From:
Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Al Gietzen
Sent: Sunday, August 06, 2006 6:45
PM
To: Rotary motors in aircraft
Subject: ***SPAM*** [FlyRotary] First
flight - Oil temp
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 J.
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 Ľ” wide, 22” long and 3 Ľ”
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 ˝” 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