Re: wake turbulence

From:         ehahn@fairlite.mitre.org (Ed Hahn)
Organization: The MITRE Corporation, McLean, Va.
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.  I heard on the radio that it is now known
   that the 757 has significant wake turbulence, just like a "heavy".  The
   report did not say, as I recall, but I again guessed it was due to engine
   thrust.  If that's the case, what sort of wake turbulence will a 777 put
   out?  The thrust on those engines is amazing!  Are there high-engine-thrust
   airplanes w/o wake turbulence problems?  Why did the 757 problem come as
   a surprise?

------
Wake Turbulence is primarily caused by wingtip vortex shedding, not
engine thrust.  The wingtip vortex strength is proportional to the
amount of lift the wing must generate to stay in the air (to the first
order).

The reason that the B757 was somewhat of a surprise is that the
particular wing design chosen for that aircraft turns out to have very
high intensity wake vortex for its weight, probably as an outgrowth of
the wing's aerodynamic efficiency.  In fact, I recall reports that the
tangential velocities of the air in the vortex exceeded any previous
measurement done on any airliner.  However, the total energy of the
vortex is smaller than that of a B747, say, because the vortex itself
occurs in a smaller volume.  However, if you happen to fly into that
small area, it appears that you are in for one heck of a ride!

ed

////////   Ed Hahn | ehahn@mitre.org | (703) 883-5988   \\\\\\\\
The above comment reflects the opinions of the author, and does not
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Really, I wouldn't kid you about a thing like this.