|
|
I wonder - is it not more correct to say: Behind the diffuser, the velocity
will be slower, density higher, therefore there will be "nearly the same
mass flow of air" thru the radiator, with same cooling, BUT "at slower
speed, higher air density, and therefore less cooling drag"?
- Drag is a function of velocity squared. The air density factor is
not squared, thus we seek a reduction of drag by cutting velocity thru the
rad.
Bernie, you are the SR-71 PW engine air duct man - am I even close to
expressing any useful and true info above?
David Carter
----- Original Message -----
From: "Ed Anderson" <eanderson@carolina.rr.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Sunday, April 03, 2005 4:15 PM
Subject: [FlyRotary] Re: Cooling -Learned a lot
You are absolutely correct, Tracy.
I did not make it clear but the diffuser does the velocity reduction and
increases the pressure in front of the core by recovery of (some) dynamic
pressure component of the air flow. This higher pressure in front of the
core then results in an increased pressure differential across the core.
This increase in pressure differential across the core, as you stated,
actually speeds up the air flow through the core itself.
My apologies for being less than careful on that point.,
Ed A.
----- Original Message -----
From: Tracy Crook
To: Rotary motors in aircraft
Sent: Sunday, April 03, 2005 12:24 PM
Subject: [FlyRotary] Re: Cooling -Learned a lot
Excellent summary Ed, correlates with my experience as well. Only
exception I would take is in the following excerpt:
"A good diffuser will reduce airflow
velocity through the core which will reduces cooling drag. Pressure
across
the core is increased which further enhances cooling."
A good diffuser will reduce velocity but the reduction occurs IN the
diffuser, not through the core. As counter-intuitive as it may sound, the
velocity through the core is HIGHER than it would have been without the
diffuser's velocity decrease (and pressure increase).
Think about it this way, How could velocity through the core be reduced
by a pressure increase? It isn't. The velocity at this point (through the
core) is increased.
This is the single most misunderstood detail in liquid cooled engine
systems.
Tracy
Subject: [FlyRotary] Cooling -Learned a lot
Too right, Jerry
My first 40 hours or so were in the marginal cooling zone. {:>). As
other
things in this hobby, there are so many variables that interact, that
what
may appear simply at first, is almost always a bit more complex. I
say(Cooling Axiom 1) if you have enough cooling surface area and air
mass
flow then it WILL cool. However, you may incur a high penalty in
cooling
drag - which may not be as important for draggy airframes (such as
biplanes)
as it is to sleeker airframes. Also a system that adequately cools an
engine producing 150 HP may not cool an engine producing 180 HP.
Picking
your cooling design point is important. Optimizing for cruise and your
will
be less than optimum for take and climb. Optimize for climb and you
will
probably have more cooling drag than required at cruise. Compromise,
compromise - cowl flaps are sometimes used to try to have the best of
both
worlds.
Some folks advocate a thinner, larger surface area core -which is great
for
slow moving automobiles stuck in traffic with low dynamic pressure
potential, but I think is not the optimum for most aircraft. Once you
trip
the airflow and turn it turbulent you have incurred most of the drag
penalty. Larger surface area cores disrupt a larger airstream and incur
more drag. Yes, thicker cores produce a bit more drag than the SAME
frontal
area thinner cores. But, with a thicker core you can use a core with
smaller frontal area.
The NASCAR radiator's average 3" thick and on the long tracks where
speeds
are higher some even go up to 7" thick. My contention is their
operating
environment is more akin to ours than regular automobiles moving at
slower
speeds. You know that the NASCAR folks will spend $$ for just a tiny
advantage - so clearly they don't use thick cores because it is a
disadvantage. But, some folks will continue to point to the large thin
radiators designed for environments with much lower dynamic pressure as
being the way to go. Will it cool? sure it will (Cooling axiom 1
above).
Is it the lowest drag option for an aircraft of the RV/TailWind type, I
am
convinced it is not.
The diffuser makes a considerable amount of difference and can made the
difference between a system that cools adequately and one which does
not.
The biggest culprit that lessens cooling effectiveness is turbulent
eddies
that form inside the duct due to flow detachment from the walls. These
eddies in effect act to block effective airflow through part of the
core.
So keeping the airflow attached to the sides of the diffusers is crucial
for
good cooling from two standpoints. A good diffuser will reduce airflow
velocity through the core which will reduces cooling drag. Pressure
across
the core is increased which further enhances cooling.
I have gone from a total of 48 sq inches opening (total) for my two GM
cores
and that provided marginal cooling - down to 28 sq inches (total) with
adequate cooling with an engine now producing more HP. Experimenting
with
the diffuser shape made the difference.
The K&W book (Chapter 12) really provided the insight to how and which
diffuser shapes provided the better dynamic recovery. The Streamline
duct
was shown to be able to provide up to 82% recovery of the dynamic
pressure.
Some folks reading the chapter misinterpreted the chart to show only 42%
recovery where there chart was actually only showing the pressure
recovery
contribution due to the duct walls and did not include the contribution
due
to the core. On the same chart, an equation (which apparently gets
ignored)
clearly shows that the TOTAL pressure recovery is 82%.
I have taken the Streamline duct as a starting point, but since I do not
have the space to provide the 12-14" for a proper Streamline duct, I did
some "creative" things to try to insure that there was no separation
even
though my walls diverge more rapidly than the Streamline duct. Won't
claim
mine are as good as a Streamline, but they clearly are much better than
the
previous design which basically just captured the air and forced it
through
the cores.
FWIW
Ed Anderson
RV-6A N494BW 275 Rotary Hours (Plugs Up)
Matthews, NC
eanderson@carolina.rr.com
----- Original Message -----
From: "Jerry Hey" <jerryhey@earthlink.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Sunday, April 03, 2005 9:27 AM
Subject: [FlyRotary] Re: phase I flight restrictions was:N19VX flys
> It was not long ago that "cooling" was the major issue. Now it seems
> that we have learned enough to make several different configurations
> work. I can't lay my finger on what it is we have learned but my
> recommendation is to use smaller radiators and EWPs. Jerry
>
>
>
>> Homepage: http://www.flyrotary.com/
>> Archive: http://lancaironline.net/lists/flyrotary/List.html
|
|