Cowl Inlet/Exhaust Area Question

Howdy Gang,

Trying to sort out something on my recently-bought/already flying RV-7.

I was wondering if there were any rough "rules of thumb" on what you should be looking for in the difference (if any) between the area of your cowl inlet vs the area of the cowl exhaust?

I have ~46 sq. inches of cowl inlet area and about 49 sq. inches of cowl exhaust area, of which the lower area is further reduced by ~6 sq. inches by the two exhaust pipes that exit there.

Just looking at it...trying to exhaust 46" of intake air into ~43" would seem to explain why I'm experiencing higher than comfortable on takeoff CHT's (420+ at 80 degrees OAT and 2-3000' DA). I'm wondering if I just don't have enough exit area for the hotter, lower pressure air to leave the lower cowl...

Like I said, I was wondering if there were any WAGS on the size/differential between cowl intake and exhaust and if the setup I have now could be driving my CHT issue.

I've already been thru checking FF, mag timing, baffle/baffle seals, etc.

I also have been thru many threads on VAF regarding adding louvers, cowl flaps, etc. The stock cowl from Van's comes with this as a built in issue? Seems strange, if that is the case.

Thanks for any insight,

Rob S.

Aggie78 said:

I was wondering if there were any rough "rules of thumb" on what you should be looking for in the difference (if any) between the area of your cowl inlet vs the area of the cowl exhaust?

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There is no rule of thumb for relative area.

Just looking at it...trying to exhaust 46" of intake air into ~43" would seem to explain why I'm experiencing higher than comfortable on takeoff CHT's (420+ at 80 degrees OAT and 2-3000' DA). I'm wondering if I just don't have enough exit area for the hotter, lower pressure air to leave the lower cowl...

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The exit is the system throttle, and determines how much mass will flow through the cowl. It can be enlarged to increase mass flow. Eventually it can be enlarged so much that the engine fin-baffle combination becomes the throttle.

There are Lycoming charts with empirical data relating baffle pressure drop, air density, resulting mass flow, and resulting typical CHT.

The usual approach to your problem is to shotgun the usual issues, based on the usual suggestions. Diagnosis requires better data, notably upper and lower cowl volume pressure, and the temperature of the intake and exhaust air.

The stock cowl from Van's comes with this as a built in issue? Seems strange, if that is the case.

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Merely a personal opinion, but I think the Vans cowls are sized as they should be for a high performance airplane, and as such require high performance sealing and baffling. Doesn't mean they can't be improved...but there are a great many of each model flying without problems.

DanH said:

Merely a personal opinion, but I think the Vans cowls are sized as they should be for a high performance airplane, and as such require high performance sealing and baffling. Doesn't mean they can't be improved...but there are a great many of each model flying without problems.

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Dan, I'm curious if your opinion on this holds even with additional obstructions in the cowl exit such as the nose gear mount and leg and exhaust mufflers (both of which presumably also cause flow disturbances). How about differences in cowl exit size between the O-320 cowl and the IO-320/O-IO-360 cowl?

Would the small lower cowl combined with nose gear and mufflers be pretty much a worst case condition, or does it not really matter? Any ideas whether data exists on this combination?

I ask because that's the combination I have on my 9A, and I've been fighting high temps for a year. This is my first rodeo, so I've read everything I could find on baffle sealing. I think my baffles are well sealed, but someone with experience might see otherwise. Just wondering at this point if stuffing all those extra things right in the exit area might simply cause a need for more gross area, or if anyone has experience indicating the stock cowl/baffles can work well with the same setup.

krw5927 said:

Dan, I'm curious if your opinion on this holds even with additional obstructions in the cowl exit such as the nose gear mount and leg and exhaust mufflers (both of which presumably also cause flow disturbances). How about differences in cowl exit size between the O-320 cowl and the IO-320/O-IO-360 cowl?

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Kurt, I don't know. One measurement is worth 1000 opinions, including mine.

krw5927 said:

Dan, I'm curious if your opinion on this holds even with additional obstructions in the cowl exit such as the nose gear mount and leg and exhaust mufflers (both of which presumably also cause flow disturbances). How about differences in cowl exit size between the O-320 cowl and the IO-320/O-IO-360 cowl?

Would the small lower cowl combined with nose gear and mufflers be pretty much a worst case condition, or does it not really matter? Any ideas whether data exists on this combination?

I ask because that's the combination I have on my 9A, and I've been fighting high temps for a year. This is my first rodeo, so I've read everything I could find on baffle sealing. I think my baffles are well sealed, but someone with experience might see otherwise. Just wondering at this point if stuffing all those extra things right in the exit area might simply cause a need for more gross area, or if anyone has experience indicating the stock cowl/baffles can work well with the same setup.

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Decowl, turn off the lights in your hanger. Grab a flashlight and shine it around the bottom of your engine while looking down from above the engine. See any light that isn't coming in through a fin? Seal it. Every bit of air that comes in the front should go out through a fin or a blast tube. If it doesn't, you are wasting cooling air.

I know I'm sure to get blasted for this but I think far to many people spend far too much time focusing on inlet and exit sizes while their baffles leak like a sieve. My cowl has no flaps between it and the ramp, no seal between the plenum lid and the cowling at all. The inlet has a gap between it and my ramp/plenum lid and you know what keeps the air from going down into the lower cowl instead of aft into the plenum? Aerodynamics. Try getting 200mph air to turn 90 degrees. There are no rubber pieces anywhere on my cooling system keeping the plenum separated from the underside....and yet my cooling is too good if anything. Because my baffles are sealed. Any air coming in...is going out through a cooling fin. I'm considering closing up some inlet and exit area to see if the reduced drag gives me any more speed.

Take a look behind number 4 cylinder. Many people leave that 1/2 inch by 3 inch gap between baffle and case unsealed. Back of number 3, by the base of the cylinder behind the oil fill tube. Around the intercylinder baffles. Around the constant speed oil line (forward). All the corners and where the baffles contact the cylinder bases or case. Seal it. All of it. Then if you are having cooling issues, look into system design but keep in mind that the design is very well proven at this point.

One of the problems with comparing data between different RV's is that there can be a lot more variation from one airplane to another than most people realize.
The reason for that is it is not something you can visually inspect (not always directly anyway).

One big variable is the quality of ring break-in to the cyl. walls.
This can have a large influence on oil temps (unintended volume of hot combustion gases blowing past the rings and heating the oil in the sump), and CHT's (blow by gases heating the cyl barrel portion of the cyl beyond what is intended).

Sometimes there is evidence of blow-by just by the level of oil consumption, but this can be a challenge to quantify because the acceptable amount vs what people common quote they have is all over the map. Some claim to use a quart every 6-8 hrs and some claim a qt every 25 hrs. My quess is the people that claim much lower oil usage also have engines that run cooler than those that have a higher consumption rate (and probably more blow by), but I have no actual data to back that up.

The bottom line is there are likely examples of nearly ever engine model/configuration, using the standard cowls and baffling, that have what would be considered normal/acceptable temperatures. There are only to explanations for this.... either those people are lying, or there is actual differences from one engine (and /or installation) to another. From my experience of working with lots of different RV's over the years, it is the second explanation (I have personal experience with airplanes that had normal and high temps).
When a particular installation does not have normal temps, it could be because of many different factors.
Poor performing cooling baffle system (leakage, etc), higher than standard compression, excessive blow-by, inferior or incorrectly sized oil cooler, advanced ignition timing, larger engine HP than recommended for a particular airframe / cowl combination (and probably many more that I am not thinking of at the moment). Or, as is probably often the case, it could be a combination of a number of these.

Thanks for the responses and replies.

Dan, your comments about the exit as a "throttle" are especially pertinent and help to explain how the inlet/outlet are related.

After "shotgunning" a few standard fixes, I went flying and saw some immediate improvements...but I want to keep working to get another few degrees cooler.

First off, when reinstalling the cowls, I was absolutely paranoid about making sure the baffle seals were where they needed to be. My limited background with this cowl setup has allowed me to go fly with incorrect placement, and I've gotten better as I get more familiar with "how things should go".

Using the trusty dremel, I cut down each of the front air dams 1/2", also I did the washer between the baffle and the back of #3 cylinder.

I also changed my climb profile and quickly accelerated to 100-110 KIAS and maintained that in the climbout.

Highest CHT was again #1, but this time peaked at only 401 degrees at WOT, and immediately started going down when I reduced to 25/25 on climbout. #3 (previously my 2nd hottest cylinder) maxxed at 381 with 2 & 4 bracketing high/low within about 3 degrees of #3.

I still have a significant amount of air dam on each side remaining and I'm considering removing another 1/2" or so before considering taking the dremel to the lower cowl exit. I found an interesting thread (on VAF, of course) in which spacers were used to "open up" the lower cowl to good effect, which will allow experimentation with this option before resorting to cowl surgery.

One goal of my add'l experimentation would be to allow more aggressive climbs for longer durations without having to immediately accelerate to 100+KIAS.

While my mag timing, etc, was right on, one interesting discovery was that my idle mixture setting was found to be giving me >200 rpm rise when leaned on the ground, so the A&P reset it to where it's only giving a 50 rpm one now. Not being familiar with fuel injection, (a.) I didn't realize there was an adjustment for this and (b.) wonder if that adjustment would have any effects on my CHT's at higher power settings? The A&P making the adjustment stated it would...

I understand the idle mixture circuit setting on a carb'ed engine apparently does not play any/all part at WOT settings, but I'm being told that's not the case with fuel injected engines? I was not able to wrap my head around the explanation given why it COULD affect things on FI vs Carburated, so any add'l info given would be much appreciated!

Thanks again, and I'll will report back after trimming the air dams down a little more..

Rob S.

Your A&P is correct. When an adjustment for idle mixture is made, it effects the mixture for all throttle settings. Edit -I didn't mean to imply that the mixture adjustment would change fuel flow at wide open throttle.. I meant that it will have some influence (that is what techs have told me any way) on more of the throttle range than just idle. Unlike with the idle mixture on a carb where it has an idle circuit that only has any influence when the throttle is at or very near the idle stop. He would have had to make the mixture leaner to correct your RPM rise at cut-off. This will have slightly leaned the entire throttle range.

Good Job.... It sounds like you are systematically working through the different things that can have an influence.

I am wondering what slower airspeed you are wanting to climb at for a longer duration, and why?
Usually the only reason to climb slow is for steeper angle (to clear an obstacle, and that only requires a couple hundred feet). The reason for this is that RV's climb so well, the rate of clime at 100 - 110 indicated has very little reduction compared to 80-90 (do your own test and see).

Because of this, the only reason I can see to climb slower is because of fast rising terrain. The way I deal with this is to modify my flight path off course a bit... since the ground speed is higher with the higher climb speed, even though I am flying a bit of extra distance, the flight time is about the same because of the higher ground speed while traveling that extra distance.

Thanks for the info on the Fuel Injection.

Strangely, he leaned it and yet on my test flight, takeoff FF went from 15.9 to 16 gph...probably statistically insignificant difference, but if it did lean it, it didn't do much.

The reason I'd like to be a little more aggressive in my initial climb other than just breaking ground and accelerating immediately is it's a little ugly off the departure ends of my airport-not for obstacles, but for engine failure options and considerations. It ain't pretty, no matter which direction I depart in off my home field.

Getting a 600-1000 feet of altitude in the bag ASAP at least gives me a fighting chance to land downwind or make a modified pattern...

Of course, daily temps in the 90's and DA's >3000' make it a continual challenge between emergency planning and treating the engine nicely...and summer hasn't really gotten underway here yet in Central Texas.

If I can open up my performance envelope to allow cooler CHT's while getting that initial altitude in the bank, I feel my margin of safety will have increased..while I'm treating the equipment better.

Thanks,

Rob

Aggie78 said:

Thanks for the info on the Fuel Injection.

Strangely, he leaned it and yet on my test flight, takeoff FF went from 15.9 to 16 gph...probably statistically insignificant difference, but if it did lean it, it didn't do much.

The reason I'd like to be a little more aggressive in my initial climb other than just breaking ground and accelerating immediately is it's a little ugly off the departure ends of my airport-not for obstacles, but for engine failure options and considerations. It ain't pretty, no matter which direction I depart in off my home field.

Getting a 600-1000 feet of altitude in the bag ASAP at least gives me a fighting chance to land downwind or make a modified pattern...

Of course, daily temps in the 90's and DA's >3000' make it a continual challenge between emergency planning and treating the engine nicely...and summer hasn't really gotten underway here yet in Central Texas.

If I can open up my performance envelope to allow cooler CHT's while getting that initial altitude in the bank, I feel my margin of safety will have increased..while I'm treating the equipment better.

Thanks,

Rob

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Very good reason.
Though with typical RV climb rates, a 1000 ft of altitude only takes 30 - 40 seconds which doesn't usually have a huge influence on whether the CHT's get overly high.

Hence, my appeal for help here at the Van's Aircraft Repository of Knowledge and Experience.

The Skyview was flashing CHT warnings to me and I was barely getting airborne, much less making it to pattern altitude, but I will admit probably a large part of that was my initial inexperience making sure the baffle seals were where they needed to be when reinstalling the cowls.

Learning that lesson and fixing that helped, but they were still too high for comfort and while I've got things headed in the right direction I will keep working it.

I am beginning to agree with your assessment regarding the multiplicity of ways/combos/setups that can make something like a single simple cowl design run the gamut of results from "no issues" to "major problems".

Rob

Change the warnings to 425F.

I fixed it!

Seriously a couple minutes per flight at 425 won't hurt anything (although, obviously, lower is better, so keep fixing things)

Aggie78 said:

Hence, my appeal for help here at the Van's Aircraft Repository of Knowledge and Experience.

The Skyview was flashing CHT warnings to me and I was barely getting airborne, much less making it to pattern altitude, but I will admit probably a large part of that was my initial inexperience making sure the baffle seals were where they needed to be when reinstalling the cowls.

Learning that lesson and fixing that helped, but they were still too high for comfort and while I've got things headed in the right direction I will keep working it.

I am beginning to agree with your assessment regarding the multiplicity of ways/combos/setups that can make something like a single simple cowl design run the gamut of results from "no issues" to "major problems".

Rob

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