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Hi Al,
I'm sitting here trying to compile a program that won't, so I might as
well give you my uninformed opinion. :)
I agree with you that (c) would be worth a try. It's sort of brute
force to get something to work, then try to clean it up later. I also
wonder if the exit scoop might not work better if the slope went up
more gradually, more like a triangle than a box. OK, I'll attach a
picture.
Bob W.
On Sun, 6 Aug 2006 18:45:05 -0700
"Al Gietzen" <ALVentures@cox.net> wrote:
> Thanks everyone for the congratulatory messages, and for the support that is
> always so helpful. No pics to post yet because my camera battery went dead
> after the first three shots, so I’m awaiting for shots from my friend who
> took hundreds (OK, only about 150) and will be editing for a while :-).
>
>
>
> The principal issue of the day was the higher than comfortable oil
> temperature; most likely due to insufficient air flow through the cooler.
> For anyone who would like to think aerodynamics for awhile and give an
> opinion on the simplest and best approach to remedy; read on.
>
>
>
> The custom cooler for this 265 hp engine is large. The core here is about 5
> ¼” wide, 22” long and 3 ¼” thick. It is located in the wing root of the
> Velocity, behind the spar, with inlet underneath and exit on the top. Alan
> Shaw, who I believe pioneered this approach, found the location worked very
> well. When I discussed the installation with him years ago, he opined that
> a scoop under the wing was probably not necessary because of a pressure
> differential between bottom and top surfaces. Since then, my investigations
> of pressure distributions, and similar installations that aren’t working so
> well, make me wonder.
>
>
>
> Photo 1 is a view under the wing showing the OC air intake, wheel well, and
> the big armpit scoop for the coolant radiator in the cowl. The inlet
> opening is about 1 1/8” wide and 23” long. There really isn’t a scoop, just
> an opening with an extended airfoil shaped lip which extends about ½” into
> the free stream. The idea was to minimize drag, and assume a more negative
> pressure at the exit would produce the necessary flow. Photo 2 shows a
> front view where you see the wheel well and the inlet – very little
> extension into the free stream. Analysis suggests that the turbulent
> boundary layer on a smooth surface at the inlet location could be about 5/8
> – 3/4” in thick.
>
>
>
> The air exit fairing is shown in photo 3; and is shaped as it is to maintain
> attached flow and cause minimal turbulence going aft. The effective exit
> area is about 1.6 times the inlet area. The thickness of the core suggests
> the need for pretty good pressure differential for adequate flow.
>
>
>
> Here are some options:
>
> a) For the first flight the landing gear was never retracted. Since the
> open wheel well forward of the inlet would likely cause significant
> turbulence; try another flight with the gear retracted to see if that
> improves the results.
>
> b) Place some VGs forward of the inlet to ‘energize’ the boundary layer,
> and see if that helps.
>
>
>
> c) Extend the ‘lip’ of the inlet to form a proper ram scoop, possible
> also with VGs forward to break up the boundary layer, and accept the slight
> increase in drag.
>
>
>
> d) Do something at the exit ( local ‘expert’ suggests there may be flow
> separation before the aft end of the fairing causing high pressure behind
> the exit). Put VGs on the top of the exit fairing and/or reduce exit area.
>
>
>
> e) None of the above.
>
>
>
> I suspect the normal aerodynamic pressure differential between the inlet and
> outlet points is minimal; especially in level flight where it could be near
> zero. Option c) seems the most sure-fire to me.
>
> Thanks for input.
>
>
>
> Al
>
>
>
>
--
http://www.bob-white.com
N93BD - Rotary Powered BD-4 (first engine start 1/7/06)
Custom Cables for your rotary installation -
http://www.roblinphoto.com/shop/
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