Rusty,
<... I figure that guys
can easily run 8 psi of boost on the street, so I should be able to do
the same in the air ...>
As you said, 8 psi is 16 in
Hg boost. If 18 k ft is the 500 mb level, that's 15 in Hg.
You could normalize to 30" with just 15" boost, and you can't file for
FL 180 so you're at 17k ft. Now, is this 8 psi _all day long_ or
in spurts? If we assume that it's loooong spurts, we might infer
that 12"-14" is available all day long. That will normalize to 30"
at 15k ft to 16 k ft. That will work fine for me. On a 60 F
day, the OAT at 16k ft is about 22 F; on an 80 F day, the OAT is maybe
42 F. These temps would tend to reduce intercool requirements and
cool the turbine which I assume (key word here) is the reason for limiting
boost to 8 psi. It might follow that one could link compressor outlet
temp to boost to get a map of conservative values for boost. Additionally,
I would be more comfortable if there was a knock sensor that would retard
timing at the onset of detonation.
<... just
how much normalization you really need ...>
I'd say somewhat less that
what we seem to have available. 30 in Hg at 15 k ft would give you
upwards of 200 kts IAS which is upwards of 250 TAS. At 250 kts and
2600 rpm you need the prop from hell. It would be FULLY stalled on
T/O (which would increase your ground roll to maybe 1500-2000ft ;o) and
would be an interesting design problem.
<... NA
Lycosaur can still produce 75% power up to 8k ft
....>
The 75% power at 8k ft convention
is on account of the 750 mb level is about 8k ft. That means, any
NA engine, balls to the wall, will produce about 75% of the power at 8k
ft that it does at SL. 75% power in this context is not a throttle
setting, it's ALL of what's available.
<...
If 6 psi is safe, then I'd be tempted to just set the relief valve there,
and try to adjust the throttle for 5 psi ...>
If 18k ft is 15 in Hg atmospheric,
then 15k ft is close to 18 in Hg. 6 psi or 12 in hg would therefore
normalize you to 15k ft. I think that's terrific. The same
6 psi would give you 42 in Hg on T/O which is waaaaay more than I would
ever deliberately deliver to the engine. For my own part, I can't
see myself operating at an airport that 32 to 34 in Hg wouldn't get me
off with room to spare. I've heard that Greg Richter's Cozy jumps
off in 1000' or so (but I don't know what his T/O MAP is.
Bottom line: By varying wastegate
at [about] WOT to regulate boost, I intuit that we can:
A. Turbo normalize to
15 k ft staying well within a 5 or 6 psi boost limit, and with manageable
(none to austere) intercooling.
B. Have all of those
benefits, if we use a stock turbo with no plumbing and dumb-as-a-stump
wastegate control, for about what we'd pay for a muffler that would deal
with the unobstructed exhaust ports of a turbo rotor housing.
Sounds pretty attractive to
me .... Jim S.
--
Jim Sower
Crossville, TN; Chapter 5
Long-EZ N83RT, Velocity N4095T
Russell Duffy wrote:
Of course you're right about residual boost
with wastegate open. My 30" take off would have to be done at partial
throttle. Am I correct in my inference that gradually closing the
wastegate as I go higher and higher will minimize intercooler requirements
and wear and tear on the turbo? I'm
counting on not needing an intercooler using this same logic, so I guess
we'll find out if my engine blows up. It would also seem like it
would save some wear on the turbo bearings by bypassing as much exhaust
as you can, but I'm not sure how significant this would be.
Additionally, I have put a 36" or so limit on MAP to safeguard the
engine. Does the turbo also have a boost limit? If the turbo
has, say, a 10" boost limit, then as ambient pressure dropped off, I would
reach my turbo normalize limit at about 20" ambient or around 10k'.
What would the boost limits be on the Mazda stock turbo? Are they
a factor in normalizing to 15k' - 18k'? Take
it away, Ed :-) Yes,
turbos have limits, but this is the part of the turbo class that I'm not
qualified to teach. As
I understand, each turbo unit has a "compressor map" that shows it's efficiency
range as a function of absolute pressure, and airflow. Absolute pressure
is basically the boost that the turbo produces above ambient pressure,
which is why I maintain that a traditional boost gauge is handy to have. My
(over)simplified way of looking at this is- I'm using the same turbo as
Mazda sized for this engine, so airflow, and boost needs to be kept in
the same range that's been proven to be durable in the cars.
The difference is the duration of the boost, and the extremely free intake
and exhaust systems that we have compared to a stock car, though many modified
cars are in about the same situation. I figure that guys can easily
run 8 psi of boost on the street, so I should be able to do the same in
the air. 8 psi gives me about 16 inches of mercury, which is 15-16
thousand feet of normalization. That's plenty for me. Another
thing to consider is just how much normalization you really need.
I don't think too many people are planning to cruise at full sea level
power. If you do, I hope you're flying a fuel tanker :-) Let's
just say that we'll use 75% like you would with a Lycosaur. A NA
Lycosaur can still produce 75% power up to 8k ft, so our turbo only
needs to make up the difference between 8k and whatever our max cruising
altitude is. Even at 18k ft, that's only 10k ft of normalization
for cruise power, or 5psi (roughly).
I really wish I knew how much
boost the 9.7 rotors would tolerate. If 6 psi is safe, then I'd be
tempted to just set the relief valve there, and try to adjust the throttle
for 5 psi. I suspect this will still be with a fully open wastegate.
Did I mention that I can't wait to actually try some of this stuff :-)Rusty
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