Re: Vortex generators on engine nacells?

From:         David Lednicer <dave@amiwest.com>
Organization: Analytical Methods, Inc.
Date:         27 Jun 96 12:56:55 
References:   1
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Maury S. Markowitz wrote:
>
>   I just took my first ride in a 737-400 (I'm typically flying longer
> flights on larger planes) and noticed what appears to be a two-hand sized
> fin on the fuselage side of the engine.  This sticks upward at about 45
> degrees, and was generating a rather noticeable vortex (visible as well
> passed through some humid layers on takeoff).
>
>   I later took a tour of the airport (the new Denver abomination) and
> noticed a similar fin on a number of planes, including DC-10's and 767's.
> What is the purpose of these?

	The location of the seperation point on the sides of a turbofan
nacelle, at high angles of attack, is rather unstable and can degrade the
wing's maximum lift coefficient if it moves too far up the nacelle.  This
effect is strongest for closely coupled nacelles.  As the 747's nacelles
are fairly far away from the wing, this effect is not strong on this
aircraft.

  Douglas originally discovered this problem on the DC-10 and to fix it,
put two "strakes", one on each side of the nacelle, on the DC-10.  These
strakes fix the seperation point and shed a vortex, which helps improve
the maximum lift coefficient of this part of the wing.

  Boeing discovered the same effect on the 707-700, a 707 reengined with
CFM56s.  Douglas had the patent for one on EACH side of the nacelle, so
Boeing used one only on the inside of the nacelle, to get around the
patent.  Boeing calls them nacelle chines has the patent on having one
per nacelle!

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David Lednicer             | "Applied Computational Fluid Dynamics"
Analytical Methods, Inc.   |   email:   dave@amiwest.com
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