Plenum chamber ... too much sealing?

[Bruce Tuncks]

This relates to a Lycoming setup on a Lancair. ( I am a Jabiru guy so please forgive my ignorance ):

 

The plenum chamber, which consists of the upper cowl plus seals, has used a lot of high-temp silicon sealant to stop up any leaks.

 

In particular, at the front of the front cylinders, (1 and 2) the floor of the plenum turns up into a flange of about 40mm height. This flange is close to touching the fins.

 

Now it seems to me that here is a good place for cooling air to transfer heat from the metal as it passes down between the fins.

 

BUT in this plane, this gap has been filled with high-temp orange silicon, which fills the gaps between the fins and all. No air can pass down there.

 

It all looks wrong to me, but as I said , I am a Jabiru guy.

 

The CHT's run hotter than I am used to. The 3 coolest cylinders run about 175 degrees, whilst no 2 runs at 190.

 

This 190 is just within the allowed limits, but it is sure hotter than my Jabiru which has the hottest at 155 degrees.

 

I reckon the gunk at the front of cylinder no 2 needs cleaning right out, but I would sure appreciate any comments first.

 

 

[derekliston]

Not much point having fins if the air can't get through them. Clean it out! Only reason I can see to have silicone there is to prevent vibration chafing of the baffle aluminium so just a very thin bead of silicone would assist that while leaving the space clear between the fins.

 

 

[Bruce Tuncks]

Thanks derek, this is a GA plane and has always ( and still is) been maintained by lames. So this is why I'm more hesitant than usual.

 

But as you say, the cooling fins need air through them.

 

I've been googling up on plenum chamber stuff, but without anything clear-cut yet.

 

 

[Yenn]

Remove the silicon and you could be in trouble, unless you are a LAME.

 

The theory of the plenum is to produce high pressure air to go past the fins of the engine and transfer heat away.

 

This requires a lowering of incoming air speed and a lower air pressure beneath the engine so that the cooling air can flow.

 

Usually silicon would only be used where there is a risk of air escaping without going past the fins, or to prevent chafing.

 

 

[Downunder]

Maybe it was an effort to even the temps out? ..... Restrict air to a cooler cyl so more air gets to a hotter one............

 

 

[Bruce Tuncks]

Nope Downunder, there is no record to support that idea. The no 1 cylinder has the same treatment, but it is closer to the inlet and so gets better cooling. I reckon the lame just noticed that the temps were in spec and moved on with his day.

 

And Yenn, thanks for the warning but it will be a bad day when I fail to do the correct safety maintenance thing because of fear of retribution. Well.... having said that, I do admit to cowardly obeying stupid and dangerous airspace lines on maps, and I feel bad about this.

 

 

[facthunter]

Seems pretty rough to me. Usually the baffles and sheet metal structure are attached to the motor and move with it. as it's on rubber mounts. Sealing to the cowl is by rubber strips attached to this and have to touch the cowl at the right way to seal well requiring some care when putting the cowl on. Blast tubes remove air to cool things like an alternator so there is always some leakage. The front heads may be part covered with baffles to prevent overcooling. The installation has to be approved for most engine providers to be satisfied as to it's effectiveness. It may be a bit more slack with some U/L's but no engine manufacturer is going to pay up if it's the cause of a problem. Nev