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I’m with you Charlie.
Many variables!
If its cooling well enough after first flight then perhaps you could leave it as is.
But having faster aircraft and therefore the possibility of reducing cooling drag for speed or economy, being able to measure whats actually going on is very helpful.
Cheers
Steve Izett
> On 22 Nov 2020, at 1:10 am, Charlie England ceengland7@gmail.com <flyrotary@lancaironline.net> wrote:
>
> You're right, and I should have been more clear about that in my posts. I started out trying to use the streamline expansion formulas from the round inlet to the beginning of the 'wedge', but I'm not sure how much (if any) survived my tweaking to get flow across the full face of the core. What I was trying to give in my previous post was a mental image of how the back of the wedge isn't straight, but is more like the flatter section of a 'French curve', where the aft end almost touches the core. Quick&dirty drawing attached, which ignores actual inlet geometry.
>
> I suspect that what Steve and I have both attempted strays away from the idealized wedge diffusers that have been documented. The ones I've seen in the texts typically show a rectangular inlet that is equal to the core in width, and some percentage of the core depth in height. The same applies to the streamline diffusers. And I strongly suspect that the actual streamline equation depends on the resistance of the core at the end of the 'trumpet' shape, for it to function optimally. With no core resistance, a pure exponential horn (which is close, but not identical) would likely be more efficient.
>
> I struggle enough with 'good enough'; 'perfect' can easily become the enemy. :-)
>
> Charlie
>
>
>
> On Fri, Nov 20, 2020 at 11:04 PM Stephen Izett stephen.izett@gmail.com <flyrotary@lancaironline.net> wrote:
> Hi guys.
>
> I think we are talking about two different beasts.
> K&W Chapter 12 describes the Streamline (Trumpet) shape.
> However we also use Oblique Flow (Wedge) diffusers as descibed by London Clopfer and Wolf.
> And we use both together, a Streamline diffuser feeding an Oblique diffuser as in Tracy’s RV8 setup that many of us modelled from.
>
> Steve Izett
>
>
> > On 21 Nov 2020, at 11:34 am, Charlie England ceengland7@gmail.com <flyrotary@lancaironline.net> wrote:
> >
> > I agree, but I'd describe it more like the core was down the centerline of a streamline diffuser, and the air feeds from the wide end. Half the original diffuser disappears.
> >
> > On 11/20/2020 8:22 PM, David COOK hoursaway1@comcast.net wrote:
> >> Wedge is kind of a poor term for what is needed, more of a bell mouth like on a trumpet with the rad. being at the large end. DRC RV6A Rotary
> >>> On 11/20/2020 9:03 PM Charlie England ceengland7@gmail.com <flyrotary@lancaironline.net> wrote:
> >>>
> >>>
> >>> Hi Bruce,
> >>>
> >>> The driver for me was trying to remain within the constraints of money, a stock cowl with minimum 'surgery', with nothing hanging out of the cowl. Another driver was due to 'standing on the shoulders of giants'; Tracy Crook does really light weight installations, and his calcs showed that at least in his RV4, he needed the weight of the heat exchangers to be up front, to keep CG in the right place. My FWF weighs almost exactly the same as an O-320, so for now, I'll keep the wedge in place. Please be aware that my setup may not survive to the 2nd flight. :-)
> >>>
> >>> In retrospect, I might well have done the same as Steve mentioned, and at least try to tuck the rad into the space behind/below the engine and feed it with a duct up to the prop.
> >>>
> >>> Charlie
> >>>
> >>> On 11/20/2020 6:59 PM, Bruce Cosgrove bcosgrove@mac.com wrote:
> >>>> Hi Steve, Charlie, Matt, Roy and others
> >>>> I am in the process making up a cooling system for a Tango 2 and was also planning to use the wedge shaped diffuser design, however the information that I have been reading from the homebuilt airplanes site discussing alternate approaches with less drag and improved efficiency? has me re thinking my design towards a belly mounted radiator. The comments under the heading Successful Auto Conversion Trials, Tribulations and Tips, I found particularly interesting - in particular input from Russell and RJ6ejGUY who both have devised systems that are working well for them Belly radiators may not be everyones idea of how to proceed but I like the idea of relatively unimpeded air flow and freeing up space in the cowling. any feedback would be much appreciated.
> >>>>
> >>>> Bruce
> >>>>
> >>>> On 21 November 2020 at 10:00:57, Stephen Izett stephen.izett@gmail.com (flyrotary@lancaironline.net) wrote:
> >>>>
> >>>>> Nice work Charlie.
> >>>>>
> >>>>> Steve
> >>>>>
> >>>>> > On 21 Nov 2020, at 6:50 am, Charlie England ceengland7@gmail.com <flyrotary@lancaironline.net> wrote:
> >>>>> >
> >>>>> > My (untested in combat) version for my radiator looks similar, but because the inlet is offset, I had to use a turning vane to get flow into the lower front of the core. The outside doesn't give a good idea on how 'tight' the inner surface is. I had to use expanding foam to fill the inside, then re-shape, then glass over the foam. As I said, the back 1/4-1/3 of the diffuser tapers tightly to almost touch the core at the back.
> >>>>> >
> >>>>> > After observing both rotary and piston installations that work, I'm beginning to think that turning vanes may be the 'good enough' vs fighting for perfection in diffuser shape. My oil cooler core has a much more elaborate vane setup, driven by the very short duct length and even greater offset to the bottom edge of the core.
> >>>>> >
> >>>>> > Charlie
> >>>>> >
> >>>>> >
> >>>>> > On 11/20/2020 4:22 PM, Stephen Izett stephen.izett@gmail.com wrote:
> >>>>> >> Hi Roy
> >>>>> >>
> >>>>> >> Here is a photo of our Mocal 44 row oil cooler with diffuser. A K&M Streamline into a wedge.
> >>>>> >> Not sure of the dimensions of hand. The Mocal web site will have specs so you could get a rough idea of size.
> >>>>> >> I tested this one recently with a leaf blower approximating 100 knot air flow and in that test it was very even.
> >>>>> >>
> >>>>> >> Having said that, I tested our water radiator wedge diffuser when I built it years ago and had it reasonably even
> >>>>> >> measuring pressures across the exit face, but now instrumenting it in flight its flow is vastly uneven.
> >>>>> >>
> >>>>> >> I have found that the flow is dependant not only on getting the diffuser shape right according to the various papers (eg. K&W, London, Oblique Flow Diffusers - Streamline )
> >>>>> >> But importantly the environment it ends up in. The papers share data for ideal conditions with for example unobstructed exit air paths.
> >>>>> >> My thoughts now are that this has a significant impact on the performance and therefore the advantage of instrumenting in flight and reshaping for actual conditions.
> >>>>> >>
> >>>>> >> The data I collected yesterday of our H2O diffuser reveals that the air flow in various points across the face are very different.
> >>>>> >> Simplified, if I average the pressures at the five points I measured, then comparatively the back is up 20%, the centre up 25% and the front down 60% from that average figure.
> >>>>> >> I’m figuring a 20% difference is probably OK, but my front to back/middle difference of ~80% down in the front top of that radiator may hold significant room for improvement.
> >>>>> >> Further, my data suggests that as IAS rises those differences are amplified.
> >>>>> >> So I am now targeting the modifications for our worst case conditions of WOT sea level ~200hp and initial climb numbers of 115KIAS.
> >>>>> >>
> >>>>> >> Cheers.
> >>>>> >>
> >>>>> >> Steve Izett
> >>>>> >>
> >>>>> >>
> >>>>> >>
> >>>>> >>> On 21 Nov 2020, at 3:37 am, ROY GLENN rglenn14@sbcglobal.net <flyrotary@lancaironline.net> wrote:
> >>>>> >>>
> >>>>> >>> Could some ne post a photo of their wedge shape and dimensions.
> >>>>> >>> Thanks Roy
>
>
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