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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 way to make up for that 200 degree extra temp differential you get from an air cooled engine. You can't fool Qdot = mdotCpDeltaT. Liquid cooling has numerous advantages. Drag reduction is not one of them. The "cooling thrust" Myth is a Myth.
At our speeds and power levels you will be wasting your time chasing Mr. Meredith. That does not mean you shouldn't do a good job on the diffuser and the nozzle to minimize drag, but you can forget about any thrust. This is true even if you dump all the heat from the exhaust into the exit air so don't bother. Just point the exhaust aft.
In a piston engine fighter there are tactical advantages to having a slender nose that you can see around. Liquid cooling allows this. It also allows greater power density in the engine because you can have heat transfer through sub cooled boiling at the hot spots in the cooling jacket. It also allows a lower frontal area from a drag standpoint, but you pay by having to reject heat at 200 deg or so less than an air-cooled engine. That is perhaps an acceptable trade off. In practice I am not sure that the big radial aircraft were not superior to the mustangs etc. There is more frontal area with a radial, but there is also more useable internal volume and the cooling drag is less. More internal fuel means more stores and more/larger ammo in the wings. Plus better resistance to battle damage. Sea Fury vs. Mustang? The Sky Raider was in use as late as Vietnam. Anyway it's all hangar flyin' at this point.
None of that applies to us. There are so many real world practical constraints (packaging, size, and weight) to a working cooling system that trying to get the "optimum" is just not feasible. This is doubly so since the gain you are going to be chasing does not exist. There is not enough heat rejection at our power levels, and there is not enough ram pressure to recover at our speeds.
Thermo is as Thermo does.
Monty
----- Original Message ----- From: "Thomas Mann" <tmann@n200lz.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Wednesday, December 23, 2009 11:55 AM
Subject: [FlyRotary] Re: Meredith Effect - Spitfire
Let's try that again
-----Original Message-----
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On
Behalf Of Thomas Mann
Sent: Wednesday, December 23, 2009 11:50 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Meredith Effect - Spitfire
I thought I would share this bit of info I ran into regarding the Meredith
Effect associated with the belly type scoop as was used on the P-51 and
Spitfire.
Enjoy.
T Mann
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