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John, don't know if these photos will help. But, like you I only have between 3 and 6" of duct distance on the radiators. You should do Ok with 20 sq inch on each opening with a good diffuser/duct. Attached are some photos of my current ducts. The openings are 18 sq inches each. I have had one opening down to as little as 10 square inches - but that was a bit marginal - so opened it back up. I have a generous exit area for the hot air including a larger 4" x 12" bottom opening as well as louvers on each side of the cowl. So you mileage could vary - but Tracy has essentially the same size opening as well as several others.

----- Original Message -----

From: John Downing

To: Rotary motors in aircraft

Sent: Monday, February 26, 2007 12:12 PM

Subject: [FlyRotary] cowl openings for water radiators

What size openings do I need for the water radiators? The Wittman Tailwind cowl I have has postal slots of 3' x 7 3/4" , which is approx. 22 1/4 sq in. on each side. Sam James for the 160 Lycoming is using 4 3/4' round holes which are 17.6 sq. inches on each side. My radiators are quite close to the opening and I plan on making the diffusers trumpet shaped, will the openings be large enough if I can stay over 20 sq. inches on each side with a decent trumpet shape. JohnD hushpowere II on order - hope to start in 2 weeks if weather cooperates.

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Attachment: Both Ducts Front Side Profile.JPG

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Attachment: Left Duct Aircraft.JPG

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Attachment: Foam Form1.JPG

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