I've read several articles that talk about making the exit smaller than the inlet. The key was not exhaust augmentation, but exit ducting that's effectively the reverse of the 'pressure recovery' of the inlet. Think of the P-51. IIRC, the CAFE Foundation articles talk about it.

http://cafefoundation.org/v1/research.htm

Try the 'local flow' articles (BTW, look at the most efficient exit shape; it's not gills or a 'reverse NACA').

I think that the old 130% of inlet ideas gained popularity before homebuilders really understood how to get air in & out of a cowling efficiently. If you look at that Mustang II in the exit area you will almost certainly see some significant changes from 'stock' below the cylinders & around the bottom of the firewall. I talked to Tracy Saylor (sp?), owner of the 180 hp, 230+ mph RV-6 about how he does it, & his mods to guide the air after it cools the cylinders & oil are pretty impressive.

Charlie

Ed Anderson wrote:

Ok, thanks, Thomas.

I understood correctly 135% OF the exit - just conveyed my understanding improperly.  Yes, if you have them already scanned I would like a copy. I hope there are a few photos as I am interested whether or not they may be using exhaust augmentation (whether they realize it or not).

Ed

----- Original Message ----- From: "Thomas y Reina Jakits" <rijakits@cwpanama.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Saturday, June 17, 2006 4:30 PM
Subject: [FlyRotary] Re: Exit area smaller than intake was External Diffusion

Ed,
there is a mistake as the intake is not 135%bigger than, but 135% of the
exit.
It still makes it 1.35 times bigger than the exit.

I posted this last in April 2005, the latest response to the subject was on
14th of december by Monty Roberts.
It always starts with a question to verify the intake bigger than exit
claim,but "no mistake" that's what it is!

Please search the archive or let me know if you want me to repost the post!
It was about Brian Schmidtbauers Mustang II being the fastest around!
Also menetioned is Dave Anders' RV-4, details in the CAFE report.
I have the essential pages of the Kitplanes article scanned, let me know if
you want them emailed....

It just shows that rules of thumb are not always the best solution. Most
likely just the most economic one to build.....

Thomas

PS: Search "Kitplanes" and go for the 14th Dec.,2005 posts!!


----- Original Message ----- From: "Ed Anderson" <eanderson@carolina.rr.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Saturday, June 17, 2006 6:35 AM
Subject: [FlyRotary] Exit area smaller than intake was External Diffusion

That's very interesting, Thomas.  I too recall seeing in several places
reference to
exit area being some multiple of the inlet with the ratios varying from
1.2 -1.7.
 There certainly could be some kind of phenomena I have not hear of or
read about, but seems strange you would ever have your intake area more

than

the exit area.  But assuming no error then it would appear to me that
external diffusion is taking place.

  What  that  indicates to me is that the exit area (what ever size
it was) provides adequate airflow for cooling flow through the engine
compartment.(assumption is the engine did not get cooked).  Enough air

mass

HAD to leave the cowling sufficient to carry away the necessary BTUs of
heat.  That said, then if the inlets were 135% larger than the exit area,
then air HAD to be spilling around the inlet or area of external

diffusion.

The air molecules in this part of the airflow (external to the cowl) then
contributed NOTHING to carrying away heat from the engine, but do add to
drag - that left only
the air that past through the core (or over the cooling fins of the

cylinder

head) to provide for cooling. Since this air has now been heated and

expands

to a larger volume, you traditionally need a larger exit area to

accommodate

this large volume of heated air.   That air must leave the engine
compartment via the exit. So I just am unable to come up with a scenario
where having an inlet larger than the exit area would be beneficial.

Having said that, it did just trigger a thought about why this might be
tried and
how it might be made to work.
.
We do know that for air exiting the cowl to provide minimum drag it

ideally

should be accelerated back to the airstream velocity before intermixing.

We

can theoretically do this by taking the larger volume of heat air and
designing an exit area
which would  accelerate the air molecules increasing the velocity of the
exiting air and reducing drag.  However, to accelerate the cowl air

velocity

to anything really meaningful,
 would require added energy.  This leads me to believe that  perhaps an
exhaust augmentation system could be used to provide increased velocity to
the exiting air using the energy in
the exhaust flow.  If the exiting airflow velocity is increased over than
normally associated with exiting air, then more air of course could flow
through a smaller opening, this would perhaps permit one to have a smaller
exit area than intake area and still
get good cooling and low cooling drag.

So with an exhaust augmentation system "helping" the air in the cowl to

exit

quicker and at a higher velocity, I can see where a smaller exit area

might

indeed be workable.
 But, without an exhaust augmentation system, I just don't see how a

smaller

exit area would be beneficial.

Any mention of exhaust augmentation??

Well that my $0.02 worth on the topic

Ed

Ed Anderson
Rv-6A N494BW Rotary Powered
Matthews, NC
eanderson@carolina.rr.com> Hi Steve,

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