Exit Area Too Small ??????

I'm thinking this is pretty good evidence (as well as the great number of RV's displaying marginal cooling on hot days) that the exit area on Van's cowling is too small. What do you guys think? I'm considering a set of these.



Building Tips / Techniques/ Mods
- Louver Install (purchased from www.AviationTechProducts.com )
...this was sent to Alex at ATP (and fwd'd to me by Tom Sampson thomas.e.sampson at comcast.net)

"Hello Alex ...
It's Wednesday night, and I am just back from the airport. The louvers arrived in today's mail, so I went out to install them and test flight.

Prepping the 2 louvers took about 20 minutes (drilling 12 holes each, deburring, etc) Cutting the two slots in the lower cowling took another 45 minutes ... as I only have a low power compressor in the hanger. Another 10 minutes to pop rivet the louvers in place, and 10 to put the cowlings back on ... so basically, and hour and a half from start to finish.

Then ... the test flight. It was 30 degrees at takeoff ... pretty warm in Portland ... just as it was last night when we flew after an oil change, with 9 gallons on fuel on board. (fyi, our aircraft is an RV9a, 0-320, 1125 empty weight)

Yesterday the oil temp ranged 205-210, peaking at 215 after climbout for a few minutes. CHTs were all over 410-420 after a climbout to 3000 feet. CHTs were 385-400 in cruise

Tonight's flight was with FULL fuel, same OAT ... so we were definitely heavier. BUT ... the MAX oil temp that we saw tonight was 185!! AND ... all CHTs were running 370-390 after climbout, and settled down to 345 to 355 in cruise.

THESE LOUVERS ARE GREAT!!!!! It was easy to see the difference, and the cooler temps are greatly welcomed! I vote for Vans to include a set of these in all kits!!!

Thanks so much for a great product, and a fast order turn around. I will mail your check and invoice tomorrow.

Regards,
Tom Sampson
N728MT"


Event News

What about all the RV's flying in stock configuration that don't have any problems?

What about Dave Anders RV-4 whose exit area was/is 10 sq inches less than the inlet?

If it solves problems or makes anyone feel better then by all means put them on. The only poeple we need to make happy are ourselves and the FAA. But, IMHO if the cooling system is thoroughly planned and executed well they are not needed.

I also thought the exit looked small compared to the entrances. I borrowed an aeronautical engineering book from a local engineer/PhD/Professor type and messured mine. The book said that the exit should be 1.2 times the size of the entrance. Mine measured about 1.01 and that was using the cross-section of the exhaust pipes. In reality the pipes come thru at an angle and take more room. I would say a more realistic number would be 0.95.

That said, mine cools fine... in Houston, in the summer. I do see some bulging in the oil filler door and another small cover I made to access the piano hinges on the top cowling at the firewall. That tells me the low pressure side is not so low pressure.

Karl

I hope that anyone installing a cooling mod such as this has done two things.

1. They have verified that the temps they are reading are valid. I am dealing with this right now. I was very concerned during my first test flight this past Wednesday (newly overhauled engine) but have since determined that the temp readings have an error of plus 30-40 degrees.

2. That they don't go making radical changes until the engine has completed its initial break-in. Actual temps can drop a lot after initial break in is completed. If you are having difficulty flying to get it broken in because of high temps, you sometimes need to resort to flights early in the morning until temps drop enough to allow flights at other times.

The cooling on RV's is a compromise.

The inlet to exit area ratios that they have seems to work to our advantage.

Other airplanes have complex systems for cowl flaps that need to be operated by the pilot. This allows for reconfiguring the cooling system for low speed/high alpha flight to get extra airflow, and then closing down the exit area when in higher speed level flight.

RV's probably have inlet areas that are just slightly on the big size for cruise flight (compared to the exit area). This is not the case for all engine sizes or engines with pumped up compression, etc., but it is true for most. Another issue all together is that we use one cowl design with the same size inlets for the full range of 150-200HP engines. The only thing that varies from one to another is the exit area.

What I believe happens with RV's, is they inlets are big enough to provide enough cooling for most RV's at high power low speed (High Alpha).
Then at high speed the slightly restricted exit area cuts down the flow which keeps us from over cooling and the side benefit is lower cooling drag (less flow through the cowl). At higher altitudes (were the air is less dens) it partially is self compensating.

The system isn't perfect...but not having cowl flaps is much cheaper, lighter,
easier for maint. etc.

Bottom line...I builders not do any mods until they have considered both of my points above.

a couple of thoughts

Assuming you have accurate CHT readings...

1) The exit area on the A model is significantly less than the taildragger

2) By adding louvers you may have cured a hit temp problem but you have also increased the cooling drag...I.e more airflow thru the cowl equals more drag.

I don't agree that the louver might be a good solution but it is wise to make sure that everything else is in order first...I.e the sides to the inlet ramps are glassed closed, the baffles seal correctly (or maybe even build a plenum) and the baffles seal around the cylinders and heads correctly.

On my Sam james setup I notice on hot days my cooling is marginal in the climb only...What would be a better solution (IMHO) is a cowl flap or a louver that can be closed after you have climbed to altitude and cooled the engine off then close it up to minimise drag.

I might get oround to this...one day....

Frank 7a

Not needed?

Yukon: I'm thinking this is pretty good evidence (as well as the great number of RV's displaying marginal cooling on hot days) that the exit area on Van's cowling is too small. What do you guys think? I'm considering a set of these.

Click to expand...

Some RV's (especially RV-4's) are over cooled, while the same engine prop in a RV-6 is more toasty? Go figure same/similar areas.

Tom, I'd avoid any cowl louvers until you fly. I think you have a O320 or O235? Rarely if ever have I heard of 320's (150/160HP) reporting high CHT issues. 150HP always run cool. It makes sense, less hp, less heat. I'd assume a O235 would run cool as well (in a cowling basically made for 180hp or more).

Per the time honored tradition, if CHT/OT temps are hot in climb, you can always lower nose and increase speed. I can see if you can't get temps down in cruise you might think about more cooling.

Oil temp should not be an issue if you use a SW (South Wind / Stewart Warner) oil cooler installed properly. The IO360 angle valve (200HP) is more of a challenge to cool apparently, needing a larger oil cooler capacity. More HP more heat, but also the 200HP angle valve uses oil more to cool the engine than parellel valve 360/320/235's.

Increasing the exit air will cool the engine, no doubt, but it will slow your cruise and top speed. I'd like to get data on the before & after louver speeds. Even after installed you could block it and flight test the delta in speed and temps. I could see making small louvers that pop open for climb only and keep the a smaller fixed exit? hummmm



From Dave Anders notes:

4) inlet to outlet ratio: what?s recommended, what works & why:

a) Stock Van?s: RV4 ratio ~ 39sq? X 60sq? = 150%
RV6 ratio ~ 44sq? X 56sq? = 127%

Designed for full throttle climbs at 90-100 mph
(gmc note: not sure but assume 100F day? Airspeed Vy?)

Results = increased drag at higher speeds

b) what works better:
Inlet 34sq" ( decreased To 30sq" no change)
Outlet 26sq" (excludes exhaust area)
Ratio 76%, could be smaller w/same ratio, CHT?s 350 max.


Dave clearly feels the exit should be 24% smaller than inlet, which is probably true in cruise. Clearly Van has larger outlet ratio, even larger than the 1.20 that RV8N Karl posted from another reference. It's clearly a moving target dependant on OAT, power, phase of flight and goal (max speed or more cooling).

A pilot controlled cowl flap is nice, but it adds complexity, weight and pilot chores. If you can do with out a cowl flap, it fits the KISS principle of the RV.

RV's have low drag and high climb/cruise speeds, so we benefit by having more cooling air to work with, than a C-172 for example, even in climb. We can climb at high speed and still out climb a C-172. In the summer you may have to lower the nose and increase airspeed sometimes. Climb rates are so good with RV's it's not a big sacrifice. This is true of many planes with out cowl flaps.

I remember flying the trusty C-172's w/ 4 people in the summer, crawling to altitude with intermediate level offs to keep OT green. Funny thing, there was no CHT gauge, so ignorance and rental planes are bliss?

No I did not abuse the above rental plane, in fact I was a CFI showing pilots how not to overheat in the summer. Most GA planes have no CHT gauge. May be they are over cooled so CHT is not an issue? May be oil temp is a good enough secondary indicator. For certification they have do a climb test and maintain temps in green. Of course green CHT per Lyc is hotter than what I want to run at. I like to keep it below 400F with out exception.

These new multi data channel engine monitors tells you if paint is flaking off the valve cover make me laugh. May be we have too much info? Kidding, I'm all for multi channel CHT/EGT, especially for experimentals. However we may worry a bit much, but CHT=400F or less is key to long engine life.


As far as oil temp, my theory about high oil temps has to do more from poor efficiency oil coolers (SW clones) and poor installation (using vans airbox kit). There is plenty of air in stock cowls, it is just how you use it.


If you have a hot IO360/200HP and operate out of death valley, louvers may be justified. I see a plus with the louvers. You can block them off in the winter. I know of a few RV-8, 200HP IO360's guys felt the urge to resort to louvers for hot days. However I think they had other issues, like using poor quality or small oil coolers for the job and/or not supplying the oil cooler with enough air.

Look at RV969WF (Alan Judy's) hot red RV-6. His exit is about 25 sq-in (not sure if its total area with or without exhaust). His RV-6's IO360/200HP inlet was cut down to nothing. I think he went with two 2.123 or 2.75" dia inlets or about 10" sq-in! This is less 1/4 of Van's stock cowl inlets. However Alan has a separate cowl scoop for his oil cooler which he can open/close from the cockpit. He stated his cooler inlet area is about 12.5 sq-in. So the total area is 22.5" and exit 25" so the ratio is 1.11. Angle valve IO360's reject more heat through its oil than parallel valve engines. Angle valve Lycs have oil squirt'ers on the piston backs to carry the heat away for one example. So his approx 10 sq-in cowl inlet area would be too small for any engine with out a separate oil cooler scoop, especially for a parallel valve engine, which needs air over the jugs.

I have 4" rings for a plan-jane O360/180hp. That's a bit over 25" sq-in. I've not decided what to do about the cowl exit area, but its going to get reduced some what. Less than 25"? Probably not. If I use Dave's 26" plus exhaust, it would be about 30"-32" total exit area. So my ratio would be about 1.24. A simple cowl flap would be cool. ANY IDEAS?

Equipment

rvbuilder2002 said:

.......
1. They have verified that the temps they are reading are valid. I am dealing with this right now. I was very concerned during my first test flight this past Wednesday (newly overhauled engine) but have since determined that the temp readings have an error of plus 30-40 degrees.
.......

Click to expand...

Scott,
What are you using that has a temperature error this large?
Thermocouples have a very defined voltage vs. temp. output, if you are using standard bayonet probes per the Lycoming spec. (there is even a MIL-Spec. number for he probes Lycoming tests with), modern electronics should be better than you quote....

gil in Tucson

Outlet size probably not the issue...

This is an interesting thread. I believe that the problem in some planes lies in the behavior of the air flow around and behind the exit area. I noticed years ago that if I slobbered oil on the belly directly behind the firewall, and within the confines of the exit area, some interesting things happened. Specifically, I saw areas where the flow was reversed, with air going into the outlet.

A while ago, we tufted the area of interest on a 7A here in town which was running towards the hot end. Look carefully at the yarns, you will see a couple that are pointing forward.

Regarding pressure in the lower portion of the cowling, I set up a manometer some years ago and found that there was something like 6 inches H2O pressure down there as compared to cabin static.

I believe the exit has enough area, but there does seem to be some misbehaving going on. I think there is much to learn from these:
[/URL][/IMG]
[/URL][/IMG]
[/URL][/IMG]
[/URL][/IMG]

Airflow...

Alex... really neat photos. It definitely seems that something is happening at the corner where the cowl meets the "exhaust tunnel"...

Did you try anything here the deflect the airflow? ... such as a lip on the side edge of the tunnel?

I presume these photos were taken in cruise... is that correct?

gil in Tucson

This is a very timely thread. Over the last six years I have done a lot of work on engine cooling with a couple of different rockets. Both had full metal plenums. I have done a lot of work on clean up the outlet air and also the inlet area. Almost all my cooling and speed gains have come from the smoothing of the inlet air. Having said that when you get the inlets working better it is then time to close down the outlets.
On my current rocket I have closed the outlet down to a slightly smaller area than the inlets and my engine runs cool at cruise, anywhere from 330 to 360 depending on power settings and outside air temps.
Last week, as a continuation of my experiments, I installed a cowl flap. My hope was that it would increase speed and help with balancing engine temperatures this winter. The cowling is not cut, the flap is inside the cowling, hinged forward and pulls up to the firewall, between the exhaust pipes. It takes a good pull to close it.
I have had three flights with the plane and I am perplexed to say the least. The system is helping to raise the engine temps but it is not helping with speed and infact may be reducing speed. The speed changes are very small and I have yet to quatify them but here is what is definately happening and I could use some input.
In level trimmed flight when I pull the cowl flap closed, the nose goes down in a very significant and repeatable way. When I trim for level flight in the flap closed condtion and I open the flap the airplane starts to climb.
My question is; Is this a drag situation in that closing the flap in some way increases drag, or is it simply that the airflow has in some way trimmed the plane differently? It is very hard to get accurate airspeed readings in trimmed before and after closing the flaps as it takes a few minutes to retrim and get stable again. I certainly intent on getting some accurate three leg gps data and also to fly beside another aircraft but the trim issue certainly surprised me and the results, at this stage, seem to be contrary to the common believe that a cowl flap should increase speed due to lower cowling airflow. Inputs are most welcome!!

az_gila said:

Alex... really neat photos. It definitely seems that something is happening at the corner where the cowl meets the "exhaust tunnel"...

Did you try anything here the deflect the airflow? ... such as a lip on the side edge of the tunnel?

I presume these photos were taken in cruise... is that correct?

gil in Tucson

Click to expand...

Gil, yes they were taken in cruise. You did notice the yarns that were pointing straight forward into the "outlet", didn't you? The ones on the outsides, laterally, to the exit do seem to bend inward a little, but the astounding ones are those pointing forward. We didn't do anything after this flight except scratch our heads.

Tom Martin said:

This is a very timely thread. Over the last six years I have done a lot of work on engine cooling with a couple of different rockets. Both had full metal plenums. I have done a lot of work on clean up the outlet air and also the inlet area. Almost all my cooling and speed gains have come from the smoothing of the inlet air. Having said that when you get the inlets working better it is then time to close down the outlets.
On my current rocket I have closed the outlet down to a slightly smaller area than the inlets and my engine runs cool at cruise, anywhere from 330 to 360 depending on power settings and outside air temps.
Last week, as a continuation of my experiments, I installed a cowl flap. My hope was that it would increase speed and help with balancing engine temperatures this winter. The cowling is not cut, the flap is inside the cowling, hinged forward and pulls up to the firewall, between the exhaust pipes. It takes a good pull to close it.
I have had three flights with the plane and I am perplexed to say the least. The system is helping to raise the engine temps but it is not helping with speed and infact may be reducing speed. The speed changes are very small and I have yet to quatify them but here is what is definately happening and I could use some input.
In level trimmed flight when I pull the cowl flap closed, the nose goes down in a very significant and repeatable way. When I trim for level flight in the flap closed condtion and I open the flap the airplane starts to climb.
My question is; Is this a drag situation in that closing the flap in some way increases drag, or is it simply that the airflow has in some way trimmed the plane differently? It is very hard to get accurate airspeed readings in trimmed before and after closing the flaps as it takes a few minutes to retrim and get stable again. I certainly intent on getting some accurate three leg gps data and also to fly beside another aircraft but the trim issue certainly surprised me and the results, at this stage, seem to be contrary to the common believe that a cowl flap should increase speed due to lower cowling airflow. Inputs are most welcome!!

Click to expand...

It would be my guess when the door is closed the internal pressure of the cowling is so great air flow is stalling at the inlets and causing major turbulence in front of the airplane and the pitch change.

I am having similar issues with a restricted exit area and am in the process of installing cowl flaps to relieve internal cowl pressure. At high speeds, I am convinced the pressure is so great internally, little air is flowing through the radiators which results in poorer cooling than flying at low speed. It's the most logical conclusion I can come up with. But I also know there is much I don't know so we shall see.

Some old Bonanza's use side cowl exits shaped like a boomerang. They have internal fences that capture air and ram it outward. Interesting how cooling issues have been dealt with over the years.

AlexPeterson said:

Gil, yes they were taken in cruise. You did notice the yarns that were pointing straight forward into the "outlet", didn't you? The ones on the outsides, laterally, to the exit do seem to bend inward a little, but the astounding ones are those pointing forward. We didn't do anything after this flight except scratch our heads.

Click to expand...

It has to be low pressure area created by the fast moving air across the cowl at the exhaust opening. I would bet if a tuft could be installed about an inch below that point, it would point straight aft. There's plenty of air coming out of the cowl, you can be sure of that or the engine would be mighty warm. But right at that corner is a low pressure spot.

I did a tuft test at the prop recently an found air coming forward from the inlet near the prop hub. It too is a low pressure area.

Blast tubes

I have several blast tubes installed (for gascolator, fuel pump, magento). Given the previous postings, I am wondering if they are actually 'blasting' and what the direction of the flow is.

Will have to look into this when I get it flying.

Alex,

Some really interesting photos!! Wonder if it could be the turbulence off the exhaust pipes causing the inward flow?? It certainly appears that the most signficant forward flow is immediately aft of the pipes?? Any fluid dynamics experts out there care to speculate?? I had always heard that a round structure out in the relative wind is the absolute worst for smooth air flow.

BTW, my experience with cooling issues on my airplane and others in S. Texas suggests that the A models tend to be over respresented among the RVs with cooling issues---although the Lycs and Lyc clones are very individual in their cooling needs. However, if you look into the air exit on a nose dragger vs a tail dragger you see the problem. The nose gear structure on the motor mount plus the gear leg itself has to create a lot of turbulance just forward of the air exit!!!

BTW, contrary to some beliefs, when I added louvers, albeit cut down versions of Alex D.'s product, there was no measurable difference in cruise speed (IAS or TAS) at specific power settings.

Thanks for the photos.

Cheers,

db

6A Cooling

I had problems with both high CHTs and oil temps. After scratching over the cowl baffel area where others said it was fine, one guy mentioned that I should clean up the inside on the low pressure side.

I had scat tubing going from the right exhaust heater muff to the left then back to the cabin intake. I removed all of them and that lowered the temp noticeably.

I then opened up the cowl around the exhaust and that helped about the same amount.

Finally I added louvers and now I have no problems.

With my rebuilt engine I am changing to a SW 8406R oil cooler with everything else the same.

As I reinstall the engine, I wonder if I can somehow round the bottom exit area like on the 8s.

Tom Martin said:

Last week, as a continuation of my experiments, I installed a cowl flap. My hope was that it would increase speed and help with balancing engine temperatures this winter. The cowling is not cut, the flap is inside the cowling, hinged forward and pulls up to the firewall, between the exhaust pipes. It takes a good pull to close it.
I have had three flights with the plane and I am perplexed to say the least. The system is helping to raise the engine temps but it is not helping with speed and infact may be reducing speed. The speed changes are very small and I have yet to quatify them but here is what is definately happening and I could use some input.
In level trimmed flight when I pull the cowl flap closed, the nose goes down in a very significant and repeatable way. When I trim for level flight in the flap closed condtion and I open the flap the airplane starts to climb.
My question is; Is this a drag situation in that closing the flap in some way increases drag, or is it simply that the airflow has in some way trimmed the plane differently? It is very hard to get accurate airspeed readings in trimmed before and after closing the flaps as it takes a few minutes to retrim and get stable again. I certainly intent on getting some accurate three leg gps data and also to fly beside another aircraft but the trim issue certainly surprised me and the results, at this stage, seem to be contrary to the common believe that a cowl flap should increase speed due to lower cowling airflow.

Click to expand...

Fascinating.

Trim: I certainly wouldn't have predicted the trim change you are seeing. It is perhaps due to the sum of two effects, and I'm not sure which effect is larger:

1. There is less air in the area behind the cowl exit, right behind the cowl flap. The lower air pressure in that area would lead to a small nose down pitching moment, as that area is ahead of the CG.
2. As suggested by David-aviator, the reduced air flow through the cowling would be changing the air flow at the inlets. This might be leading to a higher pressure on the top of the cowling, behind the inlets, which would give a nose down pitching moment.

I don't see the pitch trim changes as a problem, but they certainly are an indication that the airflow has changed significantly.

Drag: Having the cowl flap inside the cowling is certainly simpler than hacking the cowling up to make its aft edge move, but you may be shooting yourself in the foot with respect to drag. When you close the cowl flap, you now have this area behind the cowl outlet that is getting very little exit air. As the aircraft moves ahead, that dead zone must be filled somehow, and the air moving into that area may create a bunch of additional turbulence. It may be beneficial to cut a section of cowling and hinge it at the front, so that there was less frontal area when the cowl flap was closed, and no dead zone behind the cowl flap. I suspect this would also reduce the trim change a bit.

Alex

Great phots!

While doing my cowl/cooling research I really noticed how much work/research/data was put into the inlet design and almost nothing with respect to the outlet. It would seem obvious by looking at those pictures that the outet air area is very turbulent and the shape does not aid the situation at all. I believe it is important to smoothly transition the air to the cowl outlet and then smoothly transition that outlet air into the airstream.

What I didn't see

Was the suggestion to add a 2 to 3" tail to the cowl exit.

John Huft and Aj have shown significant benefits to this mod. The theory being is that the air has significant donward motion at the outlet of the standard cowl.

Extending the sides and bottom backwards forces the air to straighten.

of course the eventual aim is to accelerate the air back up to same speed as the air going over he outlet...The outlet thus must act as a nozzle to do this.

My (complete guess) is the internal cowl shows us that there is a significant downward push because the nose drops when the exit air is restricted.

I think there is merit in the idea that the air is spilling out of the inlets when the cowl flap is down.

This would point to Dave Anders work where the inlets need to be restricted in some ratio to match the exhaust.

Would seem the restricted Sam James rings are a good place to start at 24.3sq". My cooling marginal in the climb but ok after (IO360 parrallel valve motor).

The exhaust restriction on the 7a makes me wish I built a TD...

Frank

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.
The next area I look at is the cowling exit. I lay on my back and look up at the end of the cowling. Usually I can look right up the firewall, especially on RV4s, and rockets. There is a lot of air rushing down that firewall and it is going almost 90 degrees to the flow of the outside air. Extending the cowling has helped quite a bit in my installations but only if you bell the exit air into that extension.
Kevin
I suspect you are right about the dead air aft of the present cowl flap. It was a relatively easy installation that did not yield the expected results. Actually cutting the cowling would have the added benefit of reducing the frontal area of the cowling if I made the flap long enough. It would have to be built in such a way that the air did not separate at the hinge line and cause turbulence.
I never expected a trim change, and a neutral to slight loss in speed!!! 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.

Total Heat Rejection

Has anyone given any thought to the expansion taking place in the lower cowling due to the radiant heat rejection of the exhaust system? How about the heat and pressure of the cabin heat dump valve? Oil cooler heat rejection also competes for the exit area. Lot's of hot air competing for a small hole, even before cylinder heat gets there. When all this hot air finally does get to the exit, the exit area is further reduced by the diameter of two exhaust pipes, as well as the turbulence they produce protruding into the airstream.

High CHT and oil temperature is death on a Lycoming. If louvers are a quick fix, I don't see a downside. Since these airplanes have been around as long as they have, you would think the cooling system would be "plug and play" at this point. There must be a basic,fundamental flaw that contributes to these issues.

Also, all you Canucks who say you have no cooling problems.......we hear you, but the majority of RV's are well south of you, and there seem to be lots of summer time cooling issues.

On my 7A (TMX O-360 FP) I would see 425-430 on #4 during climb out, I would lower the nose and reduce power to get the CHT down, Oil temps are not a problem other than to cold using Vans standard oil cooler, Have blocked off part of the oil cooler. CHT's were not a problem in curise.
I am presently experimenting with a mod I recently made, Seems to be working as I can now climb out with full power and see 405-410 on #4 with no need to reduce power or lower the nose to increase the speed.
This is what I did.
Took the bottom plate that sandwiches the lower cowl behind the nose gear rod off and made a new one that flares out and then back about 3 inches back of the firewall. I left 1/4 to 3/8 inch gap between each exhaust pipe, them riveted on a vertical .032 piece of alumn. on each outside edge to within 1/4 inch of the bottom of the fuselage from the rear most part of the new piece to the rear lip of the lower cowl.
Not hard to do and no need to modify anything on the lower cowl. You can easily take it off and put the original piece back on.
If any of you try this please let me kown what your results are.
Only had a chance to do about 3 flights with it so far.

N742GC 7A 65 hours.
Gerry Clabots

Cowl tunnel cut

Like John, I'm here is AZ where it is hot. I was having initial cooling issues on my 7. I looked things over and determined there was not enough exit area. I then cut the exhaust tunnel 1" at at time and tested the result. At 3" off the bottom of the tunnel the cooling issues were gone. Unless pointed out, nobody has noticed this modification.

I thought about louvers but couldn't bring myself to put them on. I'm not wild about the looks and wonder about the longevity. Probably unfounded but nonetheless they aren't for me.

Today, I flew with full fuel and two people with a take off temp of 102 degrees. I climbed 800 fpm, 120 kts indicated after clearing the Phx Class B. During the climb to 7500 ft my CHT's did not get to 400 and my oil temp peaked at 188.

I've flown in temps up to 118 (not a typo) with no major cooling issues. I only have to back off the climb rate some.

Sounds interesting...

Do you have a pic you can post Darwin? That mod sounds interesting and much easier than louvers.

RV7Guy said:

Like John, I'm here is AZ where it is hot. I was having initial cooling issues on my 7. I looked things over and determined there was not enough exit area. I then cut the exhaust tunnel 1" at at time and tested the result. At 3" off the bottom of the tunnel the cooling issues were gone. Unless pointed out, nobody has noticed this modification.

I thought about louvers but couldn't bring myself to put them on. I'm not wild about the looks and wonder about the longevity. Probably unfounded but nonetheless they aren't for me.

Today, I flew with full fuel and two people with a take off temp of 102 degrees. I climbed 800 fpm, 120 kts indicated after clearing the Phx Class B. During the climb to 7500 ft my CHT's did not get to 400 and my oil temp peaked at 188.

I've flown in temps up to 118 (not a typo) with no major cooling issues. I only have to back off the climb rate some.

Click to expand...

Darwin, you increased the exit area by about 100%. Have you had a chance to check TAS since doing this. If you got away without a speed penalty, you are indeed a genious.

Oh David, come on! Since when was performance a criteria for you Subie guys!!!! . Open up that cowling and get smooth, cool hp!

Gerry,

This sounds really interesting. Any chance you could post a picture?

Gerald Clabots said:

On my 7A (TMX O-360 FP) I would see 425-430 on #4 during climb out, I would lower the nose and reduce power to get the CHT down, Oil temps are not a problem other than to cold using Vans standard oil cooler, Have blocked off part of the oil cooler. CHT's were not a problem in curise.
I am presently experimenting with a mod I recently made, Seems to be working as I can now climb out with full power and see 405-410 on #4 with no need to reduce power or lower the nose to increase the speed.
This is what I did.
Took the bottom plate that sandwiches the lower cowl behind the nose gear rod off and made a new one that flares out and then back about 3 inches back of the firewall. I left 1/4 to 3/8 inch gap between each exhaust pipe, them riveted on a vertical .032 piece of alumn. on each outside edge to within 1/4 inch of the bottom of the fuselage from the rear most part of the new piece to the rear lip of the lower cowl.
Not hard to do and no need to modify anything on the lower cowl. You can easily take it off and put the original piece back on.
If any of you try this please let me kown what your results are.
Only had a chance to do about 3 flights with it so far.

Click to expand...

Picture

Bob Brown said:

Do you have a pic you can post Darwin? That mod sounds interesting and much easier than louvers.

Click to expand...

Here is a picture of the cut tunnel. Forget about the air dam. It was an experiment that didn't work and I ground it off.

The cowl cuts were done early in Phase I so I hadn't gotten any real data on speeds. I can tell you that my 7 is pretty quick. I don't think there has been any performance degredation.

May be its not worth it? (but worth a try)

Yukon said:

Has anyone given any thought to the expansion taking place in the lower cowling due to the radiant heat rejection.......?

Click to expand...

I don't think its a matter of does it work. It's more about do you need it, at least year round. If you do, it could be a life saver in hot climates. Of course our Kay-nook friends in the hinterlands to the north are "a-boot" cool temps, Eh!

Good point about temp expansion of air. Reading the NACA report the added heat I recall is a small factor. The following quote does not really address temp, it does show what the exit is all about, in their analysis.

"The flow density in the lower plenum
is determined from the temperature rise across the engine.
This information heretofore has not been supplied by engine
manufacturers, and estimates based on experience must be used.
Typical values range from 50?C to 70?C. The exit area (5)
is then sized to accelerate the flow so that its static
pressure is the same as the local external flow (6). The
exit area acts as the system throttle in that the flow rate
and associated pressure drops will adjust so that the exit
pressure matches the external pressure. For a given flight
condition, expanding the exit area increases the flow rate,
and conversely." NASA Report

As inlet shape, position, size and internal smoothness of the plenum play a factor in efficiency of the inlet, the exit is probably more than just AREA. Shape, location and internal flow leading to it are all players.

You could have two lower louvers and reducing the main exit to just accommodate the exhaust? You could also add a movable "slider door" (cowl flap) on the louver. Its easier putting slider flaps on the louver than in stock cowl air exit area, which is busy w/ exhaust pipes & engine mount tubing.

However???? Since vertical induction Lycs already have this fwd scoop and lower bump, the exit just tucks iback n the same frontal area. Why not just use that for the exit, verses adding more exits? However you are right, if you NEED it you need it. I think most think you will not, even in the southwest usa. With the fwd facing induction and scoop-less cowl, may be the louver makes more sense.

The goal of the designer is the accelerate the exit air and make it parallel to the free airstream. It's easy to say and hard to do.

Tom Martin said:

I am perplexed to say the least. The system is helping to raise the engine temps but it is not helping with speed and in fact may be reducing speed.............In level trimmed flight when I pull the cowl flap closed, the nose goes down in a very significant and repeatable way.

Click to expand...

A couple points, observations about your cowl flap. It ONLY reduces exit area. As an observation its an exit air restrictor more than a cowl flap.........

Most cowl flaps actually increase exit area, hanging out more in the breeze when open, which promotes more airflow. Your design reduces mass air flow, which will increase engine temp as you say. I like you would think it would reduce cooling drag but in retrospect probably not, at least not with a simple hinged flap in the cowl exit. It is a cowl flap but I am just making a distinction between "classic" cowl flaps which tend to open out into the slipstream.

My observation is exit air velocity or "THRUST" was reduced. I'm sure this could cause a pitching moment change. Thinking about it, less thrust OUT the lower cowl would make the nose pitch down. Closing the cowl flap, exit area is less so cooling is less, but you lose thrust as well? I suspect air speed changes may be small, hard to measure. It would be interesting if you determine you lose SPEED.

In theory you cowl exit should be sized for MIN area and the cowl flap adds area. However this is hard to do. It is easier to make the cowl opening the max and use a flap to reduce the exit area. Nothing wrong with that set-up. I know I'm learning from your experiment. Please keep up with reports.

I was thinking along the lines of previous post, stalling the inlets as well. If the inlets "stagnation point" moves outside the inlet with the cowl flap closed, it could cause more spillage drag. There is no free lunch? It all has to work together. Clearly the exit is the throttle of the cooling system (and affects ever thing up stream in the cooling system.

Are cowl flaps on a C-182 to increase cruise speed or more about controlling temp in a climb or Hot/Cold climate operations? You'd assume it improves cruise speed, but how much? Any C-182 flyer's out there know the speed difference cowl flap closed/open in cruise? May be the gain on a RV is so small it's hard to measure?

May be Richard Vangrunsven knows SOMETHING? May be this is was why he did not use cowl flaps. He knew a well balanced and designed cowl would not gain much speed when balanced against the complexity and weight of a cowl flap? In 1993 Van wrote about the value of the cowl flaps, and difficulty executing a design with the existing RV geometry. Van has published cowl flap ideas for the RV-4. He even tried them on the RV-6. Bottom line he "did not find real noticeable benefits......"

Van also made similar observations about turbulence from a cowl flap (or exit air restrictor flap) not helping cooling drag (due to the turbulence). Since the air is not accelerated and smooth as it exit it does some harm (aka thrust or at more drag). Van also felt real cowl flaps, aka C-182, where a pain, high maintenance and required removal every time you un-cowled the engine. I think Van has tried and considered cowl flaps and just found it did not earn its way onto the design or meet the KISS total performance mission of the RV. However in hot temps or fly in wide ranges of temps, it may be a necessity.

It is like retract gear.............if your retract gear doors don't fit tight and seal well when up, its not much better than a well stream-lined fixed gear set-up. I recall a retract on a RV was good for only 6 mph. Of course when you get going real fast, say well over 250 mph, retracts become a necessity as drag increases prohibitively. May be cowl flaps are not quite right for most RV missions.

I think seasonal Cowl changes may be a good trade off in hot climates for those who need it. You can 'louver' the cowl in the summer and remove/block them in the winter?

db1yg said:

Alex, BTW, contrary to some beliefs, when I added louvers, albeit cut down versions of Alex D.'s product, there was no measurable difference in cruise speed (IAS or TAS) at specific power settings.

Click to expand...

Good to know. There's nothing like flight test. My worry was a louver may cause more drag (and less thrust) because they're not facing totally aft. There's no way you can accelerates the air, so the exit air out of the louver is slow, merging with fast outside air. In theory that is plume or mixing drag, but if the air's not shooting out, its more just leaking out, drag is minimal. However there is no thrust gain either. Which may be a fair trade off. Drag is all academic if your engine is red lined temp wise.

AlexPeterson said:

I believe the exit has enough area, but there does seem to be some misbehaving going on. I think there is much to learn from these (pics): (cowl exit tuff test)

Click to expand...

May be Alan's mod, extending the cowls exit aft, gives a net gain in light of the reverse flow tuff test pics? May be Alan will do tuff test? (Alex great pictures BTW)

Round and square exits...

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

Those local little details, good point

az_gila said:

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

Click to expand...

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.

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

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.

Yukon said:

Oh David, come on! Since when was performance a criteria for you Subie guys!!!! . Open up that cowling and get smooth, cool hp!

Click to expand...

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.

David-aviator said:

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.

Click to expand...

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 said:

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.

Click to expand...

Dan,
What does winds aloft have to do with KTAS???

Yukon said:

Dan,
What does winds aloft have to do with KTAS???

Click to expand...

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.

Yep, same here

Tom Martin said:

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.

Click to expand...

Click to expand...

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.

I did the rich mod kit installation also

Bob Martin said:

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.

Click to expand...

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

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?

Yep

RV8RIVETER said:

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?

Click to expand...

Click to expand...

This is a mock up of one extension that I tried and inside of the extension is a venturi shaped like the inside of the throat of a carb. I kept the exhaust shortened up and let the exhaust exit at the critical point prior to the venturi several inches forward of the outlet to help pull a vacuum/augment the lower cowl. Results were good as far as helping with lowering CHT?s as the manometer test showed it was working, but I could not stand the vibration, noise and the extreme heat on the floorboard. The exhaust was so hot that I couldn?t even put my feet on the floor and I was afraid of melting the bottom of the plane. I could?ve put a stainless barrier with a heat insulator sandwiched on the bottom of the plane, but like I said I could not put up with the noise so I went back to a shorter extension and put a turndown on the exhaust. I did see some improvement with cooling, but not as much as I thought. I?ll leave this mod to the RENO guys or the racers that don?t mind hot feet and a lot of noise.

The last pic is my final outlet shape and size, approximately 8" aft of the firewall with the exhaust right at the edge. Not the best design in the world but it's quiet and very pleasant with no heat on the floor.

The exhaust diameter is 3" to give anyone an idea of the outlet size.

dan said:

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.

Click to expand...

TAS=IAS corrected for installation error, temp and pressure. Accurate TAS is a necessity before GPS groundspeed and track info can be incorporated to derive winds, not the other way around.

If David has confirmed the accuracy of his computed TAS with a 4 way ground speed check, then it is what it is. I just hope I live long enough to hear the alternative community come to grips with the reality of the speed and effciency penalty these engines are displaying. I think David gets it.

On my blue HRII I had made a number of cowling exit changes to smooth the airflow and when incorporated with smooth inlets I did get a minor increase in speed but more importantly a cooler running engine.
I then tried an exhaust augumentation tube on the plane. It was about 12" long with the same outlet area and shape as the cowling air exit. The exhaust pipes were cut so that they went straight into this tube, as did all the cooling air. The size and length of the tube corresponded with some of the work done 50 years ago by the US military.
Results
1. The sound was really cool on the outside of the airplane and deafening on the inside.
2. The engine ran hotter
3. There was no increase in speed.
4. I melted the plastic grommet off my centre mounted com antennae!

The tube is in my cupboard with some of the other failed experiments.

Yukon said:

I just hope I live long enough to hear the alternative community come to grips with the reality of the speed and effciency penalty these engines are displaying. I think David gets it.

Click to expand...

Air cooled engines have few options to converge cooling airflow efficiently unlike a properly ducted coolant radiator. My experiments with a cowl flap showed a solid 5 knot gain and this is with a crude setup. With a separate duct and proper exit and flap, theory suggests we might see as much as 11 knots. Clearly reducing unneeded cooling air mass flow in cruise via cowl flaps must add some speed- how much depends on the efficiency of the design.

Archival research and modern day flight reports on the ME109 and Spitfire shown a speed difference of between 18 and 29 mph with radiator flaps open vs closed in WOT speeds. Much of the speed increases found on the Reno racing P51s like Voodoo, Strega and Dago Red have involved revised duct shapes, exit designs and internal diffusers so there are large drag reductions possible with optimized setups.

Eggenfellner is releasing a new cowling design with separate rad plenums to address the real or perceived cooling problems experienced by some of his clients. A step in the right direction for cooling but exit control probably can still be improved considerably.

Failed experiments are knowledge & Carb Jets...

Tom Martin said:

Results
1. The sound was really cool on the outside of the airplane and deafening on the inside.
2. The engine ran hotter
3. There was no increase in speed.
4. I melted the plastic grommet off my centre mounted com antennae!

The tube is in my cupboard with some of the other failed experiments.

Click to expand...

Boy is this a great thread thanks to all of the above. Thanks for the info. I was thinking of doing something like this. My old 1958 piper Apache had augmentor tubes and they do sound awesome to people on the ground.

Bob Axsom said:

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

Click to expand...

Bob Martin said:

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 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 just last nite and lowered the chts another 25 degrees to 400. needless to say, I'm very happy.

Click to expand...

Don't bother getting the 666-660* main jet; save your money ($292.91) and just drill the original jet, usually 3 drill sizes up from stock size. (As you saw the A666-660* still needed to be drilled. Just drill the one you have. You can go one or two drill #'s at a time if you want. I have the O-360-A1A and went thru this and need three drill #'s larger jet. Precision Airmotive customer support are the ones that told me not to bother with the A666 because it would still be lean. The A666* kit was for Mooney's that ran lean, offered as an optional service bulletin. RV's have better free flowing air box, filter, exhaust and fast speed means more pressure behind the induction, so RV's run lean (O320 or O360). There's no stock jet that really works perfect.

Bob Martin, I am glad your CHT's are down, but do you know the "acid test" for main jet sizing? Go to 8000 ft, WOT, Full rich, stabilize, note EGT's, lean slowly until first cyl peaks and note that EGT. The delta rich to peak (ROP) should be about +150F. Less means you are lean. You don't want to be over rich either. If CHT's are high it can be something else, often baffle gap on jug #2 and #3.

Some RV's are so lean, EGT drops almost immediately when leaning, with almost no EGT rise, meaning they are running near peak EGT all the time. It's really bad on takeoff at full power. Of course the lean test is done at 75% pwr or less. I would not use CHT as the only measure of main jet correctness. (What CHT is high, it could be baffle gap?) A second test is on the ground. Run up to say 1,200rpm and lean slowly till the engine dies. You should see 50-100 rpm rise before the engine rpm starts to drop off and shut down. Also PEAK power is made at about 160-170 ROP, so not only will a proper main jet help temps it makes more power.

Drilling the jet (a little) will not increase overall fuel burn, since you usually are not at 100% power; as long as you pull the throttle back and lean as you normally do, fuel burn is not really affected by the main jet, unless you fly WOT & full rich at sea level all the time.

BACK TO COWL EXITS.............

*Service bulletin M20-98 for Carburetor, Marvel-Schebler, done at discretion, issued 7/24/62 for M20,M20A, M20C/F (62-67)

Bell shaped exhaust plenum

Alan Judy, I am mesmorized by your bell shaped exhaust plenum.

At present I have the cowl upside down on the work bench ready to carve/shape some urethanen foam for a new exhaust outlet and incorporate a couple cowl flaps. That bell shaped figure you have - is it designed to speed up air flow exiting the cowl? Is there a science to it, or did you just make it look pretty? Would you do it again?

gmcjetpilot said:

Don't bother getting the A666-660 main jet; save your money and just drill the original jet, usually 3 drill sizes up from stock size. (As you saw the A666 still needed to be drilled.) You can go one or two drill #'s at a time if you want. I have the O-360-A1A and went thru this. Precision Airmotive customer support are the ones that told me not to bother with the A666 because it would still be lean. The A666 kit was for Mooneys that run lean, offered as a service letter. RV's have better free flowing air box, filter, exhaust and fast speed means more pressure behind the induction, so RV's run lean (O320 or O360). There's no stock jet that really works perfect.

Bob Martin, do you know the "acid test" for full rich mixture (main jet sizing)? Go to 8000 ft, WOT, Full rich, note EGT's, lean until first cyl peaks and note that EGT. The delta rich to peak (ROP) should be about +150F. Less means you are lean. Some RV's are so bad EGT drops almost immediately when leaning, with almost no EGT rise, meaning they are running near peak EGT all the time. It's worse on takeoff at full power. Of course the lean test is done at 75% pwr or less. I would not use CHT as a measure of main jet correctness. A second test is on the ground. Run up to say 1,200rpm and lean slowly till the engine dies. You should see 50-100 rpm rise before the engine shuts down.

Click to expand...

George,
Thanks for the info.
I did pass the LEAN test with flying colors. and Precision said if I passed the test, then look at baffling! I had to ask about the kit, then they said it would help. They didn't say anything further.
There is a lot of info out there on this subject, but not in one place, or not written either.
The Mooney Kit, actually has almost the same size main jet as the 10-3878.
there is restriction bushing that installs in the nozzle vent channel that enrichens the mixture, so I got the first cooling effect with very little nozzle change, maybe .0001. So I think the kit was worthwhile as it did supply me with a second main jet if the drilling was to get weird for some reason.

I'm actually kinda following AVWeb's John Deakin's articles Pelican's Perch #63, Where should I run my engine.
Under The Takeoff:
"Normal climb CHT in a well baffled normally aspirated engine is around 330F at full power and sea level, at any decent climb airspeed. CHT might be higher if you insist on low climb airspeeds. If you see higher CHT's on your engine monitor, YOUR FUEL FLOW IS TOO LOW."

Also I spoke with Bart Labonde at Oskhosh and he told me he use to drill all the 10-3878 carbs on the 0-360's until he started using the richer 10-4164-1.
He suggested drilling about .0003 at a time until it was rich enough.

This thread started as a cooling issue and I thought this information was important to the topic, but probably better suited to a new thread.
I agree with your comment that there is no stock jet that works perfect!
Why did I have to figure this out?? I have never seem this in print before you said it. This is VERY important as I believe many RV guys are running hot and don't know this information. I'm just trying to get it out there.
George, thanks again for your input, I always learn something from your posts.

Hmmm, good question

David-aviator said:

Alan Judy, I am mesmorized by your bell shaped exhaust plenum.

That bell shaped figure you have - is it designed to speed up air flow exiting the cowl? Is there a science to it, or did you just make it look pretty? Would you do it again?

Click to expand...

Click to expand...

First off I'm not an engineer, just a tinkerer, kinda trial and error and see if something works or not.
About the Bell shape outlet or whatever we want to call it. I played around with different shapes, made out of aluminum and taped it to the cowl/plane with tape, then I'd go fly and take notes. In another thread I mentioned about using oil to see where it went to see if it turned back into the cowl outlet or stayed aft. What I found with my -6 is the slight bell shape seemed to help divert the air downward and to the sides at the lip/edge. I know this doesn't sound good at keeping the exit air attached to the bottom of the plane or getting the cowl outlet air mixed smoothly with the air stream. What I found was a slightly stronger pull on the lower cowl pressure but not much and it seemed to break the air from trying to reverse at the edges and not allow the air to travel back inside the cowl.
Think of it as your flaps, when you drop them down, they divert air. I did the same thing with the outlet and probably created some drag. I could not measure any speed loss or gain, but I?m guessing for maximum speed a straight outlet is probably best. I finally got tired of trying this and that and just left it the way it is. I also don't know if the lower plenum that I made that wraps around the entire engine had anything to do with this or not. I personally like the looks of John Hufts RV-8 cowl outlet the best. It is very short in height, with an extension and it's straight as a 2x4. John was trying to keep the air up against the streamline/belly of the plane and he did a much better job then I did and his airplane if way fast, he won the Oshkosh Air Venture race this year. I've got an area about 1 1/2ft aft of the cowl that is clean, meaning no exhaust stain, then from 1 1/2ft and back the exhaust stain is attached with a very smooth pattern looking slightly different than a rectangle cowl outlet. That 1 1/2ft area is right under your feet and if the exhaust is facing straight back, it?ll drive you nuts and yes I flew like that for awhile and got tired of it. If you ever get a chance to look at Dave Anders cowl outlet do so, the last time I saw it, it was half moon shaped also, but was straight if I remember. I stayed with the half round design to match the round inlets at the front of the cowl. I could have probably seen better gains with a different design, but it seems to be working very well and I'm running cool (330F range CHT's) this time of year in Oklahoma in 105F air. I wasn?t looking for a huge speed gain, but mostly was working with balancing temps year round. Hope this helps.

Gerry was kind enough to send me some photos of this mod. I too will try this out on my airplane (looks like and easy thing to do a quick proof of concept on).

Here's the photos: http://flickr.com/photos/funkym0nkey/tags/rvcoolingmod/

Any comments? I won't be able to try this for about four weeks (out of the country and the RV will be sitting in a hangar alone)

Gerald Clabots said:

On my 7A (TMX O-360 FP) I would see 425-430 on #4 during climb out, I would lower the nose and reduce power to get the CHT down, Oil temps are not a problem other than to cold using Vans standard oil cooler, Have blocked off part of the oil cooler. CHT's were not a problem in curise.
I am presently experimenting with a mod I recently made, Seems to be working as I can now climb out with full power and see 405-410 on #4 with no need to reduce power or lower the nose to increase the speed.
This is what I did.
Took the bottom plate that sandwiches the lower cowl behind the nose gear rod off and made a new one that flares out and then back about 3 inches back of the firewall. I left 1/4 to 3/8 inch gap between each exhaust pipe, them riveted on a vertical .032 piece of alumn. on each outside edge to within 1/4 inch of the bottom of the fuselage from the rear most part of the new piece to the rear lip of the lower cowl.
Not hard to do and no need to modify anything on the lower cowl. You can easily take it off and put the original piece back on.
If any of you try this please let me kown what your results are.
Only had a chance to do about 3 flights with it so far.

N742GC 7A 65 hours.
Gerry Clabots

Click to expand...

cowl flaps

Tom Martin said:

This is a very timely thread. Over the last six years I have done a lot of work on engine cooling with a couple of different rockets. Both had full metal plenums. I have done a lot of work on clean up the outlet air and also the inlet area. Almost all my cooling and speed gains have come from the smoothing of the inlet air. Having said that when you get the inlets working better it is then time to close down the outlets.
On my current rocket I have closed the outlet down to a slightly smaller area than the inlets and my engine runs cool at cruise, anywhere from 330 to 360 depending on power settings and outside air temps.
Last week, as a continuation of my experiments, I installed a cowl flap. My hope was that it would increase speed and help with balancing engine temperatures this winter. The cowling is not cut, the flap is inside the cowling, hinged forward and pulls up to the firewall, between the exhaust pipes. It takes a good pull to close it.
I have had three flights with the plane and I am perplexed to say the least. The system is helping to raise the engine temps but it is not helping with speed and infact may be reducing speed. The speed changes are very small and I have yet to quatify them but here is what is definately happening and I could use some input.
In level trimmed flight when I pull the cowl flap closed, the nose goes down in a very significant and repeatable way. When I trim for level flight in the flap closed condtion and I open the flap the airplane starts to climb.
My question is; Is this a drag situation in that closing the flap in some way increases drag, or is it simply that the airflow has in some way trimmed the plane differently? It is very hard to get accurate airspeed readings in trimmed before and after closing the flaps as it takes a few minutes to retrim and get stable again. I certainly intent on getting some accurate three leg gps data and also to fly beside another aircraft but the trim issue certainly surprised me and the results, at this stage, seem to be contrary to the common believe that a cowl flap should increase speed due to lower cowling airflow. Inputs are most welcome!!

Click to expand...

Hi Tom and all

I am just back from a trip and getting caught up.

First, as a data point, on a C-180 I worked on speeding up in the late '90s, full open to full close (there are actuall 4 positions) made a 5 knot difference in speed.

I will take some pictures this afternoon of my 185 cowl flaps to show you, but basically there is no attempt to streamline them in the open position, but in the closed position they follow the contour of the cowl.

I think your implementation failed because you simply created a dead area when the flap was closed. Where there was airflow, now there is none, and the exterior shape remained the same. This created a large area of turbulence.

I will take a couple of pictures of the 8 to illustrate also, but I am reminded most of Fred Moreno's observation, that cooling drag is the loss of momentum of the exit air compared to the intake air. So if we can take advantage of the expansion due to heating, and create a nozzle at the exit to speed up the air, we can perhaps reduce the loss.

More later, John