(meteorobs) re:: OT query: contrail-like formations on jet wing tips--vortexes

[josephasmus at cox.net]

wow... thank you kindly, Elton for that excellent and detailed explanation...and I thank all the other list members who answered my query.

Joseph
> 
> From: "E. L. Jones" <jonee at epix.net>
> Date: 2005/03/06 Sun PM 09:27:27 EST
> To: joseph.assmus at sharp.com,  meteorobs at meteorobs.org
> Subject: (meteorobs) 
> 	re: OT query:  contrail-like formations on jet wing tips--vortexes
> 
> Joseph penned:
> 
> I was just asked a question that I can not adequately answer and since this is a quiet time on the list I thought it ok to post
> 
> - What causes the short-lived contrail-like formations that are formed on the very tips of jet engine wings?  Living near a San Diego Airport and Miramar Air Base we see these all the time...  They last for minutes or sometimes only seconds... They are curly and wispy in appearance.  
> 
> Hello Joseph.
> 
> I've flown into SD many times.  I know how close the approach is there and how easy it is to see the planes up close. What you observed is a "wingtip vortex" or a swirling motion--not unlike a miniature horizontal tornado.  They are always present but only visible when conditions are right. The air near the ground has to be humid and close to the dew point or "fog-forming temperature".
> 
> Be it remembered that the air flow above a wing is at a lower pressure than that below the wing--this is what creates lift.  All along the wing the air is trying to move from under the wing to go from an area of greater pressure to that of a lower one.   Normally, this is a flow from front to back, because the wing blocks air flow in all but one direction. On the wing tip, the blocking action is lost and the the path of least resistance becomes around the the end of the wing.  This "short cut" to the top side creates a corkscrewing mass of air or "vortex"as known in aviation.  The swirling action starts small at the forward tip of the wing and gets larger as more air is added along the length of the tip. The more air added, the faster the rotation, and the more powerful the vortex becomes. 
> 
> The disturbed air wants to equalize pressure but the swirling vortex is causing yet another low pressure zone within.   The funnel slowly expands anyway and as it reaches into air of neutral pressure, it moves this net vacuum into what was stable air with stable dew point.  This sudden pressure drop causes air at, or near saturation with water vapor to "flash cool". As air cools of course it reaches a point of saturation(dew point) and water in gas form becomes water in vapor form and visible to the eye.  As the pressure equalizes, the air re-warms, the water vapor disappears-- reverting back into a gas. This is why "seeing" a vortex is a short lived event near the ground.
> 
> You actually have this same condensation cycle occur over the whole wing. However, because the non-vortex flow is rejoining air that was just split apart the "gas-to water vapor-to gas" cycle is even more brief. You don't have the mixing of additional air like one does at the vortex.
> 
> All air foils create a wing tip vortex and they are stronger on slower, larger-winged aircraft. Well after the vapor-laden vortex becomes invisible, it maintains high momentum and slow expansion as it slowly settles towards the ground.  If a smaller airplane encounters a vortex at just the right angle, it can flip the plane corkscrew fashion-- faster than the plane can correct-- leading to a crash. (Think about flying down the funnel of a tornado).  This is why general aviation aircraft have to have a longer hold between take-offs and landings when following airliners/transport aircraft.
> 
> High altitude contrails(condensation trails) usually exist much, much longer owing to the fact that the flash cooled water vapor forms ice crystals which don't just pop back into a gaseous state as they are supper cooled and have to go through a longer cycle of sublimation to dissolve.
> 
> To move back "on topic",  it is conceivable that a meteoroid, in dark flight, could leave a legitimate visible contrail if it is 1)large enough-- and 2) has some degree of laminar/smooth flow vs. turbulent flow--e.g. an oriented or slowly rotating body.
> 
> Regards,
> Elton
> 
> 
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