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Pressure ratio boost creep.
Ok, here is what I find. The absolute
pressure ratio with boost is given by (Pa+Pb)/Pa where Pa is ambient
pressure (atmospheric) and Pb is the differential boost pressure (what we
normally see on a boost gauge).
So lets take a nominal boost figure of 7.5
psi. At sea level we have (14.7+7.5)/14.7 = 1.51. First
looking at where sea level take off would put john on the compressor chart
Eric provided. I believe John's take off rpm is around 4000
rpm. That would give a mass flow of
approx 18 lbm/min. So that point is plotted on the
compressor map as the intersection of the dashed green lines and the solid
green. I also plotted his mass flow at 5000 rpm at sea level at 7.5 boost
just so there would be a similar point of comparison at altitude, that is the
intersection of the two solid green lines..
Now John was at 11,000 feet so looking at
altitude charts it looks like Pa at that altitude would be about 9 psi. So
if one was still maintaining a differential boost gauge reading of 7.5 psi we
would have (9+7.5)/9 = 1.83 pressure ratio.
I don't recall John's rpm but I think it was
5000rpm. That would give a compressor mass flow of 27.3 lbm at sea level -
that is the intersection of the two red lines - However, that mass
flow figured needs to be adjusted for less dense air at 11000
feet that would bring it down to approx 22.7 lbm/min flow. That is the
intersection of the red and blue line.
As best I could eye ball it, that shows the
rotating assembly increased rpm by approx 12,000-15000 over the sea level rpm
just due to altitude pressure ratio creep along. The turbo had to spin
fast/work harder to maintain the 7.5 psi boost at altitude due to the lower
ambient pressure/density.
So clearly the effect of "altitude creep"
on pressure ratio (provide same boost level is maintained from sea to
altitude) is to cause the turbo to rotate faster. But, this
along does not appear to put it in a risk zone of overspeeding.
Would a BOV opening and possible adding to the rotation speed take it there
- don't know, can not show that it would or would not.
Do notice however that the rpm lines of the turbo
tend to curve very steeply downward toward the right part of the graph. So
it would not take much more mass flow to put John's operating point over
further to the right into that region where the rpm increases very dramatically
for very little additional mass flow. If Eric's postulation that a BOV
opening adds to the mass flow through the compressor is correct then would the
BOV opening under those conditions be enough to push the turbo into a danger
zone??
Don't know can show it would or would not.
But, this makes me lean more toward over speeding as a possible source of John's
turbo failures. IF this analysis is anywhere close to portraying what may
have happened, I would think it would indicate that a waste gate is a
lesser risk approach to boost control for an aircraft installation
FWIW.
Ed
Ed Anderson
RV-6A N494BW Rotary
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Matthews, NC
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