Mailing List Message #67342
From: Charlie England <>
Subject: Re: [FlyRotary] Re: Wedge diffuser
Date: Wed, 23 Nov 2022 13:19:42 -0600
To: Rotary motors in aircraft <>
IIRC, Tracy used the thickest version of the 'Sirocco' radiator for his 20B.

I'm embarrassed to admit that mine hasn't flown yet. But I am making progress; painting is in progress now. I've already done all the FWF, wiring, plumbing, etc and previously hung the wings & tail to do rigging/final drilling, so assembly is what's left after paint.

If you can get Dennis Haverlah off the tennis court, he can share some experience with his wedge diffuser on a Renesis/RV7A.  His is under the engine, but diffuser design would be similar.

Here's a link to a flow test I did after building the diffuser.

I have no idea on how *efficient* the design is, but I do get equal airflow across the entire face of the core; something that can be difficult to achieve in a lot of cases. With a round inlet offset to one edge feeding a large flat exchanger, a turning vane was essential for me. If you use a rectangular inlet that's as wide as one face of the exchanger, you shouldn't need a turning vane.

The non-intuitive thing is how 'tight' the wedge has to be at the aft edge of the exchanger. Tracy warned me, so I made mine pretty tight (~3/8" aft end) to start with. He happened to be visiting about the time I pulled it from the mold, & said that it didn't look tight enough. Sure enough, when I did a flow test, all the air was coming out the aft 1/3 of the core. I used spray foam to tighten up the gap to almost nothing at the aft end, and got even flow after that. I started out trying to 'do the math' using the K&W streamline formula to expand from the round inlet to the bigger rectangle entry to the wedge, but transitioning from round to rectangular while following the formula before starting the wedge was...not fun, so I finally just tried to approximate the expansion, then made a 'TLAR' turning vane to get some air down to the leading edge corner that is away from the inlet.

The wedge ended up having a curved wall that looks a bit like the K&W streamline wall in reverse.

Hope that's at least a little help. I can send you a link to my google cloud account with pics of the diffuser if you'd like.


On 11/23/2022 9:09 AM, Les C wrote:
Thanks Charlie.  I love the look of Tracy's set up, and if it cools adequately, I'd like to pirate it for my plane.  How did your's work out?

Les Criscillo
Nurse, Pilot, Brewmaster, Maker

From: Rotary motors in aircraft <> on behalf of Charlie England <>
Sent: Tuesday, November 22, 2022 10:49 PM
To: Rotary motors in aircraft <>
Subject: [FlyRotary] Re: Wedge diffuser
On 11/22/2022 8:49 PM, Les C wrote:
While doing some research online, I saw same good pictures of the 20b installation in Tracy's RV-8.  I am particularly interested in the wedge diffuser he used for his radiator cooling.  I've been looking on the mail list, but don't see anything referencing it or any other information about the ducting.  I have been planning on using a ventral radiator with a scoop to provide the cooling I'll need.  I'm building a BD-4 and I may have enough room in my cowling for both the wedge diffuser and cooling for my oil.  Does anyone have thoughts, suggestions or directions?  I've looked for the K&W book, but haven't been able to find it or any copies of it. 

Les Criscillo
Nurse, Pilot, Brewmaster, Maker
Hi Les,

For K&W, go to your local library & ask if they have a lending agreement with an engineering school in your state; the school will almost certainly have a copy. That's how I got access.

If Tracy doesn't see this & reply, I'll  try to send you some info on my attempt to  more or less copy his wedge diffuser.


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