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Forming aluminum for a cooling outlet fairing

This does need to solely benefit racers. If I could increase my cruise speed 10 knots or more AND get better cooling, that would be swell.
 
I know what you mean Ron

The development of the a better design than the truncated cowl outlet could benefit anyone that chose to use it.

Bob Axsom
 
Drag reduction

This whole discussion about fairings and shape ETC is all in search of the Holy Grail of speed and efficiency through drag reduction.

The link below is a good source to drag reduction information, that is well worth reading.

While the aircraft detailed here are not RV's, the concepts put forth are universal and I suspect a couple of Bobs in the mix may be interested:D

http://www.ar-5.com/
 
Just to keep this thread going with additional food for thought, see the attached

oshgosh2011073.jpg


This a shot of the outlet fairing on Jim Smith's RV-6. Look familiar Larry?? Not sure if the two are related. Jim's airplane has several aero tweaks, including wing tips designed by Paul Lipps. Jim claimed speed gains similar to what we have discussed with this mod.

And here is one way that the Man Himself -B. Rutan (not our man himself) chose to fair his exhaust on Catbird.

oshgosh2011050.jpg

Photos from Osh 2011.

Still working on some ideas, but not ready to post yet. Fly on!!
 
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My current thoughts

I like to make my speed mods so they can be backed out if they don't work. This mod has many features and it seems to me that this one can be implemented in increments. Here are my thoughts while I take a break from the photo print, book building work.

1 - A long central keel fin/separator of 0.090 attached to the cowl support (it's an A model) and at the rear on the bottom of the fuselage - alignment is critical - test but expect nothing.

2 - Two shorter 0.032 vertically tapered side flow fences extending STRAIGHT back from the sides of the cooling air outlet - test and expect some small increase in speed.

3 - Cover the space across the flow fences and center fin/separator with a screw attached plate spanning the bottom of the outlet similar to what Gary and Dan have with their single piece fairings - test and see what I get.

Subsequently the cover plate in 3 can be removed to experiment with a coanda "bump" against the fuselage starting up near the firewall and with the shape and size of the cover itself. Exhaust shroud/free stream air deflectors/suctioun creators seem likely to be tried. From Gary's tests I think there is something to be gained here and the beauty of it is the flexibility to experiment.

Bob Axsom
 
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Bob

Sounds like a very reasoned approach. I think you will find your best results will come from 2).

This is a simpler, more versatile fabrication than the Vetterman 3 / Jim Smith exit fairing. I am sure they work well, but it is a nothing or all committment.

Good luck
 
We know that the leading edge of the wing has a relatively strong suction. That might contribute to a sideward movement of the exit air as it tries to jgo there. If so, it suggests that the side flow fences ought to extend some distance aft of the wing's leading edge.

Also, if the air flow path inside the cowl is constrained with the various flow control devices and fences, don't forget to allow for sufficient cooling of the accessories at the rear of the engine.

Dave
 
That doesn't seem to be a problem

...

Also, if the air flow path inside the cowl is constrained with the various flow control devices and fences, don't forget to allow for sufficient cooling of the accessories at the rear of the engine.

Dave

I thing it stays fairly cool in zone 3 with it being isolated from the engine heated air but on trips in cruise configuration I have two blast tubes out of zone 1 (before engine heating) putting air in there. Often I have flown to fairly distant cities with a fuel stop required to get there in race configuration with the cover plates on. Fayetteville, Arkansas to Mitchell, South Dakota, then race hard to Oshkosh and return home for example. I have not measured it but I suspect it is cooler in there than with the stock setup - I know it is a lot cleaner.

Bob Axsom
 
Getin' Into It

I made up a rectangular center separator fin And worked on fitting it in tonight. The problem with some of these mods is the amount of disassembly that has to be done to do some small thing like add a platenut. Tomorrow I'll have to pull the cowl off and dissassemble the console to get to the areas where I need to add platenuts. The pressure to get ready for the race at Sherman, Texas next Saturday is not going to allow me to do this the way I thought. I really want to get relative speed numbers for the three major components but I just don't have time to dissassemble, modify, reassemble, test, disassemble, modify, reassemble, test, dissassemble, modify, reassemble, test. The penalty is I will just get one gross number.

Bob Axsom
 
Sorry Bob,
I still have a few more hours of break in on my overhauled cylinder before i feel comfortable running a test run. I am making two long flights this weekend and should be good for a test run mid to late next week! I am curious myself as I have had this fairing prepared for a few weeks now!

Dan

Hey Dan, eagerly anticipating your results on this! Any chance you've tested it yet? On a different thread, Dan, Ken and I are testing cowl pressures (upper and lower) and exit air temps. I'm considering adding this exit air fairing to the end of my testing to see if it helps lower the bottom cowl pressure.
 
Done a lot of studying and have a semi-frozen plan

You know the comon expression that each RV is unique? Well there is one area of our RV-6A that is "uniquer" than most - the bottom center of the fuselage forward of the spar. I have spent quite a bit of time today going over the photos in the build albums to come up with the best way to do this.

My first choice was to install platenuts there and attach the center/keel fin with #8 flat hear screws. I have a dimple die that came with the pitot mast mount that works great and I would just remove the fin and install the screws flush if it didn't work out. The console is so integrated into every distributed system that it would be a MAJOR operation to remove it for the installation of platenuts on the floor/skin in that area.

I considered sheet metal screws which I know work great for inspection panels and other non structural applications but I don't consider them adequate for holding on pieces that are sticking out in the wind - not to mention the pointed ends of the screws going into an area that I can't inspect.

A central feature of my approach is going to be a central fin. I'm going to mount a 3/4"x3/4"x24" piece of angle to the fuselage with blind 1/8" rivets. The angle will have platenuts on it and the fin will be attached to it with #8 flathead screws. That way the fin can be replaced at will to experiment with different shapes sizes and thicknesses.

That is the key decision right now, the shroud/flow fences/fairing is generally established in my mind but this was the hard one. There is some freedom with the rest of it for now.

Bob Axsom
 
Worked all night on the center fin

It is ready to install but I need a little sleep. Should test fly today then move on to the fairing.

Bob Axsom
 
Coming Soon

Sonny,
Let me appologize for the tardy response to posting test results. I have test flown the fairing, but haven't released the results as they were not what I was expecting, or what others here are expecting, so I was hoping to run another test run without the fairing to ensure I have a good baseline.

Waiting on a 70deg day in San Diego is normally not very difficult, but the weather has us all baffled here recently. I expect today or tomorrow I should get another run out and will post my results.

Dan
 
Got the central fin installed

IMG_5539.jpg

IMG_5540.jpg


got the fin installed but ran out of daylight for the test - hopefully I'll get it done in the morning.

Bob Axsom
 
I flew the test this morning

183.8 kts was the final number which is 0.6kt less than the best speed I've ever seen so I'm considering it "no Change in speed" and I'll move on to the fairing.

Bob Axsom
 
Bob, while speed improvement is good, can you state that cooling was impacted? I am assuming that CHT is the parameter to note.
 
Next step

I went to the hangar and visualized a few different approaches recognizing the opening race of the season is 4 days away and I have some unique opportunities and irreversible decisions to deal with. I decided to mount two more angles on the fuselage bottom with plate nuts for adding the side flow fences whil maintaining flexibility in experimenting. The first flow fences are going to be rectangles just like the center fin and the they will be 0.090"x3"x22". I'm going to test fly the open bottom and most likely that is what I will race with Saturday at Sherman, Texas (GYI). With the experience I gained on the center fin I should be able to test the open fence configuration tomorrow.

Bob Axsom
 
Exit Fairing Results!

Ok, first off again let me apologize for the tardy posting of the exit fairing results. I know this has been a 'Hot' thread with lots of anticipation.

I have a solid database from test flights before I overhauled my #2 cylinder, but I flew the exit fairing test run after the overhaul. The test flight did not produce the results I expected, so I thought it best to take another day to do a test flight without the fairing to ensure my baseline numbers were still accurate.

BLUF: (Bottom Line Up Front) My goal for the exhaust fairing was never speed, any speed gain was a bonus. My goal was to smooth the exit air, increasing the cooling flow and lower my CHTs.

I am open for questions regarding my test data or flight procedures. I try to keep as many perimeters the same as humanly possible. Nothing is perfect, but I think I have a solid base to make decisions with. All test flight profiles are flown exactly the same.

(FYI: 24/2400 numbers are climb numbers, all other MP/RPM is level flight cruise numbers)

Here is the the Baseline data: (WITHOUT Fairing!)
FairingOFF.jpg


Here is the fairing data: (TWO test flights with Fairing ON!)
FairingON.jpg


Fairing performance numbers: (Numbers in RED = Fairing produced BAD results / Numbers in GREEN = Fairing produced GOOD results!)
FairingDifference.jpg



Results:
CHT: Overall average 13 degrees HOTTER. Climb CHT increase average: 14 degrees. Cruise CHT increase average: 12 degrees.
(WITH fairing, 1st cylinder hit 400 degrees at 1min 50 sec into flight. WITHOUT fairing after 9 min 50 sec hottest cylinder was at 392 deg.) :mad:
EGT: Overal average 16 degrees HOTTER. Climb EGT decrease average: 24 degrees. Cruise EGT increase average: 30 degrees.
Fuel: Rate of burn (Both climb and cruise) increased by almost 0.5gph. :eek:

As for the speed respect of the fairing, I would not take my opinion in choosing to add the fairing. I did not fly the 'speed profile' that is described in the forums using 3 or 4 legs and TAS etc. The speed data on my forms is strictly IAS and GS for my own kicks!

I would like to take the standard "5-7mph gain" that comes with any speed modification, but my opinion is there wasn't any noticeable speed increase.

There it is guys. I hope others have better luck with the fairing. It has worked great for Gary. I think the fact that I have an "A" model with anything and everything blocking the exit and causing massive exit flow turbulence has a lot to do with exit flow performance.

JMHO

Dan
 
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Dan, Lot's of data points here with results that I will call disparate for now. I will drop you an e-mail to discuss further. Don't be discouraged, if nothing else, we're learning and you have an excuse to fly more.

Looking forward to Bob's and other's results. Latter
 
Slow going

Just got home from the airport. Spent the last 9.5 hours there and only got one flow fence made and installed. I need a little sleep and cleanupthen head back to do the left flow fence. They are all rectangles, trhe center is 4"x24" and sides are 3"x22" (approximately).

Bob Axsom
 
Built the left flow fence

IMG_5546.jpg


This photo shows the right flow fence and the center separator fin. I finished building the left flow fence today (yes building - you wouldn't believe the tedious details) but I have not installed it yet. I should get it done late tonight.

Bob Axsom
 
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I finished the left fence

I finished the left flow fence installation at 06:18. The sun isn't up yet and the plane is in semi race configuration without lights so I can't test it yet. I'm sleepy but the sky is lighting up a little I guess I'll hang around for a few more minutes and do the test.

Bob Axsom
 
Test complete - OK

IMG_5549.jpg

IMG_5557.jpg


The speed was 184.3 kts or 1/2kt faster than the test with the center separator alone. I think this is a keeper. The sound is pretty awesome as well.

Bob Axsom

P.S. for Ron Lee no effect on temps.

BA
 
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yeow Bob

Definitely getting my vote for Perseverence of the Year award!! This quote coming from you was what I wanted to hear;

; I think this is a keeper.

I know that you have some further ideas and some work to go that I think will yield more substantial results. But good enough for now and best of luck this weekend.

PS...Those puppies are not going anywhere!! I have less screws holding on my whole fairing than on one of your strakes !!
 
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good job, Bob. Now go get rested up for the Texoma race.

Is there any concern about flexing or distortion of the fences at speed? or are they pretty stiff?

I was buttoning up my temporary mods yesterday, going to test fly once the evening calm arrived. BUT got called to work unexpectedly. Hope to get out there this afternoon(leaving work early) to finish and maybe fly.
 
Bob,

Later, when you've got some time, it might be productive to extend the cowl bottom surface aft to cover some of these new strakes and make the exit duct longer.

If you do it, perhaps you could make its length adjustable.

Dave
 
That is the plan

Bob,

Later, when you've got some time, it might be productive to extend the cowl bottom surface aft to cover some of these new strakes and make the exit duct longer.

If you do it, perhaps you could make its length adjustable.

Dave

Dave - I wanted to approach this incrementally so that I would gain some first hand knowledge about the effects of the parts. So far I have the effect of a long semi-central rectangular separator/fin and that with two similar flow fences all of which are now replaceable with screws. At some point I will put a bottom on it and try various lengths and shapes - I think you can see this has essentially an infinite number of combinations that can be tested. I already have bought the "0" condition Aluminum sheet to make the bottom closure plate out of.

Brian - all three of the major test components are made of 0.090" (~3/32") thick 2024 T3 aluminum mounted on 1/16"x3/4"x3/4"x20+" 6061 aluminum angle. It is very substantial and that is another parameter I can work with.

Bob Axsom

I'm going back to bed...
 
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Maybe??

With all those fasteners holding things together, maybe the speed increase was due to a stiffer airframe. I know this works for race cars.:D
 
With all those fasteners holding things together, maybe the speed increase was due to a stiffer airframe. I know this works for race cars.:D

And I was looking at all the screws sticking out into the airflow, and thinking if he had used flush rivets, he would have gained another 1/2 knot.
 
Yes but ...

And I was looking at all the screws sticking out into the airflow, and thinking if he had used flush rivets, he would have gained another 1/2 knot.

Then they wouldn't have the experimental flexibility to continue the development. The screws can always be faired in. It is perhaps not obvious how much sequential dependence is involved now. The three new elements must now be removed to safely remove and reinstal the lower cowl. The right flow fence must now be removed to remove the center fin. Implememting this on unsupported skin in line with flight and interfacing with the lower cowl takes a lot of thought and careful progress. I still have to make fillets at the upper outboard corners to cover the small openings outboard of the flow fences plus proceed with experiments on shapes, sizes, lower fairing, etc. The screws are a blessing for now. The airplane has made a low altitude (well 500 ft agl) pass one way through the time traps at Courtland Alabama at 231 mph so it is important that parts not be lightly attached.

Bob Axsom
 
I actually realized that Mike ...

I actually realized that Mike but I jumped on it becuase it gave me a chance to point out a little more detail - we're good.

Bob Axsom
 
Dan Thompson: How did you format your Excel spreadsheets to get them to insert like that? I suspect the answer is simple, but I don't know what it is!

Thanks,

Lee...
 
Not sure what you mean?

Lee,
Not sure what you mean. I hand typed in the data after every flight. I used Excel formulas to convert some of the data and calculate averages. I can send you my spreadsheet if you would like.

Dan
 
Thanks for checking in, Dan. I meant, how did you get them to display on the VAF website. AFAK, you can't just copy and paste them. Did you make them into a photo or pdf or something and then insert them? If the later, I probably don't have the software to do that. I've been trying to insert info in a similar format on another aviation site that only handles 'off-site' photos and have had no luck at all.

Thanks!

Lee...
 
gotcha

Oh ok. I just took a picture with my cellphone and cropped the picture with MS paint and saved as jpg, then uploaded.

Dan
 
Finally the work-weather schedule allowed a little test run tonight. I had all the mods installed but figured that was not the proper way to test, SO:

Tonights run was to test the plenum seal work and the internal lower cowl outlet baffle and exhaust area cleanup. I tried to get the same conditions. But couldn't get OAT 02*C and 6650' DA I didn't realize I also had an extra 0.5" MP tonight than during my baseline run.

What does this mean?....I have no idea. I will say that that CHT's were the about the same(335-348)even though OAT was 15*C warmer. And I showed 189-192kts TAS. Baseline was 188-190.

I call it draw. It appears faster, the numbers look slightly better, but it could all be in the scatter. Next nice day, I'll install the cowl outlet extension and run again.
 
I have the bottom cover cut but ...

I Have to stop and get the taxes done before I can continue. I have decided to leave the separator/fin and the side flow fences as they are for the next iteration. The I am going to cut them at an angle from the rear of the cover up to the fuselage and retest as a next generation test.

Bob Axsom
 
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Finally back to the task

I worked woth some 0.040 6061 condition 0 material for 5 hours tonight ending at just before 0200 started making too many mistakes. I installed an angle on the center fin to attach the lower closure plate and I started finalizing the installation. The condition 0 6061 aluminum is really nice to work with for shaping things like this. I may just make a single piece shroud and pick up the flow fence mounting points, or I may brute force my way to a fast test configuration using what I have with patches (no there is no software involved).

Bob Axsom
 
Early cover temporary installation

IMG_5617.jpg

IMG_5618.jpg

IMG_5619.jpg

IMG_5620.jpg


Looks so good I hate to cut out the openings per Gary Reed's design - But I will.

Bob Axsom
 
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Just a thought.......

IMG_5620.jpg


Looks so good I hate to cut out the openings per Gary Reed's design - But I will.

Bob Axsom

You might fly it first, then cut out the scallops-------will give you a baseline figure as to the effect of the cutouts.

Also, his plane had the exhaust pipes hanging down, so the cutouts were to clear the pipes, as I recall.
 
I am very tempted

I haven't had much sleep so I'm going to take a break, finish the installation then decide. Heat damage and carbon monoxide are two concerns.

Bob Axsom
 
Bob, Looks just as I had envisioned from your explanations. Not sure how difficult it was to form the center section closure fairing, but from your explanations I know it was intended to be replaceable for experimentation.

With your configuration, particularly with your current exhaust configuration, I think I would stick with two straight cuts generously radiused into the outer strakes and central fin, making sure to get a smooth faired corner where the bottom closure meets the outer strakes.

I did mine fully by feel, and this is what feels right to me looking at yours.

Best of luck for good results!!
 
Fairly good night's work

I went to the hangar after dinner and worked until 6 am. I think it is finished but I broke a tool and decided to hang it up for the night. I woke up at 1 pm so I'm going to have my Wheaties and get back to work with a clearer head. I seem to be having screws galing in the last 20 platenuts I installed - had to remove one screw with vise grips. I'm probably adding al lot of unusual pressures in the "make it fit" mode. I'll fix it and hopefully fly to day. The idea of flying first without the cutouts is very dominant in my thinking. I will try to remember to bring my oxygen.

Bob Axsom
 
Unvented cover done and ready for test

IMG_5623.jpg

IMG_5625.jpg

IMG_5627.jpg

IMG_5629.jpg


It is on and ready to test. Waiting for clear to 6000 ft density altitude.

Bob Axsom
 
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