Re: REFLECTOR: oil temp

[john daniel cowan <jdcowan@mail.telis.org>]

the formula I have scribbled in one of my old textbooks (I can't find my
physics text right now) is Resistance =[(length)x(viscosity)x(8)]/[(radius
to the 4th power)x(pi)]  (The thick stuff we run through tubing at work
sometimes clots)

   It would seem the factors we can change the most are the length
and the radius of our oil lines.  I know it's all been hashed through
before, and that oil coolers in the wing roots are a good solution to length
and airflow problems, but has anyone come up with a solution to cabin
heating if the cooler is relocated from the nose?

   I also live a wee bit south of Death Valley and it reeeeally gets warm
here during parts of the year.  Are those little eyeball vents sitting just
over the keel keeping folks cool in warmer parts of the U.S.? anybody out
there in Phoenix or Mojave flying with this system?  I have dual 3"
snapvents in my currently flying BD-4, and when it's 115-120 degrees that
seems like barely enough.

   I have my worries about cooling a 300 hp lyc in an XL on climbout, and am
always listening for good/better solutions.  What I keep hearing is high oil
temps and people throttling back to keep cyl. head temps down.  Does that
fiberglass molded plenum system work well on 300 horses with downdraft air?
Has anyone put in an updraft system into place using the armpit scoops and
the usual aircraft baffling system? Alan?  how are you cooling you 300hp 173? 
Jean? how does yours do climbing out in really hot air temps? 
   


At 11:23 PM 1/29/99 +0100, you wrote:
>>So I thought now would be a good
>>time to replace the 1/2 inch oil lines. However, I  wonder why because the
>>entrance and exit holes from the by-pass valve have the same inside diameter.
>
>The amount of fluid of a certain viscosity that can be forced through a
>length of tubing with a certain pressure is roughly proportional to that
>pressure, inversly proportional to the length and proportional to the
>square (!) of the cross section area. That being proportional to the square
>of the diameter, the flow really is proportional to the forth power (the
>square of the square) of the diameter. (True, the engieers have a formula
>where all these exponents are weird decimal fractions like 3.765.. or
>something. That accounts for all kinds of real world spurious effects. We
>physicists live in an ideal world, where 4 is good enough).
>
>So-oo: increasing the diameter from 4 to 5 eighths increases the flow by
>the fourth power of 5/4, that is 625/256 which is 2.44. Let's say it
>doubbles the flow, all else being equal.
>
>The inner dia of fittings and the engine openings are not 5/8". Therefore
>the oil gas to flow a little faster in these places. That takes a little
>more of the total pressure available than before. So the pressure remaining
>to force the oil through the lines is a little less. So all else is not
>quite eaqual. The flow may be barely doubbled.
>
>You see, the openings in the valve do have an effect but not nearly as
>dramatic as you suspect.
>
>Why does this help with the cooling? Because twice the oil can transport
>twice the heat to the cooler.
>
>That makes sense only if the cooler is able to get rid of that heat. That
>means it has to be large enough and there has to be ample air flow. It
>seems that nobody has ever measured the airflow through our oil coolers.
>There is a lot of more or less educated guessing. The NACA scoop may not be
>shaped correctly. The porting of the air duct may not be ideal. The out
>port of the air duct may be too small and it may be in the wrong place.
>Nobody seems to know. I don't. I intend to do some real measurements as
>soon as I fly again. Brian Michalk may be quicker.
>
>Having said all that (and done all that) I have to caution you: In an ideal
>situation the oil cooler gets rid of 5% at most of the total energy that
>the engine produces. I have found that all measures that improved the
>airflow through the bottom of the engine compartment lowered the oil
>temperature more than anything I did to the oil cooler.
>
>>I have ram air going into the front of the ducts from tubes on the cabin NACA
>>duct with faired exits just before the firewall to cool the 1/2 lines
>
>That is probably going to help to get rid of more heat but I hope you don't
>fly too much in the LA basin or I wouldn't want to look inside your ducts
>in five years <g>.
>
>Best
>Simon
>
>Simon Aegerter, Winterthur, Switzerland
>
>
>
>