Scott Speirer wrote:
Paul, Mike, I don't know either if Charles did. If he did, I missed them. I've
attached the dialogue again below the dashed line.
Charles, please have a look at my questions if you have the time.
Thanks,
Scott
charles.harbert wrote:
Scott, I'm using a combination of a manual and automatic control over my
wastegate to control boost at high altitudes. I haven't flown the plane higher
than 4K' yet, but it was designed to operate effectively abv 15K'. All AC
turbo's are much larger than ground based turbo's, so that they don't overspd
to
destruction (per HMac AC section) by going over roughly a 3:1 pressure ratio.
Normal wastegate control is a feedback system that opens the waste when the
compressor (intake) side reaches a predetermined spring pressure (i.e.3-20
psi).
What do you mean by "3-20 psi"? Do you mean, "in the range 3 psi to 20 psi", or do
you mean, "a compression ratio of 3:1, or in other words, 20 psi"?
Yes, I've read about the overspeeding danger. It's interesting to note from
"Turbochargers", High Altitude chapter, that Hugh claims that as density altitude
increases, the compression ratio will increase for a constant turbine speed. The
turbo in his example could safely produce a 3.8 or 4.0 compression ratio at 16,000'
DA
where as at sea-level it would have already been surging and/or overspeeding.
Another
thing I thought quite interesting was Hugh's note that the compressor map in his
example was for 60 F, and not the standard temperature at 16K' DA. Maybe most maps
are for STP ambient conditions. Look at the section in the chapter called
"Temperature Effect at Altitude".
Do you mind my asking what particular turbocharger have you chosen, and are you
using
a stock non-turboed rotary, or a stock turboed one? (I'm thinking this might make
my
job easier.) :)
The method used to increase the intake pressure, which you will need to do at
higher altitudes, is to increase the spring pressure, or you can manipulate the
feedback pressure to the waste so that it thinks there is less pressure in the
intake manifold (which is what I did). The line from the intake side goes thru
a
small pressure regulator that I can manually adjust (based upon an AC manifold
pressure guage in the panel). For example, if you want to maintain 3 psi of
boost at sea level (and at altitude) that is really roughly 18psia (absolute),
but when you go to 15K' where the outside air press is roughly half of sea
levels 15 psia, or 8 psia, then you'll need 10 psi from the turbo ( about 3
times the boost) to get back to 18 psia. I just reduce the air pressure going
back to the waste so that it thinks it only has 3 psi and it won't go abv this.
Yes, this makes sense. Is your panel mounted manifold pressure guage measure
absolute
pressure or pressure relative to ambient/static air? Also, I'm curious to ask what
particular model waste gate are you using?
The 2 critical items are that you know the compressor map and how much you can
boost (blw 3:1), and second that you remember to screw the pressure regulator
back out when you descend.
And I assume that by the simple act of descending in altitude, without changing any
other parameters - including throttle setting - that turbocharger will be
increasing
speed, the intake manifold pressure will be increasing, and your pressure gauge
will
show the increasing pressure. You'll then increase sensed pressure via the
pressure
regulator to prevent overboosting. (Thinking out loud here.)
I've used this system a couple times in low level
flight and it works. It's a pretty simple system which I think will not fail.
I'll let you know what I find when I take it to higher altitudes, hopefully in
the next 6 months. I'm sure there are other systems that are better, but this
is
how I did it.
This all sounds great. Thanks for the info. I look forward to reading more from
you
as your flight testing continues.
Scott
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