mclaudio
Posts: 142
Joined: Fri Jan 21, 2005 7:22 am

Strange Distortion

Wed Jan 09, 2008 8:41 pm

Hi.
I would like to ask a small help so that I can understand what happened on the following photo.
I was shooting in Funchal airport in Madeira (FNC) and as a 757 from Finnair was landing I kept on shooting until the touch down. Only later I saw that strange distortion which only happened with that aircraft.
I remember that a while ago there was a topic with a similar situation, but I wasn't able to find a logic explanation to that effect.
The photo:
http://i19.photobucket.com/albums/b200/mclaudio/Finnair_ldg05_funchal.jpg
and fyi I was using a 30D plus a 100-400mm L IS
camera settings: M mode, ISO 160, and the photo was done at 340mm, f 7,1 with 1/250 shutter speed.

Is that...wake turbulence  Confused (or something similar)

thank you very much for any help
Proudly one of the 6 million Portuguese that support SL Benfica!! Champions 2009-2010!
 
Fly747
Posts: 1361
Joined: Sat Apr 16, 2005 9:03 am

RE: Strange Distortion

Wed Jan 09, 2008 8:48 pm



Quoting Mclaudio (Thread starter):
Is that...wake turbulence Confused (or something similar)

Yes, pretty much. That is the vortex coming off the tip of winglet. Weird effect.

Ivan
 
beechcraft
Posts: 731
Joined: Sun Nov 16, 2003 2:10 am

RE: Strange Distortion

Wed Jan 09, 2008 9:12 pm

That´s really weird. I´ve only seen these effects in high speed low passes of fighters.
the distorion here goes all the way up the the red sign...strange strange.

Denis
That's it! You people have stood in my way long enough. I'm going to clown college!
 
IngemarE
Posts: 281
Joined: Thu Mar 24, 2005 4:46 pm

RE: Strange Distortion

Wed Jan 09, 2008 9:27 pm

Hey Miguel!

Quoting Fly747 (Reply 1):
That is the vortex coming off the tip of winglet. Weird effect.

I agree.
The visual distorsion is due to pressure changes within the vortice.
This is probably caused by the fact that he/she is touching down with a slight crab-angle, to compensate for x-wind.
The winglet then slices through the in a manner it wasn't intended to (...'cause winglets are originally designed to decrease wingtip vortices/induced drag, remember?!) and hence creates its own wake/vortice.
In thrust I trust.
 
mclaudio
Posts: 142
Joined: Fri Jan 21, 2005 7:22 am

RE: Strange Distortion

Wed Jan 09, 2008 9:52 pm

Hi again.
Thank you for the input. It really makes sense what Ingemar wrote and if you don't mind...another question. Do you think that high humidity increases that effect on the photo, or should I say that it makes it more visible on a photo? The day was cloudy so I presume that the air was quite dense and this has some influence on the induced drag.
Note that I also cropped the photo in a way so that the windsock could be visible leaving no doubts that there was a slight x-wind.
Once again thank you very much. I am only trying to learn a bit more with the photo  Smile
Proudly one of the 6 million Portuguese that support SL Benfica!! Champions 2009-2010!
 
oly720man
Posts: 5740
Joined: Fri May 21, 2004 7:13 am

RE: Strange Distortion

Wed Jan 09, 2008 10:02 pm



Quoting IngemarE (Reply 3):
pressure changes

strictly speaking it's density changes. The refractive index of air changes with density (or temperature --> mirages in the desert). It's the same sort of effect (known in the trade as a shadowgraph) where you get light passing over somewhere warm (e.g. radiator) and you see wavy patterns on a wall behind. Alternatively if you mix hot and cold water in a glass, the interface between the hot and cold water will be seen as lines/surfaces due to the respective refractive indices and the way the light is distorted.
wheat and dairy can screw up your brain
 
777MechSys
Posts: 336
Joined: Sun Apr 30, 2006 5:49 am

RE: Strange Distortion

Wed Jan 09, 2008 10:59 pm

Here is another example:


View Large View Medium
Click here for bigger photo!

Photo © Mike Paschal

 
IngemarE
Posts: 281
Joined: Thu Mar 24, 2005 4:46 pm

RE: Strange Distortion

Wed Jan 09, 2008 11:15 pm



Quoting Mclaudio (Reply 4):
The day was cloudy so I presume that the air was quite dense

Well, that would depend on what type of clouds you were having?
Cumulus clouds usually "vacuum" the air, so to speak, of its moisture. This usually leads to good visibility and more dense air.
While stratus clouds can trap moisture between clouds and earth, usually leading to hazy conditions with rather poor visibility and less dense air.

Quoting Mclaudio (Reply 4):
Do you think that high humidity increases that effect on the photo, or should I say that it makes it more visible on a photo?

No, I wouldn't think so, but rather the propriety of ambient light created by an overcast sky. Overcast gives a more "omni-potent" light, if you see what I mean!?  scratchchin 
You know, the kind of light that makes your cam's light-meter go bananaz!  Wink

One effect that humid air have though, is the need for a higher TrueAirSpeed and/or Angle of Attack on final approach, since moist air is less dense than dry air. (Lower air density means, for a given airspeed/angle-of-attack, a lower lift-capacity.)
With a higher AoA, the wing-tip vortices will also become more intense.

Quoting Oly720man (Reply 5):
strictly speaking it's density changes

Strictly speaking #2; You don't get one without the other. Right?  Wink
Good point though!, and a couple of good examples to clarify!  thumbsup 
In thrust I trust.
 
oly720man
Posts: 5740
Joined: Fri May 21, 2004 7:13 am

RE: Strange Distortion

Wed Jan 09, 2008 11:50 pm



Quoting IngemarE (Reply 7):
Strictly speaking #2; You don't get one without the other. Right?

Depends. For a mirage effect it's all due to the temperature/density varying at constant pressure.

For vortex pics like this I wonder if it's a pressure interaction with humidity giving density changes.
wheat and dairy can screw up your brain
 
Fly747
Posts: 1361
Joined: Sat Apr 16, 2005 9:03 am

RE: Strange Distortion

Wed Jan 09, 2008 11:56 pm

Just to make a correction to my statement in reply #1, it's actually vortex coming off the flaps.

Ivan

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