Re: wake turbulence

From:         greg@bronze.ucs.indiana.edu (Gregory R. TRAVIS)
Organization: Indiana University
Date:         27 Jun 94 12:01:07 
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In <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 not, as is often believed, an engine-thrust
phenomena; it is an airframe phenomena.  Indeed, before its origins
were known, wake turbulence was referred to a "prop-wash" - early
aviators who encountered the wake erroneously believed it to be
caused by the propellor(s) of the aircraft in front of them.

In simple terms, wake turbulence results when the fuselage and
wings of an aircraft compress the air immediately below the airplane.
Since this air is at a higher pressure than the air on top of
the wings, the air attempts to "spill" out from under the airplane
and around to the low-pressure region above the wings.  As it spills,
lengthwise, from around the wingtips to the top of the wing, it is
imparted a circular motion forming two vortices behind the generating aircraft.

The wake is extremely similar to that generated by a boat on still
water.

greg