"Russell Kent" wrote in message 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?
Kent, to the best of my knowledge, this duct mod is from the Velocity
factory and I believe all (most?) current Velocitys use it. The ducts
feed a pressure cowling (also factory designed/approved?). I have
added a photo of this to the free photo web hosting site.. see
http://img87.exs.cx/img87/4545/MVC-006F6.jpg
As you can see, the cowling is a very good fit.. no gaps for
leaks..sealed around edges with RTV Si.
There is more to the story... the right-hand duct is divided about
33/66 with the 33% going to the engine air induction via an air
filter. At full power take-off I see about 27.5 inches MAP (at 800 ft
msl and measuring/reading with a Vision Microsystems 800 display).
The left duct is divided about 33/66 with the 33 going to the oil
cooler (one of two coolers... the second is up front in the nose-gear
space).
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.
....
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.
....
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).
Yes, and the increased AOA during these conditions probably make the
situation even worse.
Sid
http://img87.exs.cx/img87/4545/MVC-006F6.jpg