Miata Mailing List: February 1996, Message #210
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   From: a.mccombs3@genie.com
Subject: Re: NACA "Duck"
   Date: Sun, 4 Feb 1996 11:39:19 -0500

OK, I gotta jump in on this aerodynamic stuff.  Dr. Wm. Peter Lee notes
something about NACA = Nat'l Agency for Civil Aviation.  Well, back when it
was NACA (and I don't remember when it was founded), it was the National
Advisory Committee on Aeronautics.  It wasn't an official federal govt.
agency or administration, but more of a pure research entity.  They did
develop all kinds of good stuff, from laminar flow wings, to various safety
features, and also our NACA duct.  Later on, during our "Oh, no, the
Russkies are getting ahead of us in space" period, those wise folks who run
our country determined that we needed an organization that handled stuff
outside the atmosphere too.  Might as well expand an already existing
organization, so they did.  NACA became NASA, the National Aeronautics and
Space Administration.  Yes, they still do stuff in aeronautics, as the name
implies, and much of what they do is pretty useful, even to the extent that
Detroit and other auto manufacturers have been able to make some limited use
of some of it.

Now:  Does the NACA duct on the Miata headlight door do anything for the
Miata?  I dunno.  According to theory, they work most efficiently when there
is some slight positive pressure gradient.  The duct was developed mainly to
decrease the drag of an external scoop of some sort, which it does nicely.
But the external scoop is a bit more tolerant of different pressure
gradients than is the NACA duct.  While it appears that the Miata headlight
doors do face the relative wind (most of the time you're driving, anyway),
you'd be surprised at how the subsonic airflow behaves around a curved body
like a Miata nose.  I doubt that there is any positive dynamic pressure
there, or if any, very damn little of it.

One thing a person could do to test this, is to rig up a manometer of some
sort from that area to the cockpit.  Get some clear plastic tube and fill it
mostly with some liquid you can see, and stick one open end under the closed
headlight door and the hood (or in your NACA duct, if you have one
installed).  Tape the other end to the dash/console/whatever, and park the
car on level ground.  See where the water level is in the cockpit.  Then go
for a drive, and see whether the water level in the cockpit rises or falls
at a steady speed.  (All bets are off during acceleration and braking.)
Probably best if you try this with the top down, as the airflow around an
enclosed space tends to cause a pressure drop within the enclosure, which
could screw up the works here.  At least roll one window down, if you leave
the top up.

If you are into aviation and really want to get trick, you can borrow a
vertical speed indicator and connect your line to the VSI.  (No water in the
line this time, please.)  Soaring types can use a variometer, which will
provide all the accuracy you'll ever want for this experiment.

I'm mildly curious, but not sufficiently so to go out and mess with this
myself.  Lemme know how it works out.
                                                --Jack M. & KANSEI--