----- Original Message -----

From: Ed Anderson

To: Rotary motors in aircraft

Sent: Sunday, April 03, 2005 2:15 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
>
>
>




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