That's pretty interesting idea, to end the exhaust short and shoot it into a larger (eductor) pipe with a bellmouth on the front. I'm wondering if I even need a larger pipe, or would the inside of the subcowl act as the pipe? SO much to think about. Add some Fiberfrax and this heat barrier, and you can good over 2000F easy.
https://www.amazon.com/gp/product/B00029KC2K/ref=ox_sc_saved_title_1?smid=ATVPDKIKX0DER&psc=1

I think I'll secure the pipe as it comes past the firewall, and allow a slip joint that I'll be able to test a whole bunch of different configurations. For now, I just need to get the dam thing flying. lol. Have to stop changing things and stick to a plan.

I'm good with electronics, so I'll be building a remote bluetooth device that will have a bunch of air temp, EGT probes & pressure sensors on it. Maybe even a camera if I can figure that one out. Be able to data log all these different ideas.

- Matt Boiteau

On 1/6/2019 5:11:40 PM, Charlie England ceengland7@gmail.com <flyrotary@lancaironline.net> wrote:

There's a Lancair guy and a Mustang II guy who both post fairly often on the list. I'm sure you'd survive, as long as the subject can be tied to RVs in some way.

Did you compare the outside shape of your 'afterbody' to Vetterman's? His is much more of a convex 'egg' shape.  Have you tried to tuft test yours? I'm thinking about the horizontal equivalent of what's happening in the vertical plane in the video in that 2nd thread.

The eductors I've seen seem to have much shorter ducts, and the exhaust energizes the duct by shooting into it, rather than being submerged in it. I just found this site, which looks closer to what I was thinking.
http://nickugolini.com/website/Eductor.html
The super high velocity exhaust stream is pretty potent in dragging cooling air along with it, without the drag of all those small holes.
Another example is what was once the world's fasted RV-6, that had the pipe cut just an inch or so inside the cooling outlet (just forward of the firewall). I talked to the builder at SNF; he described cutting the pipes a half inch at a time, trying to tune for power. when he got to the exit, he decided to 'gamble' on shortening them one more time where they'd exit inside the cowl, and he saw an improvement in oil cooling after the cut.

Charlie

On 1/6/2019 11:59 AM, Matt Boiteau mattboiteau@gmail.com wrote:

Thanks guys. I browse the RV forum religiously, there is so much good information on there. I did come across and save a lot of posts from DanH but not the second one you mentioned Charlie. I don't know how they feel, if I post stuff on there without being an RV owner? The subcowl I fiberglassed, is the back half shape of a tear drop. Much like the Vetterman setup. Should help speed up the inside cowl air to align to the tip of the tear drop, where an opening will be for the exhaust pipe. Also will allow the outside air the stay attached to the bottom and sides of the subcowl.

The perforated inner tube would face aft. The idea was to  allow the slowing moving cowl air, to get sucked into the faster moving exhaust tube. But I think for now, I'll just cut the exhaust pipe short inside the bigger outer pipe. This will take place inside the subcowl. Then the bigger outer pipe will exit the tip of the tear drop, all the way down the belly of the plane. Where the pipe stops changing color from the exhaust heat, I'll cut off there. That will be my baseline.

Inside the subcowl, the front of the outer tube (eductor) will have some type of bellmouth on it. I found a few on the internet, but I may 3D print one as a mold, then fiberglass overtop. I've done that a few times now with designing parts.
https://www.summitracing.com/int/parts/vpe-10953/overview/

I have a butterfly valve on my oil outlet, which will keep temps where I want them during slow / fast speeds. Still need to make a big flap for my coolant outlet though, so it can do the same thing. This will allow me to really know if my inlets are too big / too small based on how far the flaps need to close / open to keep temps.

- Matt Boiteau

On 1/5/2019 10:49:04 PM, Charlie England ceengland7@gmail.com <flyrotary@lancaironline.net> wrote:

On 1/5/2019 11:32 AM, Matt Boiteau mattboiteau@gmail.com wrote:

*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

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