Sid wrote:
Yes, I have some literature discussing the misapplication of these
submerged NACA inlets. Unfortunately, the previous owner/builder
installed them because the original scheme(s) were apparently worse.
I am willing to tradeoff a little extra drag for better cooling... I
am just not certain yet how best/easiest to do it.

OK, Jim Morgan's comment about submerged NACA ducts are "poor at cooling
applications" is perhaps overly broad. The NACA papers actually say they
are poor choices to feed air to heat exchangers, also known as radiators and
oil coolers. They are silent as to their application for cooling air to
traditional "air cooled" (I hat that term) engines.

But nonetheless, I'd like to add my speculation (it *is* Usenet y'know ;-)

The builder's choice of submerged NACA ducts seems to have several things
working against it in this case:

1. The photo seems to show submerged ducts, but they seem exceedingly narrow
and improperly proportioned compared to my recollection of the drawings of
the original submerged NACA ducts. How likely is it that the builder's
ducts are versions of actual tested NACA ducts and not some random ducts
that the builder preferred?

2. Any air inlet, NACA or not, works best when the inlet is positioned in a
(relatively) high air pressure region, and works poorly or even in reverse
when placed in a (relatively) low air pressure region. I do not have a
pressure region map for the Velocity, but you as an owner/(re-)builder have
access to more information than I. See if you can locate a pressure map.
Just speculating, the site just aft of the point of greatest fuselage height
(where the ducts are now) seems like a local *low* pressure area.

3. Submerged NACA ducts work abysmally in areas of turbulent separation of
flow. The photo seems to indicate that turbulent separation is occuring at
the rear door seam. If you need air, and you're constrained to get it from
an area of turbulent separation, then there are only two choices: either the
thing that wants the air better be at substantially lower air pressure than
the turbulent area, or you use a scoop with a standoff to keep the turbulent
boundary layer out of the captured air.

4. The position of the existing ducts suggests that the engine uses
downdraft cooling, i.e. the cooling air is introduced on the top side of the
cylinders and pressures are used to force it out the bottom side of the
cylinders. Downdraft cooling is prone to problems at low airflow velocities
because of the tendency of heated air to rise. Low airflow velocities tend
to coincide, unfortunately, with situations that need the best possible
cooling, namely climbs (high power, low airspeed).

Regards,
Russell Kent