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----- Original Message -----
Sent: Wednesday, January 03, 2007 10:55 AM
Subject: Re: EM2 Data Logger pix
Monty,
I don't understand
what you are saying/implying about the "large diameter rad hoses" ("The reason
for cars requiring large diameter rad hoses is now less muddy in my muddy
fuddled brain."). I'm planning to run autosized rad hose and split into two
parallel smaller hoses into two evap cores, with everything (inside area of
fittings) sized to be equal to the auto hose inside area. I'm definitely NOT
going to run the smaller hoses and fittings used by most folks so far. Just goes
against my sense of "basics".
David Carter
David,
I am simply saying that in my opinion it is not necessary to
use the large auto sized hoses in our application. They are an automotive driven
requirement. The biggest resistance in the system is the cooler or the
engine block. The hoses are not the driver, unless you go to some ridiculously
small size. Take a look at an F1 car sometime, they use small hoses. The reason
for the large hoses in a street car is to cool a heat soaked engine at a stop
light when the water pump is barely turning over and there is no flow or head
pressure from the pump.
Think of a full throttle blast onto the freeway with an already hot
motor. Then you get stopped in traffic. The flow from the pump goes to nil. The
head pressure from the pump goes to nil. The heat flux from that full throttle
blast is just making its way to the coolant. At this point you need all the help
you can get. You need a big hose!
This does not happen in an airplane unless you abort a takeoff run.
Do whatever you think is best, that is why this is called experimental
aviation. ;-). It certainly won't hurt anything to use larger hoses. It is just
a little extra weight. It gives you a little more coolant volume and more
thermal mass. I just don't think that it is necessary to do so.
This data does not show that there is anything
"wrong". The steady state data shows that the cooling effectiveness is VERY
good. The air exiting the radiator is close to the exit temp of the engine
coolant after the radiator. In fact it is within about 10 deg or so!! That
means the cooler is VERY effective. The airplane is not swallowing a huge
amount of drag producing air to cool the engine.
If you use a great big thin radiator with a small
air delta T you have to swallow a lot more air which equals drag.
It also shows that there is quite a bit of thermal
mass in the system. That gives you a margin of safety. Nothing is instant. The
argument that you need some huge radiator always neglects this fact. If you use
the old NACA criteria for a fully laden air cooled bomber taking off in North
Africa, you will wind up with a huge radiator. Air cooled engines are not the
same as liquid cooled engines and we are not flying bombers in North Africa.
There is a lot more thermal inertia in our system than an air cooled engine.
Tracy had a climb duration of almost a minute. It was another 20 sec or so after
he chopped the throttle and the rpm, coolant flow, and TAS went DOWN until
the temps peaked. If you can't get over your obstacle and level off a little and
up the speed in one min, you need to stop trying to fly over Mt McKinley at
Vx.
It takes time for the heat flux to make its way
through the metal to the coolant. Then to the radiator, and finally to the air.
When the rpm, coolant flow, airspeed are all changing, you cannot make
comparisons between the various temps. It is not a valid comparison. Too many
interactions and thermal resistances to deal with.
There is also no way to know what is going on with
any kind of boiling. To do that you need a thermocouple on the surface of the
metal in the coolant passage. Information that we don't have. If you saw a jump
in the water pressure NOT related to rpm change you could surmise the
existence of some type of boiling. But to be sure you need a TC on the wall of
the coolant passage.
Monty
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