REFLECTOR: First flight - oil temps

[Al Gietzen]

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 Bill 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.

 

Rotary engines eliminate about 1/3 of the waste heat via the oil, so the
cooler for a 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 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

 

 

 

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