BIG SNIP

 I personaly feel that the NACA ducts are highly over rated. I do not

think you will

 see where any
 serious designer uses a NACA duct for critical high flow inlets. Cockpit

air, and

 possibly
 engine inlet (Carb, inj, etc) are about the only place I would consider

them. Engine

 cooling, oil coolers and radiators deserve a real inlet. Vance

Bill Freeman wrote:

I've got 800+ hrs on my main engine NACA inlet, had to partially close it
down as
the engine ran too cool.  It is a low drag inlet with SPECIFIC needs, does not
work just anywhere and can be easily misapplied.   A big ole scoop well
out into the airstream will pickup air, but at what cost in drag??  Depends on
where you can locate inlets on your airframe and where you can put outlets.
I still feel that the natural external diffusion that you get for free on
the front
of the fuselage is highly misunderstood even tho most tractor aircraft use this
type of inlet.  The difference in drag between a big old dumb hole over the
whole front of my '64 C182 and the later "advanced" inlets (about 25% of
the inlet area) of the late 70s C182s (which have cooling problems, I have
heard) is pretty darned small.   It's also why the WWII radial engined
US fighters never bothered with a spinner - the external diffusion was so
efficient that they couldn't materially improve over it.  I know the F6F
LOOKS like it should have huge cooling drag, but adding a big spinner and
closing down the inlet was tried and it was not significantly faster and was
significantly heavier and harder to inspect and maintain.  External diffusion,
efficient, natural pressure recovery.
NACA ducts are not for everyone and every application.  Use as intended
and they work well.   For instance, I tried one for my heater inlet - poorly
located, thick boundary layer (relative to the duct size) meant poor pressure
recovery.  An external diffusion "snorkel" inlet a bit like a ME109 carb inlet
gets excellent pressure and no measurable reduction in cruise speed.  Good
in one location and size, bad in another location and size, same airframe.
Bill Freeman

I think the reason NACA ducts are working on the pushers is the prop is
sucking air out of the back of the cowl.
Paul Lamar

ddunbar wrote:

I agree Paul that the air outlet is more important. This works in all static ventilation systems.
Don Dunbar

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