Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!ncar!unmvax!tut.cis.ohio-state.edu!att!cbnews!military
From: shafer@elxsi.dfrf.nasa.gov (Mary Shafer)
Newsgroups: sci.military
Subject: Re: F/A-18 modifications
Message-ID: <8710@cbnews.ATT.COM>
Date: 1 Aug 89 03:00:30 GMT
References: <8673@cbnews.ATT.COM>
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Organization: NASA Dryden, Edwards, Cal.
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From: shafer@elxsi.dfrf.nasa.gov (Mary Shafer)

In article <8673@cbnews.ATT.COM> nassio%cfassp12@harvard.harvard.edu (George Nassiopoulos) writes:

>I have gone to two airshows this summer and have noticed a slight
>modification to the F/A-18 Hornets.

>The modification i am speaking about is the addition of two identical
>canard-like objects (winglets?) to the ``chines'' of the aircraft.
>(I don't know if ``chines'' is the correct term -- i mean the parts
>of the fuselage which begin at the canopy and extend out towards the
>roots of the wings.  They provide a sort of ``hood'' over the engine
>intakes and give the F/A-18 a cobra-like appearance.)
>The ``winglets'' are bolted onto the ``chines'' almost directly
>above the engine inlets and they stick outwards almost at the same
>angles as the aircraft's twin tails.

>Does anyone know what these additions are?  They could conceivably
>be antennae/sensors, but my hunch is that they were placed there for
>aerodynamic/stability reasons.  Anyone know for sure?

They're strakes added to reduce the buffet loads on the vertical
stabilizers.  (Yes, I know they aren't really vertical :-)) The
leading edge extensions (LEXs), which are what you're calling "chines",
generate a vortex which impinges on the verticals.  The buffet from
the LEX vortes at high angles of attack is damaging the verticals by
fatiguing them.  The strakes keep the vortex burst point off of the
verticals.

If you want an idea about the LEX vortices and their impingement on
the verticals, try to find the AvWeek with the NASA Dryden F-18 on the
cover (Sometime in May, I think).  We inject smoke into the LEX
vortex, making it visible, and you will readily see where these
vortices are and why they're so hard on the vertical.

Just a quick (and probably tedious) note on nomenclature--the tail of
an aircraft is just the back end.  The rudder is on the vertical
stabilizer and the elevator/elevon is on the horizontal stabilizer.
An elevon is a control surface which provides both pitch (elevator)
and roll (aileron) commands.  If the entire horizontal stabilizer is a
control surface, it's a stabilator (combination of elevator and
stabilizer) or, particularly for larger airplanes, a flying tail.
Also on larger airplanes, an elevon is sometimes called a rolling
tail.  However, the flying and rolling tail terms are older terms and
not very common.

--

M F Shafer                          shafer@elxsi.dfrf.nasa.gov
NASA Ames Research Center           arpa!elxsi.dfrf.nasa.gov!shafer
Dryden Flight Research Facility
                Of course I don't speak for NASA