*Anybody have advice on how they run their exhaust exit systems?*

- Matt Boiteau

------ Forwarded Message --------
From: Matt Boiteau mattboiteau@gmail.com <flyrotary@lancaironline.net>
Date: 12/9/2018 10:13:56 PM
Subject: [FlyRotary] Exhaust setup's
To: Rotary motors in aircraft <flyrotary@lancaironline.net>

I'm at the stage of securing an exhaust setup. How do you guys do it?

I'll have a sub-cowl under the belly of the plane (shape of a tear drop), which will streamline the main cowl air (coolant and oil outlets).

I'm thinking I exit the exhaust (2.5" pipe) inside the sub-cowl and have a 5" wrapped around it which ends out the sub-cowl for x length. This should help create a vacuum and pump out the main cowl air.

Before exiting the main exhaust pipe, should I have perforated holes or louvers in it?

- Matt Boiteau

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Hi Matt,

I'm no engineer, but I do know a few engineers with recent hotel time. :-)

Have you read this thread?
http://www.vansairforce.com/community/showthread.php?t=68241&highlight=cowl

This is a later thread, with postings & references back to the link above:
http://www.vansairforce.com/community/showthread.php?t=159473&highlight=cowl
Watch Onewinglo's youtube video & try to visualize what the air's doing wrong as it comes out of the cowl.

It's about cooling a Lyc, but cooling is cooling. It's a long thread, but reading anything posted by DanH is worthy of respect. Note all the attention to smoothly curved entry areas, and the ultimate 'bluff body' (squared off) exit, with the external surfaces parallel to the free stream around the a/c.

I could be wrong, but I see a lot of skin drag with the tube-in-a-tube, with the exit air having to scrub on both tubes to get out. Also a *lot* of drag where the cooling outlet air enters the outer tube (reference: 'bellmouth' & 'velocity stack'). Is the perforated inner tube intended to be a muffler? If so, it's kinda like  the inner tube of a Spintech muffler, but the dimpled slots face aft in the Spintech. The spintech works great as a rotary muffler, if that's your goal.

There's lots of NACA & NASA research on eductors, if you do a little digging. Most of the stuff I've seen says that they can work great at low speeds, at the expense of excess cooling drag in cruise. A good storehouse of reading material is the CAFE Foundation. Try here:
https://cafe.foundation/v2/tech_enablingtech_dragreduction.php
and here:
https://cafe.foundation/v2/research_reports.php
with attention to the three 'Local Flow' articles in the 2nd link.

Here's hoping I'm not making things worse,

Charlie