The absolute best possible
performance would be a system that used just enough air Mdot to
heat the cooling air to the same temperature as the water
radiator exit temp ( water going back to the engine from the radiator). This
condition is impossible to achieve in practice. So you try to get as close as
you can. How close you get is called the heat exchanger effectiveness. The
closer the air and water exit temps are, the better the
effectiveness.
Monty:
I’ll respectfully
disagree on this part. It may apply if you have unlimited dynamic
pressure to work with, because it doesn’t consider radiator thickness and
air-side pressure drop. We don’t have this case
Al
Have to vote for
Monty's position on this one. His statement (I assume
intentionally) did not specifically get into radiator thickness, dynamic
pressure, etc because he was trying to get down to the basic principal
involved. In this he is correct (IMO). All these other factors are
just that, factors. They do not change the basic principal.
Limited dynamic pressure only means that the radiator has to be designed to
work with what is available. The goal remains the
same.
Dynamic pressure
data was available from the EM2 data log that I posted but I had to eliminate
it and many others for the sake of readability. In this case, dynamic
pressure on the left rad inlet was slightly higher than the full
amount available at the current airspeed (I am assuming this is due to
the benefit of prop blast into the inlet). Note: Dynamic
pressure is not a standard feature of the EM2, mine is a custom
job. You can use the TAS readout for this function though. You
just have to reverse the calculation from pressure to airspeed or get a
conversion chart.
Really finding the
discussion on the EM2 data interesting. As is obvious, interpretation of
the data is as important as the data itself. And Rusty was right, having
this capability is a two edged sword, you can spend unlimited time analyzing
data and trying to optimize various things. Identifying the "low
hanging fruit" is absolutely key if you are to do anything other that
tweak on the airplane. When Paul Lamar came here and we did a bunch of
in-flight pressure measurements, a lot of important things came to light but
we disagreed completely on what they meant and how to address them.
(discussion on 'Cooling Study' on my website describes that
effort).
Paul has done a lot
to perpetuate the idea that my cooling system is "marginal at best". I
still can't figure out where he gets this from. You do not fly an
aircraft for 12 years and 1600+ hours in Florida, fly it to the desert
southwest over 10,000+ foot mountains and win air races with a "marginal"
cooling system. Much has been made of the 'spray bar' cooling system I
used when racing. Anyone familiar with air racing knows this is common
practice. It is not necessary for normal operation and if you installed
a cooling system that you could race without the spraybar, your top speed and
MPG would suffer significantly. I wish those who have criticized the
spraybar setup would enter a race like the Sun 100 and show me how
to do it right!
Monty is also
correct about the need for studying steady state conditions. You need to
do that and the transient stuff to get the whole picture. I posted a
transient snapshot because it illustrated a few points I was interested in and
the fact that a bunch of straight lines on a graph are much less visually
interesting.
Tracy