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Combustion air inlets

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fklein(at)orcasonline.com
Guest

Posted: Sun Jun 06, 2010 3:42 pm Post subject: Combustion air inlets

On Jun 6, 2010, at 12:38 PM, Frans Veldman wrote:

 	  Quote: 	
		  While solving my cooling problems, I learned a few things:

1) Naca ducts provide no ram air, but merely "allow air to get in". 	




To all you 912S powered flyers,

Page 8-1 of the Europa "912 Rotax installation manual" states: 

 	  Quote: 	
		  The engine inlet air is fed through a NACA duct situated in the top cowling. It passes into a plenum 

chamber which feeds the air directly to the carburettors. This ensures that the coolest air available is 

used at all times, thus ensuring no reduction of engine performance. 

	




If Frans is correct, it would appear to follow that the 912S plenum does not offer inlet air to the carbs under ram air pressure.



And since the 914 engine inlet is a NACA vent also, the same would apply.

I'm surprised...having thought that engine performance benefits when the carb gulps air which is above ambient pressure.

Any wisdom to pass on to a heretic?

Fred

A194 (w/ Sube)



 [quote][b]

frans(at)privatepilots.nl
Guest

Posted: Sun Jun 06, 2010 10:22 pm Post subject: Combustion air inlets

On 06/07/2010 01:41 AM, Fred Klein wrote:



 	  Quote: 	
		   If Frans is correct, it would appear to follow that the 912S plenum does

 not offer inlet air to the carbs under ram air pressure.


	


There is quite some information available on the internet about

naca-ducts, where they were designed for, and what they can and can't do.



Naca ducts were designed for applications where you want to "let air in"

without drag penalty. They harvest air from the boundary layer, the

non-moving layer of air attached to the airplane.

If you need RAM air, you need something to protrude through the boundary

layer, a sort of scoop, or a frontal inlet.



See:

http://www.flyingmag.com/scoop-naca-scoop

" ... The conclusions of the Ames group's 1945 report were upbeat, but

they included a caution that designers have been ignoring ever since.

"The submerged inlet is essentially a high inlet-velocity-ratio type in

contrast to wing-leading-edge and fuselage-nose inlets," they wrote.

This characteristic limited its most efficient use to systems "that

require only a small amount of diffusion, such as the internal ducting

for jet motors of the axial-flow type." Submerged inlets were unsuitable

for "oil coolers, radiators, or carburetors of ... reciprocating

engines," the report continued ... "



Of course, if there is anyone willing to measure the pressure inside a

naca duct against the static port (ASI ?) we know for sure. But as far

as I can see, the 912S naca inlet doesn't provide any ram effect to the

inlet air.



Frans

fklein(at)orcasonline.com
Guest

Posted: Sun Jun 06, 2010 10:48 pm Post subject: Combustion air inlets

On Jun 6, 2010, at 11:15 PM, Frans Veldman wrote:

 	  Quote: 	
		  --> Europa-List message posted by: Frans Veldman <frans(at)privatepilots.nl (frans(at)privatepilots.nl)>



On 06/07/2010 01:41 AM, Fred Klein wrote:



 	  Quote: 	
		  If Frans is correct, it would appear to follow that the 912S plenum does

not offer inlet air to the carbs under ram air pressure.

	


There is quite some information available on the internet about

naca-ducts, where they were designed for, and what they can and can't do.

	




Frans...that's fascinating...I must find a way to remove that conditional "if" above!

Thank you...I always assumed that designers were always looking for a little ram-air boost...perhaps the low-drag benefit of the NACA inlet rules over other considerations,

Fred

 [quote][b]

Gary.Leinberger(at)miller
Guest

Posted: Mon Jun 07, 2010 10:57 am Post subject: Combustion air inlets

My understanding is that the Bing carbs on the 912S are not designed for ram air - on my Europa I am using the 914 FWD kit on a 912S  (I originally planned on a 914 but the price differences of recent years seems excessive). This has the advantage of not taking in water during rain or when on the tarmac. The air box around the NACA inlet is sealed mainly so that only cool air enters the carbs - but the air filter box is vented to the carbs so that there is no excess pressure in the air going to the carbs. The 914 is boosted, but requires numerous other gadgets to allow the Bing carbs to function properly with the excessive pressure.



Gary Leinberger

A237

Lancaster, Pa.



95% done, 95% to go.



________________________________________

From: owner-europa-list-server(at)matronics.com [owner-europa-list-server(at)matronics.com] On Behalf Of Fred Klein [fklein(at)orcasonline.com]

Sent: Sunday, June 06, 2010 7:41 PM

To: europa-list(at)matronics.com

Subject: Combustion air inlets



On Jun 6, 2010, at 12:38 PM, Frans Veldman wrote:



While solving my cooling problems, I learned a few things:

1) Naca ducts provide no ram air, but merely "allow air to get in".



To all you 912S powered flyers,



Page 8-1 of the Europa "912 Rotax installation manual" states:



The engine inlet air is fed through a NACA duct situated in the top cowling. It passes into a plenum

chamber which feeds the air directly to the carburettors. This ensures that the coolest air available is

used at all times, thus ensuring no reduction of engine performance.



If Frans is correct, it would appear to follow that the 912S plenum does not offer inlet air to the carbs under ram air pressure.



And since the 914 engine inlet is a NACA vent also, the same would apply.



I'm surprised...having thought that engine performance benefits when the carb gulps air which is above ambient pressure.



Any wisdom to pass on to a heretic?



Fred

A194 (w/ Sube)

grahamsingleton(at)btinte
Guest

Posted: Mon Jun 07, 2010 3:29 pm Post subject: Combustion air inlets

Don't think it matters Ram or not Ram, what's important is that the float chamber sees the same pressure as the inlet throat. It is important the the airflow is same velocity across the diameter. Variation will upset the mixture and smooth elbows into the carb will give varying velocity. Best if the air goes in straight from the plenum not elbowed.

Graham

From: Gary Leinberger <Gary.Leinberger(at)millersville.edu>

To: "europa-list(at)matronics.com"  <europa-list(at)matronics.com>

Sent: Monday, 7 June, 2010 19:56:00

Subject: RE: Combustion air inlets

--> Europa-List message posted by: Gary Leinberger <Gary.Leinberger(at)millersville.edu (Gary.Leinberger(at)millersville.edu)>

My understanding is that the Bing carbs on the 912S are not designed for ram air - on my Europa I am using the 914 FWD kit on a 912S  (I originally planned on a 914 but the price differences of recent years seems excessive). This has the advantage of not taking in water during rain or when on the tarmac. The air box around the NACA inlet is sealed mainly so that only cool air enters the carbs - but the air filter box is vented to the carbs so that there is no excess pressure in the air going to the carbs. The 914 is  boosted, but requires numerous other gadgets to allow the Bing carbs to function properly with the excessive pressure.

Gary Leinberger

A237

Lancaster, Pa.

95% done, 95% to go.

________________________________________

From: owner-europa-list-server(at)matronics.com (owner-europa-list-server(at)matronics.com) [owner-europa-list-server(at)matronics.com (owner-europa-list-server(at)matronics.com)] On Behalf Of Fred Klein [fklein(at)orcasonline.com (fklein(at)orcasonline.com)]

Sent: Sunday, June 06, 2010 7:41 PM

To: europa-list(at)matronics.com (europa-list(at)matronics.com)

Subject: Europa-List: Combustion air inlets

On Jun 6, 2010, at 12:38 PM, Frans Veldman  wrote:

While solving my cooling problems, I learned a few things:

1) Naca ducts provide no ram air, but merely "allow air to get in".

To all you 912S powered flyers,

Page 8-1 of the Europa "912 Rotax installation manual" states:

The engine inlet air is fed through a NACA duct situated in the top cowling. It passes into a plenum

chamber which feeds the air directly to the carburettors. This ensures that the coolest air available is

used at all times, thus ensuring no reduction of engine performance.

If Frans is correct, it would appear to follow that the 912S plenum does not offer inlet air to the carbs under ram air pressure.

And since the 914 engine inlet is a NACA vent also, the same would apply.

I'm surprised...having thought that engine performance benefits when the carb gulps air which is above ambient pressure.

Any wisdom to pass on to a heretic?

Fred

A194 (w/ ics.com/Navigator?Europa-List" target="_blank">http://www.matronics.com/N=               - MATRONICS WEB FORUMS=   -->

craigb(at)onthenet.com.au
Guest

Posted: Mon Jun 07, 2010 4:24 pm Post subject: Combustion air inlets

The  other thing worth consideration, is the longer (within reason) and smoother  the transition from plenum to carb throat the better

 the  perfomance, this allows the air to continue accelerating with minimal  turbulence, giving the best possible atomisation of the fuel

 into  the airflow. Hence the reason for the velocity stacks (trumpets) seen on a lot  of natually aspirated race engine. 

 [quote]   --

grahamsingleton(at)btinte
Guest

Posted: Tue Jun 08, 2010 12:57 am Post subject: Combustion air inlets

Yes, but note that they never go into an elbow, always straight in to the carb

Graham

From: craig bastin <craigb(at)onthenet.com.au>

To: europa-list(at)matronics.com

Sent: Tuesday, 8 June, 2010 1:29:36

Subject: RE: Europa-List: Combustion air inlets



ï»¿    DIV { MARGIN:0px;}    The  other thing worth consideration, is the longer (within reason) and smoother  the transition from plenum to carb throat the better

 the  perfomance, this allows the air to continue accelerating with minimal  turbulence, giving the best possible atomisation of the fuel

 into  the airflow. Hence the reason for the velocity stacks (trumpets) seen on a lot  of natually aspirated race engine. 

 

  [quote][b]

frans(at)privatepilots.nl
Guest

Posted: Tue Jun 08, 2010 4:02 am Post subject: Combustion air inlets

On 06/08/2010 02:29 AM, craig bastin wrote:

 	  Quote: 	
		   The other thing worth consideration, is the longer (within reason) and

 smoother the transition from plenum to carb throat the better

 the perfomance, this allows the air to continue accelerating with

 minimal turbulence, giving the best possible atomisation of the fuel

 into the airflow. Hence the reason for the velocity stacks (trumpets)

 seen on a lot of natually aspirated race engine.


	


Keep in mind that the inlet pulses on a Rotax are not balanced. Each

carb has to feed two successive cylinders, and then pauses for two

successive cylinders. This causes turbulence, uneven mixture over the

two successive inlet cycles, and a lot of other problems. We can't do

much about this (except for fitting a much larger balance tube) so

efforts to smooth the air going into the carbs is probably pretty much

wasted.



Frans

frans(at)privatepilots.nl
Guest

Posted: Tue Jun 08, 2010 4:10 am Post subject: Combustion air inlets

On 06/07/2010 08:56 PM, Gary Leinberger wrote:



 	  Quote: 	
		   My understanding is that the Bing carbs on the 912S are not designed

 for ram air - 


	


They can handle ram air fine. Just make sure that also the float chamber

vent sees the same pressure. (This means that the whole carb will see

the elevated pressure, the higher pressure due to ram air is then no

different than the higher pressure due to flying at a lower altitude.

The Bing carbs are especially designed to deal with a lot of different

inlet pressures.)



 	  Quote: 	
		   The 914 is boosted, but requires

 numerous other gadgets to allow the Bing carbs to function properly

 with the excessive pressure.


	


Not true. The carbs are the same. All the "gadgets" you see on a 914 are

for different reasons, not to make the carbs work correctly. And of

course, the float chamber vents are connected to the airbox, so they see

the higher pressure as well.



If I had a 912, I would try to feed the engine with as much ram air as

possible.



Frans

peterz(at)zutrasoft.com
Guest

Posted: Tue Jun 08, 2010 4:38 am Post subject: Combustion air inlets

re: ram-air to the carbs:  I believe there were previous threads discussing this, and the analysis showed that the ram-air effect is negligible at our speeds - but the drag penalties could be more significant.

fwiw, ymmv, etc.

Cheers,

Pete

A239

On Tue, Jun 8, 2010 at 8:08 AM, Frans Veldman <frans(at)privatepilots.nl (frans(at)privatepilots.nl)> wrote:

[quote] --> Europa-List message posted by: Frans Veldman <frans(at)privatepilots.nl (frans(at)privatepilots.nl)>

 On 06/07/2010 08:56 PM, Gary Leinberger wrote:

 > My understanding is that the Bing carbs on the 912S are not designed

 > for ram air -

 They can handle ram air fine. Just make sure that also the float chamber

 vent sees the same pressure. (This means that the whole carb will see

 the elevated pressure, the higher pressure due to ram air is then no

 different than the higher pressure due to flying at a lower altitude.

 The Bing carbs are especially designed to deal with a lot of different

 inlet pressures.)

 > The 914 is boosted, but requires

 > numerous other gadgets to allow the Bing carbs to function properly

 > with the excessive pressure.

 Not true. The carbs are the same. All the "gadgets" you see on a 914 are

 for different reasons, not to make the carbs work correctly. And of

 course, the float chamber vents are connected to the airbox, so they see

 the higher pressure as well.

 If I had a 912, I would try to feed the engine with as much ram air as

 possible.

 Frans

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