.  Molecular weight of water is 18, right?  So wet air is less dense than dry.  The air can't hold much water, though, particularly when it's cold.  You can maybe tell the difference between Phoenix and Houston, but not between Seattle and Bozeman.  I ciphered this awhile back and posted the numbers here.   
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Offline Matt Colan

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Re: Air Density
« Reply #14 on: July 13, 2011, 07:27:22 AM »
As I recall, a mole of any gas is the same size at as given temperature and pressure.  Is that right?  Matt should have learned that in chemistry class by now.  Molecular weight of dry air is about 29.  Molecular weight of water is 18, right?  So wet air is less dense than dry.  The air can't hold much water, though, particularly when it's cold.  You can maybe tell the difference between Phoenix and Houston, but not between Seattle and Bozeman.  I ciphered this awhile back and posted the numbers here.   

I learned absolutely nothing when I was in chemistry.   I had the absolute worst teacher on the PLANET!!!!!!!!!!!!!!!!!!!!!!  HB~>  I got kicked out of class because I hit my knee against the side of the desk and said ow.  That's how awful she was!
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Offline Will Hinton

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Re: Air Density
« Reply #15 on: July 13, 2011, 08:22:21 AM »
Every licensed pilot visiting this forum must be shaking their collective heads right now at some of the misconceptions of air density and density altitude!  Pay attention to Howard, for cryin' out loud!
I once had to abort a takeoff because the humidity had increased a SLIGHT amount between my morning flights and this late afternoon flight!  Failed to work the density altitude formula and paid the price.  85 gallons of liquid fertilizer was just too much by then.
Even though I didn't fly the nats this year, I saw MANY corners where the stunter simply sank during the turn.  Lift was the factor, not engine power.  Power doesn't produce lift, but I do agree that more nitro was the way to go simply for the power to remain where it needed to be and where pilots were used to having it to maintain the performance.
John 5:24   www.fcmodelers.com

Offline Jim Thomerson

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Re: Air Density
« Reply #16 on: July 13, 2011, 08:29:15 AM »
I fly near Austin at about 800 ft altitude.  Tucson is about 2500 ft.  The only difference is that the ground is about five feet higher at Tucson than at Austin.   LL~

John Leidle

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Re: Air Density
« Reply #17 on: July 13, 2011, 09:06:10 AM »
  ok Wil ,
  I beleive you, but what can I do to decrease the sink hole in Muncie I was caught off gaurd. Like I say Monday in Seattle I couldnt get my plane to stall.  I heard some guys say " increase the nitro content" .
                  John

Offline frank williams

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Re: Air Density
« Reply #18 on: July 13, 2011, 09:09:00 AM »
A meter like this read about 3400 ft. at Muncie in the afternoons.

Offline john e. holliday

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Re: Air Density
« Reply #19 on: July 13, 2011, 09:32:45 AM »
Now I know why I like grass.  Did anyone take the temp over the asphalt surface??  To make up for loss of lift for air density you need to fly faster to get the lift back.  Some guys don't realize how much better a plane will fly in the winter time than the summer time.  If you guys had taken your planes over on the grass pad you have noticed a difference, I beleive.  As I keep saying and no one listens, "How your plane flies at home will be completely different in another part of this great country of ours". H^^
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Offline Will Hinton

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Re: Air Density
« Reply #20 on: July 13, 2011, 09:39:23 AM »
Hi John,
It was good to see you at Muncie, sink hole or no.  The main defense against the sink hole is to soften things up and start the turn earlier to allow the thinner air to maintain cohesion with the surface.  This is where the weight issue becomes compounded, and the lighter ships perform noticeably better.  While less power will compound things because the ship doesn't drive through the corner, the weight issue is the biggie even for lighter ships.  We need to also remember that the prop does not perform as well either, thinner air, less thrust and thrust overcomes drag.
Here's where the relationships between thrust and drag and lift and weight become so important.  When lift is produced, an equal amount of drag is created.  More drag created, more thrust needed. and so the chain of events continues and we see frustration rear its ugly head with this year's higher density altitude.  I guess this is why stunt is such a compelling event - there aren't any easy and pat solutions, it takes several approaches to the differing conditions.  So, before this becomes a book, soften it up and start the turn earlier.
John 5:24   www.fcmodelers.com

John Leidle

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Re: Air Density
« Reply #21 on: July 13, 2011, 09:46:04 AM »
  Hi Will,
  I softened it up to a point where my squares looked like rounds.... I guess I brought a porker to Muncie & paid the price. 
  John

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Re: Air Density
« Reply #22 on: July 13, 2011, 12:35:24 PM »
Good try, then John, you knew what to do, the ship just refused to cooperate.  If I had flown this year with the ship I refinished, thee & me would have had similar shapes on the squares!!  By the way, I wasn't referring to any of your comments in my first post here, you've had spot on thoughts.
John 5:24   www.fcmodelers.com

Offline frank williams

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Re: Air Density
« Reply #23 on: July 13, 2011, 06:51:45 PM »
On Wed. afternoon, I measured 135 deg F on the blacktop with my little pocket IR meter.

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Re: Air Density
« Reply #24 on: July 13, 2011, 07:44:29 PM »
Even though I didn't fly the nats this year, I saw MANY corners where the stunter simply sank during the turn.  Lift was the factor, not engine power.  Power doesn't produce lift, but I do agree that more nitro was the way to go simply for the power to remain where it needed to be and where pilots were used to having it to maintain the performance.

     I think you overlook the advantages of additional engine performance preventing the speed reduction in the corners. Power *does* produce lift, to first approximation because it can better handle the induced drag. That's why a 15 oz/square foot airplane with an ST46 will barely get through round loops and will stall at the drop of a har, and the same airplane with a PA75 can win the 2011 NATs. Even at 3800 feet density altitude.

     The availability of almost absurd amounts of propulsion performance has tremendously improved the cornering performance of modern airplanes, and greatly enhanced the ability to handle these low-air-density situations.

    Brett

Offline Russell Shaffer

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Re: Air Density
« Reply #25 on: July 13, 2011, 08:29:36 PM »
Frank, where did you get that meter?  Did you order direct from them or through someone else, and about how much was it?  There is an address and phone number in Prescott, but I didn't see a web site. 
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Offline frank williams

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Re: Air Density
« Reply #26 on: July 13, 2011, 09:02:14 PM »
A must have toy.  The only comoplaint is that the temperature sensor is slow to stabilize and the instrument shouldn't be left in the direct sun.  Other than that, its a great tool for guessing about how much nitro to add.
http://performancetuner.net/index.html

Offline Steve Helmick

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Re: Air Density
« Reply #27 on: July 13, 2011, 09:17:18 PM »
I did an online search and dug around some, and found this interesting tidbit. If you bought one in 2005, seems it might be giving bad information. I still haven't found out how much $ they are. Probably not cheap enuf for me.  :-[ Steve   http://www.ekartingnews.com/news_info.php?n=6493

More research found a local Kart shop that had a different unit. http://www.mcssl.com/store/kart-o-ramainc/catalog/category/5073532

Also, the more deluxe wind meters will apparently give Altitude Density.   
http://www.weathershack.com/kestrel/3500.html
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Offline Garf

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Re: Air Density
« Reply #28 on: July 13, 2011, 09:22:42 PM »
I guess this answers why an airplane that had never given me any trouble in Miami, or in practice over the grass in Muncie, simply fell out of the overheads on the L pad.

Offline Howard Rush

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Re: Air Density
« Reply #29 on: July 13, 2011, 09:29:47 PM »
I wonder if the newly black-painted L-pad is hotter at five feet than it was before.
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Offline Garf

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Re: Air Density
« Reply #30 on: July 13, 2011, 09:36:51 PM »
Whose idea was it to paint the L pad black in July anyway? We need to stake them out on one of the circles.

Offline Larry Cunningham

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Re: Air Density
« Reply #31 on: July 13, 2011, 09:44:22 PM »
Wow, don't try to fly in Las Cruces or Albuquerque let alone Los Alamos!

I've been wondering when we'd see someone twirling a wet/dry bulb thermometer to calculate humidity and air density at one of the contests.. (there are easier ways nowadays). I remember flying a Cessna 152 out of Alamogordo on a hot dry day when the density altitude was over 8500'.. The trick was to lean the engine ASAP for some small extra power just as soon as the nose lifted, then exploit the ground effect and hot air rising from the runway. And of course, go hunt a thermal after you're no longer over the runway!

I know the effects for model airplanes; when we would go to Tucson from Las Cruces, where the altitude was about half of here, even when there was "Arizona dry" heat, we often referred to it as flying in "honey". (It never kept me from crashing a few times..)

L.

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John Leidle

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Re: Air Density
« Reply #32 on: July 13, 2011, 11:18:04 PM »
  I enjoyed having lunch with you on Thursday Frank. And thank you for telling me of the air density theory.
  John

Offline David Hoover

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Re: Air Density
« Reply #33 on: July 14, 2011, 04:52:49 AM »
Another instrument that will give you the density altitude without doing any calculations is the Kestrel 4000 and higher numbered weather meters. 

http://www.kestrelmeters.com/products/kestrel-4000-weather-meter

The only complaint I have with mine is that it eats the little AAA batteries pretty quickly.  Hungry little beastie!

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Offline Paul Smith

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Re: Air Density
« Reply #34 on: July 14, 2011, 05:17:28 AM »
After years of complaining about the wind at Muncie, for once the wind was calm and now, all of a sudden the problem is elevation.  Heat?  It's summer. July is hot everywhere.

Do you believe the ocean beach and Death Valley are normal and the whole rest of the world is a problem?  Carmichaels PA is some like 1100 to1300 above sea level.  I haven't heard a complaint yet.  The condition  of Muncie, wind or no wind, seems to make the same five guys win all time.  Are they complaining?
« Last Edit: July 14, 2011, 07:35:27 AM by Paul Smith »
Paul Smith

Offline Will Hinton

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Re: Air Density
« Reply #35 on: July 14, 2011, 07:04:04 AM »
"The availability of almost absurd amounts of propulsion performance has tremendously improved the cornering performance of modern airplanes, and greatly enhanced the ability to handle these low-air-density situations."

    Brett

Brett, your point is well taken, the added power does help drive through the corners, but when you say power produces lift, it's because the speed is maintained thus causing the wing to continue producing the same lift as before the radical turn is it not?  The lessor HP can't overcome the added drag produced with the added lift whereas the monsters, like our 75's, can.  (Or many of our 65's and 61's)  We are truly, as Hunt said once, in the "Good old days" of stunt.

Paul,
I don't see anyone complaining here, they're just trying to find answers on how to find better performance under different conditions..
« Last Edit: July 14, 2011, 07:37:29 AM by Will Hinton »
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Offline WhittleN

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Re: Air Density
« Reply #36 on: July 14, 2011, 07:36:26 AM »
John
If you ever feel the need to evaluate your airplane under "stressed conditions" come visit us at Salt Lake City on a 95 deg day.  Mike Haverly and Pete Peterson stopped by the field here in Layton UT last year after the NATS - what an experience - I think was the comment.  Like most of the explanations above point out you can trim around it.  Horsepower is a superior trimming tool.  The ability to trim for varying conditions separates good pilots from National Champions. 

Regards
Norm

Offline Douglas Ames

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Re: Air Density
« Reply #37 on: July 14, 2011, 09:45:24 AM »
http://wahiduddin.net/calc/density_altitude.htm

...or buy a Density altitude calculator.
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Offline Bob Reeves

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Re: Air Density
« Reply #38 on: July 14, 2011, 11:06:25 AM »
Just put the numbers in for Tulsa, result was 4081ft. Probably pretty typical for a summer day. Just for grins I calculated it for Carmichaels, PA, result was 2685ft. My 4 strokes can go from Tulsa to Carmichaels with just a slight needle tweak but it helps me understand why the airplanes seem to fly better.

Offline Peter Nevai

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Re: Air Density
« Reply #39 on: July 14, 2011, 11:55:17 AM »
Some people on this site just have to get some continuing education.

My last post specified that

"Humid air is more dense, Hot dry air less so"

I should have termed it as follows

A Humid atmosphere is more dense, a Hot dry atmosphere less so

It must be Noted that we actually operate in an atmosphere, that is composed of more than just air (@ 70% Nitrogen, 27% oxygen, 3% other gasses)

Hot air can hold far more water vapor than cold air.

Just why do you think your glasses fog up when you come in from a hot day outside to a airconditioned room?
Why does your AC unit drip water?
Why do your AC lines in your car form frost or for that matter why does frost accumulate in your freezer?

Basic thermodynamics, it is also the basis of every refrigerator, and freezer on the planet.

Pressure also effects the equation.

Compres or increase the pressure of a gas and you heat it up enabling the gas to absorb and evaporate more water (gasses are compressable water is not) Release pressure and the gas cools and water condenses out of it's evaporated state. Why do you suppose you have that water drain spigot on your paint air compressors, Why do manufacturers bother with making water traps?

Humid atmosphere is more dense because the water vapor content is far denser than the Air that contains it. Why do you think the air feels often times heavy on a Hot Humid day?

For those needing further education it works as below.

The dewpoint temperature is most commonly observed in ambient air and is also called the saturation temperature of water vapor in air. If you lower the temperature, dew will form as fog or condense on a cooler surface faster than it evaporates. This phenomenon is observed, for instance in very moist air when the dew appears as fine water droplets suspended in air (fog) and on cool beverage containers in hot weather conditions.

Conversely, if the temperature increases above the saturation temperature, dew will evaporate faster than it condenses. Fog in air and dew on surfaces will disappear under such conditions. This is commonly seen when dew on the ground vanishes as the air warms during the day after a cool night.

The dewpoint temperature depends on the air temperature, since hotter air can hold more water vapor per unit volume than can colder air.

Likewise, the value of the dewpoint at a given air temperature is also a function of air pressure. Air in Denver Colorado at 72°F can not hold as much water vapor as in Philadelphia, Pennsylvania at 72°F simply because the air pressure is less (due to altitude effects) in Denver. Denver isn't called the Mile-High City without reason and Philadelphia sits nearly at sea level.

Now that we have that out of the way, lets consider flying off of a paved black surface with air temps in the hi 80's or 90's. The surface temp of the blacktop must be very hot, perhaps over 150 degrees. That hot surface heats the air above it, that air rises (because hot air is lighter than cooler air) Cooler surrounding air rushes in to fill the void left by the rising hot air. Vortices's are formed, turbulence, down drafts caused by the cooler air coming in under the rising hot air.

This is how tornado's , hurricanes, and dust devils are formed. Any number of these factors can effect the flight characteristics of the models, are are much more pronounced and chaotic than the change of air density or pressure. Typical air pressure at sea level is @14.7 psi, it does not vary more than 1 or 2 psi due to atmospheric conditions (tornado's, Super Cells and Hurricanes Aside)
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Offline frank williams

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Re: Air Density
« Reply #40 on: July 14, 2011, 12:19:19 PM »
At a given pressure and temperature there are a fixed number of molecules in a given volume of gas, either dry or humid.  Dry air is, like you said ~70% nitrogen, ~27% oxygen, and ~3% other.   

Any water molecule, H2O (atm weight 1+1+8 = 10) must displace either a nitrogen or oxygen molecule, neglecting the 3% stuff. 

For nitrogen (N2) the weight is 7+7 = 14.  for oxygen (O2) the weight is 8+8 = 16.  So either a 14 or 16 weight molecule will be replaced by a measly 10 weight molecule in humid air. 

Therefore, humid air, at a given pressure and temperature is less dense.

Offline Will Hinton

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Re: Air Density
« Reply #41 on: July 14, 2011, 12:30:49 PM »
Good post, Frank.  Your information even goes along with all the things the FAA teaches us about air density - they must be right!   
Great to see you at the nats, by the way, sorry it was too bloody hot for me to visit much!  I hibernated under the tent most of the time.
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Offline Peter Nevai

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Re: Air Density
« Reply #42 on: July 14, 2011, 01:50:44 PM »
At a given pressure and temperature there are a fixed number of molecules in a given volume of gas, either dry or humid. Dry air is, like you said ~70% nitrogen, ~27% oxygen, and ~3% other.  

Any water molecule, H2O (atm weight 1+1+8 = 10) must displace either a nitrogen or oxygen molecule, neglecting the 3% stuff.  

For nitrogen (N2) the weight is 7+7 = 14.  for oxygen (O2) the weight is 8+8 = 16.  So either a 14 or 16 weight molecule will be replaced by a measly 10 weight molecule in humid air.  

Therefore, humid air, at a given pressure and temperature is less dense.

Water in the atmosphere is a vapor, not a gas, Water molecules in the atmosphere (Not I do not say AIR because it is a misnomer) are suspended they do not behave like mixed gasses where the Molecules of water displace gas molecules.

And even if it did then where do those displaced gas molecules go? do they vanish?

perhaps you are forgetting Daltons Law of Gas partial pressures, that state that the pressure exerted is the sum total of the partial pressures of the constituent gasses. Water is a compound that can behave like a gas, liquid or solid, but it is never anything but water. Water is a bound compound of elements Hydrogen and oxygen. Air is a mixture of gasses not a compound the various gas atoms are not physically bound together. Oxygen is not bound to the Nitrogen, etc. both O2 and N are independent element gasses which can be separated form the air mix by simply cooling the air until the gas turns liquid. No chemical, or electrical process is required as they are both independent gaseous elements.

Air pressure is the sum of the partial pressures of the oxygen, nitrogen and other inert elements. Since water is not a element nor a gas it is not figured into Gas Laws. So while vapor displaces air, the air does not go away but the heavier water replaces it. Because there is less air in the mix the density of the air VS water may be less but the atmosphere density increases.

As I noted before I should have termed it Atmosphere not AIR. But for all practical purposes we fly in an atmosphere not AIR. The thing that confuses the situation is the since water is not a elemental gas, not even when it is vaporized (It's still a compound of hydrogen and Oxygen) it is not calculated as % of humidity, independent of the AIR component. Instruments are designed to measure one or the other separately, but in real world terms they must be taken as a whole.

Just like Salt water is heavier than fresh water because of all the dissolved minerals in it. Moisture laden atmosphere is typically heavier (note not only the air it's self) is heavier because of the water dissolved in it.

If you take the items individually the above answer is correct, but since we live under several miles of atmosphere you can't just ignore certain parts of the soup.
« Last Edit: July 14, 2011, 02:23:44 PM by Peter Nevai »
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Offline Garf

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Re: Air Density
« Reply #43 on: July 14, 2011, 02:08:38 PM »
Try this: L pad, freshly painted black, 95* sunny. Here comes a thunderstorm. Rain cools the L pad and air above the pad. Humidity rises. What direction has the air density gone?

Offline dale gleason

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Re: Air Density
« Reply #44 on: July 14, 2011, 02:28:17 PM »
I think I'll side with the Boeing guys, NASA, and Lockheed guys on this one. Maybe a few airline guys, too. Also there was a guy named Koch, his name appears frequently in runway length vs. density altitude charts.

Respectfully,
dg

Offline Peter Nevai

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Re: Air Density
« Reply #45 on: July 14, 2011, 02:31:41 PM »
Try this: L pad, freshly painted black, 95* sunny. Here comes a thunderstorm. Rain cools the L pad and air above the pad. Humidity rises. What direction has the air density gone?

Atmospheric density has decreased from the 100% humidity levels of the rain. Rain, liquid water displaces the Air. So while the atmosphere becomes more dense, the AIR component is less per volume.

Water then is evaporating off of the surface, and moisture content climbs again. The ratio of AIR to water decreases per volume so again if you are measuring the Air component alone the Qty of Air is less, while the density of the atmosphere is increased.

For example the atmosphere of Venus is considered very dense because of all the stuff mixed up in it, and evidenced by the heavy continuous cloud layer. Various elements within that soup will have varying densities. Anything traveling through that atmosphere has to take into account all of the constituent elements.

Just like as a kid when you would try to run between the rain drops it never quite made it totally dry.
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Offline Paul Smith

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Re: Air Density
« Reply #46 on: July 14, 2011, 02:43:37 PM »
Piston engines, turbine engines, airfoils, and human beings react to weather in different ways.  On the other hand, rockets and electric motors don't know or care about the weather.

A day in July in the United States of America with 900' elevation, 90 degrees and 90% humidity is just an average day and nothing to be concerned about.  
« Last Edit: July 14, 2011, 03:38:04 PM by Paul Smith »
Paul Smith

Offline Peter Nevai

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Re: Air Density
« Reply #47 on: July 14, 2011, 03:09:45 PM »
I think I'll side with the Boeing guys, NASA, and Lockheed guys on this one. Maybe a few airline guys, too. Also there was a guy named Koch, his name appears frequently in runway length vs. density altitude charts.

Respectfully,
dg

When it applies to piloting full size aircraft, especially when a wrong compensation for barometric pressure can cause the apparent altitude of runways to differ from the measured heights above sea level, the values and calculations are far more critical and are corrected for a number of variables. As far as your altimeter is concerned you plug in the correction and away you go. It's all done by a calculator, chart or software now. Plug and play. Of all the items that require such calculation having your instruments calibrated to account for atmospheric conditions that is the most important so you don't accidentally fly into the ground during limited visibility conditions, or report the wrong altitude and put your self into a collision condition.

In our application, and that is where this whole topic started, is model airplanes. These effects are trivial, Unless and let me stress! Unless you are at the edge of the envelope of the wing design. (wing loading, drag, etc.) There are elements that factor much more, and effect flight performance than differing air density changes on a Hot Day vs Cool.

If your plane flies crappy in Hot lesser density air, Some thing is wrong with your model and not the air. I bet if you compared your flight against someone who had a model with a light wing loading they would not be having such problems. Garf and I both fly mainly in Florida. HOT HOT, and most times very humid. My old Vector 40 weighed in at 39.5 ozs powered by a LA40.

That plane turned like the edge of a razor blade even when the air temp was 95 degrees +. Engine runs in those temps and humidity were a challenge to get right and some of my engines at the time would not hot start after a flight in those conditions without a significant cool down period. Also had a OS 40 FSR start without even having the battery connected, I was turning it over by hand to get it primed and all of a sudden the dang thing fired up.

Engine runs, power, rpm's torque are impacted far more from atmospheric temperature, density and humidity levels. The window of the fuel / air mixture ratio is much much tighter. So small changes in temperature, density, humidity play a much more significant role. Especially since we are still using the most primitive fuel delivery and atomization system ever devised.

This was a fun and enlightening discussion though and it's great when everyone contributes something, certainly made me think and go back and do some research.

I knew I had a pretty good grasp of it as it could be a matter of life or death when going SCUBA. In diving you live and die by atmospheres of pressure, the partial pressures of gasses, Boyles law, the solubility of differing gasses in body tissues and blood stream. Effects of super saturation and and a ton of the very stuff that put you to sleep in college or high school.

Go under water and you realize the importance of all those things that you thought you would never ever use.

Did you know that Pure Oxygen becomes toxic at depths of 30ft?
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Offline Peter Nevai

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Re: Air Density
« Reply #48 on: July 14, 2011, 03:10:44 PM »
Piston engine, turbine engines, airfoils, and human beings reaction weather in different ways.  On the other hand, rockets and electric motors don't know or care about the weather.

A day in July in the United States of America with 900' elevation, 90 degrees and 90% humidity is just an average day and nothing to be concerned about. 

 y1 y1 Exactly
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Offline Will Hinton

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Re: Air Density
« Reply #49 on: July 14, 2011, 03:39:12 PM »
Apparently, Peter, you've never tried to take off in an ag plane with the same load you were using in the dryer air and discovered that "Oh crap, I forgot the humidity has risen!  I ain't gonna make it!"  I think you need to read the FAA directives and those of the meteorologists concerning cool air verses hot, humid air.

Paul, do you mean that the stunters were performing the same way at the nats as they did in April's cooler air?  Then, apparently, we don't need to ad nitro, soften the corners, or anything else, we just fly the same as always?  Even if it's someone who's never flown in those conditions?  They don't need to learn anything about compensating for conditions?  Hmm.
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