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Bill,
No doubt there are good arguments on both sides and the mission definitely dictates the design. There is no way I'd be willing to make a blanket statement on which is superior overall. They each have their strong and weak points. I just feel that the constantly repeated mantra that liquid cooling has less drag is not justified. I think it depends.
Part of the problem may be that to properly optimize the air cooled installation may give unacceptable field service. For example you may find that if you make the cooling fins small enough to take advantage of the extra temp you may have clogging problems from corrosion, or bugs, or dirt. I don't really know. The difference in air flow is on the order of 1.3-1.5 times greater (depends on assumptions) for liquid cooling vs. the "ideal" Air cooled solution. Now neither one is going to be ideal for all kinds of reasons. But even a 5% advantage is nothing to sneeze at.
So I'll just say it's going to be application specific. But people shouldn't expect a liquid cooled set up to be significantly better or even as good as an optimized air cooled solution strictly based on cooling drag. The ability to have higher power density may lead to a lighter solution for liquid cooling but I'm a skeptic. Higher power density usually means higher RPM and additional weight and complexity from a gearbox in our application. Complexity is definitely greater for LC. The ability to run LOP, and no shock cooling could tip the balance WRT fuel flow or life.
Looking at the aviation marketplace (albeit a heavily regulated and no doubt distorted one) air cooling wins.
Of course this is about piston engines not rotaries.
So with all of that arm waving I will now fly off back into the ether ;-)
Monty
Merry Christmas Everybody!!!!!!
----- Original Message ----- From: <wrjjrs@aol.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Thursday, December 24, 2009 5:31 PM
Subject: [FlyRotary] Re: Meredith Effect - Spitfire
Monte,
I believe that Tracy is correct if you want any kind of reasonable fuel flow. If WW 2 had gone on longer the. F8F might have been the most successful fighter. The big radials were the toughest engines flying. But the Merlin powered fighters were MUCH more efficient. If you want the range to fly to Berlin you don't choose a big radial. The mission decides the choice. The ability to add effective area to a water cooled engine does make it easier to cool if you work with the knowledge of the approximate volume needed to begin with. IMHO.
Bill Jepson
------Original Message------
From: MONTY ROBERTS
Sender: Rotary motors in aircraft
To: Rotary motors in aircraft
ReplyTo: Rotary motors in aircraft
Subject: [FlyRotary] Meredith Effect - Spitfire
Sent: Dec 24, 2009 2:17 PM
Tracy I also forgot to mention that you aren't quite right about the less surface area thing. Although less surface area is needed the, convection coefficient along with the surface temp dictates surface area required. That can be influenced by turbulence, fluid velocity, surface condition etc. The mass flow of air required is unfortunately directly governed by the temperature difference. No way around that one. It is simply a function of the specific heat of air, temp difference, and the amount of heat rejected. Monty ----- Original Message ----- From: Tracy Crook To: Rotary motors in aircraft Sent: Thursday, December 24, 2009 3:23 PM Subject: [FlyRotary] Re: Meredith Effect - Spitfire I'm wondering if that figure for airflow is true (2x airflow for water cooled vs air cooled). All the measurements I have seen (not many) indicate that the exit air temperature on a Lyc installation is not significantly different than on our water cooled engines. The total heat per HP is not that different so my assumption is that the CFM requirement is not much different. The only advantage the air cooled engine's higher Dt gives you is that it requires far fewer square inches of surface area to transfer a given number of BTU with a given number of CFM. Our advantage is that we can add surface area a LOT more easily than an air cooled can. You can only put so many fins on a cylinder head. But I may be missing something. Other thoughts? Tracy On Thu, Dec 24, 2009 at 3:38 PM, MONTY ROBERTS <montyr2157@windstream.net> wrote: Thomas, Though the Meredith effect is possible in theory if you actually run the numbers you find that the only time it would produce any thrust is at power levels in excess of 1000 hp and flight speeds over 400 mph. Even then the effect is very small and any gain you might get from it will be decimal dust compared to the drag from ingesting extra air. A liquid cooled engine will require ingesting roughly 2X the cooling air compared to an air cooled engine for the same power level. There is no
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