Vapour....trails that is

[QuikSanz]

Hi,

Altitude and humidity are the big factors here.If cold enough in the low presure area, the vapor can freaze, the conditions must be just right. combination air temp, wet and dry bulb, which factors in humidity. also altitude and velocity have a role and also wing loading.

Chris

[JaXanim]

Ah, the wet and dry bulb thermometer. That sounds more like the proper physics to me. Anyone care to elaborate on this? I think the truth is coming together here.

JaX

[blobrana]

Yea, i remember seeing a vapour trail coming off a rear wing on a formula one car once...
(no-where near the engine exhaust)
This was clearly the vortex creating a low pressure in which moisture condensed out of the air..

(a bubble chamber in effect)...

[JaXanim]

@blobrana

Yes, that's the transient precipitation I talked about earlier. The car didn't leave a trail right around the track did it.  If the right conditions were to prevail, it might happen, but that's highly unlikely.

So, what is it about flying that's different from the F1 scenario?

JaX

[FluffyMcDeath]

JaXanim wrote:
@blobrana

Yes, that's the transient precipitation I talked about earlier. The car didn't leave a trail right around the track did it.  If the right conditions were to prevail, it might happen, but that's highly unlikely.

So, what is it about flying that's different from the F1 scenario?

JaX

On a cold day, my car leaves a vapour trail, even when it's moving slowly. The vapour tends to drift upwards though.

Now, that's just the water from burning petrol at the rate of approx. 3 ltr/hr.

A 747 (according to this has a mac fuel capacity of 173 tonnes, for a range of 13.000 km and a cruising speed 0of 930kph.

That's a burn rate of 12.4 tonnes of fuel per hour.

Aviation jet fuels are C8-C16 hydrocarbons. As a stab in the dark say that the average is round about C about 12 and H around 22.

That works out to be about 15 tonnes of water produced per hour. It doesn't seem unreasonable to suspect that if the upper atmosphere is cold and saturated, then that amount of condensate would be visible.

[QuikSanz]

Hi, Actually blobrana is right you don't need an engine. Living in southern California I have been privlaged to see the shuttle come in.when high up with the heavy wing loading it creates an extreame low pressure and if the humidity is not too high the vapor freazes and hangs for a while. Of course you all know when the shuttle returns it is a glider. Now if you were to watch it land at florida you will see the transient vapor trails that can't hang around to long. Humidity & temps too high.

Chris

[JaXanim]

I think blobrana is getting nearer the hub (nub?) of it. I need to do some calculations myself. We need numbers like the volume of water vapour ejected at upper atmosphere conditions to get a better picture of trail generation.

Yes Chris, the Shuttle trails are the transient type caused by wing tip/lifting body precipitation. I'd suggest that you *could* see airliner trails on the same day lasting significantly longer than the Shuttle's, eh?

Another clue. Aircraft vapour trails don't usually form when the atmosphere is warmer than minus 50 Celcius (very approx).

Right, we need to use the gram molecular volume. Anyone got that to hand? I also need the air pressure at say 30-40,000 feet.

JaX

[FluffyMcDeath]

QuikSanz wrote:
Hi, Actually blobrana is right you don't need an engine. Living in southern California I have been privlaged to see the shuttle come in.when high up with the heavy wing loading it creates an extreame low pressure and if the humidity is not too high the vapor freazes and hangs for a while.

Show me a commercial jet liner that does Mach 9.

Yes, if you look out of the window when you're going through cloud you can often see condensation in the air coming off the wingtip. But this is not the mechanism that causes contrails in high altitude commercial flight.

Get a pair of binoculars and take a look next time you see a jet making a contrail. The trail starts well back from the wing and not in line with the wing tip but with the engine.

The trails that form from shocked air cling to the edge that creates them generally. Far back from the wing they just don't spring into existance.

[QuikSanz]

@ JaXanim

qoute,
I also need the air pressure at say 30-40,000 feet.

I used to know this but it's been too long. Any pilot should have the answer to that one. Any Pilots out there.

Chris

[JaXanim]

@fluffy...

That's right, contrails are caused by the engines. The wing precipitation is transient and appears within a standing wave in the low pressure zones. Once the pressure equilibrates (near the trailing edge) the fog re-evaporates.

There's a lot of useful stuff about this on the Net and we should be seeing a pretty definitive explanation of vapour trails quite soon.

BTW, the gram molecular volume is 22.4 litres, so a 747 produces quite prodigious volumes of water vapour at 35,000 feet. More later.

JaX

[QuikSanz]

@ Fluffy

By the time it gets to here it's barely over mach 1 cause it will land @ Edwards as soon as it clears the mountains. Aproxx 100 miles.

Chris

[JaXanim]

@QuikSanz

I've managed to find this on the Net HERE. Taking 35,000 feet as an average height for commercial flights, the air pressure is 179mmHg or 24 kPa. This compares with 760mmHg/101kPa at ground level.

In other words, the pressure is less than a quarter of that at ground level.

I'm gonna do some calcs using the Combined Gas Law to determine the volume of vapour expelled by a 747 at that altitude. I'll take blobrana's data as a starter. Maybe this will refine as more data comes in on aviation fuel.

Cheers,

JaX

[FluffyMcDeath]

JaXanim wrote:
@QuikSanz

Maybe this will refine as more data comes in on aviation fuel.

JaX

Maybe best to just buy some jet fuel and do an empirical test. :-o

[QuikSanz]

@ JaXanim,

AWEB won't work on that site, bummer. I'll either check it at work or my A1 when it shows up monday. As for engine related trails I belive the coditions are relativly simaler. We also get other neet shows in So.Cal. Once in a while they'll launch a military rocket from Vandenberg AF base. with the Oxygen/ Hydrogen fuel they use it makes for a great trail at least 100 mi. long. Now that stuff has to freeze.

Chris

[FluffyMcDeath]

Here's a nice little shot of the Shuttle coming in for a landing, with a nice little trail.

[JaXanim]

That's a nice picture. Note the fog precipitation in the low pressure standing wave atop the wings. This re-evaporates almost instantaneously and doesn't produce a significant trail. The tips create a short-lived trail.


@QuikSanz

Yes, that's got to be the ultimate contrail! From my reading so far, this vapour almost certainly does freeze into minute ice crystals.


@blobrana

Aviation kerosene is mainly a C10-C12 hydrocarbon mixture, but that doesn't have any significant effect on water volume. In fact I'm taking your estimates on this. The old story that a gallon of petrol creates a gallon of water isn't so far of, is it?

JaX