I think it is a
mystery, and not heating by the pump – for two reasons: 1) The heat load added
by the pump has to VERY small compared to the heat load coming from the
engine, 2) The pressure the pump is working against is basically the same as
determined primarily by the pressure regulator valve (as confirmed by his
data), and the flow is roughly the same because it is positive displacement
pump.
Al
-----Original
Message-----
From:
Rotary motors in
aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Bill Schertz
Sent: Monday, September
27, 2004 8:33
AM
To: Rotary motors in
aircraft
Subject: [FlyRotary] Re: Oil
viscosity
Some thoughts on Rusty's improved cooling
with the lower viscosity oil. If you recall, there was an earlier
discussion about drilling out the oil passage in the iron end plate and taking
the oil from there. I believe Ken Welter did this, and obtained about a (15
degree?) improvement in oil temperature. The only way this effect could be
possible is that the pump is working less hard, and there is therefore less
heat being generated in the oil by the action of the
pump.
This could relate to Rusty's experience,
namely a lower viscosity oil would require less power input to pump, and the
heating due to the pump would be less. Add to this the beneficial effect of
lower viscosity improving the heat rejection in the heat exchanger, and I can
believe the improvement is related to viscosity
change.
There is an interesting NACA paper
"Cylinder-Temperature Correlation of a single cylinder liquid cooled engine"
published in 1946 where they studied the effects of flow rate and coolant
properties on cooling effectiveness. The correlation of lots of data where
they varied the ethylene glycol concentration from 0 to 97% allowed them to
determine the relative effects of heat capacity, thermal conductivity, and
viscosity of the cooling medium. They found that high specific heat,
high thermal conductivity, and low viscosity are desirable characteristics of
a good cooling fluid. In order of importance,
Thermal conducivity (k) > heat
capacity (Cp) > viscosity (v).
Rusty is not changing k or Cp with the
lighter weight oil, and the viscosity change only affects the heat transfer
effect slightly, so I am left with the conclusion that it is the decrease in
absorbed work that makes the biggest difference.
Bill Schertz
KIS Cruiser #
4045
----- Original Message
-----
From: Russell Duffy
[mailto:13brv3@bellsouth.net]
Sent: 9/26/2004 4:39:16
PM
To:
flyrotary@lancaironline.net
Subject: [FlyRotary] Re:
Oil viscosity
5W30 seems a bit low.
>
>
I was
thinking the same thing. Leon, will an
extremely light oil be able to keep
the seperation
between the bearings and shaft? On the other hand,
will
it 'wet' the walls of the cooler better and
thereby transfer heat more
efficiently?
Since
when it 30 weight oil is "extremely light"? You might call it extremely
standard, or extremely normal :-) As I understand it, the first number is the
cold rating, which is just for cold starting, so it doesn't have any effect on
normal operation.
BTW, I
made about 3 trips around the airport (on the ground of course), and just made
it to 140 degrees. I'm convinced it's cooling much better with the lighter
(though not extremely light ) oil. Can't wait to fly it, but next Sunday is
the earliest possible opportunity, and probably more like the next weekend.
Rusty
(Airbike project for sale cheap on eBay now, gotta make room in the
garage)