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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 -----
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 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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