Aerodynamic Consequences Cowl Shaping

Hello All,

I am installing an air conditioning pump on the port fwd side of IO 390 on RV7. I have the dilemma of partially obstructing the port cooling air intake and having a leaky plenum owing to belt path, or to rotate the pump downward on its mount and into the lower cowl.

My gut tells me that ensuring proper plenum sealing and airflow path is more critical than maintaining the stock cowl profile, but I am always interested in more learned opinions.

Anyone have an opinion on the consequences of creating a 'more full' shape on the front of the lower cowl?

wawrzynskivp said:

My gut tells me that ensuring proper plenum sealing and airflow path is more critical than maintaining the stock cowl profile, but I am always interested in more learned opinions.

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I dunno about "lerned", but my gut says rotate the compressor mounting downward, seal the intake, and accept the cowl bulge.

You will find zero performance penalty from reshaping an area of the cowl with some kind of bulge, as long as the contours are smooth and don't create any aft-facing slopes.

In other words, as long as you are in the part of the cowl that is getting bigger as you go back, and your bulge creates a shape that gets bigger first quickly, then slowly, but does not anywhere get smaller, you will be fine.

A leaky cooling plenum would not be good. Always do your best to get a well-sealed upper plenum area.

thanks guys

thanks guys

You should take a look at the early design of the RV6 cowling. These had a bulge on the lower cowling section to accommodate the pulley of the altinator. Although the current design no longer has this bulge, the area of the lower cowling fabrication does not have the honeycomb structure in this area so that if a builder needs to fabricate a blister for the altinator pulley, the area can support one.

Alternate plan would be a move toward lower Vi/Vo inlets and a "front wall" style baffle like a C172, Sportsman, or Wolf Pitts cowl. That would put all the belts in front of the seals.

Can you try a smaller compressor? The early Miata compressos are very small and our cockpits are about the same size.

You will have to check but it is possible that the current production cars have electric air-conditioning compressors.

noelf said:

You should take a look at the early design of the RV6 cowling. These had a bulge on the lower cowling section to accommodate the pulley of the altinator. Although the current design no longer has this bulge, the area of the lower cowling fabrication does not have the honeycomb structure in this area so that if a builder needs to fabricate a blister for the altinator pulley, the area can support one.

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You may have seen cowls that people added bulges to during their build, but there has never been a production cowl that had one.

The reason there is no core material in the location close to the alt. pulley is to allow a bit more clearance by deleting the core thickness. If a minimal length belt is used, this generally gives more than enough clearance to allow for engine sag during high G's.

I agree with the recommendation that for an AC compressor, a bulge in the cowl would be a better choice than disrupting the cooling flow at the inlet.