Re: REFLECTOR: oil temp

[aegerter@dataway.ch (Simon Aegerter)]

>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