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Date: Sat, 18 Mar 2006 08:31:21 -0500
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From: "Bill&Sue" <5zq@cox.net>
References: <list-1037864@logan.com>
Subject: Re: [LML] Re: LNC2 cooling problem
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Jerry, I think that you missed Johannes point completely.  I believe that he 
was using the airspeed gauge to measure the differential PRESSURE above and 
below the engine, not the airspeed.

Bill Harrelson


> Gary/Johannes,
>
> I think that you are making a very common mistake in reversing what you 
> actually want.  Remember that Total Pressure is a constant, and:
>                        Total Pressure = P+1/2 x rho x Vsquared,
>
> where P = static pressure, rho = density and V = speed.
>
> The result is that static pressure increases when airspeed decreases, and 
> vice versa.
>
> So what you want is low airspeed on the forward face of the cooling fins, 
> and high airspeed behind them.  This way you have a positive pressure 
> differential which reduces cooling drag and improves the cooling.  This is 
> the opposite of what we intuitively feel, which is that high speed before 
> the cooling fins is desirable.  We generally reduce the speed by having 
> expansion room in the manifold, so that the air gets a chance to slow 
> down.  We can increase speed behind the fins by restricting the airflow 
> through a nozzle, but you do need some room for the air to stabilize 
> before restricting its outlet, and it is difficult to arrange in practice.
>
> EAA published a good explanation of this some time ago, and I posted it on 
> the List under "Engine, Prop, Cowling" in the Attachments section.  I just 
> checked and it is still there.
>
> By the way you have made a really good looking manifold.  Larry Henney did 
> something similar on his 360, and achieved excellent results.  I hope that 
> this helps.
>
> Jerry
>