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  #1  
Old 02-05-2009, 09:31 AM
Bob Axsom Bob Axsom is offline
 
Join Date: Mar 2005
Posts: 5,685
Default Larry Vetterman's sub/after cowl mod

The modification that Larry Vetterman shared in todays (2-5-09) VAF home page daily news is a major breakthrough in speed mods I believe. It is tested on an "A" model which makes it all the better from my prospective. He says that he is going to try to modify the design to work with existing exhaust system configurations but I do not believe it will be as successful as the configuration he has tested with the outboard exhaust placement. I have been thinking of something like this for some time but had done nothing to bring it past the brain to reality. His implementation is so perfect in appearance that it is hard to imagin anything better. The outboard exhaust position is required for max gain I believe but I was thinking of some short modification of the basic lower cowl with a reflex after the FAB to a tail like you see in the landing gear fairings - not faired down (up actually) to the fuselage as Larry Vetterman has done. This would require a removable section in the honeycomb structure of the lower cowl with a lot of messy accomodation work. The way he has added the fairing on behind the existing cowl is perfect. The mounting he has chosen onto the lower fuselage can easily be accomodated with platenuts and maybe some doubler plates. Man I love it! I am thinking of delaying the control surface end pocket filling till later in the racing season (www.sportairrace.org calendar of events) and try to get this in. I think a cowl flap needs to be incorporated in place of the louvers and I think the pressure recovery style may be even faster than the half teardrop approach. The beauty of this concept is you an make as many configurations as you want an substitute them just by picking up the same mounting points. All of the existing out board tank and fuel pump vents will have to be relocated away from the outboard exhaust location. THIS IS BIG!

Bob Axsom
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  #2  
Old 02-05-2009, 09:46 AM
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cjensen cjensen is offline
 
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Location: Milwaukee, WI area
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Default

I agree Bob...I saved all the pics this morning (as I'm sure many will do) to see about incorporating this to my 7 as well...VERY BIG IDEA!!

Way to go Larry!

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  #3  
Old 02-05-2009, 10:12 AM
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DanH DanH is online now
 
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Default

I suspect the fundamental gain is due to an increase in outlet velocity, or put another way, an improved inlet-outlet velocity ratio. Cooling drag is all about momentum loss. I'd love to know the new total exit area.

Need evidence? Note the increased manifold pressure (I assume an inlet snorkel in the left cheek).

The fairing may place the louvered outlet in an area of lower pressure. Don't know for sure, something worth checking. And anything with a teardrop tail is better than a flat plate tail.
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  #4  
Old 02-05-2009, 10:28 AM
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JonJay JonJay is offline
 
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Default CHT's?

I could not help but notice the dramatic increase in CHT's. Lower differential pressure, less cooling flow? It seems to be counter intuitive. What am I missing?
Guys?
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  #5  
Old 02-05-2009, 11:01 AM
Bob Axsom Bob Axsom is offline
 
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Default CHT Control

Sometime ago I experimented with cooling drag by reducing the cooling air inlet size incrementally with 1/4" slice fillers of the inboard edge of both inlets. There was no increase in speed but each reduction in inlet size increased the cylinder head temperatures seemingly directly as defined in a conceptual equation "less cooling air = hot things staying hotter". This condition can be achieved by altering the opening size at either end of the system - inlet or outlet. So, the CHT increase with the smaller outlet area - the cross section of the opening around the exhaust pipes as opposed to the cross section of the closed off opening at the rear of FAB scoop on the lower cowl - is understandable. At low speeds you need the less restricted system to get adequate air mass flow to do the cooling. By incorporating a cockpit controllable cowl flap in the after cowl fairing you should be able to provide more air mass flow to accommodate ground operations and slow flight and close the flap for high speed operations where adequate air mass flow is easier to obtain. The difference between Larry Vetterman's after fairing design and my restriction to the inlet opening is that the after fairing provides an increase in speed where reducing the air mass flow only, did not.

Bob Axsom
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  #6  
Old 02-05-2009, 11:16 AM
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JonJay JonJay is offline
 
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Default Thanks Bob

Makes a lot of sense. Is there still a concern at speed? OAT's where really low in Larry's tests. So, even at speed, add another 30-40deg OAT and we are pushing those CHT's limiting this mod to moderate or even cool weather or high altitude operations where OAT's are below 60 or 70deg.? (around here, that is almost year round!) All a good case for that Cowl Flap.
Love this stuff.
Some day I will finish my 3 and then I am going to start going after my 6 to see how far I can push things based on all of the wonderful experiments and proven mods that Larry, yourself, and others have offered up.
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  #7  
Old 02-05-2009, 11:20 AM
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Ironflight Ironflight is offline
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I really think Larry's done some exceptional experimental work here, and am excited to see a few others repeat it to see if they get the same results. I am assuming that Bob is already busy shaping some foam....

I am also interested to see how this might apply to the -8's, as we have a built-in exit ramp which changes the shape of the belly at that point - it is not flat all the way across as in the side-by-sides. Larry's comments make me wonder if van was experimenting with similar ideas when he did the -8 design - I have read that initially, the ramp was supposed to be variable (I think for cooling), but ended up fixed.

I wish I had the time that Bob does to play with things like this!

Paul
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  #8  
Old 02-05-2009, 12:00 PM
gasman gasman is offline
 
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Do a search and see how DAVE ANDERS solved this problem on his RV4 a long time ago.
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  #9  
Old 02-05-2009, 12:46 PM
RV3 Pilot RV3 Pilot is offline
 
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Location: Northern California
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Contrary to Bob's experience, I was able to increase the top speed by reducing inlet cooling area. I decreased inlet area until I noticed an acceptable increase in cht. I then resized the exit area (cut down) and voila a measureable mph increase. Effect on oil temp was negligable. A bit of trial and error but imo worth the effort.
rv3 pilot
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  #10  
Old 02-05-2009, 02:09 PM
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DickF DickF is offline
 
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Location: McKinney, TX
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Default Article

Somehow I'm not finding the article that is being discussed. Can someone provide a link?

Thanks..
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