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  #31  
Old 08-20-2007, 02:32 AM
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gmcjetpilot gmcjetpilot is offline
 
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Default Those local little details, good point

Quote:
Originally Posted by az_gila
George... one instant thing I notice on Alan's picture above and the earlier tuft testing is that Alan's exit area has rounded sides, unlike the very square sides of the stack exit.

This is interesting, since it the "errant" tufts are in the sharp intersection of the exit and the cowl bottom.... Alan does not have this sharp break.

Perhaps this is another area that could be investigated?

gil A
Yea the corners are a problem, probably a small one. The exit is very abrupt, square cut and only thick as the fiberglass. Some internal radius & rounded exit might help. There is (I guess) local turbulence in the bottom corner, causing external air to swirl around and in locally, sucking the tuff back.

After I enlarged the tuff pics and changed brightness/contrast, the thing that impressed me where the tuffs directly down stream on the belly and at the lip of the firewall. They are totaly lazy, not very tight and meandering around. That tells me the flow is slow and probably turbulent from all the junk upstream and in the way, exhaust pipes, gear leg, engine mount and lip of firewall.

When you look at the tuffs external to the scoop they are stretched straight back for dear life. The ones on the end of the cowl exit are mostly back but the corner ones are bending in slightly as discussed.

I know most of Vans kits don't provide any internal air guides/baffles/fairings near the firewall exit area except the RV-8 gets a little firewall lip radius. The builder has to make it on most RV's. Builders like RV-6A Bob Axsom have made some impressive internal guides/baffles/fairings. Alan Judy as well has gone hog wild with internal lower baffles. (and God bless them for doing it)

The model "A" nose wheel guys do have more stuff in the way than taildraggers, but "airfoil" shaped and bell mouth fairings should help. Here is a builder fairing I liked. It covers the engine mount, lower firewall lip and some tube/control cable junk, providing a nice curved surface for the air to follow. (sorry no ref/credit but nice job and you know who you are) If you can avoid putting stuff in the exit area.


Air does funny things when you suck it in, slow it down, heat it up and try to re-mix it with 200 mph air while flowing over protrusions and odd shapes.
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Last edited by gmcjetpilot : 08-20-2007 at 03:10 AM.
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  #32  
Old 08-20-2007, 05:32 AM
Bob Axsom Bob Axsom is offline
 
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Default Just got back from Greece & Turkey

Gosh I wish I had been able to follow this as it was evolving. We flew from Athens to Warsaw to Chicago to Fayetteville in a continuous ~30 hour west bound travel day, August 19, 2007, so my brain is not able to take in all this in one setting. As for the initial thought - I see no evidence that the exit area of our RV-6A is too small.

The baffling that has produced positive speed results in our airplane provides

1 - a sealed curved surface from the rear of the engine to ~1/4" below the bottom fuselage skin for the full width of the standard outlet area.

2- a sealed baffle slanting in from the side of the lower cowl to the outboard edges of the outlet.

3 - a sealed baffle outboard of the valve covers on my O-360-A1A to the lower cowl that extends from the front or the lower cowl back to the two outboard baffles in "2" above.

Until I added baffles "3" each change decreased the speed of the airplane. When I added those baffles "3" the cowl became segregated into three chambers and there was a dramatic increase in speed (approx. 6 kts over the lowest speed I had gotten down to and 4 kts over the stock baseline speed of 170.67kts at 6,000 ft density altitude. I seem to be stuck in the 174 to 175 kt. region and since this is race season I have to stick with what I have in my RV-6A until the winter ... but don't think I have stopped thinking about it.

Oh, by the way - those blast tubes really work and they do slow down the airplane. I have two race plates I cover them with and the speed change is significant over a knot in the current configuration.

I did add another baffle in the cowl below the engine to turn the air toward the cowl outlet, above the filter air box and truncated above the sealed web structure (I sealed it with a small aluminum closure plate) of the nose gear the outlet and it essentially made no difference in speed (though it did shield the lower surface of my cowl from exhaust pipe heat). However, when I spent a lot of time developing a smoothy curved pair of extensions that took this lower baffle all the way to a sealed fit around the lower portion of the NLG structure inside the cowl and a sealed fit against the sides and the lower surface of the inside of the cowl outlet I thought it looked pretty darned awesome ... a stylistic delight ... the final answer. In test there was a dramatic reduction in speed. You may recall many years ago Boeing and McDonnell Douglas competed in the YC-14 and YC-15 competition. Boeing used a technology where the air over the wing followed the smooth curve formed by the upper camber of the wing and the flaps when deployed to provide extra lift with a powerful downward flow of air. I think the same thing was happening as the air coming out of the cowl followed the upper surface of the of the baffle extensions into the air flowing under my cowl (thanks for the tuft photos Alex) and really caused a lot of disturbed air flow. I cannot ignore the current Malibu, Mooney, Columbia, Cirrus cowl outlets squeezed around the twin exhaust pipes and I was thinking of some form of embedded cowl flap but Tom Martin's experience has caused me to push this way back on the back burner because implementation on an "A" model is difficult and the benefits are at least doubtful.

For now I am focusing on the inlets.

Bob Axsom

Last edited by Bob Axsom : 08-20-2007 at 12:16 PM.
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  #33  
Old 08-20-2007, 08:03 AM
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Bob Martin Bob Martin is offline
 
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Default Fuel Flow Correct?

Not to change the direction of this thread, which I really like BTW.
But I'm addressing my overheating on Take Off/climb with fuel first, then the louvers if needed.
My setup is very typical RV-6 C/S 0-360 A1A from Vans circa 1994-95 with the M4-5 10-3878 as it came from Vans.
I was hitting 450chts on TO and limited climb angle to lower temps.
I bought the enrichment kit A666-660 from precision and installed it. The kit lowered the chts about 25 degrees. but still too hot at 425.
I then drilled the main jet out 3 to 4 thousanths just last nite and lowered the chts another 25 degrees to 400.
needless to say, I'm very happy. I still have more test/flying to do and idle setting, etc, and I may drill further........but I prefer the slow, testing approach to quick drilling.
I think there are lots of other RV's out there that are just accepting high temps and or limiting there flying or climbing due to an improper set up.
BTW my oil temp was great and the cruise temps were fine, that's what made me look harder at the TO specs which lead me to Fuel Flow. Additionally, I could take off at reduced power and reduced climb speed and keep the temps normal too, so it was easier to pinpoint the TO area to fix.
Just another angle to consider when addressing heating issues.
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  #34  
Old 08-20-2007, 09:23 AM
David-aviator David-aviator is offline
 
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Default

Quote:
Originally Posted by Yukon
Oh David, come on! Since when was performance a criteria for you Subie guys!!!! . Open up that cowling and get smooth, cool hp!
John,

No question we have a problem with internal and external drag, especially with the A model. But it is not a hopeless problem. Bob Axom is doing excellent work with his 6A. The max speed I have seen is 162 KTAS compared to his 174-175 although my speed measurements have not been as controlled as his. I simply look at what Dynon calculates with an IAS/OAT input.

As near as I can measure, the standard RV cowl provides an inlet/exit ratio of about 107.69%. This certainly is the result of much experimentation and testing by the RV factory. It is a compromise for the sake of performance and simplicity, and for the most part works very well. But, as with Darwin flying in Arizona, the exit area has to be opened to provide more air flow through the compartment to cool things in a hot-hot environment.

The Subaru challenge is compounded by trying to accomodate the liquid cooling system with the standard Van's cowl (which is dedicated to an air cooled engine). Jan has departed from that oringinal premise and has designed a cowl suited for his radiators with total ram diffuser air flow. By "total", I mean the inlet is matched to the radiator area.

My inlet air flow has a sharp drop off into a box like a water fall because the shape of the radiator is totally different than the shape of the inlet. The area is sealed but it is not a diffuser, nor can it be without major inlet modification. At present the radiator area is 35.75 inches and the inlet is 24.5. The only way I can get more air through the radiators is to increase the pressure ratio from inlet to exit and that involves speeding up the exit air flow which, I think, is accomplished by increasing exit area. Smooth air flow internally certainly will permit less exit area, but as has been mentioned by George, much easier said than done. Looking into the exit area is like looking into a junk yard of stuff from exhaust pipes to engine and NG mount structure and cooling system hoses. Air flow through that area has to slowed down a lot. The TDer is much better suited for optimizing internal air flow.
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  #35  
Old 08-20-2007, 09:35 AM
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dan dan is offline
 
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Default

Quote:
Originally Posted by David-aviator
No question we have a problem with internal and external drag, especially with the A model. But it is not a hopeless problem. Bob Axom is doing excellent work with his 6A. The max speed I have seen is 162 KTAS compared to his 174-175 although my speed measurements have not been as controlled as his. I simply look at what Dynon calculates with an IAS/OAT input.
You've got bone stock RV-7As out there with bone stock O-360-A1A + 2 mags, and they're hitting the 180-185 KTAS range. My friend Jim Percy is a shining example of that. 20+ knots is a huge difference in performance...that's 12+ percent on top of 162.

Personally I'd stop referring to the Dynon's TAS calculation (I never trust those "instantaneous" calcs as far as I can throw 'em, winds aloft data is almost never exact) and do a real TAS test run. You might find "hidden" TAS, or maybe not.
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  #36  
Old 08-20-2007, 10:19 AM
Yukon Yukon is offline
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Default

Quote:
Originally Posted by dan
You've got bone stock RV-7As out there with bone stock O-360-A1A + 2 mags, and they're hitting the 180-185 KTAS range. My friend Jim Percy is a shining example of that. 20+ knots is a huge difference in performance...that's 12+ percent on top of 162.

Personally I'd stop referring to the Dynon's TAS calculation (I never trust those "instantaneous" calcs as far as I can throw 'em, winds aloft data is almost never exact) and do a real TAS test run. You might find "hidden" TAS, or maybe not.
Dan,
What does winds aloft have to do with KTAS???
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  #37  
Old 08-20-2007, 11:00 AM
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dan dan is offline
 
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Default

Quote:
Originally Posted by Yukon
Dan,
What does winds aloft have to do with KTAS???
I was under the impression that most EFISs integrate IAS/OAT/Groundspeed/GPS track to calculate TAS & winds aloft. Some of those calculators that don't have GPS track available used to have you plug in winds aloft. Guess not? Do they just use IAS+OAT? Either way it's not as reliable (imho) as a 3-way GPS groundspeed vector calc. Especially for this "top speed" stuff.
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  #38  
Old 08-20-2007, 11:31 AM
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rv969wf rv969wf is offline
 
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Default Yep, same here

Quote:
Quote:
Originally Posted by Tom Martin
Frankh
I have extended my cowl outlet quite a bit back the fuselage and I have belled the exit inside the cowling. When I look at an installation where there are cooling problems the first thing I check are the inlets. Usually the inlet shape is wrong in that it gets a bit smaller, converges, before entering the plenum area. Also many have not done a good job with smoothing the airflow on the upper cowling aft of the inlet and sealing around the inlet.
Another thing noted is a noticeable change in sound. Not louder, just different, caused no doubt by the change in airflow over the cabin floor.
I agree with Tom about a noticeable change in sound. After all of the cooling mods that I did on my -6, there was a noticeable change. After testing and testing this and that, I never really thought much about until an old flying buddy jumped in the plane and noticed it sounded difference from the last time he was in it with stock lower cowl and no inner lower plenum. I guess you could explain it as being smoother and less vibration on the floor of the plane. One other thing I noticed on my plane is how the pattern of the exhaust stain is attached to the belly of the airplane, it looks much difference than with the rectangle outlet, not sure why, but Sam James crawled underneath my -6 at Oshkosh and told me the underneath coloring of the exhaust looked very good from the point it started and trailed aft. It just looks different than before and that's with a small turndown at the tip.

Someone mentioned about me doing tufting at the cowl outlet and providing pictures. I used drops of oil which probably isn't the best way to do it but I couldn't find anyone brave enough to take in air photos. The oil streak pattern with my final cowl outlet shape showed very straight trails at nearly all areas.
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Last edited by rv969wf : 08-20-2007 at 11:45 AM. Reason: added things
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  #39  
Old 08-20-2007, 12:24 PM
Bob Axsom Bob Axsom is offline
 
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Posts: 5,685
Default I did the rich mod kit installation also

Quote:
Originally Posted by Bob Martin
Not to change the direction of this thread, which I really like BTW.
But I'm addressing my overheating on Take Off/climb with fuel first, then the louvers if needed.
My setup is very typical RV-6 C/S 0-360 A1A from Vans circa 1994-95 with the M4-5 10-3878 as it came from Vans.
I was hitting 450chts on TO and limited climb angle to lower temps.
I bought the enrichment kit A666-660 from precision and installed it. The kit lowered the chts about 25 degrees. but still too hot at 425.
I then drilled the main jet out 3 to 4 thousanths just last nite and lowered the chts another 25 degrees to 400.
needless to say, I'm very happy. I still have more test/flying to do and idle setting, etc, and I may drill further........but I prefer the slow, testing approach to quick drilling.
I think there are lots of other RV's out there that are just accepting high temps and or limiting there flying or climbing due to an improper set up.
BTW my oil temp was great and the cruise temps were fine, that's what made me look harder at the TO specs which lead me to Fuel Flow. Additionally, I could take off at reduced power and reduced climb speed and keep the temps normal too, so it was easier to pinpoint the TO area to fix.
Just another angle to consider when addressing heating issues.
I installed the rich mod kit to the carburetor also back when I was fighting a timing problem in the initial test phase. The customer support fellow told me they tried to get van to go with that version initially. He said that Van later changed and the rich carb version is now their O-360-A1A configuration. That no doubt effects a lower temp in my plane as well.

Bob Axsom
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  #40  
Old 08-20-2007, 12:31 PM
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RV8RIVETER RV8RIVETER is offline
 
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Default Great outlet shape

Alan you have a winner. That is what I am talking about. Square corners are great drag/turbulence inducing device and would best be avoided. I am thinking along the same lines as what you have done, but mine will be rounder and extend a bit farther back to incorporate exhaust pumping.

Have you tried anything like that?
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