From: Fredmoreno@aol.com
Message-ID: <e87ddbbd.2500ad2e@aol.com>
Date: Fri, 3 Sep 1999 00:48:46 EDT
Subject: Ram air: the last frontier
To: lancair.list@olsusa.com
Mime-Version: 1.0

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Scott:

I figure 174 knots IAS should provide maximum ram pressure boost of a bit 
under 1.5 inches of Hg which is within the experimental error of your data.  
It shows that you have a good diffuser getting virtually all of what is 
available.  Some bugs that always pollute the data ointment:

1) NASA tests in the 80's measured profiles of total pressure behind a 
propeller at various radii.  These showed that one can get up to 20% more ram 
pressure than the free stream ram pressure if you chose you distance from the 
centerline correctly.  It depends on flight conditions, cowl configuration, 
etc.  So there is a bit more pressure to be had behind the prop at the right 
position. 

2) Keep in mind that the manifold pressure gage is attempting to read an 
average of a wildly pulsating instantaneous pressure within the intake 
manifold.  Depending on the details of the inlet from the intake manifold to 
the tube running to the manifold pressure gage (and resulting differences in 
inlet and outlet flow coefficients at the fitting location) the pressure read 
by the instrument can be above or below the average.  Usually it is a bit 
below because inlet coefficients are invariably lower than discharge 
coefficients (arising as the flow oscillates in and out of the tube going to 
the instrument).  It all goes to show that making accurate measurements is 
not as easy as might first seem. 

I would say your inlet system is working every bit as well as could be 
reasonably expected.  Declare victory and go flying!

Fred Moreno
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