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  #1  
Old 01-19-2010, 07:18 PM
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hydroguy2 hydroguy2 is offline
 
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Default Inlet/Outlet Ratios

I'm working on my cowl and baffles. I'm using a James cowl and plenum set-up. I have calculated the inlet area of 33.6sq.in. and the outlet area of ~37sq.in. So the ratio comes out to about 1 to 1.1.



The 4 pipe system sticks aft of the firewall about 6". I didn't cut the lip off the cowl outlet and it extends aft about 1".


I have read the threads discussing outlet velocity and seen the pic of tufting the area aft of the outlet.

So my question is: Is it worth the trouble to reshape the outlet to reduce the ratio a bit or do something to the corners to help streamline the exit area flow?
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  #2  
Old 01-20-2010, 06:27 AM
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RV8RIVETER RV8RIVETER is offline
 
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Default

From my research, unfortunately, there is no magic inlet/outlet ratio as there too many variables in individual building design. For some 75% work for others 180+ work.

I think reworking the square outlet is well worth the effort in that is has been shown to be a flow disrupter.

Just remember that we want to speed up the exhaust air to as high as posssible, resulting in lower pressure and better transition to free stream air. Also you want to make that transition as smooth as possible, and the stock square is not the best for that.
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  #3  
Old 01-20-2010, 07:18 AM
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hydroguy2 hydroguy2 is offline
 
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Reworking the square outlet is what I am thinking about.

1. extending cowl AND at the same time tapering it towards the center.
2. adding a bump aft and above the pipes, something like L Vetterman added to his -6a.
3. cutting the pipes to get them out of the airstream



Looking for opinions. what do you racers out there think?

I may just try to get this thing flying and then make the mods in order to evaluate them.
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  #4  
Old 01-20-2010, 09:55 AM
Bob Axsom Bob Axsom is offline
 
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Default It is an area I intend to work on but nothing yet

Larry Vetterman's experiment with a boat tail fairing and outboard exhaust and cooling air outlet openings were successful. I have a different idea but at this time it is only a mental concept and it may not increase the speed at all. Tom Martin has done a lot of work in this area on his EVO Rocket and he is the fastest one out there so it is an area that is definitely worth working on.

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  #5  
Old 01-20-2010, 10:27 AM
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Default Cowling Inlet/Outlet Ratio(s)

If you are really serious about this (and it appears that you are), I would suggest you study what Dave Anders did on his RV-4. I don't know if his work is readily available except by talking to him. I remember talking to him at Oshkosh several years ago and he told me the 1-to-1 ratio he read about in the literature didn't work for him.

He ended up with something like 1.33.(P.S. I just looked it up...there's an article on the Oshkosh365 web site about the TriAviathon contest...he had an inlet area of 34 sq in and an outlet area of 24.7 sq in for a an apx. 4-to-3 inlet to outlet ratio) I didn't talk to him about cooling issues and now wish I had. I do remember that he was using a semi-round shaped outlet with a 4-into-1 exhaust resulting in an exhaust "augmentor."[Not to be confused with "augmentors" used on certified twins in the 1950's; that's a different "augmentor."] If I understand that right, the engine exhaust actually helps to scavenge the cowling exhaust so that could have a considerable influence on the chosen inlet/outlet ratio. I'm really on the edge of what I remember so don't take this as fact until you talk to someone like Dave who did a lot of work in this area. I don't think Dave was the first to use an "augmentor," but he may have been the first to use the concept on an RV. No one can argue that he didn't have one of the fastest RV's in existence at that time. (Notice...he later modified his exhaust system with mufflers to compete in a noise reduction contest; the speed mods discussed here were in the 1994-1997 time frame.)

With your four exhaust pipes, your cowling inlet/exhaust ratio goal of 1-to-1 might be a better choice. Whatever you chose, I would suggest you be prepared to make several tweaks in order to optimize your design.

I remember being very impressed with Dave's study of the then existing literature relating to drag reduction of general aviation airplanes. I believe he still lives in California, but I haven't heard too much about him lately. Maybe someone else with knowledge of the details of his much modified RV-4 can contribute information about Dave Anders' cowling design.

Keep us posted on what you do. I've read that 15-20% of the total drag on a typical general aviation airplane is due to air flow through the cowling. I'd like to think that Van's stock design is much better than that. I can't recall that anyone has ever tried to quantify those losses on an RV.

Interesting stuff.
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Last edited by rv7boy : 01-22-2010 at 01:28 PM. Reason: Changed exhaust "eductor" to "augmentor." Second edit corrected the inlet to outlet ratio.
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  #6  
Old 01-20-2010, 12:22 PM
Bob Axsom Bob Axsom is offline
 
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Default Lower Cowl Baffling

I don't know what results anyone else had but after a significant amount of direct experimentation I increased the speed of my RV-6A 4 kts with baffling in the lower cowl. The baseline speed before the current configuration was achieved was 170.67 kts. Most of the modifications I did resulted in decreases in speed until I hit upon the current configuration. One could try a lot and achieve nothing if the program isn't organized and the testing isn't methodical and consistent.

John Huft is a very successful racer flying an RV-8 that he built for speed and he has a website revealing all.

Bob Axsom
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  #7  
Old 01-20-2010, 01:34 PM
Paul Thomas Paul Thomas is offline
 
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Brian,

IIRC, Sam James told me that the ratio should be 1:1. This way air is leaving the cowl just as fast as it entered it. The air enter at 170+mph, slow down so 15-17 mph as it moves through the baffle needs accelerates back up for the exit.
Sam is always willing to talk shop, pick up the phone and ask him. If you can't get a hold of him, I see him once a month. I'll print out your pictures and ask.
If you're looking for speed, I'd cut those pipes down since they're adding drag and long as there is not some unintended consequences.
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  #8  
Old 01-20-2010, 01:53 PM
jrs14855 jrs14855 is offline
 
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Default Dave Anders

Airmans records lists Dave in Cottonwood CA. As far as I can determine Daves RV4 still holds the triaviathon record.
The true augmentor exhaust was probably designed by John Thorp. It is totally different than the four into one exhaust. The augmentor is a very large collector tube that is open on both ends. The individual pipes feed into the large tube and end well short of the aft end. The augumentors were used on most of the light twins in the fifties but fell out of favor because they are VERY noisy.
Google "Dave Anders RV4" and you will find a wealth of info on Dave and his airplane.
Another builder who has done a lot of research on cowling /cooling is the Lancair builder in the Bay area. He has written some articles in Sport Aviation.
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  #9  
Old 01-20-2010, 02:04 PM
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I'll probably just keep these mods on the shelf and get this thing flying. Once I have a baseline, I can start fiddling again. It was wishful thinking to have someone say, "yes do this or that and things will be better"

gotta start somewhere.
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  #10  
Old 01-20-2010, 02:38 PM
Danny7 Danny7 is offline
 
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Quote:
Originally Posted by jrs14855 View Post
Another builder who has done a lot of research on cowling /cooling is the Lancair builder in the Bay area. He has written some articles in Sport Aviation.
you're probably thinking of paul lipps, i believe he posts here under "elippse" or something similar
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