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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
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