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wiper airfoils

>>From: Gary H <garyh@domain.elided>
>>Date: Thu, 8 Dec 94 1:30:36 PST
>>Subject: E30 M3 Wipers / M3 Performance chips

>>Has anyone noticed that the wind deflector is only present on the driver
>>side wiper.  Why? 


This can be explained by the "diffluence" of the airstream
as it flows across the windshield -- imagine the airflow coming 
straight up the center of the hood, and then straight up across 
the center portions of the windshield; along the driver's side, 
the airflow curves upward and outward toward the driver's A-pillar, 
while along the passenger side, it curves upward and outward toward
the passenger's A-pillar.  Hence the diffluence.  (there may be
divergence as well, but that's not important now...)

Now imagine the sweep path of the driver's wiper arm/blade.  At all
times, its axis is somewhat perpendicular to the air flowing upward and
outward toward the driver's A-pillar.  This can create "lift" and
"skipping" of the blade assembly (similar to how air flowing across
an aircraft wing creates lift).  An airfoil serves to offset
this phenomenon.

An airfoil is not needed on the passenger-side wiper arm because
during the wiping motion, the axis of that wiper arm/blade is
generally parallel to the airflow across the passenger side of
the windshield.  Lift is insignificant.  Or at least it is
to me...I don't have an M3.

Scott Bachmeier
NASA Langley Research Center
Hampton,  VA
'85 325e (complete with a driver's-side wiper airfoil)