Re: Meridith Effect and Radials
From: Corky Scott (charles.k.scott_at_dddartmouth.edu.retro.com)
Date: 02/11/05
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To: sci-military-moderated@moderators.isc.org Date: Fri, 11 Feb 2005 12:33:51 -0500
On 9 Feb 2005 21:30:28 -0800, panzerboy@gmail.com (Jeremy Thomson)
wrote:
>The P51 was probably the most efficient fighter design of WWII to
>exploit the Meridith Effect. This is the thrust generated from the
>radiator acheived by varying the size of output duct to optimise the
>exit area with the speed through the air and temperature.
>Pix of F4U Corsairs seem to show 'frill' of doors behind the engine.
>I'd always assumed this was the exhaust system. But I guess the air
>cooling the engine has to be ejected somewhere, so maybe they are for
>that?
>Was this ring a simple full-open, half open system like the output of
>a Spitfires radiator?
>I think this most likely.
>Still It wouldnt be too hard to open/close these doors with an air
>pressure sensor and temp sensor like the Mustang.
>Assuming the F4U did vary the exit area of these spill doors could the
>Meridith effect have been increased by added the exhaust gasses too?
>I've havent heard of the F4U using a turbo supercharger so it would
>seem simple to combine the exhaust gases and cooling air.
>
>Maybe I've got it all wrong and this 'frill' behind the engine of the
>F4U was fixed?
The "frill" you are describing around the cowling of large radial
engined airplanes are called "cowl flaps". They are used to assist
the engine in cooling.
They are normally opened for ground operation and during climb. At
cruise they are normally shut to clean up the nacelle and reduce drag.
At that point the engine is throttled back for cruise and is producing
much less heat. In addition, cruise is an airspeed that is a lot
faster than climb so less flow through is required to cool the engine.
This has nothing to do with the Meridith Effect, it's a simple means
to increase airflow past the cylinders when they most need it.
The drag of open cowl flaps can be considerable. My father (Naval
aviator during WWII flying PB4Y-1's) told me he heard a group of Navy
fighter pilots talking about flying formation during long flights.
They said that they would set their engine and prop to the standard
cruise settings in general get themselves flying at the proper speed
to hold formation and then make minute changes in airspeed using the
cowl flaps. Gross changes of position required large power changes,
but to just adjust by a foot or two they could crack open the cowl
flaps and drift back into position, then close them again.
The Meridith Effect was about ducting in the freestream air to the
cooling duct and slowing it down by expanding the duct size gradually
so that the air slowed down enough to actually absorb some heat as it
passed through the various heat exchangers fighter used. The Mustang,
for instance, enclosed a coolant radiator and oil cooler together in
the same duct.
Meridith figured that heated air expanded. The air brought in to the
coolant duct was slowed down and heated. The heated air was now
directed through a narrowing exit flute during which the air
accelerated considerably. The heated and accelerated air was directed
out the rear of the airplane as thrust. Meridith postulated that
properly designed, such a system could greatly negate the drag of the
cooling system itself, thus increasing the overall speed of the
airplane and it's efficiency.
There have been some exaggerated claims about just how much thrust the
Mustang's system produced, but those who designed it say that at no
point did the thrust produced actually totally negate the drag of the
system, but it came pretty close.
The maximum thrust came at relatively high altitude and engine power.
Corky Scott
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