Re: wake turbulence

From:         barr@netcom.com (Keith Barr)
Organization: Netcom Online Communications Services (408-241-9760 login: guest)
Date:         22 Jun 94 16:57:49 
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In article <airliners.1994.1334@ohare.Chicago.COM> "Fred Christiansen" <fredch@quad4.phx.mcd.mot.com> writes:
>Seeing the amount of thrust that the PW4084's can put out on the 777 prompts
>me to ask about wake turbulence.  It is my understanding that wake turbulence
>has been a problem with the "heavies" -- DC-10, MD-11, 747, 767, TriStar.
>My guess was that this was due to the large amount of thrust put out by
>the engines of these aircraft. 

Wake turbulence (more correctly called wake vortices due to their structured
nature) is not caused by thrust, but by a wing that is creating lift.  For
a wing to create lift, it must have circulation about it.  Circulation is
sort of a measure of the vorticity about the wing, and vorticity is more or
less the angular velocity of the fluid around the wing.  Circulation can
have no end, so at the wingtip it appears as a rapidly rotating cylinder of
air. (n.b.  many people and books will tell you that high pressure air
below the wing trying to get to the low pressure air on top of the wing is
what creates wingtip vortices, if this is true, why does it only happen at
the wingtips, and why does it form well structured vortices...something
that doesn't just "happen"?).

As the angle of attack (the angle between the wing chord line and the
relative wind) increases, circulation increases, thus so does the strength
of the vortices.  A slow, heavy aircraft with flaps up will produce
stronger vortices than the same aircraft when it is going faster or has
the flaps down.

So, yes, the 777 will produce some strong wake vortices.  As for whether
the jet blast from the 777 will be much faster, I do not know for sure, but
I suspect they get the massive amounts of thrust from accelerating a larger
volume of air rather than accelerating a "normal" amount of air through a
greater change in velocity (I guess this from the 112" fan disk diameter).
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