-------------- Original message from "Ed Anderson" <eanderson@carolina.rr.com>: --------------


> 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"
>
>
> > 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