-------------- Original message --------------
From: "Ed Anderson" <eanderson@carolina.rr.com>

> Thanks Charlie, I'll read the Low Flow Articles.
>
> Its no secret that you reduce exit airflow drag by accelerating it to the
> outside airstream velocity. We also know that if we make an area smaller
> that the air velocity will speed up. I will be very interested in how they
> do it without incurring drag. It would take a higher pressure area in front
> of the narrow exit in order to force the hot air through a smaller area.
> Since mass flow will remain constant, you can increase the velocity and get
> the same amount of air through a smaller exit area or increase the area and
> get a larger volume of air through at a slower velocity.
>
> But when you decrease the exit area you generally will experie nce pressure
> increase in front of the narrow area (which in this case is inside the
> cowl). That pressure increase in back of a core or fins would tend to
> oppose the air flow coming in to the determent of good cooling. If they had
> exhaust augmentation then I could see how that would work.
>
> But, I should stop speculating and read the articles and see if that
> improves my understanding of how a smaller exit improves cooling.
>
> Ed
>
>
> ----- Original Message -----
> From: "Charlie England"
> To: "Rotary motors in aircraft"
> Sent: Saturday, June 17, 2006 9:00 PM
> Subject: [FlyRotary] Re: Exit area smaller than intake was External
> Diffusion
>
>
> > I've read several articles that talk about making the exit smaller than
> > the inlet. The key was not exhaust augmenta tion, 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"
> >>
> >> To: "Rotary motors in aircraft"
> >> Sent: Saturday, June 17, 2006 4:30 PM
> >> Subject: [FlyRotary] Re: Exit area smaller than intake was External
> >> Diffusion
> >>
& gt; >>
> >>> 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"
> >>>
> >>> To: "Rotary motors in aircraft"
> >>> 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
> >& gt;>
> >>>> 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
> &g t;>>> 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
> >>>> t han
> >>>> 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,
> >>>
> >
> >
> >
> >
> > --
> > Homepage: http://www.flyrotary.com/
> > Archive and UnSub: http://mail.lancaironline.net/lists/flyrotary/
> >
>
>
>
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