Re: Trailing edge wedge?

Date:         19 Apr 99 02:22:37 
From: (Mark Drela)
Organization: Massachvsetts Institvte of Technology
References:   1
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In article <airliners.1999.378@ohare.Chicago.COM>, writes:
> In the latest issue of Av Week (8 March 99) there is an article on
> Boeing's plans.  Included for the 747-400 is something called a
> 'trailing edge wedge'.  Apparently this would have given fuel savings
> of 7% on the MD-11, and works even better on the 747.  "The trailing edge
> wedge was great on the MD-11, but it is stunning on the 747".
> It seems surprising there are still relatively easy 7% improvements laying
> around.  Does anyone know what this wedge is, and the basic idea behind
> it?  Why is it only being introduced now?

The TE wedge is a somewhat more sophisticated
version of the Gourney flap.  Preston Henne,
formerly of Douglas, did an even more sophisticated
design which he called the "Divergent Trailing Edge".
This was in an AIAA paper about 5 years ago.

These things dramatically improve transonic airfoil
performance, PROVIDED the airfoil has little aft
camber to begin with.  I suspect the 747 airfoils
have relatively little aft camber, which wasn't
in vogue quite as much back then.  Hence, the
TE wedge is beneficial.  The 777 airfoil would
see little if any benefit, since it has a large
aft camber to begin with.

One drawback of the TE wedge is that it increases
the airfoil's pitching moment quite a bit.  This
is certainly an important consideration in a large
plane like the 747, which would likely see a
significant added structural twist from the wedge.
As with all aeronautical things, it's a tradeoff.

  Mark Drela                          First Law of Aviation:
  MIT Aero & Astro          "Takeoff is optional, landing is compulsory"