PPRuNe Forums


Tech Log The very best in practical technical discussion on the web

Reply
 
Thread Tools Search this Thread
Old 3rd Sep 2017, 03:50   #1 (permalink)
Thread Starter
 
Join Date: Oct 2003
Location: Canberra Australia
Posts: 1,304
Increasig Atmospheric Pressure

Has anyone calculated the extent of the rise in average sea level atmospheric pressure caused by the combined weight of all aircraft flying?
Milt is offline   Reply With Quote
Old 3rd Sep 2017, 05:40   #2 (permalink)
 
Join Date: Jan 2008
Location: Denver
Posts: 686
I'm not sure aircraft weight in flight increases atmospheric pressure (weight) on the ground. But using the analogy of the weight of people in a building increasing the pressure the total building + people puts on the ground...

1) Only aircraft in flight at a given instant count. That would be on the order of 10000 aircraft at any one instant. Could range from 8000 to 12000 depending on time of day (and where "day" is at any instant - U.S.? EU/Africa? Asia?).

2) Weight of those aircraft - C150 to A380? Let's take the 739ER as a reasonable average: MTOW = 71350 kg

x 10000 = 713500000 kg or 7.135 x 10ˆ8 kg

3) Weight of the atmosphere = 5.1 x 10ˆ18 kg

4) aircraft weight adds a bit over one 10-billionth (or ten 1000-millionths) to the total.

5) In millibars, average atmospheric pressure with no planes flying = 1013.25 millibars

- with planes included: 1013.25000017418 millibars

A microscopic difference, in other words....

(Assumptions and math open to criticism).

Last edited by pattern_is_full; 3rd Sep 2017 at 12:36. Reason: spelling
pattern_is_full is offline   Reply With Quote
Old 3rd Sep 2017, 16:37   #3 (permalink)

PPRuNe Person
 
Join Date: Jun 2001
Location: see roster
Posts: 1,242
I'll pass this one on to my sister-in-law who interviews physics candidates for a well-known university.

Thinks: surely the atmosphere just expands a bit, there is no lid on it, after all?
overstress is offline   Reply With Quote
Old 3rd Sep 2017, 16:46   #4 (permalink)
 
Join Date: Feb 2009
Location: Virginia
Posts: 248
I'm pretty sure the atmosphere's already big enough to hold the aircraft when it's sitting at the gate.
Chu Chu is offline   Reply With Quote
Old 3rd Sep 2017, 16:56   #5 (permalink)
 
Join Date: Nov 2015
Location: Farnham, Surrey
Posts: 708
Mean density rises, but I'm not sure that pressure does = I'll have to think about it!
PDR1 is offline   Reply With Quote
Old 3rd Sep 2017, 16:59   #6 (permalink)
 
Join Date: Dec 2001
Location: FL, USA
Posts: 1,920
Why would an aircraft add to the weight of the atmosphere as it's developing lift and and not actually resting or sliding along an area of air excerting pressure on it?

How much do mid to high altitude clouds add to the pressure on the ground?
B2N2 is online now   Reply With Quote
Old 3rd Sep 2017, 17:00   #7 (permalink)

PPRuNe Person
 
Join Date: Jun 2001
Location: see roster
Posts: 1,242
Quote:
I'm pretty sure the atmosphere's already big enough to hold the aircraft when it's sitting at the gate.
It is, but it's not supporting it at that point.

edit: just thought about that again, I've written rubbish. I think.
overstress is offline   Reply With Quote
Old 3rd Sep 2017, 17:35   #8 (permalink)
 
Join Date: Sep 2016
Location: USA
Posts: 97
Quote:
Originally Posted by B2N2 View Post
Why would an aircraft add to the weight of the atmosphere as it's developing lift and and not actually resting or sliding along an area of air excerting pressure on it?
As the aircraft develops lift, it exerts a force downward on the airmass equal to its weight. So it is "actually resting or sliding along an area of air excerting pressure on it."

But the atmosphere under the aircraft is not sealed on the side, so it escapes sideward and upward until it reaches equilibrium.
Vessbot is offline   Reply With Quote
Old 4th Sep 2017, 00:04   #9 (permalink)
 
Join Date: Jan 2008
Location: Denver
Posts: 686
Some points:

- If the atmosphere expands in radius, the air column above any given square meter or centimeter of the earth's surface gets taller and thus weighs more = higher pressure at sea level.

- for aircraft not flying, their weight is supported directly by the ground - and would be, even in a vacuum. Not a factor in this thought-experiment about air pressure.

- clouds are equivalent to lighter-than-air balloons. Water vapor and humid air are less dense than dry air. So they float, most of the time.

http://images.slideplayer.com/14/441...s/slide_15.jpg

- don't confuse "volume" with "weight." An unpressurized aircraft is just a tube with equal atmospheric pressure/density inside and outside the thin skin. A pressurized aircraft has higher air density inside, and thus a slightly higher weight - but I left that out as being an even less significant effect than the total aircraft weight(s), which are already insignificant.

The atmosphere is really big - all the aircraft flying are just bacteria by comparison:

https://pix-media.priceonomics-media...ash_plane.jpeg

- I'd like to hear from overstress's sister-in-law, too.
pattern_is_full is offline   Reply With Quote
Old 4th Sep 2017, 05:11   #10 (permalink)
 
Join Date: Mar 2005
Location: N/A
Posts: 1,679
This is an extension of the question if a cargo load of birds take to the wing would a flying aircraft weigh less, or no change.

Quote:
In this month's edition of NTVN, the Physics Journal for members of the Dutch Society of Physicists, atmospheric scientist Peter Siegmund and geophysicist Laslo Evers report on such measurements. Using an array of microbarometers laid out at surface, they obtained transient pressure readings from flying airplanes in line with the Kutta-Zhukowsky description of flight. They managed not only to detect airplanes and determine their position, the measurements even allowed them to weigh the airplanes in full flight
How Airplanes Fly - The Real Story (With Experimental Verification!) | Science 2.0

You could request a copy of the scientific paper here, probably cost.

https://www.nnv.nl/en/english-home/
megan is offline   Reply With Quote
Old 4th Sep 2017, 07:40   #11 (permalink)
 
Join Date: Jun 2006
Location: Brisbane
Posts: 610
What a fascinating question.

I like to think about it this way. The atmosphere exerts a certain force on the surface of the earth. That force is caused by gravity pulling on the mass of the atmosphere. This is known as the weight of the atmosphere. It is this weight that causes sea-level atmospheric pressure to be what it is.

If you picture an aircraft sitting on the ground, the total weight of the atmosphere plus the aircraft will be the same as the total weight of the atmosphere plus the aircraft if the aircraft is airborne. However, in the airborne case, the atmosphere must transfer the weight of the aircraft to the surface rather than the aircraft's wheels. The only way the atmosphere can do this is to increase its MSL pressure slightly.

The bit where I get stuck is that the atmosphere doesn't have a "lid". If you kept pumping air into the atmosphere (from a hypothetical worm-hole), I don't think it would increase the atmospheric sea-level pressure. The excess air just escapes out the top into space.

It that is true, does getting an aircraft airborne actually increase MSL pressure, or does it just displace some air out the top into space?
Derfred is offline   Reply With Quote
Old 4th Sep 2017, 08:08   #12 (permalink)
 
Join Date: Jan 2014
Location: N5109.2W10.5
Posts: 444
Quote:
The bit where I get stuck is that the atmosphere doesn't have a "lid"
True. But the atmosphere remains attached to the planet due to gravity. If your "worm hole" supplied sufficient air to double the mass of atmosphere here already, then the sea level pressure would double.

In old money, sea level pressure of 15 lbs per square inch is due to a column of air weighing 15 lbs over every square inch of the earth.
Goldenrivett is offline   Reply With Quote
Old 4th Sep 2017, 11:25   #13 (permalink)
 
Join Date: Jun 2006
Location: Brisbane
Posts: 610
GR, you are correct. I was mistakenly under the belief that Earths atmospheric volume was limited by the escape velocity of atmospheric molecules. In fact, on Earth, this only happens to Helium and Hydrogen. Earth is capable of supporting a much larger and denser atmosphere. I have just learned something.

So yes, I believe MSL pressure is increased proportionally by the weight of all airborne aircraft (and birds)

As an aside, I believe the water pressure at the bottom of the ocean also increases for every floating vessel, but in this case it is not proportional to the weight of the vessel, because the ocean walls are not vertical.
Derfred is offline   Reply With Quote
Old 4th Sep 2017, 12:05   #14 (permalink)
 
Join Date: Feb 2015
Location: North Up
Posts: 305
Doesn't it depend whether the conveyor belt is in service?

Doesn't it also depend whether a lorry full of budgerigars startles the birds into flight and off their load-bearing perches?

We need to know these things. We also need to let the kiddies have the wanted answer in the ATPL question bank of their preferred crammer for slow learners.
Cazalet33 is online now   Reply With Quote
Old 4th Sep 2017, 12:09   #15 (permalink)
 
Join Date: Feb 2009
Location: Virginia
Posts: 248
As an extension of the nautical analogy, imagine a toy submarine at neutral buoyancy circling, submerged, in a bucket of water on a scale. Now replace some of the air inside the submarine with lead, but set the dive planes so it continues to circle at the same level.

The reading on the scale has to increase by the amount of lead added, but the water level in the bucket won't change. The answer seems to be that the hydrodynamic effects of the dive planes increase the dynamic pressure at the bottom of the bucket. I'd guess that the same is true for an aircraft flying in the atmosphere, even though the effect would be too small to measure.

I assume the effect would be localized as well -- after all, atmospheric pressure isn't the same everywhere to start with. Imagine a helicopter hovering above a set of truck scales.
Chu Chu is offline   Reply With Quote
Old 4th Sep 2017, 14:36   #16 (permalink)
 
Join Date: Feb 2013
Location: 60 north
Age: 53
Posts: 203
Rain

A storm drops 100cm of rain over 1000 square km .
No NET water vapor is added or subtracted anywhere else, for arguments sake.:
Pressure must drop, due to mass of atmosphere ( rain ) resting on ground.
BluSdUp is offline   Reply With Quote
Old 12th Sep 2017, 17:14   #17 (permalink)
 
Join Date: Aug 2005
Location: Estonia
Posts: 840
Quote:
Originally Posted by Derfred View Post
What a fascinating question.

I like to think about it this way. The atmosphere exerts a certain force on the surface of the earth. That force is caused by gravity pulling on the mass of the atmosphere. This is known as the weight of the atmosphere. It is this weight that causes sea-level atmospheric pressure to be what it is.
Not exactly.
Consider a water bottle with broad bottom and tall neck, which is full of water.
The water pressure on the bottom is high, and over the wide bottom sums out to a large force. Yet the weight of the water column in the tall but narrow neck does not sum up to so much.
The matter is that the water also exerts pressure on the upper part of the bottle, below the neck. That force is applied down to the water, and transferred to the bottom.
Likewise, if there is something pressing on air from above, such as a landed plane taking up volume from atmosphere but resting on wheels, that´s extra air pressure which need not be weight of air.
chornedsnorkack is offline   Reply With Quote
Old 13th Sep 2017, 03:21   #18 (permalink)
 
Join Date: Jun 2006
Location: Brisbane
Posts: 610
I don't know what you are saying with your water bottle, but if you are suggesting that the landed aircraft adds to atmospheric pressure as does an airborne aircraft, then no, I disagree.
Derfred is offline   Reply With Quote
Old 13th Sep 2017, 15:16   #19 (permalink)
 
Join Date: Aug 2005
Location: Estonia
Posts: 840
Adds, but not equally.
Take a cylindrical vessel of water. Drop a stone in it.
Water level will rise due to the volume displaced by stone.
The pressure of the stone on the tiny contact points to the bottom will be the weight of the stone minus the weight of water displaced by the stone.
The pressure of the water on the bottom is the weight of the water actually in the vessel plus the weight of the water displaced by the stone.

Ditto about Earth atmosphere.
Reenter a shuttle into atmosphere so that it is no longer supported by centrifugal force but by aerodynamic force, and the pressure of air will be weight of atmosphere plus weight of shuttle.
Land the shuttle, and the pressure of air will be weight of atmosphere plus weight of air displaced by shuttle.
chornedsnorkack is offline   Reply With Quote
Old 13th Sep 2017, 17:30   #20 (permalink)
 
Join Date: Jan 2008
Location: Denver
Posts: 686
But no aircraft except spacecraft "enter" the atmosphere - they start out already in the atmosphere, from the moment they start to be assembled in the factory.

...or are you saying that the moment Boeing or Airbus finally seal every new fuselage with the last rivet, the world's air pressure jumps a tiny notch?

And spacecraft of earthly origin must reduce atmospheric pressure slightly when the leave the atmosphere to begine with, so when they reneter, they are simply restoring the original conditions.
pattern_is_full is offline   Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT. The time now is 22:30.


© 1996-2012 The Professional Pilots Rumour Network

SEO by vBSEO 3.6.1