----- Original Message -----
From: Finn Lassen <finnlassen@netzero.net>
Date: Tuesday, December 23, 2003 8:56 am
Subject: [FlyRotary] Re: dual EWP plumbing?

> Russell Duffy wrote:
>
> > Haven't I heard some of you say that you've had difficulty
> getting the
> > water to flow evenly though parallel cores?
>
> Yes, especially if they don't flow the same. I'm effectively only
> flying
> on one radiator (just use the other one for drag :).
>
> Finn
>
>
But that is with only one 'motivating force'.  Here's my jumbled thinking on the matter, derived from what I know of resistive electrical circuits.

With a single pump before the split, the water will mostly tend to go to the rad with the least resistance.  The total flow will be set by the pumps head vs the back pressure of the combined rads and the engine. For the purposes of the rads, the engine backpressure can be ignored since it'll be exactly the same for both.  The low volume rad's flow will be set by the backpressure created by the high volume one.  As the flow through the main one increases, it's backpressure increases;  however, radiators are designed for low backpressure, so the second one will never see much flow.

Plumbing the pumps after the split, each rad with have an independant push.  The flow  through each side will be determined by the pump's head pressure against the backpressure generated by the rad and the engine.  If one side is more efficient, it will have a higher share in causing the back pressure in the engine, but it would take an order of magnitude difference in the rad flow rate for one to significantly effect the other.

A couple batteries and a handful of resisters should illustrate the concepts convincingly. (Uh-oh! There's a project for my budding teenager!)

On another note, using the information provide by Ed and Al, my calculations indicate that only half the cooling is for 75% cruise at 180mph, compared to full power climb at 120mph.  Loosing one on climbout would simply cause some heat soaking till you get to cruise. I think this arrangement would be about as close to perfect as you could ever desire.