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Airflow under a cowling

Actually flying meant really “actually flying”. There is a grand total of one test-bed Panthera flying and the VW configs are all experimentals.

Here is a more advanced aero engine cooling system

Nice! But wasn’t this thread about air-cooled engines?

EBZH Kiewit, Belgium

When flat engines were in their infancy a number of different configurations were tried, including uncowled and updraft. Updraft cooling was used on the Navion (designed by same people as the Mustang mentioned above, 2600 built), the Globe Swift (1500 built) and the Luscombe Sedan (~100 built). Cooling drag and efficiency can be improved on those with a modern cowling.

The function of the plenum under the cylinders is to produce a volume of (roughly uniform) lower pressure, which can be enhanced by a lip on the lower aft edge of the cowl. Flow is pushed vertically through all cylinders, with the same pressure differential present across all of them. The pressure drop exiting the cowling will not be high because the velocity is relatively low in the plenum.

Last Edited by Silvaire at 02 Oct 16:14

Actually flying meant really “actually flying”. There is a grand total of one test-bed Panthera flying and the VW configs are all experimentals.

Limbachs are certified engines. More than 6000 have been made. Thousands are still flying.


On a tangent, what do people recommend as sealant between the baffles and the engine?
I really dislike the aesthetics of the standard orange heat resistant silicone.

Forever learning

I used two different materials for the seals. The Teflon coated “stiff” type for the upper seals (which have to push up against the upper cowling) and the “floppy” type for the lower seals (which lie, using gravity and air pressure, against the side of the lower cowling). I wrote it up here somewhere, with photos…

One thread is here

The upper stuff came from MacFarlane and the lower stuff came from Browns. Some pics here

This made a huge improvement in the CHTs.

Shoreham EGKA, United Kingdom

In terms of sealant, you can get exactly the same silicone in clear, black, white, “aluminium”. The preferred is Dow Corning 732, apparently. I very much respect the view of under-cowling aesthetics – and personally like to see no silicone at all. Easier with some installations than others.

An earlier question was why take the air up into a chamber and then down vertically through the cylinders? Why not front to back?

You want cooling air to be as cool as possible. If you went front to back, the cooling air for the second bank of cylinders will have been warmed by the first bank and so on. Not good if you have an IO-720!

The basic idea is to convert dynamic pressure to static pressure in the inlet plenum. Then run the air across the fins into the exit plenum and exhaust the air backwards. This should be done as efficiently as possible with as little drag as possible.

Here are some very good reading:




Here are some very good reading:

Nice to read for me who built in RV8 from very beginning the closed plenum under upper cowl. The plenum was al-sheet construction and the first one from 2420 Al. This was too hard and supported only from one point in middle and perimeters naturally and got soon damages from the pressure fluctuations. So we built second one from softer / not so brittle Al 7056 and made low profiles to support the plenum from corners to middlesupport. Happy with this with low CHT’s and drag although two dozen screws to fix it.


The baffle seal pics in my post above are now back. These show the use of two different materials.

It took quite a while (months) for the full benefit on CHTs to show up. This suggests what I suspected: the air will escape through even the smallest gap (mm) and it takes a while for the baffle seals to “settle down”.

Shoreham EGKA, United Kingdom
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