I'm building and installing cowl flaps on my Bearhawk. I've been planning to install them on the cheeks on my cowl similar to a Carbon Cub but will make them adjustable from the cockpit.



I recently learned about another BH that has cowl flaps on the cheeks of its cowl but they open inward. They look really nice and would be a fair bit easier to build and install. I'm concerned they my not be as effective since the flaps doesn't extend into the air stream.





Any thoughts on these two cowl flap styles?

whee wrote:I'm building and installing cowl flaps on my Bearhawk. I've been planning to install them on the cheeks on my cowl similar to a Carbon Cub but will make them adjustable from the cockpit.



I recently learned about another BH that has cowl flaps on the cheeks of its cowl but they open inward. They look really nice and would be a fair bit easier to build and install. I'm concerned they my not be as effective since the flaps doesn't extend into the air stream.





Any thoughts on these two cowl flap styles?

I think you’ll get better cooling at less drag with the inward opening option.

When I was fighting oil temps, I was going to put some like this in my cowl. The sides of the cowl have a much lower pressure around them at higher fuselage angles (like climb out), when cooling is most important.

If your boot cowl “bulges” outward from your cowl, you may want to put them further forward on the cowling to put them in the lowest air pressure possible.

Depends on what you need.

Cowl flap bulges create suction to enhance the pressure gradient for cooling. They are often not mere variable outlet area devices. There are tons of NACA circulars on the subject, and a large number of pretty good articles in Sport Aviation going back to the 70's as well.

The cowl flap sticking out into the flow creates a low pressure behind it that draws the air flow you are after. Since you are designing it to be pilot operable, you'll have the ability to close them in cruise when you are less likely to need them. The flap that deflects inward will not perform as well, and as Lesuther stated there is a lot of aerodynamic studies on the web about this.

Thanks guys.

It’s been a couple years since I read up on cowl flaps and came up with a plan. None of the information was interesting enough for me to put in my permanent memory banks though I enjoyed reading the old military studies. When another Bearhawk owner sent me the pics of the inward opening close flaps I became intrigued and was trying to shortcut learning about the difference by asking here.

Rather than make a last minute change I’ll just stick with my original plan.

Wow...was my first thought when I saw the picture of inward opening cowl flaps.. I wouldn’t have thought of that! My mind went directly to the kinetic forces or energy created by placing a cowl flap in an area of high speed smooth air flow. Less drag for sure. With enough width, top to bottom that style creates a good air pump, with little to no drag. Well not exactly a pump like the kinetic energy created by a properly designed eductor. That design uses friction to accelerate air flow coming out of the cowl...doesn’t create a low pressure area...the more I think about it...the better I like it.. thing is...it’s not as effective at climb out speeds...but might be good for reducing overall cooling drag...perhaps omitting the lip that deflects airflow ahead of the cowl tunnel. Something to noodle about..


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