exhaust exit fairing dimensions

Made a fairing, took the pressure recover wheel pant and used it as a basic outline for the fairing but it looks way longer than Larry Vetterman's. Does anyone have any dimensions out there? Just don't want to make it too long or short. Mine is 24".

I've contemplated the same, but haven't made one yet. If I make one I would use the oil/exhaust pattern on my belly to size the fairing.

Instead of trying to fair a lump, why not just get rid of the lump?

Increase exit air velocity and smooth airflow

Dan, not sure what you mean. Bob axom's testing showed marginal gains from side skirts. Vettermans design seems to smooth out the airflow. I'll post a pic showing turbulence.

sorry for the large pic....moderators feel free to resize

sorry about the size of pic...i thought medium would be fine but it is too large.

which would be better

a teardrop fairing without a lump or one with one. I guess it would be best to just make two and test. The one without the lump would be different than Bob's as its sides would taper and curve in a teardrop shape. In theory which would provide greater speed?

Carl,

That's not "turbulence", at least not per a textbook definition. Your photo (somebody re-size it please) shows one half of a classic Von Karmen vortex street, the unsteady separation of flow behind a blunt body.

Adding a tail can reduce or eliminate the alternating vortices. For example, in the case of the cylinder shown in the illustration, adding material to form the classic teardrop, or even just adding a single center fin with a length of more than the diameter of the cylinder will kill the alternate shedding.

The blunt body in your case is the giant laundry chute used as a cowl exit. It has a supply of air into the wake which is independent of the external flow, which offers another way to kill the alternate shedding.

Consider a bit of accounting. Separate form drag (a function of frontal area and wake) from cooling drag (the energy necessary to re-accelerate the exit air to freestream velocity).

Reducing the size of the blunt body reduces frontal area. It can also reduce the exit area, which increases exit velocity, which reduces cooling drag. A smaller blunt body with no great velocity difference between freestream and exit flow produces no vortices, i.e. a reduction in wake (pressure) drag. These three facts say the first modification to consider is a reduction in the size of the blunt body, with a corresponding reduction in exit area.

Note that fairing the tail end of the body for wake reduction only improved one of the three drag sources.

Carl,

Below is a link to a short video taken to check operation of a variable exit area door. Note that when frontal area is large and exit velocity is low, the yarn tufts are quite unorganized. However, at 14 seconds I close the door (partially, the system needed more structural work to get full sealing, search "The Shrinking Exit"). Even with just partial closure, the messy wake disappears, both at 120 knots, and later at 180 knots.

http://youtu.be/nA5PY7PYBsU

The door structure was later stiffened, so now it closes pretty well. There is ~4 knots between open and closed, in cruise.

http://youtu.be/aIBXAE2Ezn4

thank you Dan

Thank you for your explanation as I understand what is happening now. I was wondering too if the two exhaust pipes are also creating von karmen vortex streets as well and wondered if creating a fairing around the exhaust pipes in the lower portion of the cowl would increase exit velocity. I'm going to take a look at my exhaust and see how I can reduce my cowl laundry chute shape and reduce my outlet size. Look at a cowl flap too. Initially seems like the easiest thing to do would be to add a teardrop but overall this is just mitigating the problem and not fixing the root problem. I think this is going to be a fun lengthy project.

Carl,

I'm a Shrinking Exit disciple too, and have some thoughts for ya. Concur with Dan on the multi-faceted (multi-drag-type reduction) benefits of shrinking the exit. Larry Vetterman's after cowl was effective for him, but in reading his site, and comparing his pics, it seems it was a good solution for an A-model, where he may have had limited ability to reduce the cowl exit dimensions, due to the nose gear components. His pics show two variations, one with wide exhaust pipes exiting the cowl via individual bluf bodies, and one with the ex pipes closer together and exiting via the after cowl. In both cases, he used louvers to get the exit airflow needed for cooling. So the design of the teardrop after body requires much thought and experimentation with size, shape and exit air control (louvers, etc) to find a solution. Pics of Larry's work, for ref:





Garry Reed and others used an after fairing of a different design, to straighten the flow aft of the exit. Bob Axsom used that concept as part of his experiment in this area…what you called side skirts. The reverse flow seen in your pic with the oil could also be partially due to chaotic flow at the edge of your exit, but its hard to see that in the pic. The blunt body sure does seem to have an effect on the flow as well, as pointed out by Dan. Here is a picture of Gary's exit fairing, which he said increased cooling and increased speed…meaning he increased mass flow and still gained speed (typically they work inversely, so he may have really increased the efficiency of his flow with this fairing). I still have Gary's patterns, and can pass them along if desired.



Even with Gary's good results, and Bob Ax's extensive and meticulous testing of side skirts and a bit of a venturi-creating bump on the cowl tunnel bottom (documented in the thread "Forming Aluminum for a Cooling Outlet Fairing"), these measures seem to have lower return on investment than shrinking the exit and increasing the efficiency of the exit flow. Since you have a 7, not a 7A, you should be able to do both of those last two things, though its definitely work, and a gut-check to cut into the cowl of a flying airplane (especially if its painted…AMHIK!).

I studied "The Shrinking Exit" thread quite a bit, and consulted with Steve Smith quite a bit along the way (thanks mucho fellas!). Steve told me one of the most effective measures to increase exit flow efficiency is to add a firewall radius, like the radius that the 8 has. This is also described in a thread called "RV-7 Cowling Exit Bump", with lots of pics. I chose to make mine look like an 8s radius, only slightly larger. Others have made large-radius shields further up into the engine mount area. I also took Dan's advice and made my radius of SS and continued it onto the belly as a heat/fire shield:



Then I reshaped my 6 into 2 exhaust pipes to bring them closer together, to allow the shrinking of the exit. Here are a few pics that compare before and after. Per Steve's great advice, the goal was to make the new tunnel smaller, with all faces more inline with the free stream airflow than before, and have it extend slightly behind the firewall (a bit of the side skirt concept, which I still intend to extend a bit in future experiments).






New pipes, old cowl:


New pipes, new cowl:


I picked up good speed, and my temps did increase, as expected when reducing cooling mass airflow (and cooling drag!). I'm at the point where cooling is adequate in cruise and in racing, but in lower airspeed regimes (slow climbs or formation flying), cooling is not quite adequate. Solutions range from installing a cowl flap to installing a louver, which could be exchanged for a flat plate plug for racing. Likely my winter project! Alan's (Anti-Splat Aero's) EZ-Cool Flap looks like a nice solution. A concern I have with a cowl flap is the impact that the side walls and actuator might have on the exit airflow when the door is closed. When open, the mass airflow is large, and increasing cooling is the concern, rather than cooling drag or speed. When closed however, I wonder if those sidewalls and the actuator, when raised up into the cowling, might become impediments to exit airflow, and cost speed. That is why I'm thinking of trying the removable louvers first. Its also why I would think that the internal exhaust pipe fairing that you mused on in your last post might not be an effective tool…but I'm unsure about that. I'd be very interested in other's thoughts on this concept. If it wasn't for the obsession with tip-top speed, I'd go for the cowl flap…I'm studying more on that in prep for the mod. Dan's cowl flap is very elegant and solved the sidewall issue…and would require more exhaust and cowl changes...it remains grail-like!

Good luck with the project…you're right about it being fun and challenging!

Cheers,
Bob

turbosaaber said:

I was wondering too if the two exhaust pipes are also creating von karmen vortex streets as well and wondered if creating a fairing around the exhaust pipes in the lower portion of the cowl would increase exit velocity.

Click to expand...

As a very general rule I think it relates to fineness ratio of a body (length/thickness), the sweet spot for lowest Cd being a fineness ratio about 3 for a two-dimensional body like a pipe.

As the angle between flow and pipe axis is decreased, the apparent fineness ratio is increased. Quick sketch, 90, 45, 30, and 15 degrees:



Plenty of detailed data out there somewhere; real aero guys kindly jump if if I'm wrong.

Update on my progress (or lack of).

I have made many changes on the airplane - 9:1 ported cylinders and a new prop. In the process I had a long fight with high CHT's that I ultimately got under control with some extensive baffle mods, timing adjustments and fuel flow adjustments. As a result, any effects of the work that I did on the exit was pretty much lost in the clutter.

I still run the basic outlet fairing and a bottom firewall rounding fairing. I have removed the center fairing body from the original design. My original goal was the smoothing of the confused flow illustrated in Carl's photo. There is no doubt in my mind that this fairing works for that purpose (you will see very streamlined oil streaks) and that smoothing of this flow improves the overall situation, but it is very hard to quantify.

I agree with Dan that the "most efficient" method for working these issues is a reduction in outlet size. If you can do that with a reduction in frontal area and form drag, even better.

I have often considered streamlining the exhaust pipes. If you study the work done in WWII, you will see many instances where the pipes themselves were formed into aero shapes. Never actually tried it and I suspect the gains would not be measurable.

Have not had a chance to SARL this summer. Too much other stuff going on. Hopefully back in the game next year. Short of instrumenting the inside of the cowl as Dan has done, performance measurement is about all that is left as a means of determining effectiveness, and it is a poor second choice to instrumenting. Too many variables.

Good luck and have fun.

I like the idea of side exit for cooling air, as used on Grumman Bearcats and Focke-Wulf 190a. My Thorp T-18 uses this as well.
When I was flying my RV-8 with the turbo rotary engine, I installed a set of louvers on the right side of the cowl for the oil cooler exit air. When the oil thermostat opened, I could feel the airstream warm if I held my hand down into it (while ground running). The cowl sides seem to have a low pressure area for most of the cowl length behind the air inlets.
I like Ken Ken H's idea of small air exit on the bottom.
For additional cooling in slow or climbing flight, I will retain my side louvers. These will be throttled with a sliding guillotine behind the louver. The ideal lower exit ramp would not be compromised with cowl flap hinge, side plate or actuator.
I'm installing a Lycoming now, so the lower cowl is in for major surgery with the removal of the radiator scoop and the addition of a forward facing injector air scoop. I can rework the air exit ramp and throttle the louvers while I'm gooping around with pour foam & fiberglass.

Bob, have you read this thread by Professor Horton??

http://www.vansairforce.com/community/showthread.php?t=68241

You could have both size exits 'Ya know.......just a few screws to R/R the bottom.



And then there is his servo operated outlet setup.......................

Mike S said:

Bob, have you read this thread by Professor Horton??

You could have both size exits 'Ya know.......just a few screws to R/R the bottom.

And then there is his servo operated outlet setup.......................

Click to expand...

Oh yes, to be sure. As I said, I'm a disciple of this concept, and Dan's work and cowl flap is the stuff grails are made of…

How to incorporate a center-hinged cowl flap into a 6 cowl, and how to shape the opening around the ex pipes (no exit recess like 8's have), and how to minimize frontal area with the current James inlet are cowl shaping issues I have contemplated a lot. This cowl rev 2 was a first step. As my glass fab skills develop (hopefully) through trial and error, error, sand-fill, trial…and then I get more courage to cut into the cowl…

Still learning', for sure!

Cheers,
Bob

I had the idea for swappable exit panels early on, while building the airplane. I just wanted to be able to install anything down there later without cutting or repainting the cowl itself. That includes the intake too, although I've never changed it...so far.

Anyway, I flew three progressively smaller exits while tinkering with cooling (mostly oil), all while gathering data. Then built a fourth with an even smaller primary exit and the variable door. Slow but sure.

I've always admired the vision you showed while building, as well as the study that goes into your decisions. I have a couple pics like yours above, but they came in steps, and represent two, "OMG, what have I done?well, no turning back now" moments! Couldn't figure out how to do both at once without the cowl falling completely apart!

2012


2013


Slower, and a bit less sure-er ?but sure learnin' a lot!

Cheers,
Bob

rvmills said:

Solutions range from installing a cowl flap to installing a louver, which could be exchanged for a flat plate plug for racing. Likely my winter project! Alan's (Anti-Splat Aero's) EZ-Cool Flap looks like a nice solution. A concern I ha

Click to expand...

Nasty that's a pretty good idea. If installed with the small camlocks the louvers/plate can be swapped out in a minute or two. A few days ago I was doing some math running LOP and what the true savings in speed/time/money really is. For a 300mi trip losing 7 knots added 4 minutes to the trip. Most of the time you can run the louvers and for the times where speed and efficiency really matters put the flat plates in.

rocketbob said:

Nasty that's a pretty good idea. If installed with the small camlocks the louvers/plate can be swapped out in a minute or two. A few days ago I was doing some math running LOP and what the true savings in speed/time/money really is. For a 300mi trip losing 7 knots added 4 minutes to the trip. Most of the time you can run the louvers and for the times where speed and efficiency really matters put the flat plates in.

Click to expand...

Kinda what I'm thinking Pooner. It's a sacrifice in the immediate flexibility of a cowl flap in exchange for being able to keep the cowl exit smooth and clean for racing. It could be overkill analysis, but perhaps worth the experiment.

Not trying to bash Allan's cowl flap at all?looks like a great mod, and he and I talked about it while it was in development. Still may end up there. However, with the effort expended to smooth, reshape and resize the exit area, I'm hesitant to place anything in the flow path that might block, slow, or cause separation within, the exit flow that we are trying so hard to reaccelerate to free stream velocity.

Really bad home-brew illustration here:

The side locations may be less of an impact than the center-mounted location?air is earlier in its acceleration (I believe), and there is more room to flow around the cowl flap side walls and actuator. Down in the tunnel section, seems that it would be more critical to keep flow smooth and fast.

So I may try a pair of louvers that are just undersized for the hole that Allan's cowl flap fits in?if it works (cools well for slow work), and maintains the top speed when replaced with flat plates, a good experiment. If not, next phase is a call to ASA!

Or...make a pair of louvers that has the same cutout size, joggle shape and hole pattern as the joggle in the ASA EZ-flap, and make the two interchangeable for some testing?

Butch is thinking' again?always trouble!

Thoughts?

Cheers,
Bob

2 questions and musings

Hey Bob,
I think you are like our old friend Bob Axsom, posting at 3AM! Beautiful artistry with the digital flow lines there!.

Couple of questions - 1. are you using internal curtains like Alan Judy and Bob? And do you think they provide some help?




2. what did you use for that slick inside coating of your cowl.

I am kinda deep in "all things cooling" on my build now and looking at the NACA reports, and DanH's silicone sheets around the cylinders and head, it seems like there is not much flow area out the bottom there. And that is all the air coming through the engine (except the oil cooler). The remainder of heat picked up at random from all the non-heat transfer designed surfaces. Assuming AJ has the same heat rejection to head/cylinders as the rest of us (may not be true) then he is using waaaay less air than that Lycoming cooling flow chart with those 2-1/8" inlets. Thinkin'out loud - Surely, someone out in the world has taken a head/barrel assy, taped off areas and run it on a flow bench to record CFM (M-dot) vs Delta-p is across there. If so, we can understand better what to do with that air. Now, we slow the air and dump in a huge box on top of the engine, squeeze it through the head, barrel and cooler (sped up), then dump in back into a "toy box" (slow again) and try to make it flow smoothly and play nice on the exit as it is reaccelerated (fast again).

edit: Oh - - why is it no one but DanH has the flow blocked behind the spinner?

rvmills said:

...I'm hesitant to place anything in the flow path that might block, slow, or cause separation within, the exit flow that we are trying so hard to reaccelerate to free stream velocity.

The side locations may be less of an impact than the center-mounted location…air is earlier in its acceleration (I believe), and there is more room to flow around the cowl flap side walls and actuator. Down in the tunnel section, seems that it would be more critical to keep flow smooth and fast.

Click to expand...

Steve Smith was kind enough to offer some guidance in that regard. If I understood correctly, when an enclosed volume at some pressure higher than ambient is offered a simple exit, the acceleration of air is practically all within the exit itself, or very close to it. As a side note, with a stock RV 6, 7, or 9, the sharp edge of the firewall probably causes some flow loss (top illustration).

I'm using an exit bell. In this case, the air is accelerating over a longer distance, as it passes down the converging duct. Although there may be some advantage in getting the airflow nicely organized (for lack of a better phrase), the bell itself has a lot of surface area, and the additional skin friction may slow the flow as compared to an exit with less skin area (middle).

The addition of a radius at the base of the firewall may be the best compromise, and in fact it's the standard on an RV-8 (bottom).

Notice I've used a lot of "may" and "probably" words in the above descriptions. All design is a matter of making intelligent compromises. Nobody knows precisely, for sure, what is best here...but given an aerodynamics issue I'd bet on Steve. The bell has some other attributes.

Returning to additional variable exit area, certainly everywhere inside the cowl has flow, in considerable volume. However, we're probably well advised to not worry too much about flow in the slower areas, and worry about it a lot in the high velocity areas. I suspect that adding flow impediment (a door mechanism) in the outboard rear corners of the lower cowl wouldn't actually be much of an impediment.

BillL said:

Couple of questions - 1. are you using internal curtains like Alan Judy and Bob? And do you think they provide some help?

Click to expand...

I have doubts Bill. Yep, the textbooks all illustrate theoretical cooling flows with converging ducts, but (1) note previous comments about duct skin friction offsetting flow gains, (2) a big converging duct under the engine is chock full of stuff...wires, pipes, hoses, all flow impediments, and (3) regular maintenance becomes more difficult.

Assuming AJ has the same heat rejection to head/cylinders as the rest of us (may not be true) then he is using waaaay less air than that Lycoming cooling flow chart with those 2-1/8" inlets.

Click to expand...

We all probably have roughly the same total heat rejection for any given HP (oil to air and cylinder head/cylinder to air combined), but the parallel valve heads appear to have less fin area, more widely spaced. They also tend to run higher CHT for a given HP. I think parallel valve heat transfer from the heads and barrels is somewhat lower than from an angle valve.

Nothing wrong with small inlets, if you have the space for proper diffuser shapes behind them. Might even flow more than you think at first glance. Have you read Chris Zavatson's work?

http://n91cz.com/

DanH said:

<snip> Have you read Chris Zavatson's work?

http://n91cz.com/

Click to expand...

Yes, read several papers and squeezing for detail. Is there a particular part to which you refer?

Still, a little disappointing that the information provided by Lyc is lacking details about fraction to oil and conditions.

Curtains - looking under the engine had me wondering if they worked at all. May be a good heat shield. (if needed)

Why has no one (on VAF) but DanH used the prop spinner seal? That seems to be a key sealing area to manage.

BillL said:

Yes, read several papers and squeezing for detail. Is there a particular part to which you refer?

Click to expand...

I was just thinking about the good diffuser photos Chris posted. The paper you sent me was excellent.

Why has no one (on VAF) but DanH used the prop spinner seal? That seems to be a key sealing area to manage.

Click to expand...

Actually there are several. For example, Tom Martin's Rocket is reputed to be the fastest in SARL. He seals his propshaft opening with a foam ring.

The fast Continentals simply run the front baffle wall below the propshaft, as they don't have an alternator and starter down there.

SHIPCHIEF said:

I like the idea of side exit for cooling air, as used on Grumman Bearcats and Focke-Wulf 190a. My Thorp T-18 uses this as well.

Click to expand...

I noticed at Oshkosh this year that a lot of Glassair IIIs have a louver on the right side. Here's a picture

I attended a forum on engine cooling and bought the book from a lady engineer. I'll post her name when I get home from work. She said the sides were a great location and cited the T-18 as a great way to move the hot air out with minimal drag. In looking at the T-18's it looks like they have quite a bit more square inches of exit area on the sides than we have on the bottom.

CAFE Reports

For those who don't already know, go to the CAFE Foundation's site to find three articles from the Miss. State aerodynamics research papers.

Lots of good information and well worth the time and effort.

Glenn Wilkinson

BillL said:

Why has no one (on VAF) but DanH used the prop spinner seal? That seems to be a key sealing area to manage.

Click to expand...

Are there any pictures of this? I read about it a while back but never found any...

rmartingt said:

Are there any pictures of this? I read about it a while back but never found any...

Click to expand...

Been experimenting with this one 350+ hours now. A fine use for Van's black baffle rubber...

When new:





Lower cowl pressure pushes it against the ring bear carrier. Wears a smooth strip on the paint, and a feather edge on the seal.

BillL said:

Hey Bob,
I think you are like our old friend Bob Axsom, posting at 3AM! Beautiful artistry with the digital flow lines there!.

Couple of questions - 1. are you using internal curtains like Alan Judy and Bob? And do you think they provide some help?

2. what did you use for that slick inside coating of your cowl.

edit: Oh - - why is it no one but DanH has the flow blocked behind the spinner?

Click to expand...

Bill,

No 3am posting here...our clocks are 2 hours apart. It was a bit late tho! On your Q's:

1. I don't have internal curtains like Alan, or any baffles like Bob Axsom. Those techniques would seem to help isolate upper and lower cowl plenum areas, prevent reverse flow around the engine, and help direct flow towards the exit. However, it is art and science, as Bob found out. Some baffle configurations slowed his plane, and his final configuration appears to have increased his speed. It was a lot of trial and error, and placement and design of the baffles did not appear to me to be intuitive at all. John Huft's super-fast 8 didn't have zone separation baffles (as Bob called them) but did have ducts (fences) on the lower cowl that curved towards the exit opening. As I reshaped my lower cowl and exit, I had numerous conversations with Steve about the project, and as Dan said below, he expressed concern about skin friction. It's why I'm trying to keep things as uncluttered as possible. It's very difficult to say which method is best.

2. The inside needed a lot of work. I ended up sanding it clean (even removed a little material in some really thick spots, then smoothing with Superfil, and painting with primer and paint left over from a neighbor's Bearhawk build. The tape is Van's heat resistant tape, with fiberfrax underneath.

As Dan said, Tom Martin also sealed his cowl at the prop shaft. He glassed-in the front opening to within 1/4", then sealed it with a foam strip. Jason Rovey sealed his gap using Dan's method. Maybe each of them will post their results here.

DanH said:

Steve Smith was kind enough to offer some guidance in that regard. If I understood correctly, when an enclosed volume at some pressure higher than ambient is offered a simple exit, the acceleration of air is practically all within the exit itself, or very close to it. As a side note, with a stock RV 6, 7, or 9, the sharp edge of the firewall probably causes some flow loss (top illustration).

I'm using an exit bell. In this case, the air is accelerating over a longer distance, as it passes down the converging duct. Although there may be some advantage in getting the airflow nicely organized (for lack of a better phrase), the bell itself has a lot of surface area, and the additional skin friction may slow the flow as compared to an exit with less skin area (middle).

The addition of a radius at the base of the firewall may be the best compromise, and in fact it's the standard on an RV-8 (bottom).

Notice I've used a lot of "may" and "probably" words in the above descriptions. All design is a matter of making intelligent compromises. Nobody knows precisely, for sure, what is best here...but given an aerodynamics issue I'd bet on Steve. The bell has some other attributes.

Returning to additional variable exit area, certainly everywhere inside the cowl has flow, in considerable volume. However, we're probably well advised to not worry too much about flow in the slower areas, and worry about it a lot in the high velocity areas. I suspect that adding flow impediment (a door mechanism) in the outboard rear corners of the lower cowl wouldn't actually be much of an impediment.

Click to expand...

Very much on the same page with you here Dan.

I chose the radius for simplicity of fabrication and ease of installation in a very crowded firewall area. The friction of the bell's surface was considered, but I would think that the bit of extra friction might be countered (in terms of pros and cons) by moving air towards the exit that otherwise might find its way to the firewall and all the clutter back there. Is that what you mean by other attributes?

I do think that the air accelerates quickly at the exit, and there is far less movement further upstream. Steve and I were texting about the placement of the louvers or cowl flap(s), and he concurs that the cowl flap side-walls and actuators may have little impact in the side positions of the bottom of the cowl, but may be impacting in the center of the tunnel. How much is unsure. Are you aware of any flow studies within a typical cowl? I believe quite a bit of study has been done on differential pressures, but I would be very interested in flow visualization within a cowl. I believe you and Steve are right about the flow action being focused around the exit...or at leakage spots like around the prop.

A related question...if the flow is less in the corners, does that make the cowl flap less effective there than in the tunnel?

In my case, I want to add cooling capacity with minimal impact to the race configuration. Louvers may do that, but the flexibility of the cowl flap adds the ability to maintain improved cruise performance while adding cooling for climbs and formation work. I'm contemplating a pair of louvers that match the size and hole pattern in the EZ Cool flap, adding nut plates to each, and making them interchangeable for comparison testing to a flat plate plug.

Hey Allan...want to join in a comparison test?!

Great discussion!

Cheers,
Bob

rvmills said:

Maybe each of them will post their results here.

Click to expand...

Minor side note...the shaft seal, like many other features discussed here, is an individual component of a system. Added at random, individual components may not show any significant change by themselves.

The friction of the bell's surface was considered, but I would think that the bit of extra friction might be countered (in terms of pros and cons) by moving air towards the exit that otherwise might find its way to the firewall and all the clutter back there. Is that what you mean by other attributes?

Click to expand...

The primary additional attribute is its effect on the oil cooler duct exit.

A related question...if the flow is less in the corners, does that make the cowl flap less effective there than in the tunnel?

Click to expand...

Nope. Don't confuse velocity with pressure. Air moves from here to there because of (1) pressure differential, (2) inertia, (3) entrainment, or (3) all of the above.



My guess is that lower cowl internal flow with a single center exit looks like the example on the left. The primary driver is the difference in pressure between the enclosed cowl volume and the outside air. Air moves from high to low, which sets up a flow to the exit. Air in the rear corners is entrained along "A" and circulates, doing nothing very useful (same thing happens in the wheelwells of cars). There is some energy loss. You could wall off those dead spaces and eliminate the circulation (a big duct extending from the underside of the engine, as previously discussed), but skin friction from the walls would also be an energy loss.

Two additional small exits offer new ports from high to low, so the air goes there too. Velocity at all exits will decrease.

I'm contemplating a pair of louvers that match the size and hole pattern in the EZ Cool flap, adding nut plates to each, and making them interchangeable for comparison testing to a flat plate plug.

Click to expand...

My man!

DanH said:

Minor side note...the shaft seal, like many other features discussed here, is an individual component of a system. Added at random, individual components may not show any significant change by themselves.

Click to expand...

Yes...Tom did say his original intent with prop sealing was speed, but the result instead was increased cooling, which allowed him to shrink his exit (actually just keep his cowl flap closed), which likely increased speed...

...all very interrelated!!

DanH said:

The primary additional attribute is its effect on the oil cooler duct exit.

Click to expand...

Looked back at The Shrinking Exit thread, and that effect is due to the oil cooler exit duct being routed to a part of the bell that forms a bit of a Venturi, thus creating a bit of low pressure to increase the flow out of the oil cooler, is that correct?

DanH said:

Nope. Don't confuse velocity with pressure. Air moves from here to there because of (1) pressure differential, (2) inertia, (3) entrainment, or (3) all of the above

My guess is that lower cowl internal flow with a single center exit looks like the example on the left. The primary driver is the difference in pressure between the enclosed cowl volume and the outside air. Air moves from high to low, which sets up a flow to the exit. Air in the rear corners is entrained along "A" and circulates, doing nothing very useful (same thing happens in the wheelwells of cars). There is some energy loss. You could wall off those dead spaces and eliminate the circulation (a big duct extending from the underside of the engine, as previously discussed), but skin friction from the walls would also be an energy loss.

Two additional small exits offer new ports from high to low, so the air goes there too. Velocity at all exits will decrease.

Click to expand...

This makes good sense. Decreased exit airflow speed, but increased overall mass flow, with the three openings. Better cooling, but increased cooling drag. Close the cowl flaps, and the "stuff" sticking up into the lower cowl may have min impact in that entrained air in the corners.

DanH said:

My man!

Click to expand...

Gulp...committed now!

Oh Allannnnnnnnn........



Cheers,
Bob

Looked back at The Shrinking Exit thread, and that effect is due to the oil cooler exit duct being routed to a part of the bell that forms a bit of a Venturi, thus creating a bit of low pressure to increase the flow out of the oil cooler, is that correct?

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That's what I had in mind when I started, but it doesn't work out that way. The pressure drop wasn't all that great...static pressure measurements were still well above freestream at the point where the oil cooler duct entered the bell.

A series of static pressure measurements along the length of the bell did suggest that the bell resulted in a pressure gradient (a drop from the high pressure in the cowl to the low freestream pressure) with some length (A to B below), rather than the short, more abrupt gradient expected with a conventional exit.

I made a variety of oil cooler system changes, all to the good, but still not quite as good as I had in mind. I stopped fooling with it when I started thinking about how and where to place a variable exit door. Lots of test flying with the various exit sizes said I didn't need much additional area for full CHT control, but it would be nice to have more dynamic range for oil temperature control.

So, here's what I'm flying now. When the door is closed, the oil cooler duct exit is subjected to pressure less than lower cowl pressure, meaning it flows more than if I just dumped the oil cooler air into the lower cowl in the conventional manner. However, it's still higher than freestream; the flow is damped.

When I tip open the door, the gradient moves forward and shortens. The oil cooler duct exit is now subjected to pressure pretty close to freestream. It's almost like I extended the oil cooler duct all the way outside the cowl.

The effects are really pronounced at low airspeed. I can stagger along at 3 knots over stall in formation with an ultralight, nose up, with power. CHT won't get out of a reasonable range (325-365) no matter what I do with the door, but oil temperature is hugely responsive to door position.

Yeah, I know, everybody thinks I do all this stuff for speed...but that's not the Vans design mantra, is it?

A couple of notes on my experience.

I suspect that Dan's drawing of lower cowl flow is correct. The back corners are low speed, high pressure areas. An anecdote about that -

I also have a Dan H style spinner seal. While at a SARL run after installing that, I taped the top gear intersection fairings to my gear leg fairings. As the run proceeded, I developed a yaw to the right that took considerable rudder to correct and as you might expect, speed dropped. Upon inspection, I found that the trailing edge of my right gear leg fairing had split open at the epoxied seam. The pressure from the back corner of the cowl was no longer escaping through the seam at the upper fairing to leg intersection (that was now taped shut). This re-routed the high pressure from the back corner of the cowl into the fairing and split the seam in the trailing edge of the leg fairing open.

This is logical and mentioned by Tom in one of his threads. He sealed the openings in the back corners of the cowl where the gear legs pass through the cowl and noticed a significant change in cowl flow.

Also, if you look at Tom's cowl and Huff's cowl and Allan's cowl, they have built internal diffusers to isolate these corners and smooth flow to the exit, right along the exit flow lines in Dan's sketch. All of these examples are very fast and cooling efficient airplanes.


This all makes me believe that placement of variable exits in this area would be a very effective way to turn that high pressure area into a very efficient source of cooling flow by opening it to the relatively low free stream pressure via louvers or a door.

Wish I had more time to spend with these mods. Can't wait to retire and take up hobbies full time!!

Scott,

John Thorp felt the exits in the "traditional" Thorp cowl were too large. He was fine with them for asthetic reasons By his (quite conservative) guidelines, the exits should only be around 69.3 sq in for an O-360. Generally, (not specifically T-18) one concern about side exits "over the wing" is reduction in lift, or a beneficial improvement in stall origination.

For those who mentioned it, I believe that Tom and Mark and some other Rocket guys have been down the side cowl exit path. There are threads somewhere documenting their efforts.

Marc Bourget said:

By his (quite conservative) guidelines, the exits should only be around 69.3 sq in for an O-360.

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No doubt sized for WOT and low speed (climb), as a designer must do with a fixed exit area. It's well over twice the area required for cruise.

rvmills said:

Or...make a pair of louvers that has the same cutout size, joggle shape and hole pattern as the joggle in the ASA EZ-flap, and make the two interchangeable for some testing?

Butch is thinking' again?always trouble!

Thoughts?

Cheers,
Bob

Click to expand...

Just ordered a custom louver for the Rocket from these guys: http://rodlouvers.com/

Price was very reasonable and they were easy to work with.

gereed75 said:

A couple of notes on my experience.

<snip> . . . Upon inspection, I found that the trailing edge of my right gear leg fairing had split open at the epoxied seam. The pressure from the back corner of the cowl was no longer escaping through the seam at the upper fairing to leg intersection (that was now taped shut). This re-routed the high pressure from the back corner of the cowl into the fairing and split the seam in the trailing edge of the leg fairing open. . . <snip>

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Now THAT is very interesting ! I had been wondering about that area of the cowl. Also sealing the perimeter so that it would not have to be taped.

Thanks for that. Just how was this corner sealed anyway? Foam attached to the cowl?

BillL said:

Now THAT is very interesting ! I had been wondering about that area of the cowl. Also sealing the perimeter so that it would not have to be taped.

Thanks for that. Just how was this corner sealed anyway? Foam attached to the cowl?

Click to expand...

I had the same thing happen, and at the open place in the middle the trailing edge it opened up further in flight. A photo of the formation is how I discovered it. I don't believe it was caused by high pressure in the cowl as the top of the fairing was foamed and completely sealed.

Of all of the things that I have done, I believe that sealing the top of the gear leg faring from the cowling ranks about as high as any other single issue.
The air is trying to get out of the lower cowling, and it really does not care how it gets out. The opening in the cowling at the gear faring is usually "U" shaped,allows air to escape out around the upper faring, the gear leg, lower gear faring and around the wheel pant opening. All this air squirting out causes drag. Foam in the gear faring will stop some of this movement but it will not stop the internal pressure around the upper gear faring. I believe this pressure explains why you see the aft edges of a lot of upper gear farings bent outwards after repeated flights when they were a perfect fit when first installed.

Here is my method.
Remove lower cowling.
Slide upper gear faring up until it is in the flight position and secure it there
Take a large piece of baffle seal and cut it so that it nests smoothly in the shape of the upper faring, it should be large enough to more then cover the "U" cutout in the lower cowling.
RTV/bond etc, this baffle piece to the engine mount and firewall. Seal all holes. Make sure that you do NOT bond the baffle seal to the upper faring!
When the glue has set, remove upper gear faring.
Now when you put the lower cowling in place the baffle piece will rest nicely inside the cowling and will provide a nice tight seal in that area.
Mine has been in place for about five years now.

As has been stated in this and many previous threads, no one modification will provide a significant change. Some modifications will not work unless other areas are worked on.

Your goal is to get the cooling air to go straight and smoothly into a sealed upper plenum and straight and smoothly out of a sealed lower plenum. As it leaves the lower cowling it should be going in exactly the same direction as the surrounding air. I do not believe that louvers are for lovers of speed!

We've all read about how efficient and low drag the NACA scoops are.

Has anyone tried to use small scoops in a reversed position in the high pressure areas Dan highlighted, I would think one on either side of the cooling exit would be the better place... they may be better that louvers.

Glenn Wilkinson

Few build a "proper" NACA scoop. They're efficient as intakes in high pressure locations, usually where the cross-sectional area is growing.

Multiple exits, unless well positioned and well designed, usually add add vortex generators. (not absolute, just trying to prompt thinking!)

Peter Garrison designed a cowl for the T-18 where tufting the side exits demonstrated an almost perfect "after cone". Visualize a five foot "hemi-teardrop" with the last 18" cut off. The air "filled in" for the missing tail of the tear drop.

mjb

Dan,

Another point, supported by CR-3405 is that you can oversize the inlet with little or no additional drag IF it's designed correctly. As I mentioned in another post proper exit design gets you an efficient "after body" or after cone.

Maybe it's not the size but how you design it? :^)

I sent you a PM did you see it? I'd like to backchannel some questions.

mjb

Marc Bourget said:

Another point, supported by CR-3405 is that you can oversize the inlet with little or no additional drag IF it's designed correctly.

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Well, I've said that more than a few times......

As I mentioned in another post proper exit design gets you an efficient "after body" or after cone.

Click to expand...

I really think the RV-8's slot exit is quite good, after eliminating all the cowl exit chute hanging below the belly skin line.

I sent you a PM did you see it?

Click to expand...

No PM here.

I really enjoy threads of this type...I learn something every time.

For example, I had never considered that air loss through the gear leg holes in the cowl would be significant but now I know better.

Glenn Wilkinson

Lots of great info, for sure!

Dan, thanks for the amplification on the cowl flap's (variable-size exit's) effect on oil cooler performance. Very interesting. Perhaps related, as I don't have a ducted oil cooler, but after shrinking my exit, the impact on oil temp was also more marked than the impact on CHT, though both are higher during slower operations than before. However, both still move with speed changes, whereas it sounds like your CHTs are fairly steady. Still thinking through this effect.

Tom, great description of the corner seals...I'm all over that! Any chance you can post a pic? I think I get it, but you know what a picture is worth! As for louvers, I completely agree. My thought with them is to make them interchangeable with flat plates, so for anything but racing, cooling is improved...and racing performance is maintained with little impact from cowl flap internal parts clutter...kind of a KISS "cowl flap" version. Then again, it's kinda like a ground adjustable prop...it leaves a lot to be desired in terms of flexibility. A variable exit also maintains cruise performance...and heck, every flight has some low-speed elements that could use additional cooling.

Testing appears to be on the horizon...good lead on the custom louvers Pooner.

Neat info!!

Cheers,
Bob

Video in flight RV-6

Couple of videos to watch.

https://www.youtube.com/watch?v=dtIwhaSiu_4 7,500 ft. WOT

https://www.youtube.com/watch?v=2zwiMMBEMH8 3,000 ft. WOT

This last summer I was at a flying event in Natchez, MS and I did some low altitude WOT testing at 800 ft. turning the engine 2,680 rpm and picked up a significant speed gain over WOT at my home airport of 3,000 ft and the manifold pressure was the highest that I'd ever seen it, 31.4" All temps are great, many times I have a hard time to keep cylinder temps up. All I know is that the cooling mods that I've done seem to be working, I know some of the mods are not perfect and could use some attention, the oil cooler exit is in a terrible place but I just don't have any room to make it better. I just go fly it and don't mess with anything anymore other than doing an annual and oil changes, if it works leave it alone and go fly.