Sender: <marv@lancaironline.net> (Marvin Kaye)
To: flyrotary
Date: Mon, 24 Mar 2003 18:18:56 -0500
Message-ID: <redirect-2085146@logan.com>
From: <rogersda@cox.net>
Subject: Re: [FlyRotary] Thick or Thin? 
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Hi All,

   Would someone please help me better understand the physics involved with this?  

   It appears to me that, especially given the airspeeds we're talking about, as long as there is significant temperature differential between the air and the tube/fin at the trailing end of the air path, the radiator is working.
Isn't it the case that our biggest concerns with thick cores are (1) saturation of the airstream, (2) weight per transferred BTU, (3) drag?

   So, for a given transfer rate, one might expect a thick core radiatior to be heavier than a thin one, but offer less drag.  If the load due to additional weight is less than the drag of the lighter thin core, aren't we ahead?

Dale

> David Leonard wrote:
> 
> > More air CAN cause less drag if it is going slower. Again the argument
> > lies in the fact that drag is a function of velocity CUBED (not squared
> > as I previously stated - I think maybe laminar flow is velocity squared)
> > but mass of air to only the first power.  A quick search found this
> > classic discussion of parasitic vs. induced drag which illustrates the
> > velocity cubed point:
> > http://www.paraborne.com/how_airplanes_fly_3.html
> >
> > Drop the velocity of the air, the drag drops by a 3-fold factor.
> > Increase the mass, and drag only increases by a one-fold factor.
> > More air has the added benefit of not requiring each packet to gain as
> > much heat, allowing an overall higher temperature gradient.
> 
> Tracy replies:
> 
>  OK here is the guts of what I blathered on for two hours about:
> 
> You are correct about the importance of the velocity of air in the drag
> equation.  But the air velocity through the rad is of no (or very minor)
> importance.
> 
> The velocity that matters is the velocity of the air at it's *source*.
> That, of course, is the speed of the air rushing past the airplane that we
> snagged to run through our cooling system.  Since this speed is the same
> regardless of what radiator is sitting inside the cowl, the *only* factor
> that makes any difference is how much of that air we used.  More air = more
> energy = more drag.