See, I told you this was the most miss-understood detail in water cooled
engines! : )
FWIW, here is my summary of the discussion so far:
Ed, you are being way too modest. Yours has been the most coherent
real world analysis so far while still tying it to the theoretical
groundwork. Your brief departure into the irrelevant
molecular velocity (for this discussion), was in response to someone
else, not your fault.
Jim, excellent point, we must separate the irrelevant and 'things we can't
do anything about' from the discussion about building a actual honest to god
flying machine.
Doug's 'rule of thumb' post was based on real physics but left out some
factors that made it not universally applicable. What works for an RV will
not necessarily work for a Pietenpole, even if they had the same size
engine. And while the military must design for "Std air + 40 deg F" (100
degrees), we have the advantage of not needing to design to that standard.
On those very hot days, I can accept the limitation of using a lower power
setting in climb in order to have other advantages. I'm not scrambling to
intercept MiGs at 40,000 ft. But Doug's point of using rules of thumb
(when accurate and applicable), is very true. Very few builders have the
time or inclination to fully absorb K & W (which he has done pretty
well).
Tracy
Ok, Jim
I agree that the theory part gets out of hand at
times (my fault). Theory only counts if it works in practice
{:>). However, I think these question naturally arise when we start
talking about some of this stuff - the old "how it do that".
The only part that really counted was
understanding what was necessary to keep air flow from separating from the
walls in a diffuser. If you eliminate that problem you have done
probably 90% of what you can do to achieve optimum diffuser performance (my
opinion of course). So you can have otherwise adequate core surface and
volume, but if you have a poor duct design with lots of flow separation and
eddies then your system may fail to adequately cool.
Ed A
Is it possible we're dismissing some important factors
getting a little out of our depth here? Dynamic pressure in the cores
and across the cores would seem to be so highly dependent on surface
friction and core density and passage size as to be impossible to estimate,
much less quantify accurately.
If the purpose of the plenum is
pressure recovery (converting dynamic pressure into static pressure) and
it's the static pressure drop that drives the mass of air through the
radiator core, why not just forget about the molecular, boundary layer and
core passage size considerations for the moment since we can't quantify any
of that anyway. As Ed has stated so many times in so many ways, a good
inlet/plenum design does a better job of converting dynamic pressure to
static pressure than a bad one, and he's found out pretty much what he has
to do to make a bad one good.
If we measure static pressure at
the forward and aft face of the radiator and we've got the pressure drop
across the core. Period. We know how close we are to Ed's
plenum. Then adapt the stuff that Ed has pioneered for us to make it
better An Airspeed indicator I find is handier and more accurate than
a water manometer. The Pitot connection on the upwind side and the
Static connection on the downwind side should give me upwards of 100, maybe
120 kias drop across the radiator at cruise. More is better. If
I don't have sufficient pressure drop across the radiator, I probably need
to improve my intake and plenum to get rid of the eddies Ed alludes
to. That is what I've got the most influence over. If I don't
get enough pressure recovery, I study Ed's findings and approach implement
them better.
I think all this molecular stuff is more
appropriate to the ACRE list where nothing ever really has to fly.
This list (to me) is the guys who actually FLY. A sound
qualitative analysis of the issues involved (which we already have)
will lead me to a workable solution. That is very nice since an
acceptably accurate quantitative analysis is not possible. To
that end (to coin a phrase) I don't have to know how it works or why it
works, I only have to know what I have to do to MAKE it work. And I
have been blessed that Ed has found out most of this.
Are we PVORT.
again? ... Jim S.
David Carter wrote:
----- Original Message -----
From: <jbker@juno.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Monday, April 04, 2005 7:07 AM
Subject: [FlyRotary] Re: A lot to learn ! Re: Cooling -Learned a lot
Charlie E wrote:
At the risk of embarrassing myself with a display of misunderstanding
the physics of it all, should your pressure sensors be measuring dynamic
pressure or static? Seems like I remember Tracy's measurement pics
having foam chunks over the pressure sensors to remove the dynamic
component of the pressure measurement. I couldn't remember if your setup
has that (& I really don't know if it should, either).
Charlie
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