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Author Topic: Air Density  (Read 22427 times)

Offline BillLee

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Re: Air Density
« Reply #50 on: July 14, 2011, 03:47:17 PM »
Talk about STUNT PHYSICS, this is "Physics beyond imagination"!   LL~ LL~
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Offline Randy Cuberly

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Re: Air Density
« Reply #51 on: July 14, 2011, 06:03:25 PM »
y1 y1 Exactly

Peter and Paul,
Come fly in Tucson in August after a rain the previous day when the temp is 108+ and the relative humidity is 50% and the Density Altitude reaches 7,000+ ft.
That will give you a lesson in Thermo dynamics anddensity altitude...
You're all wet if you think it's all the same everywhere!

Randy Cuberly!

PS:  Peter, nice try but I'm afraid your "Physics" is a little short on the reality of gas theory and aerodynamics!
       It just ain't so!
Randy Cuberly
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Offline Will Hinton

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Re: Air Density
« Reply #52 on: July 14, 2011, 06:31:34 PM »
Randy, you just said it better in very few words than many of us, me included, have with lots of words.  With what you & Frank have said, this should settle it.  (But I betcha it doesn't for them.)
John 5:24   www.fcmodelers.com

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Re: Air Density
« Reply #53 on: July 14, 2011, 06:33:56 PM »
  Norm,
  Your point is well taken  thank you.
  john

Offline Jim Thomerson

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Re: Air Density
« Reply #54 on: July 14, 2011, 07:21:40 PM »
http://www.natmus.dk/cons/tp/atmcalc/atmoclc1.htm

Here is a lot more than you want to know about water in air.

Offline john e. holliday

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Re: Air Density
« Reply #55 on: July 15, 2011, 08:00:48 AM »
Let me tell you for sure you don't want to repaint the L-Pad white.   LL~ LL~ LL~
John E. "DOC" Holliday
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Online Brett Buck

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Re: Air Density
« Reply #56 on: July 15, 2011, 09:43:53 AM »
Let me tell you for sure you don't want to repaint the L-Pad white.   LL~ LL~ LL~

    I think just about everyone would be on board with that plan. The black is very difficult to see, particularly early in the morning at a low Sun angle.

    Brett

Offline Steve Fitton

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Re: Air Density
« Reply #57 on: July 15, 2011, 12:15:41 PM »
Having flown enough on the L Pad last week, I think the biggest issue, easily trumping density altitude, would have been the wind, or lack thereof most of the time.  Getting steady air in the morning practice and officials was pretty much a matter of luck.  With even light winds from a steady direction my plane flew just fine, running the same % nitro as home (actually I could have gone down a bit on some mornings).  I had crisp corners, plenty of power, etc etc.  But just as quickly you could find yourself with zero wind, a (fill in the blank)° wind shift, or a nasty thermal and your plane would be complete junk.  It was the erratic light air that shot down Doug on top 20 day, and had Brett displaying impressive agility and speed as he backed up like mad on most of the maneuvers on a qualifying flight the day before.
  This isn't a corroboration of the phony physics from a few earlier posts, just my own small observation that performance hinged on much more than density altitude at Muncie this year.
Steve

Offline Paul Smith

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Re: Air Density
« Reply #58 on: July 15, 2011, 12:31:04 PM »
Google Earth issue:

I thought that I was able to get elevations off Google Earth in the past, but now I can't.  Are they still there or has the elevation be dropped from the current revision?

PS: The carrier and combat guys didn't have any complaints about Muncie this year, other than wanting a bit of breeze for personal comfort.

AMA was looking at a property in Hutchinson Kansas in the mid 1970's.  It was 400 feet higher (all the time) and 15 degrees hotter in July, plus the wind was generally over 20 MPH.  That being said, there was plenty of CL activity in the area.  The plan was scrapped when the F3A TT was scheduled there and cancelled due to wind.   
Paul Smith

Offline Jim Thomerson

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Re: Air Density
« Reply #59 on: July 15, 2011, 03:01:20 PM »
One reason grass is cooler than concrete or blacktop is guttation.  Guttation is a plant pumping water out of the ground and dripping it out the leaves.  Cut grass drips from the end of the cut blade. If you have ever laid anything down on grass and wondered why it got wet, guttation is to blame.  So a green grass circle has water continuously evaporating in the heat, thus cooling the area.   D>K

Offline Mark Scarborough

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Re: Air Density
« Reply #60 on: July 15, 2011, 04:04:49 PM »
Have you guys ever heard the expression "the tension in the air was so thick you could cut it with a knife"?  I wonder if all the tension of the flyers had anything to do with the air density. No, really. Honest. H^^ D>K LL~ LL~ LL~ LL~ LL~ LL~ LL~
Have you guys ever heard the expression "the tension in the air was so thick you could cut it with a knife"?  I wonder if all the tension of the flyers had anything to do with the air density. No, really. Honest. H^^ D>K LL~ LL~ LL~ LL~ LL~ LL~ LL~


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Ty's comment=PRICELESS
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Offline Garf

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Re: Air Density
« Reply #61 on: July 15, 2011, 07:49:46 PM »
Let me tell you for sure you don't want to repaint the L-Pad white.   LL~ LL~ LL~
Now, why would that be?

Offline frank williams

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Re: Air Density
« Reply #62 on: July 15, 2011, 09:32:53 PM »
Well I made a mistake in my description of the elements in moist air.

I remembered the atomic number of the elements and not the atomic weights ..... weights are N=14 O=16 H=1 .....

... so the N2 molecule weighs 28,    the O2 molecule weighs 32, and the water vapor molecule weighs 18 .....

But the result is still the same, due to Avagadro's law, a molecule of weight 18 will be displacing one of either 28 or 32 unit weight.

The result is still the same .... humid air at a given temperature and pressure is less dense than dry air at the same conditions.


Offline john e. holliday

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Re: Air Density
« Reply #63 on: July 16, 2011, 09:32:14 AM »
Now, why would that be?

Up here in short order you would call it snow blindness.   D>K
John E. "DOC" Holliday
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Offline Steve Helmick

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Re: Air Density
« Reply #64 on: July 16, 2011, 01:28:23 PM »
One thing that works well is to dust the asphalt surface with Portland Cement. Makes the asphalt a nice grey, not quite like concrete, but a lot better than fresh black asphalt. Our driveway in Pullman was done that way, in the late '50's. Too steep to do in concrete, but Mom didn't want asphalt, a good compromise. I heard that at least one of the circles in Christopher Columbus Park was treated this way, because the asphalt was defective and sticky like glue.   f~ Steve 
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Offline Howard Rush

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Re: Air Density
« Reply #65 on: July 17, 2011, 06:56:42 PM »
OK, I did some ciphering.  Not really. I used a couple of Web pages, http://wahiduddin.net/calc/calc_da_rh.htm and http://www.psychrometric-calculator.com/HumidAirWeb.aspx , and iterated a little.  I looked at four cases at sea level standard day (SLSD) pressure:

15% relative humidity, 59 degrees F
95% relative humidity, 59 degrees F
15% relative humidity, 100 degrees F
95% relative humidity, 100 degrees F

I calculated two things: the effect of humidity on density, expressed both as density ratio to standard day and the popular-but-not-too-useful density altitude, and the effect of humidity on the amount of oxygen in the air.  The former affects how planes fly; the latter affects how much nitro you need to add to your fuel.  I made up a new unit for the latter, "oxygen density altitude", defined as the standard, dry-air altitude that has the same amount of oxygen per unit volume.  The upchuck is below.  Going from 15% to 95% relative humidity at 59 degrees F doesn't do much, but at 100 degrees F it's worth 663 feet of density altitude and 2473 feet of "oxygen density altitude", which is nontrivial.  Please check my calculation.
« Last Edit: July 17, 2011, 07:31:10 PM by Howard Rush »
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Offline Tim Wescott

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Re: Air Density
« Reply #66 on: July 17, 2011, 07:54:57 PM »
Humid air is more dense
From Wikipedia: "Both an increase in temperature and, to a much lesser degree, humidity will cause an increase in density altitude."

http://en.wikipedia.org/wiki/Density_altitude.

Water vapor has a molar weight of around 18 (two hydrogens and an oxygen), air is pretty close to molecular nitrogen, at 28.  So water vapor is less dense.

But I'm just picking nits, because there's not all that much water vapor on a percentage wise basis, even at 100% humidity, unless the temperature is getting close to 212F.
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Offline Jim Thomerson

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Re: Air Density
« Reply #67 on: July 18, 2011, 01:39:43 PM »
I read someplace that the maximum amount of water in air is about 4%.

Online Brett Buck

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Re: Air Density
« Reply #68 on: July 18, 2011, 02:01:44 PM »
OK, I did some ciphering.  Not really. I used a couple of Web pages, http://wahiduddin.net/calc/calc_da_rh.htm and http://www.psychrometric-calculator.com/HumidAirWeb.aspx , and iterated a little.  I looked at four cases at sea level standard day (SLSD) pressure:

15% relative humidity, 59 degrees F
95% relative humidity, 59 degrees F
15% relative humidity, 100 degrees F
95% relative humidity, 100 degrees F

I calculated two things: the effect of humidity on density, expressed both as density ratio to standard day and the popular-but-not-too-useful density altitude, and the effect of humidity on the amount of oxygen in the air.  The former affects how planes fly; the latter affects how much nitro you need to add to your fuel.  I made up a new unit for the latter, "oxygen density altitude", defined as the standard, dry-air altitude that has the same amount of oxygen per unit volume.  The upchuck is below.  Going from 15% to 95% relative humidity at 59 degrees F doesn't do much, but at 100 degrees F it's worth 663 feet of density altitude and 2473 feet of "oxygen density altitude", which is nontrivial.  Please check my calculation.

    I calculated the wet density altitude for the conditions on Sunday (jul 3) for the NATs and found about 300 feet delta between the wet and dry density altitude. I had never considered the oxygen ratio, that actually explains, nicely, the results I got this year - didn't have to change my handle spacing but the engine was hurting.

    Brett

Offline Howard Rush

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Re: Air Density
« Reply #69 on: July 18, 2011, 03:28:19 PM »
My RO-Jett was happy there with 15% nitro.  I use 10% at home. 
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Online Brett Buck

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Re: Air Density
« Reply #70 on: July 18, 2011, 03:32:40 PM »
My RO-Jett was happy there with 15% nitro.  I use 10% at home. 

  I was running "new" 15 and it wasn't putting out like it had been in 2009.

    Brett

Offline PJ Rowland

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Re: Air Density
« Reply #71 on: July 18, 2011, 07:04:48 PM »
I found I had more power that I had back home.

Which really threw me out when I first arrived, I planned for less, but had more.

I think overall these Nats were a great test - the altitude and density proved difficult for many setups but the cream of the Top 5 rose to the top.
If you always put limit on everything you do, physical or anything else. It will spread into your work and into your life. There are no limits. There are only plateaus, and you must not stay there, you must go beyond them.” - Bruce Lee.

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Offline Russell Bond

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Re: Air Density
« Reply #72 on: July 18, 2011, 07:17:05 PM »
Hey Ty,
If you are in a Cessna 172 with full fuel and four passengers, you are way over weight and shouldn't be trying to take off.

That's the problem with entry level Cessnas and Pipers, they can't have full fuel and all seats filled..... ~^

I know, you were only using it as an example...just having fun.  :D

I remember flying a Piper Cherokee 6, 300hp with 6 people with luggage and 3/4 fuel. I took off from an outback airstrip where the temperature was 110f. I used all the runway and climbed out at about 100 ft/min. Very hairy!! :o
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Re: Air Density
« Reply #73 on: July 18, 2011, 08:27:28 PM »
 I  wonder if the sureface was painted white if the sun would reflect much like it does when snow sking?
  John

Offline BillLee

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Re: Air Density
« Reply #74 on: July 19, 2011, 06:08:07 AM »
I  wonder if the sureface was painted white if the sun would reflect much like it does when snow sking?
  John

You better believe it!

The circles at Scobee Field in Houston are continuous poured concrete (i.e., no joints). When they were new, and whenever the county pressure washes them back to original condition, they are like standing in the middle of a snow field. The reflected light is strong and you really have to protect against bad sunburn, just like you do when skiing.
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Offline frank williams

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Re: Air Density
« Reply #75 on: July 19, 2011, 10:08:01 AM »
I measured the temperature on the concrete at Scobee this last Sunday around noon at 105.  The noontime pavement at the NATS was 135.  Cooler, but you need to put sunblock under your chin.

Online Brett Buck

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Re: Air Density
« Reply #76 on: July 19, 2011, 10:44:59 AM »
I measured the temperature on the concrete at Scobee this last Sunday around noon at 105.  The noontime pavement at the NATS was 135.  Cooler, but you need to put sunblock under your chin.

   As an OT aside, after spending all week in the mid-90s/50% humidity of Muncie on the blacktop, I went to visit my parents in Hot Springs, AR. Where it was as high as 106 in the shade - right next to a huge lake. Heat index from my Dad's weather gadget = 118. I left town at 4:45 in the morning at it was in the upper 80s.

    Brett

Offline Steve Helmick

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Re: Air Density
« Reply #77 on: July 19, 2011, 01:32:13 PM »
  I was running "new" 15 and it wasn't putting out like it had been in 2009.

    Brett

You should have used Howard's home-brew fuel. Legend has it that Howard's 15% is more than your 15%.  LL~ 

If your fuel was made by weight instead of volume, that would be actual fact, but of course you know that.  PJ's bonus power might also be explained by the method by which the fuel components were measured for his brand in Ozzie and whatever brand he got in Muncie.  H^^ Steve
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Online Brett Buck

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Re: Air Density
« Reply #78 on: July 19, 2011, 04:46:18 PM »
You should have used Howard's home-brew fuel. Legend has it that Howard's 15% is more than your 15%.  LL~ 

If your fuel was made by weight instead of volume, that would be actual fact, but of course you know that.  PJ's bonus power might also be explained by the method by which the fuel components were measured for his brand in Ozzie and whatever brand he got in Muncie.  H^^ Steve

  The "new" 15% is supposed to really be 15% by volume.

    Brett

Offline Howard Rush

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Re: Air Density
« Reply #79 on: July 20, 2011, 09:35:59 AM »
I use a 100 ml volumetric flask to weigh fuel to check nitro content.  It is helpful for the many jugs of mystery combat fuel in my shed.  It is helpful that it all pretty much has 20% oil. 
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Offline Kestrel Meters

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Re: Air Density
« Reply #80 on: July 22, 2011, 09:06:22 AM »
Hi Guys, thanks for the mention of our Kestrel Meters, if you'd like to try one out, please use coupon code:

LoveStuntHanger

For 5% off any orders of $100 or more at http://www.kestrelmeters.com

Enjoy!

Offline Mark Scarborough

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Re: Air Density
« Reply #81 on: July 22, 2011, 10:54:24 AM »
Hi Guys, thanks for the mention of our Kestrel Meters, if you'd like to try one out, please use coupon code:

LoveStuntHanger

For 5% off any orders of $100 or more at http://www.kestrelmeters.com

Enjoy!
So which of these units do you reccomend for being the most versitile and functional in our useage?
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Offline skyshark58

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Re: Air Density
« Reply #82 on: July 22, 2011, 11:39:59 PM »
No one has mentioned propeller efficiency? Prop pitch is right next to engine performance on the list. It seems to me if the hot humid air was affecting performance you would want a bit more pitch, that is if you still had enough power left to make use of it.
mike potter

Offline Skip Chernoff

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Re: Air Density
« Reply #83 on: March 13, 2021, 06:32:06 AM »
From my Enduro Karting days....air density is always greatest early in the morning and later in the afternoon. So in the morning we richen the mixture and lean the mixture as the day goes on ,until later in the day when we would richen again.

Regarding flying a Brodaks. I found that I needed to carry less fuel than here in Philly or have an engine over run.

Offline Chuck_Smith

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Re: Air Density
« Reply #84 on: March 13, 2021, 07:34:44 AM »
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. 

Wow. Nothing to be concerned about? My experience is quite different. IAS is a function of the air density. That's why we fly in IAS but navigate in TAS and they can be significantly different.

BTW, electric motors do care about ambient temps as well. Magnet strength is a function of temperature.

Also, I can speak with authority that rockets operated in the atmosphere do indeed care about weather and ambient conditions. The thrust is dependent on the atmospheric back pressure and how it allows full expansion of the exhaust at the end of the bell.  That's why rocket engines used in space are different than those used in the atmosphere. Take the SpaceX Raptor for example. It has different configurations depending on the whether it gets used in vacuum or atmosphere.

And yes, I just realized that Skip had just responded to a post from almost 10 years ago, lol!

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Offline Martin Quartim

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Re: Air Density
« Reply #85 on: March 13, 2021, 06:44:39 PM »

One thing I never understood is why 4 Stroke Engines change very little, if any,  with Density Altitude.
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Online Lauri Malila

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Re: Air Density
« Reply #86 on: March 14, 2021, 06:28:44 AM »
One thing I never understood is why 4 Stroke Engines change very little, if any,  with Density Altitude.

At least Beringer's venturi has an adjusting screw for that. Choke area, that is.
And in general, it's better to be able to compensate with choke area than just needle setting as air gets warmer/thinner during the day. Or with nitro% if you wish. L

Offline Martin Quartim

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Re: Air Density
« Reply #87 on: March 14, 2021, 04:14:47 PM »
At least Beringer's venturi has an adjusting screw for that. Choke area, that is.
And in general, it's better to be able to compensate with choke area than just needle setting as air gets warmer/thinner during the day. Or with nitro% if you wish. L

Hi Lauri,
My venturi was not adjustable and I only used 20/20 fuel.   I flew at Brodaks and in Brazil in different cities from 500m to 1400m msl from winter to summer.  Always, 90ml, needle unchanged, 6:30 min flight,  always the same results.  Maybe my laps did vary a bit but it was not noticeable. Only in extreme conditions, I felt the need to change prop with more or less pitch.  In high D.A. and no wind then I used the same prop model but with more pitch, and when it was very windy I used a reworked prop with less pitch at the tips.

If I had your skills I would be making a compact and lite 4S  ;D

Martin
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Offline bob whitney

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Re: Air Density
« Reply #88 on: March 14, 2021, 06:32:36 PM »
 When flying Clown racing we aim for the most laps with a one once tank. after setting it up here in Florida.at Muncie we need to remove at least one head shim or it will quit the second you remove the battery no matter how much you worm it up, and most of our other race stuff runs better by removing a head shim .back in Fla we start blowing plugs unless we put the shim back
rad racer


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