A photo of a sonic boom. [Archive]

RantyDave

1st October 2007, 20:38

Is this actually a 'sonic boom'... the white 'cloud' is the compression (rarefraction) of the moisture in the air causing it to condense into what we see as a 'cloud'.
Yes it is. What you see here is a Prandtl Glauert singularity which, I am pleased to say, I spelt correctly first time through. It's actually the shockwave that builds up on anything astonishingly fast (not just aircraft) and which it has to pass through in order to go properly supersonic. The cone is caused by a region of low pressure forming behind the shockwave causing the water vapour to (briefly) condense.

I love them. They give me the horn and remind me of happy days when I gave a lot of a shit about fluid dynamics.

Here's a much cooler picture of someone having a lot more fun than me. Bastards.

Dave

oldguy

1st October 2007, 21:07

check the linkhttp://www.youtube.com/watch?v=-d9A2oq1N38<object width="425" height="350"><param name="movie" value="http://www.youtube.com/v/-d9A2oq1N38"></param><param name="wmode" value="transparent"></param><embed src="http://www.youtube.com/v/-d9A2oq1N38" type="application/x-shockwave-flash" wmode="transparent" width="425" height="350"></embed></object>

Curious_AJ

1st October 2007, 23:12

TAKE MY BREATH AWAAAAY!!

*come on... don't say you don't know what I'm on about...*

Bren

2nd October 2007, 07:17

TAKE MY BREATH AWAAAAY!!

*come on... don't say you don't know what I'm on about...*

I think you are on about Maverick on a 1985 Kawasaki GPZ 900 Ninja

Bass

2nd October 2007, 08:49

Yes it is. What you see here is a Prandtl Glauert singularity which, I am pleased to say, I spelt correctly first time through. It's actually the shockwave that builds up on anything astonishingly fast (not just aircraft) and which it has to pass through in order to go properly supersonic. The cone is caused by a region of low pressure forming behind the shockwave causing the water vapour to (briefly) condense.

Dave

Well, there you go. I had always assumed that the condensation cloud marked the region of high pressure of the shock wave itself. I had reached this conclusion because while I hadn't thought too deeply about it, I always drain a fair bit of water out of the receiver of my air compressor. So I assumed that compressing air increased the relative saturation and so caused condensation.
Now you tell me that decompressing the air does this and I can't disagree because, firstly, I had always thought that the shock wave should form further forward on the aircraft than the cloud does and secondly, I have often seen condensation trails on the top surface of the flap tips of landing 737's (obviously a low pressure area) in conditions of high relative humidity.
Somewhere around here, I have a psychrometric chart which probably explains this, but since I can't find it at the moment, can you tell me why a pressure shift in either direction appears to increase relative saturation?

Curious_AJ

2nd October 2007, 09:52

I think you are on about Maverick on a 1985 Kawasaki GPZ 900 Ninja

well, I was just quoting a song from the movie, but close enough.. lol

Ocean1

2nd October 2007, 10:07

Well, there you go. I had always assumed that the condensation cloud marked the region of high pressure of the shock wave itself. I had reached this conclusion because while I hadn't thought too deeply about it, I always drain a fair bit of water out of the receiver of my air compressor. So I assumed that compressing air increased the relative saturation and so caused condensation.
Now you tell me that decompressing the air does this and I can't disagree because, firstly, I had always thought that the shock wave should form further forward on the aircraft than the cloud does and secondly, I have often seen condensation trails on the top surface of the flap tips of landing 737's (obviously a low pressure area) in conditions of high relative humidity.
Somewhere around here, I have a psychrometric chart which probably explains this, but since I can't find it at the moment, can you tell me why a pressure shift in either direction appears to increase relative saturation?

There's water in your reciever because of the pressure drop as it moves from the compressor into the larger volume. There's also some thermodynamic stuff going on there, along with the fact that there's a lot of cold surface area inside the reciever. I always get confused between Boyle's law and Charles's law... looks... http://en.wikipedia.org/wiki/Gas_laws

I have heard of instances where the local dew-point has been right on the limit for fog and the the shock wave and subsequent local pressure drop has tripped the whole area into a whiteout. Must be both spectacular and fucken scary.

Bass

2nd October 2007, 10:40

There's water in your reciever because of the pressure drop as it moves from the compressor into the larger volume. There's also some thermodynamic stuff going on there, along with the fact that there's a lot of cold surface area inside the reciever. I always get confused between Boyle's law and Charles's law... looks... http://en.wikipedia.org/wiki/Gas_laws

I have heard of instances where the local dew-point has been right on the limit for fog and the the shock wave and subsequent local pressure drop has tripped the whole area into a whiteout. Must be both spectacular and fucken scary.

OK.....Ummm.... I don't think that the ideal gas law helps much here and I have long forgotten most of my thermo. Also, if I go outside to the big factory compressor which is an oil flooded screw machine, it is basically a positively sealed progressing cavity pump. This means that the pressure in the screw elements never gets much higher than the receiver pressure. Also, the receiver gets quite warm. Yet I still drain lots of water out.

For the whiteout phenomenon you describe, I wonder if it's a supersaturation one. You are probably aware that air will supersaturate if it is very clean and very still, so that the moisture has no nuclei to coalesce on. This is what causes the long lasting vapour trails behind high flying aircraft when supersaturated air is both disturbed and polluted by their passage.
I wonder if your whiteout is a low altitude example of the same thing.
Either way, it would be a trifle inconvenient, hard out, mid corner on the bike.

avgas

2nd October 2007, 10:50

English sucks - Boom = sound
Unless you are 2 and then everything looks boom.
Onamatapaia (dammed if i can spell it properly)
However Clap and the Clap are exceptions to the rule. hehe.
I wonder if the pilot farted?

Ocean1

2nd October 2007, 11:00

OK.....Ummm.... I don't think that the ideal gas law helps much here and I have long forgotten most of my thermo. Also, if I go outside to the big factory compressor which is an oil flooded screw machine, it is basically a positively sealed progressing cavity pump. This means that the pressure in the screw elements never gets much higher than the receiver pressure. Also, the receiver gets quite warm. Yet I still drain lots of water out.

Nonetheless, the pressure in the reciever is lower than that in the compressor. If the air is close to saturated then any drop in pressure will cause condensation, there's just less mass there to saturate. Also, warm as the reciever is it's still way cooler than the incoming air, the compressor has just squished it into a tenth of it's previous volume. I can't be bothered looking it up but it'll be fuggen hot.

For the whiteout phenomenon you describe, I wonder if it's a supersaturation one. You are probably aware that air will supersaturate if it is very clean and very still, so that the moisture has no nuclei to coalesce on. This is what causes the long lasting vapour trails behind high flying aircraft when supersaturated air is both disturbed and polluted by their passage.
I wonder if your whiteout is a low altitude example of the same thing.

Seems likely, reports seem to be from low altitude at transition to supersonic. I heard of one such incident as an F18 flew past a carrier at under 40ft, the whiteout was instant and apparently extended for hundreds of metres. Temperature was sub-zero, don't know if that had any bearing on the outcome.

terbang

2nd October 2007, 11:08

Well, there you go. I had always assumed that the condensation cloud marked the region of high pressure of the shock wave itself. I had reached this conclusion because I hadn't thought too deeply about it, but I always drain a fair bit of water out of the receiver of my air compressor. So I assumed that compressing air increased the relative saturation and so caused condensation.
Now you tell me that decompressing the air does this and I can't disagree because, firstly, I had always thought that the shock wave should form further forward on the aircraft than the cloud does and secondly, I have often seen condensation trails on the top surface of the flap tips of landing 737's (obviously a low pressure area) in conditions of high relative humidity.
Somewhere around here, I have a psychrometric chart which probably explains this, but since I can't find it at the moment, can you tell me why a pressure shift in either direction appears to increase relative saturation?

Ranty Dave is quite correct about the visible moisture with the formation of a shockwave. The trails that you see off the flaps on a 737 are of a much lower speed nature though created by the same principle. A lowering of the pressure of a parcel of air cools it (Adiabatic process) as conversely, increasing its pressure heats it. Put your finger over the end of a bicycle pump and give it a good pump and watch your eyes water. As a parcel of air cools the relative humidity (RH) increases and when the RH reaches 100%, you will get visible moisture (cloud). Thats why we tend to get lots of clouds on cold days and clear skies on the warm days. Ever notice that it often clouds over in the evenings as the day cools.
Using a bit of instructors licence rather than scientific, to produce lift an aeroplane wing creates a low pressure zone over its curved upper surface and like any fluid, higher pressure areas tend to move toward, to 'fill in', low pressure areas. Thats why the high pressure air in yer tyres is always trying to get out rather then in. Rather successfully in my case.
Higher pressure air (though only marginally above ambient air pressure) on the wing's under surface attempts to move toward the lower pressure on the upper surface. Because the wing is there in the way, it imposes a force on the wing giving us a lifting force. Viola, we fly! There are also those that say the wing is sucked into the air by lower pressure above it. This is not the explanation that aviators like to use though. So nuff said on that. The higher pressure air under the wing, in it's relentless and natural quest to fill in the low pressure areas on the top, tends to also have a spanwise component (or along the wing) toward the tip, attempting to rotate around the wing tip to 'fill in'. Essentially the wing moves on due to the forward movement of the aircraft so this leaves a rotating vortex off the wingtip (or tip of the flap if they are down). Like a tiny tornado, the wingtip vortex has a low pressure core. Applying the old adiabatic process and if the atmosphere has already got a fairly high RH, our little tornado could easily be further cooled to reach 100% RH. Quite spectacular visible moisture!

007XX

2nd October 2007, 11:08

Awesome pics and great reading about the technical explanations of it...Even though I do not have the technical knowledge to keep up with this discussion and participate with gobsmacking clever comments, I still find it very interesting...Cool stuff guys! :yes:

Bass

2nd October 2007, 11:27

Nonetheless, the pressure in the reciever is lower than that in the compressor. If the air is close to saturated then any drop in pressure will cause condensation, there's just less mass there to saturate. Also, warm as the reciever is it's still way cooler than the incoming air, the compressor has just squished it into a tenth of it's previous volume. I can't be bothered looking it up but it'll be fuggen hot. .
This doesn't gel for me.
The absolute humidity is fixed by the condition of the incoming air.
The relative humidity determines whether or not condensation occurs.
We appear to be agreed that lowering the pressure increases the relative humidity - does it therefore follow that raising the pressure decreases RH?
Certainly the heat of compression raises the temperature and so decreases RH, so if the increased pressure also decreases it then we have air in the receiver that is at a lower RH than the inlet air both for pressure and temperature reasons. How can it condense? Yet it does - and this is not explained by the temperature hump it has gone through - especially in the case of the screw compressor because the oil acts as a continuous intercooler and so the air entering the receiver is at almost the same temperature as the oil. This is of course way cooler than for a piston compressor.
Sorry, but I think we are missing something here.

Seems likely, reports seem to be from low altitude at transition to supersonic. I heard of one such incident as an F18 flew past a carrier at under 40ft, the whiteout was instant and apparently extended for hundreds of metres. Temperature was sub-zero, don't know if that had any bearing on the outcome.
Yep, supersaturation usually occurs when saturated air is cooled, so low temperature is a factor.

terbang

2nd October 2007, 11:34

I always get confused between Boyle's law and Charles's law... looks... http://en.wikipedia.org/wiki/Gas_laws

Mr Bernoulli had something to say here (http://www.grc.nasa.gov/WWW/K-12/airplane/bern.html) as well.

Bass

2nd October 2007, 11:43

Ranty Dave is quite correct about the visible moisture with the formation of a shockwave. Applying the old adiabatic process and if the atmosphere has already got a fairly high RH, our little tornado could easily be further cooled to reach 100% RH. Quite spectacular visible moisture!

So far, so good. This is pretty standard Bernoulli stuff. Now Mr Bernoulli was a pretty bright cookie, but in the end, he simply integrated Euler's equations and there are some aspects of flight which Bernoulli does not explain fully, momentum and drag in particular. More modern standing vortex theory does rather better but has no place in a motorcycle forum.

As for vapour trails, they require supersaturation. This is witnessed by their lifetime. If the air is not supersaturated, then the moisture would evaporate again and the trails would disappear as the tip vortex dies away.
The life of the vortex is measured in minutes but the trails can last in the order of hours.

Bass

2nd October 2007, 11:50

Mr Bernoulli had something to say here (http://www.grc.nasa.gov/WWW/K-12/airplane/bern.html) as well.

Good find - thanks for that.
It does however point out that Bernoulli's equation is at heart simply an expression of conservation of energy and that you have to be careful with the boundary conditions.

terbang

2nd October 2007, 12:52

So far, so good. This is pretty standard Bernoulli stuff. Now Mr Bernoulli was a pretty bright cookie, but in the end, he simply integrated Euler's equations and there are some aspects of flight which Bernoulli does not explain fully, momentum and drag in particular. More modern standing vortex theory does rather better but has no place in a motorcycle forum.

As for vapour trails, they require supersaturation. This is witnessed by their lifetime. If the air is not supersaturated, then the moisture would evaporate again and the trails would disappear as the tip vortex dies away.
The life of the vortex is measured in minutes but the trails can last in the order of hours.

Vapour trails and wing tip vortex are not the same. Vapour trails, the ones you see way up in the atmosphere are produced by an aircrafts engines. These would be more correctly called Ice crystal trails as they consist mainly of Ice crystals. I have climbed through many 'vapour trails' in the troposphere and, like cirrus clouds, you can hear a 'rustling sound' over the cockpit as you pass through them. Climbing further into the stratosphere (in a Gulfstream not a Boeing) I notice that our own 'vapour trails', viewed from our own external vigilance cameras, don't exist anymore.
The, sometimes visible, tip vortex is created as a 'byproduct' of lift. Mainly at low speed. The wing tip vortex is actually considerably larger than the core that you often see, as much as half of the aircrafts wing span. Wing tip vortex has extremely high local velocities that are root of the wake turbulence danger that we are faced with every day. Wing tip vorticies also add to the aircrafts drag profile and much effort is put into reducing it wirh all sorts of devices like winglets, fences and so on. Nice picky here of a light aircraft using smoke to highlight the vorticies.

http://web3.knallgrau.at/static/twoday_stable05/bmworacleracing/images/737px-airplane_vortex_edit_bmwPreview.jpg

Not only do winglets serve to decrease drag, thus increasing range, but they also look damn sexey.

http://www.bjsco.com/images/gulfstream.jpg

avgas

2nd October 2007, 13:14

Sorry, but I think we are missing something here.

Have you considered the concept of the surface temperature venturi effect.
While you are getting a pressure change on the inlet of your compressor you are also getting a change in air speed. Faster air = more air = more air hitting the sides of the bottleneck....which in turn is cold.
Lots of air hitting cold surface.....condensation.
Similar to air ram systems on a cold night.
Sorry cant be more technical than that, i only did mechanical stuff for a few years. Their wasn't enough flashing lights for me :lol:

Ocean1

2nd October 2007, 14:12

This doesn't gel for me.
The absolute humidity is fixed by the condition of the incoming air.
The relative humidity determines whether or not condensation occurs.
We appear to be agreed that lowering the pressure increases the relative humidity - does it therefore follow that raising the pressure decreases RH?
Certainly the heat of compression raises the temperature and so decreases RH, so if the increased pressure also decreases it then we have air in the receiver that is at a lower RH than the inlet air both for pressure and temperature reasons. How can it condense? Yet it does - and this is not explained by the temperature hump it has gone through - especially in the case of the screw compressor because the oil acts as a continuous intercooler and so the air entering the receiver is at almost the same temperature as the oil. This is of course way cooler than for a piston compressor.
Sorry, but I think we are missing something here.

Yup, incoming air at ambient pressure will contain a quantity of water vapor, that doesn’t change. Saturation occurs when the ratio of partial pressure of water vapor exceeds the saturation vapor pressure of the other gasses in the mix,(Nitrogen, Oxygen etc). That’s more or less the definition of relative humidity. The relative humidity increases with both an increase in temperature and a drop in pressure. Both these things happen inside a compressor system, repeatedly, in spades. Enough to cause precipitation even when the ambient humidity is reasonably low, (on a reasonably dry day). Pressure changes continue to happen throughout the reticulation system too. That’s why it’s good practice to install a drain leg below the air feed to any equipment, traditionally with an increased diameter to force a further pressure drop.

I think the observed effect is exagerated by the speed at which precipitation occurs, much faster than evaporation, so the accumulated effect of all of the pressure changes throughout a compressed air system is to make water. Even on a well set up system you can see evidence of water (or ice) around any high volume exhausts, (ever used a pneumatic breaker?).

Bass

2nd October 2007, 14:31

Vapour trails and wing tip vortex are not the same. Vapour trails, the ones you see way up in the atmosphere are produced by an aircrafts engines.

DOH!! Where is my head at today?
You are of course quite right and it is particulates from the engine exhausts that provide coalescence nuclei. However, supersaturation can be condensed at lower altitudes (much rarer phenomenon) by turbulence such as the tip vortex.

These would be more correctly called Ice crystal trails as they consist mainly of Ice crystals. I have climbed through many 'vapour trails' in the troposphere and, like cirrus clouds, you can hear a 'rustling sound' over the cockpit as you pass through them. Climbing further into the stratosphere (in a Gulfstream not a Boeing) I notice that our own 'vapour trails', viewed from our own external vigilance cameras, don't exist anymore.

I can't explain that - can you?
There must be a limit to the altitude at which the required conditions occur. I suspect it stems from from those conditions being generated by rising air which is then cooled past its dew point without condensation.
Is the tropopause a natural limit to the distance that air can rise due to thermal currents?

The, sometimes visible, tip vortex is created as a 'byproduct' of lift. Mainly at low speed. The wing tip vortex is actually considerably larger than the core that you often see, as much as half of the aircrafts wing span. Wing tip vortex has extremely high local velocities that are root of the wake turbulence danger that we are faced with every day. Wing tip vorticies also add to the aircrafts drag profile and much effort is put into reducing it wirh all sorts of devices like winglets, fences and so on. Nice picky here of a light aircraft using smoke to highlight the vorticies.

Not only do winglets serve to increase drag, thus increasing range, but they also look damn sexey.

Ummm decrease?
Lots of work in recent years on adapting the way that a wing sheds the tip vortex. If the vortex can be kept outside the wing span, then the drag is hugely reduced - very hard to do though.

terbang

2nd October 2007, 15:01

I can't explain that - can you?
There must be a limit to the altitude at which the required conditions occur. I suspect it stems from from those conditions being generated by rising air which is then cooled past its dew point without condensation.
Is the tropopause a natural limit to the distance that air can rise due to thermal currents?
Not too sure either, but it will have something to do with the moisture content up there I guess though I have observed shockwaves on various parts of the airframe up there as well. The normal weather that we know is supposed to stop at the tropopause which is the boundary (essentially a temperature inversion layer) between the troposphere, which is where we live and the stratosphere (where we only visit). Though I have encountered very low (outside the normal theory) temperatures and also strong winds at 45000'. I'll have to dig up me notes and have a wee think.

Ummm decrease?
Lots of work in recent years on adapting the way that a wing sheds the tip vortex. If the vortex can be kept outside the wing span, then the drag is hugely reduced - very hard to do though.
Argh yes decrease.

Korumba

2nd October 2007, 15:04

A few more picks..

Bass

2nd October 2007, 15:08

Yup, incoming air at ambient pressure will contain a quantity of water vapor, that doesn’t change. Saturation occurs when the ratio of partial pressure of water vapor exceeds the saturation vapor pressure of the other gasses in the mix,(Nitrogen, Oxygen etc).

Not sure of your meaning here.

That’s more or less the definition of relative humidity.

I have always defined relative humidity as the absolute humidity now, divided by the absolute humidity at saturation at the same temperature and pressure. In other words, as a percentage, how close it is to saturation or simpler yet, how humid it is, relative to its saturation value

The relative humidity increases with both an increase in temperature and a drop in pressure.

Now that, I just can't go along with. The RH goes down as temperature rises and does so rapidly because the saturation humidity goes up. Hot air can hold more water. It's the reason that driers work.
Similarly, if the RH rises as pressure falls, perhaps it drops as pressure rises. Both these effects are counter to your argument of what goes on in a compressor.

Both these things happen inside a compressor system, repeatedly, in spades. Enough to cause precipitation even when the ambient humidity is reasonably low, (on a reasonably dry day).

Well we agree that it gets wet - I guess that's a start

Pressure changes continue to happen throughout the reticulation system too. That’s why it’s good practice to install a drain leg below the air feed to any equipment, traditionally with an increased diameter to force a further pressure drop.

Yes, but that's the pressure down, RH up effect and that is the second thing we agree on so far.

I think the observed effect is exagerated by the speed at which precipitation occurs, much faster than evaporation, so the accumulated effect of all of the pressure changes throughout a compressed air system is to make water. Even on a well set up system you can see evidence of water (or ice) around any high volume exhausts, (ever used a pneumatic breaker?).

Yep but same as above plus a bit of Messeurs Joule and Thompson
God save us - if we can't get this straight, imagine the thread if we took say, hydrogen into account. You know what I mean here? Ambient temperatures are above the critical point for hydrogen and so the Joule Thompson effect is reversed i.e. it heats as it EXPANDS and vice versa. Crack the valve on a hydrogen cylinder and it can catch fire.

Bass

2nd October 2007, 15:13

The normal weather that we know is supposed to stop at the tropopause

That would explain it.
Supersaturation occurs in cooling and rising air - if the air cannot rise past the tropopause then neither can supersaturation
That's not to say that the air is perfectly dry - it will still contain moisture and so shock waves will be visible - only that vapour trails will not

Ocean1

2nd October 2007, 15:54

God save us - if we can't get this straight, imagine the thread if we took say, hydrogen into account. You know what I mean here? Ambient temperatures are above the critical point for hydrogen and so the Joule Thompson effect is reversed i.e. it heats as it EXPANDS and vice versa. Crack the valve on a hydrogen cylinder and it can catch fire.

:laugh: Dude you're confusing me, a task I like to reserve for myself, when the sun's below the yardarm, (and that's just plain bewildering). Time to look instead of shooting from the hip... http://www.domnickhunter.com/tech_Centre.asp?chapter=1&section=2_Water-Vapour_4_3.htm&getIndex=false

Looks like the higher pressure squishes the water out but the effect's temporarily offset by the increase in temperature. Suppose that explains why all the rain happens in the reciever.

Bass

2nd October 2007, 16:22

:laugh: Dude you're confusing me, a task I like to reserve for myself, when the sun's below the yardarm, (and that's just plain bewildering). Time to look instead of shooting from the hip... .

Agreed!
Besides, it's band practice night and I need to go and get some beer.
Until the morrow.....................

Bass

2nd October 2007, 18:00

I figured it out on the way home.
Honestly. some days I think that my brain has taken a sabbatical.
We had the effect of pressure reversed. The effect of increasing pressure is to REDUCE the humidity at which saturation occurs (or increase the RH).
However the other key factor is that the effect of temperature on saturation is MUCH more pronounced than the effect of pressure and this is important.
So when we compress air in the compressor, it wants to reject moisture. But it doesn't happen immediately because the increased temperature overrides it. So it's pretty much as you said and the water is squeezed out in the receiver when the air cools.

Well, how do we get condensation in the LOW pressure area behind the shockwave (a la Ranty Dave)? If the pressure is reduced, then it should hold MORE water, not less.
Again it's the temperature effect overriding the pressure effect.
A shock wave has some pretty vigorous pressure changes, and I will bet my left bollock (I've had the sports model conversion, so that's not as big a wager as you might think), that if we could put a thermometer in the low pressure area behind the shock wave, that we would find that the temperature momentarily drops low enough that the air goes below its dewpoint, despite the pressure change pulling it the other way.

The clues were all there. Terbang mentioned the temperature drop with pressure. I said that the saturation humidity changed rapidly with temperature and you suggested that the higher air pressure effectively squeezed the water out.
Just took a while to put it together.
Sorry about that.
Just to complete the picture, it also explains why I only see condensation trails off flap tips when the RH is really close to 100%. It also means that you need drains around your compressed air reticulation because the air is cooling, not because the pressure is dropping.

PS Just looked at your web reference, only to discover that I've largely repeated what he said.

R6_kid

2nd October 2007, 18:19

A few more picks..

heh, the F-14 really does know how to do things the best!

terbang

2nd October 2007, 18:26

Here is what Meteorologists and Aviation people refer to as the Adiabatic process (http://en.wikipedia.org/wiki/Adiabatic_process) to explain a lot of what happens in the atmosphere.

Korumba

2nd October 2007, 20:36

heh, the F-14 really does know how to do things the best!

I agree with that.

avgas

5th October 2007, 11:46

So correct me if i'm wrong but velocity of the air is irrelevant of the pressure? As i thought the 2 were linked when you involve a surface.
Also what about the roll off effect of the air curling back on itself after it hits the jet? does this condense all the air around the jet.....hense the big halo?

Skyryder

5th October 2007, 16:33

Well it's nice to know that the geeks have a bit of competition.:niceone:

Skyryder

peasea

5th October 2007, 22:14

Vapour trails and wing tip vortex are not the same. Vapour trails, the ones you see way up in the atmosphere are produced by an aircrafts engines. These would be more correctly called Ice crystal trails as they consist mainly of Ice crystals. I have climbed through many 'vapour trails' in the troposphere and, like cirrus clouds, you can hear a 'rustling sound' over the cockpit as you pass through them. Climbing further into the stratosphere (in a Gulfstream not a Boeing) I notice that our own 'vapour trails', viewed from our own external vigilance cameras, don't exist anymore.
The, sometimes visible, tip vortex is created as a 'byproduct' of lift. Mainly at low speed. The wing tip vortex is actually considerably larger than the core that you often see, as much as half of the aircrafts wing span. Wing tip vortex has extremely high local velocities that are root of the wake turbulence danger that we are faced with every day. Wing tip vorticies also add to the aircrafts drag profile and much effort is put into reducing it wirh all sorts of devices like winglets, fences and so on. Nice picky here of a light aircraft using smoke to highlight the vorticies.

http://web3.knallgrau.at/static/twoday_stable05/bmworacleracing/images/737px-airplane_vortex_edit_bmwPreview.jpg

Not only do winglets serve to decrease drag, thus increasing range, but they also look damn sexey.

http://www.bjsco.com/images/gulfstream.jpg

I wish I had a plane with winglets. I'd invite my favourite kb'ers to come party on it with crates of Champers, a handful of hookers and saucerfuls of cocaine. Ahh, the good old days.