Re: Leading Edge fences

Date:         01 Oct 97 14:08:45 
From:         ehahn@mitre.org (Edward Hahn)
Organization: The MITRE Corporation
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In article <airliners.1997.2330@ohare.Chicago.COM>, howard@perth.dialix.com.au (Howard Jones) wrote:
>k_ish (kenish@ix.netcom.com) wrote:
>: rosevax wrote:
>: > Regarding wing fences:
>: > Do the rather wide "pontoons" (sorry, don't know the correct name) located
>: > on the lower surface of the wings of A320s & 757s serve the same purpose?
>:
>: These are called "flap track fairings", and house the tracks and some of
>: the drive mechanisms that the flaps slide on.   Since they are on the
>: trailing edge of the wing, I doubt they have much benefit in stopping
>: spanwise airflow.
>
>an english aerodynamicist, dieter keusner(sp?) used something like this
>nicknamed Keusner's Carrots (seriously) to squeeze and invigorate the airflow
>to overcome a problem on an english jet. they may be doing more than just
>housing the tracks.
>(sorry to be vague, I read his book back in the 80's)

Hmm.  I think you're referring to "Kuechemann Carrots" (spelling?), which
were also installed on the trailing edge of the Covair 990.  They were
installed to help with the area-rule distribution on the C990, which had
a problem with wave drag.

In an aircraft with a high mach cruise speed (like the C990 ~ .90), the
wave drag (which is caused by compressibility of the air and the geometry
of the vehicle) can build up to be unacceptable for fuel economy if the
body is not shaped appropriately.

The optimum wave-drag shape profile is a javelin-like body with a specific
mathematical shape (actually, it looks more like two bullets placed
back-to-back).  The describing formula relates cross-sectional area to
the longitudinal coordinate of the body.  (If the body coordinates are x,
y, z, with the longitudinal axis formed by x, the formula relates:  Area
in the y-z plane  = f(x).)

It turns out that you can get nearly the same wave drag performance out
of an arbitrarily shaped body as long as you keep to a cross-sectional
area of the fuselage and any appendages (wings, tails, etc.) similar to
the optimum distribution.  This is called the "area rule".

In the C990's case, the total cross sectional area distribution through
the body stations near the wing was insufficient to adhere to the area
rule.  The solution was to mount these "carrots" on the trailing edge,
which made up for the deficit in total cross-sectional area - this was
definitely a "tack-on" fix, and was not part of the original design.

Other aircraft of the time period (such as the F102) had the reverse
problem - the cross sectional area through the wing had more area than
optimum, and thus required the famous "coke bottle" shaping of the fuselage
to bring the cross-section area distribution back into line (the revised
F102 became the F106).

With regards to modern jets, the area rule distribution is taken into
account in the design of the aircraft: while the B757's flap fairings are
part of the overall cross-sectional distribution, they are not tack-on
fixes like the original Kuechemann Carrots.

ed
(PS.  The above is a hazy recollection of this subject from my undergraduate
days.  I invite corrections to the above.)

>>>>  Ed Hahn    |    ehahn@mitre.org    |    (703) 883-5988  <<<<
The above statement is the opinion of the author.  No endorsement
or warranty by the MITRE Corporation is expressed or implied.
Really, I wouldn't kid you about a thing like this.