RV Snorkle Needed?

[MJC]

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

[Corky Scott]

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

[MJC]

Thanks Corky, that seems to explain why no one has done it yet on an RV
that I know of.
The reason I was thinking of that is because the forward facing sump
option on the 180HP Lycoming is $2200, and you pick up even more speed than
the ram effect from the FFS gives you because you can get rid of the snorkel
entirely. I was just wondering if there were a way to streamline things
better without digging deeper into near-empty pockets.

MJC

"Corky Scott" > wrote in message
...
> 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
>
>

[Charlie]

The more pragmatic reason is that the updraft carb or throttle body &
air filter box extend below the line of the bottom cowl. A bump is
required so putting the snorkel inlet on the front of the bump gives the
slight pressure recovery mentioned below. It's doubtful you could
measure the difference in speed because most of the variants extend back
to the back edge of the cowl, forming a 'bluff body' that helps extract
cooling air. Cooling efficiency is improved & exit air fills the 'hole'
in the air dug by the snorkel/filter box cover. (Snorkel-less cowls do
*look* faster, though...)

Charlie
flying RV-4, -7 in the oven


MJC wrote:
> Thanks Corky, that seems to explain why no one has done it yet on an RV
> that I know of.
> The reason I was thinking of that is because the forward facing sump
> option on the 180HP Lycoming is $2200, and you pick up even more speed than
> the ram effect from the FFS gives you because you can get rid of the snorkel
> entirely. I was just wondering if there were a way to streamline things
> better without digging deeper into near-empty pockets.
>
> MJC
>
> "Corky Scott" > wrote in message
> ...
>
>>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
>>
>>
>
>
>