Re: Smoke Trail

From:         jtalbot@extro.ucc.su.oz.au (jts)
Organization: Information Services, The University of Sydney, NSW, Australia
Date:         03 Jul 96 01:23:51 
References:   1
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In article <airliners.1996.1078@ohare.Chicago.COM>, laewell@iastate.edu
says...
>
>
>To Whom it May Concern,
>
>        I recently rode on a United Airlines 737
>(-300 I think) and I noticed something peculiar during
>take-off.  Just prior (approx 3sec) to the front
>wheels leaving the ground, a thin (approx 10cm 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 phenomena?  More
>specifically, does anyone know what the cause of this
>is?

At high speed the increased speed of the airflow reduced the density of
the air.  This in turn lowers the condensation temperature of the water
vapour in the air.  The result is a stream of "smoke" which basically
is a cloud.  The speed of the aircraft at rotation precludes any type of
engine emission coming from the front of the engine, any combustion
smoke would come of out the nozzle.

This can also be seen from other parts of the wing on aircraft,
especially around the ends of the flaps which the pressure coefficients
are high.

I also remember taking-off at DFW in an AA DC-10 during a storm with low
cloud.  Once the aircraft reached a reasonable speed a thin "cloud"
formed over the wing root and thickened considerable at rotation when
the lift was increased.  This was also seen on other aircraft on the
day.


Justin Talbot-Stern
University of Sydney

jtalbot@extro.ucc.su.oz.au