Re: Smoke Trail

From:         stephen@genesis1.physics.YALE.EDU (Stephen B. Selipsky)
Organization: Yale University, Department of Computer Science, New Haven, CT
Date:         03 Jul 96 01:23:51 
References:   1
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In article <airliners.1996.1078@ohare.Chicago.COM>
 laewell@iastate.edu (Lars A Ewell) writes:
> ...
>    I recently rode on a United Airlines 737 (-300 I think) and I noticed
> something peculiar during take-off.  Just prior (approx 3 sec) to the
> front wheels leaving the ground, a thin (approx 10 cm diameter) stream
> of smoke was emitted from the front of the engine (I could only see
> one engine).  This stream of smoke traveled over the top of the wing
> and then back toward the tail of the plane.  It lasted about ten
> seconds.  The sight reminded me of pictures I have seen of objects
> being tested in a wind tunnel where smoke (or something similar) is
> used to make visible the airflow direction.  The same scenario was
> repeated on my return flight.
>    Has anyone else seen a similar phenomen[on]?  More specifically,
> does anyone know what the cause of this is?

   Not truly knowing the answer, I'll make a physicist's guess:
   That sounds like moisture ("fog"), not smoke.  The wings and
especially the engine nacelles perturb the airflow, and the local
pressure/temperature changes can cause moisture to condense if the
humidity is high enough-- just as fog and dew can form at night when
the temperature drops.  (Lower pressure over the lift-generating
wings, thus lower temperature).  It isn't surprising that this
occurred around take-off, when the wings' angle of attack and hence
pressure differences were likely to have been highest.  I have
sometimes seen a sheet of fog flowing across a whole airplane wing,
evaporating at the back as it returns to ambient pressure.

   You can look for the same thing to happen with other large pressure
changes; for example, when landing flaps or air brakes are deployed,
or when military planes perform extreme maneuvers and make vapor
condense in wing tip vortices.  (Not to be confused with aerobatic
planes intentionally attaching smoke generators.)  Note that real
smoke lasts a while, but your condensation is momentary.

   Vapor trails left by stratospheric planes are somewhat different;
I believe the air must be "supercooled" above 100% humidity, and then
engine exhaust can provide nucleation centers for ice crystals.
Anyone out there know for sure?
---
Stephen B. Selipsky             stephen@genesis1.physics.yale.edu
Yale University Physics Dept.   Phone: 203/432-6923
P.O. Box 208120                 Fax:   203/432-6175
New Haven, CT  06520-8120       Home:  203/782-2065