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Tracy,
I'm no expert, but I do follow cooling issues with
great interest and keep notes on previous attempts and successful outcomes. From
these notes I conclude that with different core fin densities and size one
core likely to steal from the other core from the available air available.
As you described in your own trials, so it's logical to have separate air for
each core.
It's also logical to know that higher deltaT in air
cooled engines require smaller inlets - trade off is larger tolerances
etc.
I'm at a loss to understand similar exit air with
the larger delta T, given similar conditions. Seems unlikely to
me.
Finally 2 x times the cooling of air cooled engines
does seem a rather large difference, unless we are talking 1.5 cu" times
the HP and 3 cu"times the HP . I do believe even 1.5 is a very low number. So I will settle for 2
times the HP for air cooled and perhaps with exceptional efficiency 2.5 times
the hp for water cooled.
I believe that anything under that limit will
result in marginal performance in high HP and exceptionally hot days. As Lynn
said there is no such thing as excess cooling and that's true, however the
penalty is excess drag. So I'm happy to be given new 'rules of
thumb' in the event your results are different. Being conservative I will
probably stick to the '3 times' rule of thumb - but I in no way put a
mocker on your attempts nor ridicule your endeavours.
George (down under)
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 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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