Re: wake turbulence

From:         plisner@mti.mti.sgi.com (Peter Lisner)
Organization: Silicon Graphics Inc.
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> wrote:
>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?

The 757 has an incredibly high rate of climb. A takeoff in one of
those (especially a noise abatement one!) is probably the most fun you
can have in an airliner. They also descend much faster than most other
aircraft on final approach. So this makes it harder for following
traffic to stay above the flight path of a 757. This last statement
sounds counter-intuitive, but what happens is that even though a
following aircraft may be above the preceding one, the first aircraft
was actually higher than the second one when it was at the same
distance from touchdown earlier. So the second aircraft is below the
wake still there from the first. There is a very good article about
this in the latest (July I think) edition of "Flying" magazine,
written by Richard Collins. Get a copy, and you will get a much better
explanation than my clumsy attempt !

All opinions are mine, not SGI's.

--
Peter Lisner                    
e-mail: plisner@mti.sgi.com
phone : (415)390-4419
Silicon Graphics Inc.