Those streams of 'white' trailing smoke from Commercial Jets in Blue Sky...Vortices?
 

I was taking a stroll in an park with a female friend close (a rare break from the ATPL). The park was close to City Airport.

There was clear blue sky when looking at 2 passing Jets - there was a trail of white I can only describe as 'white smoke' trailing from behind the jets.

There was a third jet that passed and there was no 'white smoke' - knowing that I am seeking a career in commercial aviation - the female friend quipped 'What is the trailing smoke behind the 2 jets and why is there none on the third passing jet?'

My immediate thought process lead me to believe these may be wing tip and/trailing edge vortices? Which I gave a suitably educated guess at the time.

As I have just done the Chapter on Aircraft Axis and Spanwise Flow - I am still thinking they are tip vortices

Would I be correct or could it also have been something else?

Ta!

Hi Scoobster,

the reason, why some jets leave those white trails, while others not, is altitude. The hot and humid exhaust from jet engines mixes with the atmosphere, which at high altitude is of much lower vapor pressure and temperature than the exhausted gas. The water vapor contained in the jet exhaust condenses and may freeze, and this mixing process forms a cloud very similar to the one your hot breath makes on a cold day.

Depending on a plane's altitude, and the temperature and humidity of the atmosphere, contrails may vary in their thickness, extent and duration.

Mustn't mention the Che****l word....

As soon as I saw the post I thought oh oh, this could get good

Remember: A dimmer sky is a safer sky.

Rob

Thanks Rob, I nearly spat out my wine over the keyboard. ;)

Yeah, I see a number of those old planes getting around.
They'll probably need bearings as well as a ring job.

In the meantime, a can of STP helps, I believe.

I'm assuming that the aircraft you were watching were approaching to land at City Airport, rather than passing at high altitude.

You were right about the vortices. They are generated primarily at the wing tips, and at the edges of the flaps.

If a vortex is powerful enough, it will lower the pressure at it's centre to a point where any moisture in the air will temporarily condense into water droplets, and cause the 'white trail' to appear.

The presence, or otherwise, of the white trail will depend on the humidity of the air, and the strength of the vortices.

The reason that the trails are there some days and not others, is that some days are more humid.

The reason that, on humid days, some produce trails and some don't, is that some produce more powerful vortices than others.


MJ:ok:


Ps.

STB. Quite right. This is no place for that kind of silliness. :rolleyes: