|
Hi! Guys
I' m showing my ignorance but you all lost me in the cooling
discussion. Correct me if I'm wrong , If coolant temperature on an RV on
climb out at 100mph full throttle stay within limits wouldn't a slow
airplane with a top speed of say 125MPH be able to use the same cooling
system set up? or those the fact that an RV has less drag come into
it?
Georges Boucher ( notice " No fiddly things" at the bottom of the
page)
-------Original
Message-------
Date: 04/05/05
07:34:41
Subject: [FlyRotary]
Re: A lot to learn
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
---------------------------------------------
>> Homepage: http://www.flyrotary.com/
>> Archive: http://lancaironline.net/lists/flyrotary/List.html
|