Re: B757 "Heavy Jet"?

From:         tim@me.rochester.edu (Tim Takahashi)
Organization: University of Rochester, School of Engineering
Date:         24 Jun 96 12:24:49 
References:   1 2
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>     Hello I was wondering if someone can help me understanding why the
> FAA decreed the 757 as a Heavy Jet.

Dfwmech@concentric.net writes :
>I have heard it is because of the large amount of wing vortex
>turbalance the 757 creates during takeoff and landing.

Strictly speaking, vortex strength != turbulence; although
the seat of the pants feel in the following aircraft is similar.

Gary Moffitt writes :
>Therefore the FAA has called the 757 a heavy jet so that
>controllers will maintain a greater degree of separation between the 757
>and other aircraft

A wise move.... as one travels downstream from the aircraft,
the wing vorticies roll up and translate downwards (the after-effect
of downwash). Greater interaircraft spacing will allow additional
time for the vortex to translate out of the flight path.

C. Martin Faure writes :
>I believe its category was changed because of its wingspan and wingtip
>vortex characteristics.  The 757, like the 767, was given very long wings
>to maximize its efficiency.  These wings can generate fairly strong tip
>vortices (sp?).

A point of confusion is the net strength of the far field wing vortex
with the strength of the near field wing-tip vortex. In theory, the
higher aspect ratio wings should have lower induced drag, and a weaker
wingtip vortex for a given net wing loading. Another point to
consider is that aircraft, such as the 757, have a significant
amount of taper and twist in their wings... the span loading should
diminish towards the wingtip resulting in vorticity being shed
over the entire span (not just at the wingtip).

John Wangermann notes :
>Operational experience and some experiments have shown that while the
>wake vortices of 757s are not unusually strong, they have a very tight core
>that is very persistent

This may be due to a number of factors... my personal hunch being
the lack of thrust gates in the high-lift system on the 757.
Research at NASA/Ames during the 1970s (Rossow, Corsiglia, et al)
demonstrated that the far-field wake vortex behavior can be
modulated by changes in the flap configuration. Experiments made
with the then new B-747 had revealed that unconventional flap
rigging could produce vorticies which interacted in the near
field so that they would actually disperse in the far field.

The 757 has a very clean high-lift system, with essentially
an inboard and outboard flap edge vortex for each wing. The
chances for a favorable interaction of these vorticies a-la
Rossow and Corsiglia seems remote to me. In any case, the
induced drag of the 757 high lift system is lower than some
other aircraft... the good points of this design are better
takeoff and climb performance with the flaps deployed.

-tim