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Resolved High CHT's, But....

N

N787KV

Member
I have a 7a with an ECI IO-360, duel Pmags and a 4 pipe exhaust now with 61 hours. I have had my CHT's wanting to blow passed 400 deg. on take off and a hard time keeping them below 380 deg. in cruise. Oil temp is 180. My baffling is very good and run the A curve on the Pmags.

What I've found from looking at some local tail wheel 8's, is that I have much less cowling exit area because of the four pipes and the nose gear. And no, I did not measure, but it's obvious. After talking with Allan from Antisplat, I placed nylon spacers on both sides of the exit opening the lower rear cowl and created a wedge like gap on either side. The spacers lowered the cowl next to the exit 3/4". I went for a test flight and I could leave the throttle wide open and the highest CHT was 381 deg. and the oil temp also came down causing me to close the butterfly valve to the oil cooler to almost full closed. To me, it is clear that I need to increase my exit area and I don't believe louvers are the answer.

These are what I see as my options:
1. Open the exit area by lowering the trailing edge (the easiest)
2. Open the exit area by widening
3. A combo of the above
3. Adding a cowl flap in conjunction with one of the above for takeoff

Thanks
 
N787KV said:
These are what I see as my options:
1. Open the exit area by lowering the trailing edge (the easiest)
2. Open the exit area by widening
3. A combo of the above
3. Adding a cowl flap in conjunction with one of the above for takeoff

Thanks
Click to expand...

Im not sure what you mean by lowering the trailing edge. But take a 1/2" cut off the fiberglass off the opening and go fly it. Only if your 100% convinced you have a clean and tight top plenum area
 
Kahuna said:
Im not sure what you mean by lowering the trailing edge. But take a 1/2" cut off the fiberglass off the opening and go fly it. Only if your 100% convinced you have a clean and tight top plenum area
Click to expand...

LifeofReiley said:
I took 3/4" cut off the fiberglass exit and it solved my heat/temp issue.
Click to expand...

So when you guys talk about cutting off the opening, are you saying that you cut 1/2-3/4" off the trailing edge of the cowl scoop so that it is no longer even with the firewall but actually forward of it?
 
3/4 Cut ??

What does that really mean ? Did you take 3/4" off the outlet lip and did that make it even with the firewall. Mine comes back of the firewall maybe an inch and I have wondered if taking it off might help a little bit.
 
alpinelakespilot2000 said:
So when you guys talk about cutting off the opening, are you saying that you cut 1/2-3/4" off the trailing edge of the cowl scoop so that it is no longer even with the firewall but actually forward of it?
Click to expand...

That is exactly right. Just take enough off that your happy with the temps in your worst case scenario. Remember that You could take out many inches and get lots of cooling. But your giving up speed. You only need to take enough off to make you happy and NO MORE.
 
Anyone have pics of the outlet after modification? I have a -6A with O-360 and in the summer high CHT's and Oil temp are a problem. My upper plenum area has good baffle sealing.
 
RVDan said:
Anyone have pics of the outlet after modification? I have a -6A with O-360 and in the summer high CHT's and Oil temp are a problem. My upper plenum area has good baffle sealing.
Click to expand...

Before you reach for the saw, read what Mike said one more time. Every increase in mass flow will increase drag. If that's cool with you (pun intended), then yes, increasing exit area will reduce temperatures.

Quick sketch..cut back the cowl exit chute as shown:

2quowi1.jpg

1zoxh8x.jpg


Variable exit area allows mass flow control. Note that variable area does not mean standing up some sort of wall or spoiler in the airstream. It merely means installing some kind of system to vary the size of the exit hole(s).

Ken Kopp and I did slightly different versions. His is hinged at the forward end, much like a Cessna, while mine is center hinged to reduce required force at the actuator. Both worked very well. It is nice to flow as much cooling air as desired for unlimited climb, then transition to cruise, toggle the door shut, and watch the speed roll up a few more clicks.

http://www.vansairforce.com/community/showpost.php?p=634763&postcount=1

http://youtu.be/LetuLfkR4Vo

http://www.vansairforce.com/community/showpost.php?p=778137&postcount=174

http://www.vansairforce.com/community/showpost.php?p=781564&postcount=175

Think you have good upper plenum sealing and cylinder baffling? Measure your cowl outflow temperature and see the truth. The goal is to (1) reduce leakage and (2) increase heat transfer, the result being hotter exit air.

Air which leaks past rubber seal strips never gets near the hot parts, so it doesn't pick up any significant quantity of heat.

Most aluminum baffle wraps allow air to leak out the sides. Air allowed to flow in the general vicinity of fins at low velocity doesn't pick up nearly as much heat as air forced entirely between fins, for as much length as possible, with enough velocity to be turbulent.
 
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Thanks for the pictures. Have been following this, as I am sure many readers have... and I have some difficulty translating the text into a diagram in my head.
Something interesting I noticed this week. Had removed the vacuum system and plugged the firewall hole temporarily with aluminum tape. During the first flight, I noticed a blast of air below the panel that rivaled my shop vac.
Sure enough, the tape blew off both sides of the firewall right away. What surprised me was the positive pressure in the area. The vacuum inlet was in the standard postion on the firewall... which is a fair distance north of the cowl outlet. It is now plugged and sealed... but interesting that I got to see some evidence of airflow I had not expected.
Considering cutting back the outlet to increase outflow as shown above. Cylinder repairs are pricey, and I can sacrifice a bit of speed for lower CHT's.
 
DanH said:
Before you reach for the saw, read what Mike said one more time. Every increase in mass flow will increase drag. If that's cool with you (pun intended), then yes, increasing exit area will reduce CHT.
Click to expand...

I understand this to be true theoretically, Dan, but do you have any sense for how much speed is lost in a practical sense? E.g. Does cutting the scoop back, say 1/2 in., translate into a speed loss that most of us would consider significant? Just curious if there is a way to predict the downside of enlarging he exit area.
 
alpinelakespilot2000 said:
I understand this to be true theoretically, Dan, but do you have any sense for how much speed is lost in a practical sense? E.g. Does cutting the scoop back, say 1/2 in., translate into a speed loss that most of us would consider significant?
Click to expand...

For an old racer, giving up a few knots would be anathema. That said, no, it won't be a huge loss.

Peter offered a pretty good experiment in the first post:

I placed nylon spacers on both sides of the exit opening the lower rear cowl and created a wedge like gap on either side. The spacers lowered the cowl next to the exit 3/4".

Do the same, fly it, and if you like the result, calculate the additional area provided by the spacers. Cut the chute to get the same area increase without spacers.

Uggg ;)
 
Peter,

One other thing.

Check the timing on the P-mags - we had high CHT's and had a couple of kickbacks on start.

After speaking with Brad, we retimed 3? after TDC.

2 advantages - no kickbacks and our CHT's came down by around 40f.
 
My full exit area

Here's a pic of my cowl exit and it is full. You can also see my nylon spacers used for my test.

9k35tw.jpg


I'm concerned that cutting the cowl forward of the firewall, with all that is going on in there, may not cause a low presser area to help the air flow. At this time the exit matches the firewall.
 
N787KV said:
I'm concerned that cutting the cowl forward of the firewall, with all that is going on in there, may not cause a low presser area to help the air flow. At this time the exit matches the firewall.
Click to expand...

It doesn't work that way.

I've flown mine with the exit panel removed (equivalent to cutting the entire exit chute from your cowl) just to get a baseline with a huge 17" pressure drop across the engine and cooler. The result was as theory predicted; huge mass flow, CHTs way too cold, airspeed low.
 
Peter,

Search the sight for high oil temp and look for my mod.

I sliced the lower cowl exit and dropped the whole thing 1 1/2 and it helped a lot.

I did have a plenum so the cht's were fine but dropped oil temp 30*

I hope this helps.
 
N787KV said:
Here's a pic of my cowl exit and it is full. You can also see my nylon spacers used for my test.

9k35tw.jpg
Click to expand...

Interesting photo. Area aside, it nicely illustrates the futility of attempting to optimize exit velocity on an A-model, given the forest of stuff in the way.

Instead of the same monkey-see, monkey-do, when is an A-model builder going to remove the entire center exit chute, fully fair the nosewheel structure back to nothing at the firewall, and move the outlets outboard? You know, just like those fast Lancairs.....

So far, the only guy I know is Bryan Milani, with his RV-10 cowl.

28tchf5.jpg
 
If my memory serves me, Larry Vetterman tested something close to this concept on his 7a -- and then abandoned it. He moved the exhaust outboard, faired them in, and faired in the center section by installing a tear drop aft of the firewall. Maybe someone else can recall the negatives that caused him to abandon this concept???? Could it have been interaction of exit air with the gear legs--and therefore increased drag---sounds familiar but I can't find any documentation.

"monkey see, monkey do" ---- come on Dan -- you can do better!:D

Cheers,

db
 
DanH said:
Interesting photo. Area aside, it nicely illustrates the futility of attempting to optimize exit velocity on an A-model, given the forest of stuff in the way.

Instead of the same monkey-see, monkey-do, when is an A-model builder going to remove the entire center exit chute, fully fair the nosewheel structure back to nothing at the firewall, and move the outlets outboard? You know, just like those fast Lancairs.....

So far, the only guy I know is Bryan Milani, with his RV-10 cowl.

28tchf5.jpg
Click to expand...
Well Dan, I have been talking to you for the past two years now about doing that very thing with my 9A. I have talked to Larry Vetterman and Clint about building me a new exhaust that will allow me to redirect the exhaust to the outboard trailing edge of the lower cowl and then totally enclose the current center opening. Unfortunately some more pressing life issues have interfered with my abilities to do that anytime in the near future but it is still a priority for me to get accomplished. At present I do not have the CFO's authority to do it nor the physical capability to take on the project. Someday it will happen though.
 
db1yg said:
If my memory serves me, Larry Vetterman tested something close to this concept on his 7a -- and then abandoned it. He moved the exhaust outboard, faired them in, and faired in the center section by installing a tear drop aft of the firewall. Maybe someone else can recall the negatives that caused him to abandon this concept???? Could it have been interaction of exit air with the gear legs--and therefore increased drag---sounds familiar but I can't find any documentation.

"monkey see, monkey do" ---- come on Dan -- you can do better!:D

Cheers,

db
Click to expand...
I have talked to Larry Vetterman at length about his setup and doing changes that Dan suggested (see my previous post) on my 9A. Larry does not have his exhaust outboard but he actually does indeed have a "boat tail" fairing extending his exhaust exit aft of the firewall. You can see pics on his website here:
http://www.vettermanexhaust.com
He has stated to me he has seen a measurable increase in speed with his exhaust fairing mod.
 
Larry did several different versions built around the boat-tail add-on. The closest to what I described is this one...

2wpttft.jpg


....but it's not at all the same. The entire cowl chute is still there, with all its frontal area, although I'm sure the boat tail does wonders for reducing its overall drag. The boat tail retains some cooling exit area via louvers. The outboard exhaust exits also have some cooling exit area. Between the three exits there is no significant attempt to optimize exit velocity, certainly not like the Lancair cowl work of Andy Chiavetta:

2n1ypex.jpg


That said, my compliments to Larry...he builds ideas, then flies them with a careful eye toward gathering good data.

One more detail. Note the Aerochia exits are actually well aft of the firewall, and as such simplify the hinge point and linkage requirements for variable area exits. Oh yeah, note how little area is used to cool an IO550.
 
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I like where this is going

Kevin, I really like your idea and would like to do something like that in another phase. For know, I need to make the best of what I have.

If I cut the bottom forward, will that as effective as dropping exit down? My concern is the blockage the pipes create. I would like to only open up the exit just enough for cruise cooling and add a cowl flap for take off and climb.
 
...and that would be poor Laura trying to coax Mike out from under the airplane.

"Here Mikey, come here...that's a good boy, yes, come here...." ;)
 
Thanks Dan--the first pic in post #21 is the Vetterman experiment that I was remembering---just don't know what he got in performance changes.

Ref Bryan's RV10---does that center fairing in Mike's pic go into the cowl and up and encompass the gear structure?? Also looks like the exit area is significant around the exhausts. Any performance data on that mod?

Cheers,

db
 
db1yg said:
Ref Bryan's RV10---does that center fairing in Mike's pic go into the cowl and up and encompass the gear structure?? Also looks like the exit area is significant around the exhausts. Any performance data on that mod?
Click to expand...

No idea about fairing inside the cowl, or performance. The exits (or the canoe fairing) may not be as large as the lens suggests.
 
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