On Thu, 19 May 2005 12:37:37 -0500, "MJC" wrote:

Since I admit that I have no background in the physics of aerodynamics,
I'd like to throw out this question to the group.

Those of you familiar with the RV series know that a standard Lycoming
engine without the forward facing sump requires that little forward facing
snorkle inlet at the bottom of the cowl.
However, I'm guessing that the snorkle creates some drag there that
costs a few mph. At least those who seem to know suggest that.
So what's the possibility of feeding the updraft sump with air by
instead installing a flush NACA duct at the same location?

Just wondering out loud.

MJC

That type of inlet opening is known as a "pitot" (pronounced pee -
toe) type opening and they are built that way because they have proven
over the years to provide the maximum inlet pressure possible. It's
kind of a low grade supercharger. Extremely low grade, but better
than a flush inlet.

The problem with a flush type inlet, even a NACA inlet is that air has
mass and a certain amount of viscosity and it wants to get out of the
way of objects hurtling through it. So it parts, and begins to part
well ahead of the approaching object. By the time the cowling
arrives, the air is by now trying very hard to get out of the way and
a lot of it will flow right over the flush inlet without ducking in.
Some does duck in of course, because the inlet is connected to the
intake manifold which operates at a lower pressure than atmosphere, so
it's always trying to suck air in.

The engine would run even if there were no carburetor inlet opening,
it would just draw on the air inside the cowling, but a certain amount
of added power can be realised by pressurizing the inlet, by whatever
means.

Making the inlet a pitot type, protrudes the opening out into the
incoming air before it starts dodging the bluntness of the cowl
(essentially) which allows the air to pile straight down the inlet
thus mildly pressurizing it.

This does not impart much drag, if any, because the inlet is within
the flat plate area of the fuselage, and is facing directly forward.

By the way, at the speeds we're talking about, a mildly rounded inlet
lip works better than something sharp because you want the air to
attach to the sides of the inlet rather than be broken off by a sharp
edge and tumble into turbulent flow.

So the short answer, in my opinion, is that a flush inlet, any flush
inlet including a NACA type, will very likely produce less power and
therefore less speed than the already tried and proven pitot type
inlet, as long as this pitot type inlet is sized and designed
properly. And the pitot type inlet should not produce any more drag
than the flush type.

Corky Scott