REFLECTOR: Wing-root cooler re-visited

[Al Gietzen]

I have mentioned here in the past that in hot weather I had marginal oil
cooling on my 20B rotary, and that I was somewhat disenchanted with the
wing-root cooling idea.  Well, if you do it right, it works very well.

 

At the time I did my installation, I assumed (based on input from Alan Shaw)
that the scoop and diffuser for the wing-root cooler was not critical
because the pressure difference from bottom to top of wing would result in
good flow.  That was not to be, and I quickly learned I needed an effective
scoop to make the large oil cooler required for the rotary work effectively.
Adding the scoop definitely helped, and later some attempt at shaping the
diffuser (the entry between the scoop and surface of the cooler) without
removing the wing improved it a bit more, and everything was fine except in
hot weather.

 

With coming of summer, I decided to install a compact oil/water heat
exchanger to drop the oil temps for extended climb.  At the same time, I
decided to redo the scoop and inlet to the wing-root radiator (still in its
original, inefficient configuration) to add capacity for the cooling system.
I installed a MoCal oil/water heat exchanger, which I calculated would
handle ¼ to 1/3 of the total oil heat load. (1st Photo)

 

Doing that installation required removing the engine; which then allowed
removing the wing-root radiator (without taking the wing off). After some
consideration, I decided to mold a new scoop and diffuser in accordance with
the K&W coordinates for optimum pressure recover (modified for the flow
directional change required); that I could install into the existing cavity.
In doing so, I could also allow for a boundary layer bleed so ingesting the
BL would not mess up the flow in the diffuser section. Oh, and BTW, prior to
all this I had made a BL rake (2nd photo) to measure the flow distribution
prior to the entrance.  That told me that removing about ¼” of BL would
allow reasonably uniform flow into the scoop.

 

The 3rd and 4th photos shows the shape of the new scoop/diffuser; and the BL
bleed which simply takes that portion of the flow to the upper ½” or so of
the radiator.

 

The results of the changes surprised me. The drop in the oil temp is about
what I expected (hoped for) and I can now continue extended climb at 1500+
fpm on an 85 to 90F day.  What surprised me is the dramatic improvement in
the effectiveness of the wing-root radiator.  Even with the added heat load
from the oil, the coolant temp stays below 190 during such a climb, and
likes to stay at about 140-145 at cruise.  

 

I had also installed an in-line 170F thermostat in the wing-root rad loop,
but I drilled some small holes to maintain some flow.  At some point I am
now going to replace the thermostat with one that only has the small air
bleed hole.

 

Al

 

 

 

 

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