Why do inner contrails spread more than outer contrails?

DLH440 36,000 ft. Airbus A380-841, wales uk.
Anybody know why the inner trails spread so much more than the outer?

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Trailspotter

Senior Member.
Anybody know why the inner trails spread so much more than the outer?
I think this is due to the wake vortices, which deform the inner trail differently than the outer ones. I have an unpublished video of a symmetrical evolution of A380 contrails, showing the above. I'll dig up and post at the weekend.
 

Trailblazer

Moderator
Staff member
Nice video, but I am not entirely convinced that spinning holds things together..
Looking at the video the outer trails appear denser because they are folded into more of a tubular shape. You can also see that the more persistent trails are behind the outer engines, while the inner ones seem to dissipate more quickly.
 

Mick West

Administrator
Staff member
I think it's to do with the wing vortices.

The outer trails get twisted around by the wing vortices. The inner trails get spread by the vortex sheet.



The area of low pressure coming off the wing of a plane is not just two vortices, it's a sheet of low pressure that begins immediately to fold up at the ends. You can see this in that aerodynamic contrail:


The exhaust contrails are injected into this sheet, and the spreading and curling of the sheet is what creates the initial shape:
 
I think you have explained how two trails can roll-up one, but I am not sure it explains why it is thicker within one plane's length. If it spreads horizontally it should thin vertically, and I have not seen this... yet.
 
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