Re: wake turbulence

From:         rdd@netcom.com (Robert Dorsett)
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.  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.  

No, thrust can provide a threat to aircraft and personnel (and buildings)
on the ground, with very high wind velocities, but in-flight wake turbulence
(and on-runway wake turbulence) is primarily the result of the *lifting*
capability of the wing.  When the wing is producing lift, vortices are 
spilled off the wingtips.

>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 issue is covered surprisingly well in the July issue of _FLYING_.
The bottom line is that different people are saying different things.  The
FAA is proceeding in one direction.  NOAA researchers are saying 
others.  And the NTSB has axes to grind in a third direction.

Historically, the 757 issue went something like: NTSB starts muttering about
the 757 in mid-1993.  NOAA's comments are taken out of context by the press 
in late 1993.  FAA starts damage control in early 1994.  NTSB publishes wake 
vortext report this spring.

Everyone's saying something a bit different, but the bottom line is that
the 757's wing-flap geometry can produce *relatively* heavy wake vortices,
but not *spectacularly* heavy for airplanes of its weight.  The *risk* of
the vortices comes from the 757's performance: it can out-climb just about
every other transport out there by a factor of 1.5 to 2.  This means that
classic restrictions on vortex separation may not work: you can't stay "at
or above" another airplane's wake if you can't stay at or above the other
airplane.  

To make things even more complicated, the NOAA research, which wasn't 
primarily concerned with the 757--as a research issue, at least--also showed
that the characteristics of wake turbulence in general may be more complicat-
ed than once thought: i.e., the effects may stay around a lot longer than
the 500 fpm sink rate used as a rule of thumb.

It's probably important to note that the relationship between the NTSB and
FAA seems to be at an all-time low.  This adds a *political* dimension
to the equation.

Lastly, I'm interested in seening the NOAA report.  I've only seen references
and rehashed versions of it, none of which have a complete citation.  Any
pointers would be of interest.




--              
Robert Dorsett                                                       
rdd@netcom.com