Engine cowl vents

[Vev]

G'day guys,

 

Keen to hear your thoughts?

 

I was looking at a 430 Jab a few weeks ago which had a fixed louvered outlet vent inserted in the rear section of the top cowl.

 

The owner claimed this modification allowed the hot air trapped inside the cowl to escape after shut down, which he said was good for extending the life of the electronics.

 

He also said the vent helped reduce the pressure inside the cowl which increased air flow volume through the cowl providing a reduction in engine and oil temps.

 

Has anyone come across this approach and does it make sense … what do you think?

 

Regards

 

Jack

 

 

[Guest Maj Millard]

Years ago at an airshow in the States I was lucky enough to have an encounter with the legendary aircraft designer, Elbert Rutan. He and about 50 others, were gathered around his latest design (Catbird) which he had just flown in.

 

It had louvered vents all over the top of the top cowl. Somebody asked him why ?. His reply was " When your sitting waiting to take off, and your No 4 in line in the middle of summer, in the desert, heat risers, and I want it out !" Made sense to me.

 

The vent type arrangment on the Savannah top cowl seems to work well, except that if you did get an inflight engine fire (rare) it's going to come straight out onto the windshield. Then you better know how to slip well to keep it from melting the windshield I suppose ??...I do believe they would have to get rid of some heat though..........................................

 

 

[facthunter]

Cowl venting.

 

Heat does rise but oil and flames may do so too. On a multi-engine where the engines are not in a position to ruin your vision if they eject fluids (which they constantly do. Look at the common oily belly of the Jab Breather) this may be possible and a good idea as it has some potential for efficiency (drag reduction).

 

Most "flat" engines have the cowl area divided into 2 sections, normally pressured above the engine and not pressured or even reduced pressure underneath Seals and baffles restrict and direct the cooling airflow around the appropriate areas of the engine to ensure some control over where the heat is extracted from, hot areas being given the required attention. IF you RUN such an engine with the top cowl removed the airflow over and down thriough the engine will not happen, and it will unevenly "cook" itself in no time, even though the engine is exposed. The top is..

 

It is generally considered that to reverse this flow so that air enters at the bottom and exits through the top would have advantages in a "purist" design. Close cowls do present a problem after the engine is shut down and the airflow ceases. It is hard on engine seals and other components that are under the cowl like batteries alternators magnetos etc many of which have air jets directed on them when the engine is running, but nothing after it stops and the motor is a big heat sink. I am all for cowl gills so that you can open them up after landing and get some of the heat out of the engine before you shut it down. Nev

 

 

[Vev]

Maj/Nev... Thanks for the input .

 

 

I did ask the question of the owner re oil on the screen and he said that he hadn’t experienced any so far. He did say however that a major oil leak could be another story but also said a major oil leak would easily find its way between the cowl joints??? I didn’t think of the fire consequences, but mind you, I think any fire in the front end of a jab wouldn’t be a good look anytime.

 

 

May be the answer is to just pop the oil dip stick inspection door after landing to let the heat out, although this doesn’t address the taxi holding the Major mentioned or the inflight cooling?

 

 

Cheers

 

Jack

 

 

[Yenn]

When I had a 1600 jab engine in the Starlet it used to get so hot standing after flight on a hot day that I had to remove the top cowl to cool the ignition modules. They got too hot to work.