High CHT's resolved and what worked.

[Titan-Xpert]

Originally Posted by Toobuilder

"This modification" addresses the significantly restricted/zero airflow at the point of zero fin depth on some Lycoming cylinders. Yes, there are hundreds of thousands of hours flying with restricted or zero airflow to the bottom fins, but are you suggesting this is an acceptable practice? Why go through the trouble of adding the wrap around baffle if there is no airflow?

I am suggesting that this design has worked very well for over 50 years and it was purposely designed to have a small regulated airflow in this relatively cool part of the head (Front of # 2, rear of # 3 head area). Baffling in this area of the head should not be so tight fitting that it totally blocks the flow. That?s why most use a washer between the head and baffle where it attaches to the head. The only way it could have zero airflow, would be if this area was silicone shut.
Most Lycoming type engines and their clones go to TBO. They would not if they were not being cooled properly. This cooling design is well proven, something must be working right.

[Toobuilder]

Originally Posted by Titan-Xpert

Most Lycoming type engines and their clones go to TBO. They would not if they were not being cooled properly. This cooling design is well proven, something must be working right.

The problem is, it is up to the airframe designer to create the baffles. Many airplanes have the baffling well below the mid point on the cylinder and allow plenty of airflow to the "bottom" fins. As we know with Vans baffles, the airflow IS essentially stopped up for the left fwd and right aft cylinders. The fact that washers are used as a crutch for this deficiency should be warning enough that the basic design is flawed. We also can assume that these bottom fins need airflow since they are quite large and are wrapped with a baffle.

It's obvious that the Lycoming is fairly tolerant of baffle design "sins", but we should not dismiss this tolerance as "successful". There are far more effective ways to cool a Lycoming than the kit supplied parts Vans puts out.

[DanH]

Originally Posted by Titan-Xpert

I am suggesting that this design has worked very well for over 50 years and it was purposely designed to have a small regulated airflow in this relatively cool part of the head (Front of # 2, rear of # 3 head area).

Quick sketch, just to illustrate the issue...

Baffling in this area of the head should not be so tight fitting that it totally blocks the flow. That’s why most use a washer between the head and baffle where it attaches to the head. The only way it could have zero airflow, would be if this area was silicone shut.

Apparently you do feel some step must be taken to ensure "regulated airflow" to the fins below the restriction (red circle). A washer or two on the baffle mount bolt (the bolt hole is seen here below the black rectangle) does indeed space out the baffle, allowing lots of airflow through an area with no fins. That's a useless leak, fine for GA slugs with huge inlets and exit areas, but not so spiffy for efficient cooling.

Most Lycoming type engines and their clones go to TBO. They would not if they were not being cooled properly. This cooling design is well proven, something must be working right.

Have to agree. There's a pretty good argument for each cylinder being an individual engine. Temperature deltas between cylinders probably don't compromise TBO, assuming the hot one isn't too hot.

Break.

Bobby, could you please identify the source document for the pressure measurement drawing in the previous post? The baffle buttons are straightforward, but I can't quite tell what is intended with the Section A-A probes, or how they are mounted.

For those interested in such things, CR3405 made an excellent comparison of probe types and arrangements. See pages 14 through 20, and Figs 11, 12, and 13. The investigators preferred baffle buttons for upper plenum pressure and piccolo tubes for the lower plenum, as they found long piccolos in the upper plenum read slightly low compared to buttons. That's not a surprise, given that the buttons are not shielded against local flow velocity.

I chose foot-long piccolos for both upper and lower plenums, for several reasons. They are easy to make, mount, and plumb, in an identical fashion, important if global, easily comparable results are expected from a group of homebuilders. Two, they work pretty much the same regardless of plenum volume as they shrug off dynamic pressure. Three, they are self-averaging over the majority of the plenum volume.

http://www.vansairforce.com/communit...7&postcount=40

[Hack]

I have a SJ Cowl on Plennum on my -4, and have heating problems that require a step climb. In my research and efforts on trying to make things cooler I have noted the necessity to keep the differential (4" H2O). Wouldn't this fix as described in the opening to this discussion create more pressue in the lower area and thus restrict downward air flow?
My hotest cylinder is always #2.
Any other ideas for us SJ users?

[Bavafa]

Originally Posted by DanH

Quick sketch, just to illustrate the issue...





Apparently you do feel some step must be taken to ensure "regulated airflow" to the fins below the restriction (red circle). A washer or two on the baffle mount bolt (the bolt hole is seen here below the black rectangle) does indeed space out the baffle, allowing lots of airflow through an area with no fins. That's a useless leak, fine for GA slugs with huge inlets and exit areas, but not so spiffy for efficient cooling.





Have to agree. There's a pretty good argument for each cylinder being an individual engine. Temperature deltas between cylinders probably don't compromise TBO, assuming the hot one isn't too hot.

Break.

Bobby, could you please identify the source document for the pressure measurement drawing in the previous post? The baffle buttons are straightforward, but I can't quite tell what is intended with the Section A-A probes, or how they are mounted.

For those interested in such things, CR3405 made an excellent comparison of probe types and arrangements. See pages 14 through 20, and Figs 11, 12, and 13. The investigators preferred baffle buttons for upper plenum pressure and piccolo tubes for the lower plenum, as they found long piccolos in the upper plenum read slightly low compared to buttons. That's not a surprise, given that the buttons are not shielded against local flow velocity.

I chose foot-long piccolos for both upper and lower plenums, for several reasons. They are easy to make, mount, and plumb, in an identical fashion, important if global, easily comparable results are expected from a group of homebuilders. Two, they work pretty much the same regardless of plenum volume as they shrug off dynamic pressure. Three, they are self-averaging over the majority of the plenum volume.

http://www.vansairforce.com/communit...7&postcount=40

I am going to check this gap on my engine to see if there is any space but I though this type of fins are limited to a certain cylinders and not all.

My temps over all is not bad, in the 360-370 range ROP and summer time and the delta between the hottest and coolest is about 10F or less. But I am limited on climbs and would love to improve that.

My hottest cylinder is #3

[Titan-Xpert]

Originally Posted by DanH

Quick sketch, just to illustrate the issue...





Apparently you do feel some step must be taken to ensure "regulated airflow" to the fins below the restriction (red circle). A washer or two on the baffle mount bolt (the bolt hole is seen here below the black rectangle) does indeed space out the baffle, allowing lots of airflow through an area with no fins. That's a useless leak, fine for GA slugs with huge inlets and exit areas, but not so spiffy for efficient cooling.





Have to agree. There's a pretty good argument for each cylinder being an individual engine. Temperature deltas between cylinders probably don't compromise TBO, assuming the hot one isn't too hot.

Break.

Bobby, could you please identify the source document for the pressure measurement drawing in the previous post? The baffle buttons are straightforward, but I can't quite tell what is intended with the Section A-A probes, or how they are mounted.

For those interested in such things, CR3405 made an excellent comparison of probe types and arrangements. See pages 14 through 20, and Figs 11, 12, and 13. The investigators preferred baffle buttons for upper plenum pressure and piccolo tubes for the lower plenum, as they found long piccolos in the upper plenum read slightly low compared to buttons. That's not a surprise, given that the buttons are not shielded against local flow velocity.

I chose foot-long piccolos for both upper and lower plenums, for several reasons. They are easy to make, mount, and plumb, in an identical fashion, important if global, easily comparable results are expected from a group of homebuilders. Two, they work pretty much the same regardless of plenum volume as they shrug off dynamic pressure. Three, they are self-averaging over the majority of the plenum volume.

http://www.vansairforce.com/communit...7&postcount=40

Section AA is a copper tube soldered to a flat piece of steel. It attaches to an intake manifold bolt to hold the tube up inside the fins in between the valves. I have had this drawing for so long I forget where it came from; it most likely is a Lycoming service instruction

[Toobuilder]

Originally Posted by Bavafa

...I am going to check this gap on my engine to see if there is any space but I though this type of fins are limited to a certain cylinders and not all...

It appears that some cylinder types have more fin depth in this area than others, but the simple fact remains that when cylinders are "paired", the "deep fins" on the exhaust side of one cylinder provide an ample flow path for both. However, when you have the "shallow" (intake side) fins up against a baffle (as with the left fwd and aft right), then this flow path is almost completely gone. Several of us are simply trying to replicate this flow path so the engine can uniformly cool as designed.

[DanH]

Originally Posted by Titan-Xpert

Section AA is a copper tube soldered to a flat piece of steel. It attaches to an intake manifold bolt to hold the tube up inside the fins in between the valves. I have had this drawing for so long I forget where it came from; it most likely is a Lycoming service instruction

Most likely from Installation Design for Engine Cooling, an old in-house Lycoming manual. I've requested a copy in the past with no success.

A vertical, open-end tube would measure total pressure, which more or less indicates the investigator was spending his days next to a dyno, i.e. vertical cooling flows. No surprise.

Fig 11-1 has a few problems as instrumentation for in-flight research. First, there is no provision for lower plenum pressure. The system as shown assumes current static in its measurement of pressure drop, which isn't true in any decent cowl. Let's assume there is some other drawing detailing a lower cowl pressure tap setup, as drop can be determined by comparing an upper cowl-static port measurement against a lower cowl-static port measurement.

The other drawback is a bit more subtle. When lower cowl exit area is increased, overall mass flow is increased. When exit area is decreased, mass flow is decreased. It's an intuitive result; reducing lower cowl pressure causes more air to flow through the engine fins.

Here's the measurement problem. Consider the case of increased exit area and more mass flow. The increased inlet ratio (inlet velocity must rise to accommodate the increased mass) almost always comes with loss of pressure recovery and internal drag loss, unless the inlet's internal shape is very good. Because a total pressure tube responds to dynamic pressure as well as static pressure, the higher internal velocities (which are quite likely turbulent flow) would tend to show higher pressure on the instrument, while a static pressure probe (like a picciolo tube) would tend to show lower pressure.

Take a good look at CR3405's Fig13. Probe type(s) 2, 4, and 5 (total pressure tubes) are all over the plot, as the individual locations are subject to different local velocities. The dotted line is static pressure measured with a piccolo.

Why is the difference important? It was established way back (see the NACA papers) that the front of a cylinder (or top in our case) with the usual half-baffle is mostly cooled by random turbulence. It is static pressure that pushes air down through the baffled fins and out a reduced exit area.

Bottom line? I'd suggest tubes like Section A-A should be used to examine local flows, read individually (example, a hot cylinder problem), not ganged together for an average like in Fig 11-1. As previously noted, I think piccolos are a better choice for investigating upper and lower pressures in RV cowls with various inlets.

Bobby, I sure hope you'll make it a point to join us at the Oshkosh RV Social. I'd really enjoy talking with you.

[DanH]

Last weekend I noticed these baffles on a Grumman Tiger.

Left front (#2) cylinder:



Right rear (#3) cylinder, from top:



Right rear (#3) cylinder, from rear:

[Toobuilder]

Originally Posted by DanH

Last weekend I noticed these baffles on a Grumman Tiger...

Looks like Grumman recognizes the need to get air to the bottom fins and provided ample flow.

While better than the typical Vans baffles, is still falls short of a comprehensive airflow management plan - The upper fins are still left to their own and are essentially just radiating in nearly still air of the upper plenum.

I have some changes to the Rocket in work which serve to pull air completely through the fins of the head and cylinders using some intercylinder baffles I have fabricated. In theory, forcing incoming air through all the fin area available should result in much more effective cooling. We'll find out shortly if theory and reality agree.