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I'm currently working on my engine cowling as one of my final steps before taking my Waiex to the airport. One thing I've not yet been able to figure out is how I should cut the bottom of my engine cowling. The plans have dimensions that might apply to a regular Aerovee or Jaibru, but don't seem to make sense with the way the turbo exhaust exits the engine compartment.

My main questions are these:

#1: Should the opening (airflow exit) in the bottom of the cowling be as centered as possible with the engine, despite the fact that the exhaust comes out on the right side. Maybe this doesn't matter?

#2: Is there a certain square-inch dimension of open space I should be aiming for.

#3. Do I need to add any sort of exit baffling to the cowling (as opposed to simply having a cutout in the fiberglass.)

I'm a total newbie to both fiberglass and the dynamics of engine cowlings, and since I'm going "off plans" with my Turbo installation I could use some guidance.

Hi Ryan,

I'd be happy to offer my suggestions but please don't take this as a gospel truth. It's just my two cents:

1. The main purpose of the exit airflow area is for cylinder head cooling, and as such I highly recommend you keep the exit area centered as called for in the plans. It seems to work very well. You do need to cut a round hole in the right side of the cowling for the exhaust pipe but that's a different opening as compared to the cowling airflow exit area.

2. I seem to recall the plans call for around 50 sq/in of exit area but from what I recall, there's nothing scientific about that dimension. Like many of others, I have enlarged my exit area to around 80 sq/in to help in cylinder head cooling and it does seem to help. I know of others who have taken the recommended size as called for in the plans and doubled the exit area which may be useful if you are routinely flying in areas of the country where the weather is hot.

3. A very important part of exit airflow is having the baffle ramps installed on the bottom of the ramps to help "pull" the air out of the cowling. The ramps can help create a vacuum to pull the air out of the cowling. Refer to your plans on how to assemble the ramps. They're super easy to build! That's really all you need though.

Does this help at all? Best of luck getting your project to the airport soon!

Actually, there is something "scientific" about the size of the cooling air outlet. Studies done by John Thorp and others indicate that the outlet area has to be at least 25% larger than inlet area. (i.e., 1.25 to 1). Most of our cowlings on the Sonex line end up having about 1.5 to 1 outlet vs. inlet, which seems to work pretty well.

It is important to have a pretty pronounced "lip" on the cooling outlet. This aids in creating the low pressure area that allows the cooling air to exit the cowling.

Thanks guys, that's very helpful!

Here's a few pics of the factory airplanes I took. and Jeff Schultz seems to have some good info on his website under Improving engine cooling http://www.sonex604.com/ I wonder if there's an advantage to going the way of the first picture? I think that's from one of the earlier Sonex's, so maybe not?

mike20sm wrote:I wonder if there's an advantage to going the way of the first picture? I think that's from one of the earlier Sonex's, so maybe not?

The first picture is the Jabiru 3300 cowl on the prototype Sonex (S/N 1). That cowl is not applicable to the AeroVee-powered airplanes.

I just discovered that the main reason I felt at a loss for how to design the exit for airflow in the cowling is because it wasn't included in the Turbo Aerovee manual that was provided in my engine kit. I looked up the regular Aerovee manual that's published online and it does have info for both Jaibru and Aerovee installations, which should be of some help to me. However the Turbo Aerovee manual is missing this information.

Specifically, Appendix A: Sonex Cowl Openings, is missing from my Turbo Aerovee Manual.

Sonerai13 wrote:

mike20sm wrote:I wonder if there's an advantage to going the way of the first picture? I think that's from one of the earlier Sonex's, so maybe not?

The first picture is the Jabiru 3300 cowl on the prototype Sonex (S/N 1). That cowl is not applicable to the AeroVee-powered airplanes.

Is there any reason why the jabiru cowling outlets are not compatible with the turbo Aerovee? The lower outlets on the jabiru seems, by looking at the pics, to be larger and more aerodynamically designed for suction.

If I'm not mistaken that cowling was the original design, When the 2 into 1 exhausts came out they were next to each other instead of separated. The universal cowl was designed to fit all the engines and the exit was cut in the center which simplified the construction a bit. I upgraded my AeroVee to a Turbo and use the same cowl. I simply cut a new hole on the right side for the exhaust and now the entire space of the exit is unobstructed. I actually have lower temps than when normally aspirated. After my first few flights I opened my cowling exit by one inch forward thinking the motor mount cross tube may be reducing the actual exit area.

How large is you actual exit outlet now Bill?
Was it the standard size before?

-Brett