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Tracy, I'm not certain what the airspeed
probe in the cowling is really telling you , except that the velocity (dynamic
pressure) of the air stream as measured by the probe is near that of
the outside air.
It would seem to me that it depends on
how the airspeed indicator is oriented. If it is pointing into the
airstream coming through the cowl duct then the airspeed probe itself is a
stagnation point (regardless of what's happening around it). If on the
other hand, if the airspeed probe is oriented perpendicular to the airflow and
is being used more as a static port then I would agree that indicates the
pressure build up is close to that of a radiator core stagnation.
If we look at Q = Mv(Th-Ta)*cp. We know we
have higher temp cylinder heads with a Lycoming than with our
core. That being the case, it would seem that you might not need the
same mass flow to conduct away the same Q. However, while the
cylinder head temps are higher, the$64 question is whether the heated
departing air temp increased any more (or less) than it would through a
core. It seems pretty clear that regardless of whether cylinder head or
core that slowing the air down (within limits) provides more time for heat
transfer to the air and should result in better cooling in either case.
But, it may be that given the higher temps of the cylinder head that you
may not require as much pressure recovery to cool as with a core. Just
some speculative thinking.
New Hangar is due to arrive tomorrow!
Ed
----- Original Message -----
Sent: Saturday, March 11, 2006 10:14
AM
Subject: [FlyRotary] Re: NACA's, Cooling
and Sport Aviation Mag..
Just a comment on one often repeated point: "There is less pressure
differential [on an aircooled engine] than with a radiator". This
factor is a major one in the decisions/arguments made regarding cooling
of aircraft engines.
The problem is that I have seen absolutely no empirical evidence to
support it and some which refutes it. For example, some Lycoming
powered RV flyers locate a backup airspeed indicator pickup inside the
cooling plenum over the cooling fins. They report that it reads within a
few MPH of the primary ASI fed from the pitot tube. This indicates that
almost full dynamic pressure is being recovered from the airstream and that
pressure differential is at least as much as seen on radiator
installations.
Bottom line is that reduced backpressure is NOT one of the advantages of
an aircooled engine. At least that is the working premise I go on when
making cooling decisions on my airplane. If I'm wrong, I'd like to
know. Anyone have data supporting/refuting this?
Tracy
----- Original Message -----
Sent: Friday, March 10, 2006 1:25
AM
Subject: [FlyRotary] Re: NACA's,
Cooling and Sport Aviation Mag..
Group,
The problem with submerged inlets, and Buly is correct to mention that
means flush with the surface, is that they do not handle back pressure well.
Ed A posted the original NACA data and their conclusion was that submerged
inlets don't work well with RADIATORS. The comments PL has been making are
only to re-publish the data. If you do a smoke tunnel test on submerged
inlets you will find that once enough pressure is built up they will "flip"
and hardly take in any air at all. The actual NACA ducts also have the
carefully designed lips, or rounded edges to train the boundry layer into
the inlet. The full profile defined by the NACA is rarely used. Most of the
inlets we see are some attempt at looking like a NACA inlet, without the
trouble of actually BEING a NACA inlet. We used to call this "eyeball
engineering." Aircooled engines do work better with NACA inlets as there is
less pressure differential than with a radiator. This doesn't mean they will
never work, just that the NACA didn't recomend their use with a
radiator/heat exchanger.
Bill Jepson
In a message dated 3/9/2006 8:24:30 PM Pacific Standard Time,
atlasyts@bellsouth.net writes:
John, would
you stop calling it a NACA scoop. Remove the big raised
lip and
make it flat. Than come and report to us. Your inlet is half
submerged and half raised scoop. NACA is a flush with the
surface
SUBMERGED inlet.
Buly
On Mar 9, 2006, at
10:44 PM, John Slade wrote:
> Dave,
> My only cooling
intake is the plans Cozy IV NACA.
> Cooling has never been a
problem.
> Regards,
> John
>
> David Staten
wrote:
>> At the risk of invoking PL's name, anyone else read
this months
>> Sport Aviation mag from EAA, and notice an
article on cooling that
>> seems to indicate that NACA's
are acceptable and adequate for
>> aircraft cooling needs?
I have no idea regarding the authors
>> credentials, and I
no longer monitor PL's "newsletter".. I was
>> curious
more than anything else... Pauls reaction, others
>>
reactions, etc.
>>
>> Translation.. yes.. I'm stirring
the pot/Trolling... I figure if
>> we are using NACA's on
the Velocity, that makes us somewhat of a
>> NACA
supporter..
>>
>> Dave
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