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Performance Problem Analysis

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martin(at)gbonline.com
Guest

Posted: Fri Apr 21, 2006 7:31 pm Post subject: Performance Problem Analysis

Dave,

I have an RV8 with a James Cowl and plenum and a IO-390 engine.  I believe 

that it contributes and average speed increase of 10 knots over a standard 

Vans cowl.  I have over 1300 hours with this set up.  I have also talked to 

as many others with this set up as possible, and the general consensus is 

the same as mine.  Also, cooling problems are usually not a problem with 

this cowl.

Dick Martin

RV8 N233M

the fast one

---

luckymacy(at)comcast.net
Guest

Posted: Sat Apr 22, 2006 6:47 am Post subject: Performance Problem Analysis

WRT James cowl, is it basically just the differently shaped air inlets and their inlet area that make the difference?  What's the diameter of the openings?  In other words, if one took Van's cowl and redid the openings would that theoretically lower drag??  



Or is there a fundamental difference in the air outlet area on the bottom too?



Anyone ever take a side by side photos and otherwise do a meaningful compare?



Just curious and looking for rainy day projects....

-------------- Original message -------------- 

From: "dick martin" <martin(at)gbonline.com> 



[quote] 

 

 Dave, 

 I have an RV8 with a James Cowl and plenum and a IO-390 engine. I believe 

 that it contributes and average speed increase of 10 knots over a standard 

 Vans cowl. I have over 1300 hours with this set up. I have also talked to 

 as many others with this set up as possible, and the general consensus is 

 the same as mine. Also, cooling problems are usually not a problem with 

 this cowl. 

 Dick Martin 

 RV8 N233M 

 the fast one 

 ---

khorton01(at)rogers.com
Guest

Posted: Sat Apr 22, 2006 8:01 am Post subject: Performance Problem Analysis

On 22 Apr 2006, at 10:41, lucky wrote:



 	  Quote: 	
		   



 WRT James cowl, is it basically just the differently shaped air  

 inlets and their inlet area that make the difference?  What's the  

 diameter of the openings?  In other words, if one took Van's cowl  

 and redid the openings would that theoretically lower drag??



 Or is there a fundamental difference in the air outlet area on the  

 bottom too?



 Anyone ever take a side by side photos and otherwise do a  

 meaningful compare?



 Just curious and looking for rainy day projects....


	




Lucky,



As I understand it, the performance increase comes from three places:



1. The cowl comes with a plenum chamber, which means more of the air  

that comes in the inlets actually cools the engine.  Baffles leak, so  

you need to bring in extra air to allow for that leakage.  The fact  

that the plenum chamber has less leakage means less air is needed, so  

the inlets are smaller.  The less air you bring in the inlets, the  

less cooling drag there is.



2. The contour of the inlet is designed to try to have laminar flow  

as the air expands, which results in the least amount of drag, and  

the greatest amount of pressure once the air has expanded.



3. The inlet shape is round.  Round inlets have the smallest  

circumference for a given area, so there is less frictional drag  

along the walls of the inlet.  Any frictional drag results in some  

pressure loss in the cooling air, which reduces the cooling  

effectiveness, and means you probably need a bit more air to get the  

same amount of cooling.  This is probably a very minor effect.



I believe you could achieve most of the performance increase by  

having a plenum chamber that has a good seal at the inlet, and  

reducing the size of the inlets.  Many people add plenum chambers,  

but I believe most of them don't have a good seal at the inlet, so  

there is quite a bit of leakage there.  I also believe that most  

people don't reduce the size of the inlets.  If you don't reduce the  

size of the inlets, you won't realize any performance increase.



Kevin Horton         RV-8 (finishing kit)

Ottawa, Canada

http://www.kilohotel.com/rv8

kboatright1(at)comcast.ne
Guest

Posted: Sat Apr 22, 2006 8:13 am Post subject: Performance Problem Analysis

Sam James used to sell (and may still sell) round inlets that you can 

retrofit into your stock cowl.



I looked into it when I was building my plane and decided that 1) It would 

be a lot of work.  2) Even worse, the work would be *serious* fiberglass 

work. 3) It would be a shame to mess up the nice factory cowling.



If I recall correctly, Sam's recommendation was that without a plenum (which 

he sold), the cowl mod probably wasn't worth the effort.



KB

---

luckymacy(at)comcast.net
Guest

Posted: Sat Apr 22, 2006 8:45 am Post subject: Performance Problem Analysis

Any engineers out there care to hazard a guess what the percentage of gain is for each of the 3 referenced sources?

ie, for the 10 knot gain in using the Sam James setup, 70% from the combo of the smaller area openings and use of a integrated/sealed plenum and 30 percent from exterior cowl shape?

If it was the other way around, I wouldn't even consider it a worthwhile project....

-------------- Original message -------------- 

From: Kevin Horton <khorton01(at)rogers.com> 

 	  Quote: 	

 On 22 Apr 2006, at 10:41, lucky wrote: 

 > 

 > 

 > WRT James cowl, is it basically just the differently shaped air 

 > inlets and their inlet area that make the difference? What's the 

 > diameter of the openings? In other words, if one took Van's cowl 

 > and redid the openings would that theoretically lower drag?? 

 > 

 > Or is there a fundamental difference in the air outlet area on the 

 > bottom too? 

 > 

 > Anyone ever take a side by side photos and otherwise do a 

 > meaningful compare? 

 > 

 > Just curious and looking for rainy day projects.... 

 Lucky, 

 As I understand it, the performance increase comes from three places: 

 1. The cowl comes with a plenum chamber, which means more of the air 

 that comes in the inlets actually cools the engine. Baffles leak, so 

 you need to bring in extra air to allow for that leakage. The fact 

 that the plenum chamber has less leakage means less air is needed, so 

 the inlets are smaller. The less air you bring in the inlets, the 

 less cooling drag there is. 

 2. The contour of the inlet is designed to try to have laminar flow 

 as the air expands, which results in the least amount of drag, and 

 the greatest amount of pressure once the air has expanded. 

 3. The inlet shape is round. Round inlets have the smallest 

 circumference for a given area, so there is less frictional drag 

 along the walls of the inlet. Any frictional drag results in some 

 pressure loss in the cooling air, which reduces the cooling 

 effectiveness, and means you probably need a bit more air to get the 

 same amount of cooling. This is probably a very minor effect. 

 I believe you could achieve most of the performance increase by 

 having a plenum chamber that has a good seal at the inlet, and 

 reducing the size of the inlets. Many people add plenum chambers, 

 but I believe most of them don't have a good seal at the inlet, so 

 there is quite a bit of leakage there. I also believe that most 

 people don't reduce the size of the inlets. If you don't reduce the 

 size of the inlets, you won't realize any performance increase. 

 Kevin Horton RV-8 (finishing kit) 

 Ottawa, Canada 

 http://www.kilohotel.com/rv8 

Any engineers out there care to hazard a guess what the percentage of gain is for each of the 3 referenced sources?

ie, for the 10 knot gain in using the Sam James setup, 70% from the combo of the smaller area openings and use of a integrated/sealed plenum and 30 percent from exterior cowl shape?

If it was the other way around, I wouldn't even consider it a worthwhile project....

-------------- Original message -------------- 

From: Kevin Horton khorton01(at)rogers.com 

 -- RV-List message posted by: Kevin Horton <KHORTON01(at)ROGERS.COM>

 On 22 Apr 2006, at 10:41, lucky wrote: 

  -- RV-List message posted by: luckymacy(at)comcast.net (lucky) 

  WRT James cowl, is it basically just the differently shaped air 

  inlets and their inlet area that make the difference? What's the 

  diameter of the openings? In other words, if one took Van's cowl 

  and redid the openings would that theoretically lower drag?? 

  Or is there a fundamental difference in the air outlet area on the 

  bottom too? 

  Anyone ever take a side by side photos and otherwise do a 

  meaningful compare? 

 &g

 t; <BR

 	  Quote: 	
		    Just curious and looking for rainy day projects.... 

 Lucky, 

 As I understand it, the performance increase comes from three places: 

 1. The cowl comes with a plenum chamber, which means more of the air 

 that comes in the inlets actually cools the engine. Baffles leak, so 

 you need to bring in extra air to allow for that leakage. The fact 

 that the plenum chamber has less leakage means less air is needed, so 

 the inlets are smaller. The less air you bring in the inlets, the 

 less cooling drag there is. 

 2. The contour of the inlet is designed to try to have laminar flow 

 as the air expands, which results in the least amount of drag, and 

 the greatest amount of pressure once the air has expanded. 

 3. The inlet shape is round. Round inlets have the smallest 

 circumference for a given area, so there is less frictio

 nal dr

ag 

 along the walls of the inlet. Any frictional drag results in some 

 pressure loss in the cooling air, which reduces the cooling 

 effectiveness, and means you probably need a bit more air to get the 

 same amount of cooling. This is probably a very minor effect. 

 I believe you could achieve most of the performance increase by 

 having a plenum chamber that has a good seal at the inlet, and 

 reducing the size of the inlets. Many people add plenum chambers, 

 but I believe most of them don't have a good seal at the inlet, so 

 there is quite a bit of leakage there. I also believe that most 

 people don't reduce the size of the inlets. If you don't reduce the 

 size of the inlets, you won't realize any performance increase. 

 Kevin Horton RV-8 (finishing kit) 

 Ottawa, Canada 

 http://www.kilohotel.com/rv8 

 ======

================================================= 

&g

 t; <BR

 	  Quote:

rvbuilder(at)sausen.net
Guest

Posted: Sat Apr 22, 2006 9:31 am Post subject: Performance Problem Analysis

Kevin pretty much hit it on the head.  You are looking for a couple of things in a cowl.  One is to have easy bends for the inlet and outlet air to pass through so you are not imparting much of the airflow's energy into the airframe (more drag).  Also you want the best pressure recovery ratio you can get.  Essentially whatever the pressure is of the air entering the cowl you want to try and make it the same number as it exits.  Any deviation from 1 to 1 will cause drag and/or cooling problems.  You then use the venturi effect to either slow or speed up the airflow inside the cowling to increase or decrease the amount of heat transfer from the engine.  These two areas are where the James cowl makes the difference.



Michael Sausen

-10 #352 Fuselage



--

rvbuilder(at)sausen.net
Guest

Posted: Sat Apr 22, 2006 9:31 am Post subject: Performance Problem Analysis

From what I have been told most of it is from a combination of streamlining the inlets, reducing their size, and from pressure recovery.  The plenum helps but is mainly to manage the cooling.



Michael Sausen

-10 #352 Fuselage

Do not archive



--

rv8ch

Joined: 10 Jan 2006
Posts: 250
Location: Switzerland

Posted: Sat Apr 22, 2006 10:02 am Post subject: Performance Problem Analysis

 	  Quote: 	
		   Any engineers out there care to hazard a guess what the percentage of

 gain is for each of the 3 referenced sources?

 

 ie, for the 10 knot gain in using the Sam James setup, 70% from the

 combo of the smaller area openings and use of a integrated/sealed

 plenum and 30 percent from exterior cowl shape?


	


The cowl shape has got to be a lot.  It is much smaller at

the front, and since it is 4 inches longer, the transition

to the full fuselage is much smoother.   They have some

pics on their website that kind of show what I mean:



http://www.jamesaircraft.com/Our_Products.html



However, when you see it in person, compared to the standard

Van's cowl, the difference is striking.

-- 

Mickey Coggins

http://www.rv8.ch/

#82007 finishing

do not archive

_________________
Mickey Coggins
http://www.rv8.ch/

Dan.Beadle(at)hq.InclineS
Guest

Posted: Fri Apr 28, 2006 7:05 am Post subject: Performance Problem Analysis

Dick,

I am very interested in hearing your speed numbers.



I am building RV8 with IO390, James Cowl and Plenum (per your

suggestion) and A/C Prop.  I am thinking the Grove landing gear for

slippery legs.



This configuration is very close to yours - what speeds are you getting.

My target is 190K or better at 8,000'



Dan

--

bicyclop(at)pacbell.net
Guest

Posted: Wed May 10, 2006 5:18 pm Post subject: Performance Problem Analysis

Tracy Saylor, who's got a very fast 6, modified an old style Van's cowl

with inlet rings from Dave Anders and a homemade plenum. He was having

heat problems which he tried to solve with a cowl flap. That didn't help

so he took it off and the increased exit area with the flap removed was

just the ticket. I guess this principle would be: 1.a - less inlet area

+ extra outlet area makes for reduced pressure under the engine and more

efficient air movement = less cooling drag and improved cooling. One

could also try an extractor lip on the bottom of the cowl to help the

air exit better. Look at the CAF foundation reports.



Pax,



Ed Holyoke



--

LessDragProd(at)aol.com
Guest

Posted: Wed May 10, 2006 7:23 pm Post subject: Performance Problem Analysis

It does pay to look very closely at Tracy Saylor's RV-6, and notice what is  

NOT there.

 

If I remember correctly, Tracy has 4 individual exhaust pipes pointing aft  

that end just behind the carburetor inside the cowl.  I believe the  

installation is still this way.  However, things keep changing on Tracy's  RV-6.  

 

Regards,

Jim Ayers

 

In a message dated 05/10/2006 6:23:24 PM Pacific Daylight Time,  

bicyclop(at)pacbell.net writes:






Tracy Saylor, who's got a very fast 6,  modified an old style Van's cowl

with inlet rings from Dave Anders and a  homemade plenum. He was having

heat problems which he tried to solve with a  cowl flap. That didn't help

so he took it off and the increased exit area  with the flap removed was

just the ticket. I guess this principle would be:  1.a - less inlet area

+ extra outlet area makes for reduced pressure under  the engine and more

efficient air movement = less cooling drag and improved  cooling. One

could also try an extractor lip on the bottom of the cowl to  help the

air exit better. Look at the CAF foundation  reports.



Pax,



Ed Holyoke

bicyclop(at)pacbell.net
Guest

Posted: Wed May 10, 2006 9:14 pm Post subject: Performance Problem Analysis

Yup. He started out with longer pipes and cut 'em off a little at a

time. He said that each time he shortened them, he got a little faster.

The exhaust gas is helping to accelerate the cooling air exiting the

cowl or the cooling air is helping to extract the exhaust. Either way,

it's helping him go faster.



Pax,  



Ed Holyoke



--

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