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O-360 RV-8 cylinder fins covered

werxcv01

Well Known Member
Working on the condition inspection with an I/A he commented about the orange rubber wrapped around each cylinder. I bought this flying -8 last year and am still learning about it, even though I was a partner in a -6A for 10 years, also bought as a flying aircraft. This -8 has an O-360 and fixed pitch prop and an aluminum plenum. Anyone familiar with this wrap? It appears to cover from the 2 o?clock to 10 o?clock positions on the inner fins and from the 3 o?clock to 9 o?clock positions on the outer fins, with the top of the fins uncovered.
 
CHT’s seem normal, perhaps a bit lower than I saw in the -6A with an O320. Maybe 335-360. It has a 4 cylinder CHT/EGT gauge from Electronics International.
 
My IO360M1b parallel valve is wrapped about that much. Wrapping the cylinders will make no difference to the CHT measurement or very little, but shifting the airflow will. It does not mean that nothing is happening, just not reflected in the CHT. Removing them completely might even increase the CHT due to increasing air demand by cylinders dropping plenum pressure, thereby lowering mass across the heads.

What engine is this, parallel or angle valve?
 
Is it something like this?

drag-reduction-05.jpg


https://www.kitplanes.com/so-youd-like-to-go-faster/
 
So the cylinders are wrapped, or the heads or wrapped, or both?

Pictures?
 
Both cylinders and heads are wrapped. My original post did not use the correct terminology.
 
I haven't seen this approach before and am confused (seriously- not kidding).

It doesn't "fit" with my prior experience.

What is the benefit from blocking cooling air access to the "top" of the cylinders??

Plenty of support in the NACA cooling papers that argue this approach increases cooling drag and lowers heat transfer - especially on the "bottom" of the cylinders.
 
What is the benefit from blocking cooling air access to the "top" of the cylinders??

Plenty of support in the NACA cooling papers that argue this approach increases cooling drag and lowers heat transfer - especially on the "bottom" of the cylinders.

It's not blocked, merely channeled between fins rather than relying on random upper plenum air motion.

Note there are differences when discussing cylinders and heads. I'm not aware of any NACA documents seriously measuring heat rejection from heads with varied baffle arrangements. In any case, they would have been radials, not much like the heads on our flat engines.

There are some docs dealing with cylinders, and with fin size and shape. As I recall, all the wrap arrangements tested (NACA 620) result in a warm spot at the exit where the two flows attempt to join. The researchers were looking for even temperatures, and didn't really find them.

Cooling drag = (momentum loss x mass). The stubby exits under our cylinders are less than optimal, but building really goods ones would be very hard to do. Nothing about the wraps extending to the upper side suggests cooling drag issue. If anything, their purpose is to transfer more heat to less air, allowing a mass reduction, i.e. less cooling drag.
 
RV-8 cylinder fins covered

Viewing this intuitively, it looks wrong. Maybe I?m missing something, but I wouldn?t do this. Why block airflow from the upper plenum through the main cooling fins? It certainly will reduce mass airflow, even if it just reroutes it. I?m concerned with cooling, not cooling drag. Also what are the rectangular strips on the foreground fins? How does that help cooling ?
 
Guys, that is Dave Anders angle valve engine.

He had/has excellent data on the speed and cooling. There is a lot more than just sticking the stuff on there. The upper plenum pressure has to be correct, the head and barrel temps are known, and if you plane is going as fast as Daves, then it may well have excess pressure so the reduction in mass flow gets it back to the regular RV level.

It is just a picture, we need the OP's data and pictures of the installation.
 
What the wrap does is force air to enter the cooling fin channels along the sides and flow down between the cylinders within the fin channels, which is what you really want.

(although I think there is an argument to be made for allowing a small amount of air around the outside of the channels to provide a reservoir of cooler air to mix with the channel flow, which is getting heated a lot as it moves along the channel)

Those rectangular strips on the head fins are elastomeric dampers.
 
Why block airflow from the upper plenum through the main cooling fins? It certainly will reduce mass airflow, even if it just reroutes it. I’m concerned with cooling, not cooling drag. Also what are the rectangular strips on the foreground fins? How does that help cooling ?

Airflow is not blocked, just channeled in a specific path between fins.

Our real interest is not maximum flow, but what that mass flow does for us, which is carry away heat. If a fella can arrange things so the air picks up more heat during its journey through the cowl, we can cool the installation with less air. A key measure of the experiment's progress is exit temperature.

In a perfect installation, we would like to maximize heat transfer with little loss of pressure across the engine. A lot of restriction is not desirable because high lower cowl pressure boosts exit velocity, i.e. minimizes momentum loss. Balancing the desire for highly turbulent flow between the fins with low pressure loss is why you see lovely diverging ducts on all the theoretical cylinder wraps in the old research papers.

Those blue strips look like ordinary fin stabilizer combs. I suspect Dave is experimenting with tripping the flow, which would increase heat transfer. Or maybe he just suffered a cracked fin or two in the past and said, like the raven, "nevermore".
 
I responded with a gut level feeling from what I saw in the pictures. Actually, The empirical method is the only way to find an optimal arrangement. The maximum transfer of heat is the goal. Maintaining higher velocities through the fins, by restrictions, may locally increase heat transfer. On the other hand, reducing the total mass flow too much can reduce heat transfer. And of course the many possible geometry variations complicate the problem.
 
Looking at the photos I took (sorry but I’m not sure how to provide a link to the photos) the cylinders are wrapped so the fins on top are exposed for about 2.5 to 3 inches, and most of the upper side of the cylinder heads are exposed. It appears the intent was for air from under the plenum to flow under the wrapping and through the fins.
 
Here is Bill's 8. It appears to have an aluminum plenum cover, not sure about the diffuser section but it appears to have standard Vans cowl.

Bill, if you could post some observations for climb/cruise CHT's, oil temps, and if you are seeing any issues it would be very helpful.

Are you just curious or is there some flight problem you are trying to solve?


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Bill, thanks for posting the photo I sent you. I’m not seeing any CHT or other temp issues. In fact, this engine seems to be running cooler than the O-320 in my -6A. I was curious since my A/P-I/A had not seen this before, and neither had several others I showed.
 
Folks have been doing similar silicone/glass wraps for a long time.

The above setup looks like 9 oz plain weave filled with ordinary orange Permatex between two sheets of plastic, then placed on the fins and cured with the top sheet in place. If rolled hard between the plastic, it's possible to make wraps with less silicone content. Note these were so wet when applied that silicone was squeezing out between the fins. Not a big deal.

In the photo above, cyl #2 has no bypass for the zero-fin-depth area on the intake side of the head. Does #2 run a little warmer than the others?

I've been flying silicone/glass wraps about 10 years now, although mine don't clock around the cylinders quite as far. They are more like gaskets to ensure zero leakage along the sides of the aluminum wraps, which are typically a leaky mess. Note that it is important to use a little bead of sealant betwen the silicone/glass wrap and the aluminum wrap. Don't want air leaking between them, where it would cool nothing.

P1010009.JPG


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Cylinder%20Wrap%20Sealing.jpg


Optimized cylinder baffles would look something like this:

Optimum%20Baffles.jpg
 
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