I have posted here as it relates to the "red cube". A recent post by Lan Vinh Do (http://www.vansairforce.com/communit...d.php?t=149243
) showed he relocated his flow transducer to FWF downstream of the mechanical pump. This (along with other changes) stabilized his fuel pressures at altitude.
We have long talked about the fuel flow difference and the tiny pressure drop. While the pressure drop from end to end is relatively low, Dan has shown the flow area of the interior. A 3/8" fuel supply tube (.297" ID) at 12 GPH enters at ~11 in/sec, but accelerates to 49 in/sec in that small diameter. The Bernoulli calculator
shows that this is a 0.08 psi change in local pressure. Maybe enough to cause bubbles if it is near the vapor pressure due to temps already.
I had hoped this calculation would give some insight as to why it is likely to result in fuel pressure issues at altitude (boost pump off) when located prior to the mechanical pump, but with the corrected values, it is only a small benefit. At least, now (with bubbleheads assistance) we have quantified this parameter.
Edit: The values above have been corrected - I missed a critical division by 12in/ft in the velocity and input the wrong values. Thanks to bubblehead for checking and sending me a nice email.