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  #31  
Old 07-15-2017, 06:40 AM
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Quote:
Originally Posted by N941WR View Post
How is your exit opening on the bottom cowl?
It should be even withe the firewall, even a little forward. This will help draw air out of the bottom cowl.
Cutting the exit further forward is unlikely to lower local pressure just aft of the exit opening. Trimming forward of the firewall requires a slant cut, which will increase exit area. Increasing exit area practically always lowers CHT, the mechanism being decreased pressure in the lower cowl volume, thus increased deltaP across the cylinder fins. Just keep making it larger, and it will run cooler and cooler, until the exit is so large that lower cowl pressure is near zero. Unfortunately, it will also run slower, which doesn't seem to be what Paul had in mind.

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  #32  
Old 07-15-2017, 07:09 AM
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Originally Posted by DanH View Post
...Just keep making it larger, and it will run cooler and cooler, until the exit is so large that lower cowl pressure is near zero. Unfortunately, it will also run slower, which doesn't seem to be what Paul had in mind.

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Yes Dan, but... if the cut only slants forward an inch, what is the speed loss? 2 knots, maybe 3? Well worth the tradeoff, if all else fails. Also, IL looks better than adding louvers or a lip and simpler than adding a cowl flap.
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  #33  
Old 07-15-2017, 07:17 AM
GTWreck GTWreck is offline
 
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If this is still a problem, you may want to consider getting your injectors balanced. In my stock Lyc IO-540, I had a similar issue where #5 & #6 CHTs ran hotter than the rest, and would easily hit 400 on climb out.

I took my plane to Airflow Performance and they did an injector balancing for me. We found that #1 & #2 were too rich, and #5 & #6 were too lean (which accounted for the higher CHTs). We installed new restrictors that balanced things out. All of my high CHT issues went away.

While I know it'd be difficult for you to take your plane to them, they can do this by shipping you restrictors and you do the experimenting until you find the right combination.
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  #34  
Old 07-15-2017, 12:17 PM
BillL BillL is offline
 
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Default Not easy to see, but .. .

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Originally Posted by paul330 View Post
Just to keep this thread going as I haven't totally solved the issue.......
Paul, A careful meticulois 10-friend said he had his baffles all sealed, absolutely. Having had my under cylinder baffles off and measured, I looked at his. Huge gaps. You may have already checked these, but if not look carefully from many angles (all) . Mine (and his) did not fit well. Hope this helps . . .
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  #35  
Old 07-16-2017, 02:30 AM
paul330 paul330 is offline
 
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Thanks to everyone for the input - especially Dan for his informative tutorial.

I'm not exactly cooking the cylinders at the moment so there is no rush to get this done. As Dan surmises, I don't want to do anything that costs speed as I want to race the aircraft again next year. What I want is to get the no 6 down about 30F to match the others so that I can experiment with a lower fuel flow and see if I can get a bit more power without getting the CHTs too high.

One thing I can't agree with is that the oil duct doesn't "steal" air. The argument that the pressure equalizes I don't think holds water. Since the air is a moving compressible fluid, the pressure is going to be different all over the baffles, so air going through the duct can for sure cause a lower pressure (and therefore less cooling) over the no 6 cylinder.

I'm going to try just blocking some of the duct with AL tape and see if it makes any difference. I'll also have a look at re-working some of the sealing material at the front of the engine where there may possibly be some leaks around the intakes - I'm pretty sure everything aft of that is fine. Next time I have the bottom cowl off, I'll also have a check on the lower baffle seals to make sure all is in order. However, as the cooling is good for all the other cylinders I doubt that I have a sealing problem.

As discussed earlier, I don't think that the no 6 has a weak injector or it would be peaking EGT before the others.

I'll report back - it may be a while.......
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  #36  
Old 07-16-2017, 05:09 AM
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Quote:
Originally Posted by paul330 View Post
One thing I can't agree with is that the oil duct doesn't "steal" air. The argument that the pressure equalizes I don't think holds water. Since the air is a moving compressible fluid, the pressure is going to be different all over the baffles, so air going through the duct can for sure cause a lower pressure (and therefore less cooling) over the no 6 cylinder.
And you could be right, so measure and let's see.

The piccolo below is 12", but 6" would probably work ok, just model airplane aluminum tubing. Or you might try a few aquarium air stones, since you're trying to measure a point source. I just ordered a six-pack of the small ones, as I have some similar measurements on my own to-do list.

http://www.drsfostersmith.com/produc...SABEgJonfD_BwE

Inexpensive Chinese manometers seem to work fine.

http://www.ebay.com/itm/LCD-Digital-...0AAOSwlMFZKUh9

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  #37  
Old 07-16-2017, 05:27 AM
BillL BillL is offline
 
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Quote:
Originally Posted by DanH View Post
And you could be right, so measure and let's see.

The piccolo below is 12", but 6" would probably work ok, just model airplane aluminum tubing. Or you might try a few aquarium air stones, since you're trying to measure a point source. I just ordered a six-pack of the small ones, as I have some similar measurements on my own to-do list.

http://www.drsfostersmith.com/produc...SABEgJonfD_BwE

Inexpensive Chinese manometers seem to work fine.

http://www.ebay.com/itm/LCD-Digital-...0AAOSwlMFZKUh9

[/url]
Dan, Confused - what are exactly are you proposing to measure? Static pressure is clear, but where and why, to illuminate what? Pressure at each individual cylinder?

BTW-Paul - at these velocities and pressures the compressibility of air is not a factor. It is acting like an incompressible fluid.
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Last edited by BillL : 07-16-2017 at 06:01 AM.
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  #38  
Old 07-16-2017, 08:38 AM
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az_gila az_gila is offline
 
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Quote:
Originally Posted by N941WR View Post
Yes Dan, but... if the cut only slants forward an inch, what is the speed loss? 2 knots, maybe 3? Well worth the tradeoff, if all else fails. Also, IL looks better than adding louvers or a lip and simpler than adding a cowl flap.
Slight shift from RV-10's but same issue --

That would be my question too, but for a 4 cylinder RV. Is is a lot or only a knot or two?

Trying to get cooler CHTs in climb in the AZ heat.
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  #39  
Old 07-17-2017, 06:53 AM
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Quote:
Originally Posted by BillL View Post
Dan, Confused - what are exactly are you proposing to measure? Static pressure is clear, but where and why, to illuminate what? Pressure at each individual cylinder?
Spot pressures in various locations...making a 3D map really. The subject of interest is how the oil cooler duct location might affect the adjacent cylinder's cooling. Since all flow is driven by pressure delta, we could survey pressures at the locations of interest to gain some insight. For example, in Paul's case one might zip-tie a bubble rock to the aft side of the #6 exhaust pushrod tube, then fly the changes in duct opening that he proposed. Since it is now winter in Oz, perhaps a quick turn around the patch with the cooler duct entirely blocked would be possible. It's a parallel valve, not as dependent on the cooler as an angle valve motor with squirts.

I'd also locate one well inside the duct, perhaps at the cooler face, then another on the exhaust side of the cooler. Perhaps another over #4. The bubble rocks come in packages of six, so place 'em all as desired, and run a bundle of tubing back to the cockpit with the ends marked. A fella with a test plan might learn a lot in a few flights with a helper to swap tubes on the manometer and record data on a clipboard.

Quote:
Originally Posted by az_gila View Post
That would be my question too, but for a 4 cylinder RV. Is is a lot or only a knot or two? Trying to get cooler CHTs in climb in the AZ heat.
Gil, you gotta do what you gotta do. Maintaining acceptable temperatures is the first priority.

How many knots? You'll hear opinions, but the number of folks who flew before and after NTPS triangles when cutting a cowl outlet is very low, and even then, results would vary based on (a) how much they cut, and (b) baffle leakage.

Ballpark? The variable exit on mine costs 2 to 4 knots when open, depending on conditions. There should be quite a few RV-6 owners with an Anti-Splat cowl flap; ask them for input. And what is the speed penalty for an open cowl outlet on a good 'ole C-182?
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  #40  
Old 07-17-2017, 06:59 PM
alpinelakespilot2000 alpinelakespilot2000 is offline
 
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Quote:
Originally Posted by az_gila View Post
That would be my question too, but for a 4 cylinder RV. Is is a lot or only a knot or two?
Quote:
Originally Posted by DanH View Post
How many knots? You'll hear opinions, but the number of folks who flew before and after NTPS triangles when cutting a cowl outlet is very low, and even then, results would vary based on (a) how much they cut, and (b) baffle leakage.

Ballpark? The variable exit on mine costs 2 to 4 knots when open, depending on conditions. There should be quite a few RV-6 owners with an Anti-Splat cowl flap; ask them for input. And what is the speed penalty for an open cowl outlet on a good 'ole C-182?
Hi Gil-
Just for a reference point, I just installed an AntiSplat cowl flap a couple days ago. It increases the exit air by about 11.2" square inches and in 65% power cruise it seems to have about 2-3 knots of cruise penalty. That said, I generally wouldn't need it at cruise (just climb) and, additionally, since the flap hangs down in the slipstream my un-expert opinion would surmise that just cutting back the cowl exit would have less of an impact on speed than does a cowl flap. Again, though, I wouldn't really know for sure.
Hope this helps.
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